Antarctic Ice Mass Controversies

Like many others, I was interested in the recent controversy arising from findings of Zwally et al 2015 that there had been ice mass gain gain of ~112±61 Gt/year over 1992-2001 and ~82±25 Gt/year over 2003-2008.  Zwally’s findings obviously contradict a widely held contrary belief, expressed, for example, in IPCC AR5’s assertion there was “high confidence” that the Antarctic Ice Sheet had been losing mass for the prior two decades and that the rate of loss had “likely increased” to ~147±75 GT/year over 2002-2011 or in NASA’s widely cited statement that “the continent of Antarctica has been losing about 134 billion metric tons of ice per year since 2002”.

I had no prior interest in the literature, but was intrigued by the dramatic contrast between Zwally and IPCC on such a widely covered topic.  This quickly led into a voluminous technical literature, which is the subject of today’s post.   The issues were not only about interpretation of satellite data, but quickly led into thorny interpretations of the history of the entire Holocene.

Warning: the following post is very lengthy, but I think that the details are worth paying attention to.

An important and extremely interesting subplot to the Zwally controversy is the remarkable difference between trends in East Antarctica and West Antarctica (plus Antarctica Peninsula).   Most of Antarctica (East Antarctica, which makes up over 80% of the area, in particular) has been gaining ice mass.  On the other hand, there is quite dramatic ice mass loss on the Antarctic Peninsula and localized (but highly publicized) areas of West Antarctica, especially the Pine Island and Thwaites glaciers,  the solid blue areas to the bottom left of Zwally et al 2015 Figure 6b shown below.

zwally 2015 fig 2b

Figure 1.  Zwally et al 2015 Figure 6b, showing IceSat dH/dt for 2003-2008.

In my opinion, the most (and only) convincing explanation for this peculiar localization takes one back to the Last Glacial Maximum and changes throughout the Holocene – interesting paleoclimate topics.

Understanding the controversy also leads quickly into complicated problems of glacial isostatic adjustments to gravity surveys (GRACE), as the size of these adjustments turns out to be more or less equivalent to the size of the mass loss itself – hardly a desirable property of the method.  Over the past decade, the size of generally accepted glacial isostatic adjustments has fallen quite dramatically, with estimates of mass loss falling in conjunction.   In retrospect, IPCC AR5 can be seen to have adopted mass loss estimates that were far larger than up-to-date technical literature.

At the end of the day, it is up to specialists to determine whether ice mass gain in East Antarctica exceeds the large ice mass losses in localized areas of West Antarctica or not, but the inconsistency between the two areas is something that ought to interest non-specialist readers.  Behind that topic,  I think that readers ought to wonder what proportion of the large but localized ice mass loss in West Antarctica is a present manifestation of a long-term Holocene pattern, a phenomenon that is attractive to alarmists but one that will continue to take place regardless of climate policy.


AR5 took a far more aggressive line on Antarctic ice mass loss than AR4, which, in retrospect, was rather cautious.   AR4 reported that there had been attempts to measure Antarctic ice mass balance using a variety of techniques: radar and laser altimetry,  the GRACE gravity surveys and input-output balances (using climate models to estimate accumulations and satellite data to measure glacier flow over their grounding lines. In its summary, it was unable to reach a conclusion as to whether the mass balance was positive or negative:

Assessment of the data and techniques suggests overall Antarctic Ice Sheet mass balance ranging from growth of 50 Gt yr-1 to shrinkage of 200 Gt yr-1 from 1993 to 2003.

AR4 was considerably more candid than AR5 about the continuing impact of the end of the last Ice Age.  In a section entitled “ Ongoing Dynamic Ice Sheet Response to Past Forcing”, AR4 contained an interesting comment that “retreat of the West Antarctic grounding line in response to the end of the last ice age” was estimated to contribute “about 90 Gt yr-1” to ice mass loss:

Because some portions of ice sheets respond only slowly to climate changes (decades to thousands of years or longer), past forcing may be influencing ongoing changes (Box 4.1). Some geologic data support recent and perhaps ongoing antarctic mass loss (e. g., Stone et al., 2003). A comprehensive attempt to discern such long-term trends contributing to recently measured imbalances was made by Huybrechts (2002) and Huybrechts et al. (2004).[1] They found little long-term trend in volume of the Greenland Ice Sheet, but a trend in antarctic shrinkage of about 90 Gt yr-1, primarily because of retreat of the West Antarctic grounding line in response to the end of the last ice age.

This is not an incidental number as it is almost exactly equal to the total Antarctic estimate in AR5.  However, discussion of post LGM long-term effects over the Holocene vanished entirely in the AR5 (chapter 4) discussion of the cryosphere, which used the word “Holocene” only once in passing.

AR4 to 2011

Subsequent to AR4,  interest in Antarctica mass loss increased tremendously, not least because of the availability of new satellite data measuring gravity, altimetry and grounding lines.  The GRACE gravity surveys had become available in mid-2002 and have continued to the present. The high resolution IceSat laser altimetry satellite became available in 2003 and operated until 2009, but, prior to Zwally et al 2015, does not appear to have resulted in a technically published estimate of Antarctic mass loss (though estimates were noted in the Supplementary Information of Shepherd et al 2012, as discussed below).

In contrast, between 2006 and 2011, there were numerous articles estimating Antarctic mass loss using GRACE data. Articles in this period used “early” models for glacial isostatic adjustments (Peltier 2004; Ivins and James 2005), estimates which were dramatically reduced in 2011-12.  Although not evident in most GRACE articles, the size of the glacial isostatic adjustment was the same order of magnitude as the ice mass loss itself (and even larger).  Velicogna and Wahr 2006 (somewhat anomalously) reported both items in their calculation, showing that the “uncorrected” GRACE trend was actually positive and that the estimated mass loss of 152 km3/year (~139 Gt/year) arose from the glacial isostatic adjustment of 192 km3/year (~176 Gt/year):

We subtracted this PGR [post-glacial rebound: the viscoelastic response of the solid Earth to glacial unloading over the past several thousand years] contribution from the GRACE-minus-leakage ice mass estimates … The PGR contribution (192 ± 79 km3/year) is much larger than the uncorrected GRACE trend (39 ± 14 km3/year)…  The best-fitting linear trend, and our final estimate of the decrease in total Antarctic mass between the summers of 2002 and 2005, is 152 ± 80 km3/year

Velicogna and Wahr 2006, almost uniquely, contained a figure (excerpted below) showing the difference between unadjusted and adjusted mass in the figure shown below. Before glaciostatic adjustment, the GRACE data had a slight positive trend (blue circles); after glaciostatic adjustment, there was a dramatic decline (red-orange crosses):

velicogna wahr 2006 figure 2Figure 2. Original Caption: GRACE monthly mass solutions for the Antarctic ice sheet for April 2002 to August 2005. Blue circles show results after removing the hydrology leakage. Red crosses show results after also removing the PGR signal. The latter represent our best estimates of the mass variability. The error bars include only the contributions from uncertainties in the GRACE gravity fields and represent 68.3% confidence intervals (13). Also shown is the linear trend that best fits the red crosses.

Velicogna et al (2009) extended the estimate of mass loss to the period from April 2002 to February 2009, this time emphasizing the “acceleration in mass loss” which was said to have increased dramatically in a short period:

the mass loss increased from 104 Gt/yr in 2002– 2006 to 246 Gt/yr in 2006–2009

There were numerous other contemporary estimates using GRACE data, most of which were later quoted in IPCC, but I’ve shown Velicogna’s because she was subsequently an IPCC author and results from Velicogna et al 2009 closely anticipate IPCC AR5’s subsequent “assessment” estimate. They showed a fit using a quadratic, which they claimed to be “statistically significant
at a 99% level.

The F-test show that the improvement obtained with the quadratic fit is statistical significant at a very high confidence level (99%).

In the diagram below (first blink), I’ve plotted the linear fit as well as the quadratic fit.  Using a standard anova test on digitization of the diagram, their confidence claims cannot be replicated.

Velicogna et al 2009 did not show their results prior to GIS adjustment. They stated that their GIA adjustment was “removed… as described by Velicogna and Wahr [2006a, 2006b]” and reported a GIA correction of 176 Gt/yr, somewhat reduced from the 190 Gt/year in their earlier article.   In the diagram below (second blink), I’ve added back the GIA adjustment to reverse engineer the unadjusted GRACE data (green), which, as before, shows a slight increase prior to GIA adjustment.  I’ve also shown a quadratic fit to the data with the (linear) GIA adjustment added back. The R2 of the cumulative mass loss after GIA adjustment arises almost entirely because of the linear GIA adjustment.

Unadjusted mass loss is (somewhat surprisingly) greater in the early part of the data. As a result, a quadratic fit to unadjusted data is slightly concave downwards.  When this slight concavity is combined with the cumulative linear GIA adjustment (which is negative sloped down), one gets a concave downward quadratic, which looks like “accelerated” ice mass loss.  However, this “acceleration” is an artifact of the peculiar method.


There were numerous other contemporary estimates of Antarctic ice mass loss using GRACE data with some even higher results (e.g. Cazenave et al 2009’s estimate of 198 Gt/year for 2003.0-2008.0; Chen et al 2009’s estimate of 178 Gt/year from 2002.3 to 2009.1).

Rignot et al 2008 carried out fresh input-output calculations over much (but not all) of the Antarctic coastline, but including all the locations of rapid ice flow, extrapolating from the observed to the total – a calculation subsequently criticized by Zwally.  They used satellite data on outflow and estimated snowfall using climate models.  They estimated mass loss of 112 ± 91 Gt/yr in 1996, increasing to 196 ± 92 Gt/yr in 2006, with East Antarctica being in balance, with large and increasing mass loss in West Antarctica and the Antarctic Peninsula, with most of the mass loss coming from the highly publicized Thwaites and Pine Island glaciers in West Antarctica.

Prior to 2012, the new laser altimetry data does not appear to have been used to estimate overall Antarctic ice mass loss, though it was used in some local studies.

Mackintosh et al 2011

Mackintosh et al 2011, unfortunately not cited by AR5, was a thorough re-evaluation of evidence on changes in the Antarctic ice sheet through the Holocene, incorporating the considerable accumulation of information on ice sheet recession.  They confirmed the remarkable (in my opinion) scenario of an extremely high West Antarctica in the LGM, with massive reductions of up to 3 km ongoing through the Holocene to the present, but negligible change in East Antarctica – if anything, slight accumulation (see an excerpt from their Figure 5 below).   According to their Figure 5b, ice mass loss in parts of West Antarctica has been up to 3 km over 14 kyr, i.e. an average of 21 cm/year over the 14,000 years, though the changes were heavily front-loaded as shown in their Figure 5a, which shows that nearly all the Antarctic melt since the LGM has occurred in West Antarctica, with contributions continuing almost to the present (at reduced levels).

mackintosh 2011

Figure ^. Excerpt from Mackintosh et al 2011 Figure 5.


Zwally et al 2011

In 2011, Zwally and coauthors attempted reconcile the various estimates, with Zwally et al 2011 being, in effect, their opposing answer to the question framed by Bamber in his realclimate article; they described their purpose as:

a critical assessment of estimates of the AIS mass balance in IPCC07, along with more recent estimates as listed in Table 2. Our purposes are to find areas of agreement among the methods, to identify outliers, and to provide a rationale for a narrowed range of estimates and a preferred estimate

First, they discussed three different estimates from radar altimetry estimates:  Wingham et al 2006 had estimated mass gain of +27 Gt/year for the Antarctic ice sheet for 1992-2003, whereas Zwally et al 2005 had estimated a mass loss of 31 Gt for 1992-2001.   Davis et al 2005 had estimated a mass gain of 45 Gt/year in East Antarctica, 29 Gt/year more than Zwally, and, if extended to the AIS, would have been about break-even overall.    Zwally et al attributed the differences to different assumptions on the density of the snow/firn/ice as the altitude changed, noting that the Zwally estimates were based on density of 900 kg/m^3 (the density of ice) whereas the Davis and Wingham estimates were based on density of 350 kg/m^3 (the density of snow).   Bamber argued that Zwally et al 2015 differed from prior studies on this issue, but, in reality, the issue had been clearly articulated in Zwally et al 2011 and, in this article, calculations using the higher density value led to more negative results, undermining Bamber’s line of criticism.

Second, Zwally et al 2011 critically analysed input-out analyses from Rignot and coauthors (including Bamber).  They levelled a number of technical criticisms, of which they singled out extrapolation as an area of largest concern.  Rignot’s observations covered a large fraction of the coast, but not all.  Much of Antarctic glacier discharge occurs in relatively fast-flowing channels; Rignot et al stated that their observations included all fast-moving channels.  Zwally et al argued that such parameters would over-estimate discharge in the non-observed (NOBS) portion in which there were no fast-moving channels. Zwally argued that the differences were substantial, enough to potentially tip the balance from net mass loss to net mass gain, rendering the method inconclusive:

Compared to R08, area scaling reduces the NOBS [not observed]  outflow from 624 Gt/year to 410 Gt/year (columns m and o) and the total outflow (O+) by about 10% from 2191 Gt/year to 1977 Gt/year, which would change the net mass balance from a loss of 136 Gt/year of R08 to a gain of 78 Gt/year…

we believe there is insufficient evidence in the IOM [input-output method] results for either an acceleration or deceleration.

I have no views on the merits of the respective sides of this dispute. AR4 had said that there were very large uncertainties attached to input-output estimates;  Zwally does not appear to have withdrawn his criticisms; one would have hoped that the issue would have been assessed in AR5, but they did not confront it, a topic that I’ll return to below.

Zwally et al itemized a long list of major uncertainties in the GRACE calculations, leading with the GIA adjustments, but including numerous other issues, some of which are also very large:

uncertainties in the GIA correction from model and density errors, uncertainties in the various terms in the spherical-harmonic expression of the Earth’s gravity field, uncertainties in the corrections for variations in the atmospheric mass, errors in the calculations of the scaling factor (”kernel”) to account for the effective size of the measurement region or limited spatial resolution of GRACE satellites, and ”leakage” of signals from mass changes outside the measurement region.

Again, one would have hoped that these points would have also been addressed in AR5.

New GIA Estimates: Whitehouse et al 2012, Ivins et al 2013

New calculations (Ivins et al 2013; Whitehouse et al 2012) dramatically reduced estimates of Antarctic GIA adjustments from previous methods.  The impact was illustrated in the IMBIE SI Figure S8 which showed ICE-5G GIA adjustments of ~170 Gt/year at plausible upper mantle viscosities and Ivins and James 2005 GIA adjustments of ~100 Gt/year at plausible viscosities.   The newer estimates of Ivins et al 2011 ((IJ05_R2) and Whitehouse et al 2012 (W12a) lowered estimates to ~50 Gt/year (!), down 120 (140) Gt/year from assumptions of Velicogna et al 2009 (Velicogna and Wahr 2006).

shepherd 2012 figure S8

Figure S8. The optimum IJ05_R2 and W12a model corrections vs. the past IJ05 and ICE-5G corrections employed for GRACE Antarctic ice mass balance analysis. Model GIA calculations of IJ05 and ICE-5G assume two lithospheric thicknesses (h = 120, 65 km) and two upper mantle viscosities; çUM = 1 x 1020 Pa s and çUM = 1 x 1021 Pa s. (See Fig. S6 for the differences in melt history). Error bars for the open circle and diamond are ± 4 Gt/yr, based upon scatter in the filtering methods that were tested in the course of the IMBIE. The blue star is placed at (41) recommended ‘stiff’ Earth structure model, and the red star, near the structure model advocated by (39) based upon global inversions. The two new optimum models are discussed in the text of this SOM. (Adapted from (32)).

Ivins et al 2013 also observed (once again) that the GRACE mass balance without correction was slightly positive over the 2003-2012 period:

ivins 2013 excerpt

They also noted that implementation of the revised GIA adjustments would dramatically reduce prior estimates of Antarctic ice mass loss.

Ivins et al 2013 similarly estimated positive (though considerably lower) unadjusted mass gain over 2002-2013, but its GIA adjustment was less than one-third of the estimate in Velicogna and Wahr, resulting in a dramatically lower estimate of ice mass loss than used in the NASA webpage or IPCC AR5:

GRACE monthly solutions from the Center for Space Research Release 04 (CSR-RL04) release time series from January 2003 to the beginning of January 2012, uncorrected for GIA, yield an ice mass rate of +2.9± 29 Gt/yr. The new GIA correction increases the solved-for ice mass imbalance of Antarctica to −57±34 Gt/yr.

In 2011, Thomas et al  (GRL) attempted to reconcile observed uplifts from then recent GPS information with uplifts predicted from GIS models and reported systemic errors. They concluded that the GIA models were “unreliable” and GRACE estimates of ice mass loss were biased high except for a few regions:

part from a few regions where large ice mass loss is occurring, the spatial pattern of secular ice mass change derived from Gravity Recovery and Climate Experiment (GRACE) data and GIA models may be unreliable, and that several recent secular Antarctic ice mass loss estimates are systematically biased, mainly too high

Whitehouse et al 2012 (Geophys. J. Int.) attempted to reconcile these inconsistencies through estimating paleoclimate glaciation through the Holocene and forward modeling to present uplift.

Implementation of their GIA model in King et al 2012 which dramatically reduced prior GIA estimates and, with them, estimates of ice mass loss from GRACE data:

After applying the model to 99 months of GRACE data spanning Aug 2002 to Dec 2010, we estimate a continent-wide ice mass change of -69±18 Gt yr-1 (+0.19±0.05 mm yr-1 sea-level equivalent). This is 36-48% of the most recently published GRACE estimates2,5 that cover a similar time period but are based on older GIA models. …

They reported that West Antarctic ice loss was limited to basins along the Amundsen Sea (and the Antarctic Peninsula) and that the rest of West Antarctica was more or less in balance, with gains in East Antarctica:

We resolve 26 independent drainage basins and find Antarctic mass loss, and its acceleration, is concentrated in basins along the Amundsen Sea Coast. Outside this region we find West Antarctica is nearly in balance (-10±7 Gt yr-1 in total) and East Antarctica is gaining substantial mass (60±13 Gt yr-1 in total).

The SI to King et al 2012 noted that the W12a GIS model contained serious inconsistencies along the Siple Coast of the Ross Sea (West Antarctica) where there was negligible observed uplift:

the relative sea-level change to be nearly zero (-0.3 mm yr-1) over our GRACE  data  period. This suggests substantial over-prediction of the magnitude of the major GIA uplift centre along this coast in W12a.

They observed that the inconsistency could result from Late Holocene (recent millenia) accumulations and attempted ad hoc patches.

The potential impact of recent (last two millennia) ice sheet advances and retreats is the topic of ongoing analysis by this group.  Nield’s thesis (entitled “The Effect of Late Holocene Ice-Mass Changes on Glacial Isostatic Adjustment in West Antarctica”), partly reported in Nield et al 2014 (Earth Plan Sci Letters), points to the likelihood that explanation of present-day uplift requires knowledge of late Holocene loading (e.g. Little Ice Age) loading.  Nield observed that mantle viscosity under West Antarctica was low, entailing rapid response to recent loading and the requirement to consider recent history for GIS adjustment in West Antarctica and the Antarctic Peninsula:

Conversely in West Antarctica, where mantle viscosity is lower, the Earth responds much more quickly to changes in surface loading so that present-day GIA is likely to be dominated by recent ice-mass changes rather than LGM ice history. Therefore currently unmodelled Late Holocene ice-mass changes are a potential source of large errors in GIA models of West Antarctica.

Similar issues arise in Bradley et al 2015 (another article by the same group) concerning GIA in the Weddell Sea area (which drains some West Antarctic ice), which reported that its observed uplift “rates are noticeably smaller than many current GIA model predictions for Antarctica (Peltier, 2004 and Whitehouse et al., 2012b)”, noting a mean bias of 3.4 mm/year.   Bradley et al considered a variety of ice histories, forward modeling them to estimate present uplift.  Whereas prior models, including Whitehouse et al 2012, had assumed that deglaciation was monotonic and had tapered off some time ago, they considered scenarios which included retreat and re-advance in the late Holocene (a pattern for which there is paleoclimatic support.) They improved model performance “by revising the Late Holocene deglaciation pattern within the Weddell Sea to include an early retreat behind the grounding line defined in the W12 model (at 6 kyr BP) and a relatively long stillstand followed by a short readvance that continues to present day”.  While they did not carry their analysis forward to its impact on GIA adjustments to ice mass loss, they noted:

Finally, the revised Holocene ice-loading history proposed in our study might have important implications for the GIA correction applied to the GRACE data, with a likely reduction in the GIA correction producing a smaller estimate of the present-day ice loss around the Weddell Sea than previously suggested (King et al., 2012).

For comparison to previous GRACE work that used older GIA models, replacing W12a with ICE-5G19 produced total rates of estimated Antarctic mass loss that were greater by 90 Gt yr-1

IMBIE (Shepherd et al 2012)

In preparation for AR5,  there was an attempt by specialists in the field in 2012 purporting to reconcile the wildly discrepant estimates: the Ice Sheet Mass Balance Exercise (IMBIE), reported on in Shepherd et al 2012 (pdf and especially its SI).  Co-authors included Zwally as well as IPCC Chapter 4 coauthors Velicogna and Rignot, and Chapter 4 Coordinating Lead Author David Vaughan.

Their Table S2 summarized their collation of estimates from different methods: note, in particular, the much reduced estimated showed mass loss of 57 ± 50 Gt/year from GRACE data.

Shepherd et al 2012 Table S2 (Antarctic Gt/year)



The GRACE estimate of 57 ± 50 Gt/year (2003.8-2009.0) is derived from six analyses of GRACE data commissioned for IMBIE, each carried out by one of six different groups of specialists.  Each of the various groups (including Velicogna’s) appears to have used the more recent (and much reduced) estimates of GIA adjustment, as these values are much lower than (for example) Velicogna et al 2009 or Chen et al 2009 (see above.)   The uncertainty of the above estimate appears to be the standard deviation of the six analyses, each using the more recent (and reduced) GIA adjustments and does not incorporate uncertainty/error from changing GIA adjustments, about which IMBIE noted uncertainty of “up to 130 Gt/year” (though this uncertainty was not carried forward into the mass loss uncertainties.)

the use of GIA models has in practice introduced considerable uncertainty (up to 130 Gt/year) into ice-sheet mass balance estimates derived from satellite gravimetry (31-33).

For the input-output method, they simply applied Rignot’s method to somewhat updated data, reporting an estimated mass loss of 142 Gt/year for a common period of 2003-2008,  negligibly different from the results of Rignot et al 2008 interpolated to this period.   For East Antarctica, they estimated a mass loss of 30 ± 76 Gt/year, whereas estimates of laser altimetry, radar altimetry and even gravity were all positive – especially laser altimetry.  As noted above, Zwally et al 2011 had severely criticized how Rignot and coauthors had extrapolated from observed areas (which included all fast flowing glaciers) to non-observed area (which necessarily had no fast flowing areas.)   Shepherd et al made no attempt whatever to rebut Zwally or otherwise refute his criticism.  I have not been able to figure out how they purported to calculate uncertainties for this method, but it is my strong surmise that there is no allowance whatever for potential issues in respect to extrapolation, since they did not rebut or comment on Zwally et al’s critique or explicitly include Zwally’s alternative calculations.

Although the IceSat satellite had operated from 2003-2009, at the time of the IMBIE intercomparison in 2012, there (remarkably) does not appear to have been any formal publication of IceSat laser altimetry results. Zwally et al 2015 appears to be long overdue in this respect.  For the IMBIE intercomparison, four centers (Louise Sørensen and René Forsberg (SF) at the Technical University of Denmark, Hamish Pritchard (HP) at the British Antarctic Survey, Donghui Yi and Jay Zwally (YZ) at NASA Goddard Space Flight Center, and Benjamin Smith (BS) at the University of Washington) were commissioned to estimate mass change from the IceSat data.  Their estimates of volume change were reported in Shepherd et al Table S7  and mass change in Table S8, both in the SI.  All four centers estimated volume gain in East Antarctica exceeding the volume loss in West Antarctica plus Antarctica Peninsula. The Zwally (YZ) estimates of volume increase was greater than, but similar to, volume changes from other groups, with Sorensen and Forsberg (SF) estimates being particularly similar. Both Zwally (YZ) and Sorensen-Forsberg got similar mass gains for the AIS as a whole, with subsector results by the other two groups being similar as shown in their Table S8, excerpted below.

Shepherd et al Table S8 (Antarctic):  Ice mass change 2003-2008 (Gt/year)

shepherd table S8


Zwally also reported the above IMBIE estimate at the SCAR ISMASS Workshop in Portland, OR on July 14, 2012 (added to NASA’s Technical Reports server on September 7th, 2012).  Zwally’s workshop presentation was noticed by Anthony Watts, who drew attention to it here, pointing out, unsurprisingly, the inconsistency with GRACE estimates of massive ice loss.   Part of Zwally’s presentation is online at WUWT.  In this presentation, he said that it had been his expectation that IceSat laser altimetry data would have put an end to the controversy (“we would launch IceSat and that would be it”, but acknowledged that it hadn’t, “at least some large negative values [are] going away”.

Zwally’s IMBIE estimate in (49 Gt/year mass gain for 2003-2008) is somewhat less than the later estimate of Zwally et al 2015 (82 Gt/year for 2003-2008), but both show mass gain.  In addition, though Bamber’s realclimate article attempted to isolate Zwally et al as a unique outlier, both the Sorensen-Forsberg group and Benjamin Smith (supplemented with a plausible Antarctic Peninsula estimate) also show mass gain, though less than Zwally.  So there appears to be a larger issue with the discrepancy between laser altimetry and gravity survey results that cannot be blamed on Zwally being an outlier.

This survey still left a very large range between the different methods, ranging from estimated mass loss of 142 ±  86 Gt/year from input-output calculations  (with only a single representative with unclear uncertainty calculations) to mass gain of 21 ± 71 Gt/year from laser altimetry.   Shepherd et al then appear to have simply averaged these wildly discrepant results, but do not report let alone show any attempt at actual reconciliation of the differences.  (The term “reconciliation” is an accounting term and requires that the differences be explained.)    This papered-over average is carried forward into Table 1 of the main article, where the mass balance estimates are evidently derived from the average of Table S2 (over varying time periods.)

shepherd table 1

In addition, although the differing methods revealed a very wide range of estimated mass change (both gain and loss), Shepherd et al showed a lower uncertainty (±  43 Gt/year) for a simple average than for any of the individual methods.   This seems highly unjustifiable to me.

In addition, their methodology gave disproportionate weighting to the single Rignot-style input-output calculation, which showed much larger mass loss than the other two methods. If Zwally’s alternative calculation had been given equal consideration, the average would have been close to zero, with an enormous uncertainty.  The resulting average of laser altimetry and gravity methods would have yielded a much lower mass loss of ~20 Gt/year over 2003-2008 with wider uncertainty.  So there is an enormous effective weight on the very uncertain and problematic IO calculation in the IMBIE composite.

While Shepherd et al entitled their IMBIE report as “A Reconciled Estimate of Ice-Sheet Mass Balance” and used the term “reconciled” repeatedly in the article,  the title, in my opinion, is a misrepresentation, since the article did not reconcile the discrepancies, but merely collated results of inconsistent approaches without confronting the discrepancies.


Despite Zwally’s optimistic belief that new data had resulted in “at least some large negative values going away”, IPCC AR5 did nothing of the sort, reviving some of the estimates that Zwally had believed to be discredited.  Two advocates of high negative values were AR5 Lead Author and Contributing Author: respectively, Eric Rignot, also a coauthor of the Copenhagen Diagnosis, and Isabel Velicogna, who was prominently featured in the SKS Denial series.

For the period 2005-2010, IPCC estimated mass loss  of 147 ± 74 Gt/year, almost double the corresponding IMBIE estimate of 81 ± 37 Gt/year:

Overall, there is high confidence that the Antarctic Ice Sheet is currently losing mass. The average ice mass change to Antarctica from the present assessment has been –97 [–135 to –58] Gt yr–1 … over the period 1993–2010, and –147 [–221 to –74] Gt yr–1… over the period 2005–2010. . …

In the Chapter 4 Executive Summary, IPCC stated:

The average rate of ice loss from Antarctica likely increased from 30 [-37 to 97] Gt yr-1… over the period 1992-2001, to 147 [72 to 221] Gt yr-1 over the period 2002-2011 … [4.4.3, Figures 4.16, 4.17]

Even though the IMBIE study had included leading authors in the field, including IPCC coauthors, and had been completed in time for the IPCC cycle (and was referred to by IPCC), IPCC AR5 presented its own calculations (described as the “present assessment”) instead.  The methodology of IPCC’s calculations was nowhere explained, but appears to be a combination of Rignot and Velicogna’s own prior results.

In their Appendix 4.A, they listed ten articles as sources, but do not describe how they went from the articles to the values presented in Figure 4-16 and the running text.  Eight of the ten listed studies were based on GRACE gravity surveys (and thus limited to mid-2002 on.) The average of the eight gravity survey studies in IPCC Appendix 4A is 144 Gt/year  (for varying coverage of 2002-2011), 2.5 times higher than the IMBIE gravity survey average of 57 Gt/year for 2003-2008.  All but one of the GRACE studies used older GIA adjustments and thus had much higher values than in IMBIE.  One of the GRACE studies was Velicogna et al 2009, results of which were stated in Appendix 4.A as having been “extended to 2012”.

For Antarctica, they asserted instead that there was “no significant difference” between their results and IMBIE results:

The recent IMBIE intercomparison (Shepherd et al., 2012) for Antarctica, where the GIA signal is less well known than in Greenland, used two new GIA models (an updated version of, Ivins and James, 2005; for details see, Shepherd et al., 2012; Whitehouse et al., 2012). These new models had the effect of reducing the estimates of East Antarctic ice mass loss from GRACE data, compared with some previous estimates.  For Antarctica, Shepherd et al. (2012) estimate an average change in mass for 1992–2011 of –71 ± 53 Gt yr–1… For the same period this present assessment estimates a loss of 88 ± 35 Gt yr–1 at the 90% confidence level. Averaging across technique ensembles in the present assessment, rather than individual estimates, yields no significant difference.

This claim was completely untrue. As noted above, the IPCC estimate of mass loss in the period 2005-2010 was nearly double the IMBIE estimate for the corresponding period. And for the GRACE data separately, the IPCC estimate was more than double the IMBIE GRACE estimate.  In fact, it would be more accurate to say that there was “no significant difference” with the Velicogna et al 2009 estimates using its obsolete and too high GIA adjustments.  This is shown in the annotated version of IPCC Figure 4.16 below, in which the IMBIE estimate (red) clearly diverges from the IPCC estimate, while the Velicogna et al 2009 estimate of ice mass loss over 2002-2009 closely matches the IPCC estimate.


Ironically, Velicogna appears to have submitted a lower estimate of ice mass loss to IMBIE, but IPCC appears to have reverted to the higher earlier estimate.  Velicogna does not appear to have ever published the extension as used in IPCC. Instead, the subsequent Velicogna et al 2014,  using recent GIA adjustments, reported much lower mass estimates than used in AR5. AR5 did concede that the size of the GIA adjustment was “similar in magnitude” to the estimate of ice mass loss and had very large (±80 Gt yr-1) uncertainty, but, as elsewhere, the GIA uncertainty was not carried forward into the uncertainty of the ice mass loss:

In Antarctica, the GIA signal is similar in magnitude to the ice-loss signal, with an uncertainty of ±80 Gt yr-1 (Velicogna and Wahr, 2006b; Riva et al., 2009; Velicogna, 2009).

Appendix 4.A listed one laser altimetry study as being considered in the IPCC calculation, but did not use Zwally’s IMBIE results which showed a slight mass gain. Instead, they used results from a Chinese language article (Shi et al., 2011), about which they had commented: “Methodology and error budget incompletely described.”

In addition, none of the IPCC numbers for mass loss over named periods reconcile precisely to the information in Figure 4.16, as illustrated here.

IPCC discussed IMBIE in both in its section on Greenland and in its section on Antarctica.  In their section on Greenland, they stated that there was “good agreement” between the different methods and the combination of methods “considerably improves the overall uncertainty”:

A reconciliation of apparent disparities between the different satellite methods was made by the Ice-sheet Mass Balance Intercomparison Experiment (IMBIE) (Shepherd et al., 2012). This intercomparison combined an ensemble of satellite altimetry, interferometry, airborne radio-echo sounding and airborne gravimetry data, and regional atmospheric climate model output products, for common geographical regions and for common time intervals. Good agreement was obtained between the estimates from the different methods and, while the uncertainties of any method are sometimes large, the combination of methods considerably improves the overall certainty.

While there may have been “good agreement” between methods for Greenland (I haven’t parsed these results), this was obviously not true for Antarctica.     While IPCC did not directly say that there was “good agreement” for Antarctica,  if “good agreement” was worth reporting for Greenland, then surely disagreement for Antarctica ought to have been reported, but it wasn’t.

IPCC Lead Author Eric Rignot was also an author of the Copenhagen Diagnosis, which, unsurprisingly reported the most lurid estimates of ice mass loss, without mentioning that specialists had begun the process of walking back these estimates.

Velicogna et al 2014

In her newest analysis, IPCC Contributing Author Velicogna dramatically reduced the estimated mass loss from the earlier Velicogna et al 2009 and AR5 estimates, but neglected to even mention the change.

The figure below shows their estimate of cumulative Antarctic mass loss (blue; smoothed as green), for which they diagnosed “acceleration”, which they discuss at length.  I’ve overprinted Velicogna et al (red) and IPCC AR5 (magenta), both displaced to line up in mid-2002.  The revised estimate of mass loss, as estimated in Velicogna et al 2014, is a fraction of the estimated mass loss published in AR5 the previous year.  One presumes that the Velicogna submission to IMBIE in 2012 was along the lines of Velicogna et al 2014, but, for some reason, Velicogna and IPCC coauthors reported much larger estimates of ice mass loss as the IPCC “present assessment”.

In this figure, I’ve also overprinted my estimates of the pre-adjusted GRACE measurements (black) – Velicogna et al 2014 did not report the quantity, but said that they used the adjustment of Ivins et al 2013, which, by difference, appears to be 60 Gt/year, which is added back as above.  The unadjusted mass increases somewhat in the first part of the period and decreases slightly in the second part, giving a very weak concavity.


Figure ^.  Annotation of Velicogna et al 2014 Figure 3a, showing reported ice mass loss after GIA adjustment (blue; smoothed in green); ice mass loss (smoothed) of Velicogna et al 2009 (red) and IPCC AR5 (magenta). Black – estimate of pre-GIA adjustment, showing linear and quadratic fit.

Zwally et al 2015

Although the IceSat laser altimetry data had provided unprecedented detail on height of the Antarctic ice sheet from 2003 to 2009, Zwally et al 2015 was the first fully published journal presentation of an estimate of Antarctic ice mass loss using this data.  (As noted above, it had been used for IMBIE contributions, but these only showed final results and otherwise had been used in some detailed glacier analyses, but not for a fully published Antarctic estimate.)  By the time that it was presented, it was criticized for being out of date – raising an obvious question of why there was such a delay between availability of data and its technical presentation.

Zwally et al 2015 contained a somewhat higher estimate of mass gain of 82 Gt/year, with losses of 29 Gt/year in the Antarctic Peninsula and 25 Gt/year in West Antarctica being offset by a gain of 136 Gt/year in East Antarctica. Zwally drew attention to an interesting aspect of West Antarctic mass change: while the mass loss in the Pine Island and Thwaites glacier area had attracted wide attention,  this was substantially offset by mass accumulation in the Kamb Ice Stream sector of West Antarctic, where slowdown of the ice stream had led to substantial accumulation (observed in the increase in height of the ice sheet in the Kamb sector).

In his 2012 workshop (and IMBIE contribution), Zwally et al had estimated slightly lower mass gain during the IceSat period of 2003-2008 of 86 Gt/year for AIS excluding the Peninsula (versus 111 Gt/year in the subsequent Zwally et al 2015), which was said to be similar to his revised results for 1992-2001 from radar altimetry.  This was a change of sign from the previously estimated loss of 31 Gt/year for 1992-2001 (Zwally et 2005, re-iterated in Zwally and Giovetti 2011).

The largest contribution to changes from the ice mass loss estimates of Zwally et al 2005 and Zwally and Giovetti 2011 came from implementation of the newer glacial isostatic adjustments, which also impact the estimates from altimetry, though the impact on altimetry estimates is less than 20% of the impact on gravity surveys, as shown in Zwally’s Table 8, an excerpt of which is shown below:

zwally 2015 table 8 excerpt

Zwally’s Table 8 contained an instructive reconciliation of the impact of changing GIA adjustments on his calculations. Zwally et al 2005 (and 2011) had used an exceptionally high GIA adjustment, even higher than other contemporary estimates.  This had the effect of disguising the difference between Zwally’s results and other results.  Had Zwally et al 2005 simply excluded the Huybrechts (2002) GIA adjustment from his composite, he would have shown a mass increase of 16 Gt/year, rather than a mass loss of 31 Gt/year (as the GIA adjustment would have been 16.5 Gt/year, rather than 62 Gt/year.)  Zwally et al 2015 used the even lower Ivins et al 2013 GIA estimate, which, had it been used in the earlier articles, would have led to a somewhat higher estimate of mass gain.

zwally 2015 table 8


Zwally converted changes in altitude of the ice sheet to mass through a model of changes from snow to firn.  Through information from ice cores, it is well known that density increases downhole, with snow being transformed to ice as it is subject to increasing pressure.  Zwally has being doing this sort of calculation for many years and his procedures do not appear to have been seriously challenged by other specialists.

Zwally observed that nearly all of the difference between his results and GRACE results occurs in East Antarctica. He noted that GIA model improvements had led to positive GRACE values in East Antarctica in the most recent studies, though still less than his East Antarctica estimate of 136 Gt/year:

GIA model improvements have been a primary cause of AIS mass estimates from GRACE becoming less negative, and those for EA becoming more positive. Recent GRACE estimates of dM/dt for EA are +35 Gt a-1 (Shepherd and others, 2012), +60±12.8 Gt a-1 (King and others, 2012) and 62.8±8.1 Gt a-1 (Luthcke and others, 2013). Additional convergence may occur as the full implications of long-term growth of EA are taken into account in the ice loading history.

Zwally observed that it would take only a small change (1.6 mm/year) in the estimate of East Antarctic GIA adjustment to reconcile the remaining difference:

a change in the modeled dB/dt for EA downward by 1.6 mm a-1 (from the small average uplift of +0.4 mm a-1 (Table 2) to -1.2 mm a-1) would bring the two GRACE-based dM/dt estimates of 60±12.8 Gt a-1 and 62.8±8.1 Gt a-1 in line with a corresponding adjustment of our ICESat value of 136±8 Gt a-1 at ~150 Gt a-1 for both methods.

Zwally also observed that present knowledge of ice unloading history supported the possibility of further revision of East Antarctic GIA adjustments in the indicated direction.  Zwally posited that there had been ice thickening in East Antarctica over the past 10,000 years which would result in negative, rather than positive, GIA in East Antarctica:

The small modeled uplift of +0.4 mm a-1 averaged over EA implies a history of ice unloading used in the GIA model. The ice-loading histories typically treated in the models are episodic ice unloadings during a glacial-interglacial transition, which cause relatively rapid isostatic adjustments, followed by a much slower residual response rate decaying over thousands of years. In contrast, our finding of a stable dynamic thickening of 1.59 cm a-1 over EA (Table 2), along with our interpretation as long-term dynamic thickening for 10 ka since the early Holocene, implies a slow long-term ice loading resulting in the addition of 159 m of ice averaged over EA in 10 ka. Using 6 for the ratio of the density of mantle rock to the density of ice implies an additional bedrock depression of 27 m continuing at a rate of -2.65 mm a-1 assuming full long-term isostatic adjustment. Therefore, the -1.6mm a-1 needed to bring the gravity and altimetry dM/dt estimates into agreement is only 60% of the full isostatic adjustment rate, and therefore within the range of what can be expected if the ice loading implied by the long-term dynamic thickening is accounted for in GIA models.

Zwally re-iterated their previous critique of Rignot’s input-output calculations, taking particular aim at his East Antarctic results, where Rignot had estimated a slight mass loss. Zwally once again observed that this estimate depended on a disproportionate extrapolation of ice mass loss to non-observed areas and that equally (or more plausible) extrapolations led to I/O estimates of East Antarctic mass gain consistent with his results:

The estimate in Rignot and others (2008) for EA was near-zero at -4.61 Gt a-1, which was based on an input estimate of 1131 Gt a-1, an observed output of 784 Gt a-1 and an extrapolated non-observed output of 349 Gt a-1. Therefore, a disproportionate 31% of Rignot and others’ total output from EA came from only 15% of the area that was non-observed. If the non-observed output had been scaled in proportion to the area as stated, then the non-observed output would be only 173 Gt a-1 (i. e. 15% from 15% of the area) and the net IOM balance would be +174 Gt a-1. Or if the non-observed slower-moving areas were only 70% as effective at discharging ice as the faster-moving observed areas, then the modified non-observed output would be +121 Gt a-1 and the net IOM balance estimate for EA would be +226 Gt a-1 (Zwally and Giovinetto, 2011).

As a reader who is familiar with reconciliation procedures in other fields, I was favorably impressed by Zwally’s effort to actually address and reconcile discrepant results,  an effort that seems far too unique within the field. On the other hand, the very long delay between availability of IceSat results (2003-2008) and Zwally’s presentation of results to IMBIE in 2012 and their eventual publication in late 2015 –  far longer than the usual cycle of publication of satellite data e.g. the numerous GRACE articles – is disquieting. Zwally was clearly worried that his results would “give fodder to the skeptics” and/or “dilute the message” (to borrow terminology from Mann’s correspondence about hiding the decline), worrying to Nature that “some of the climate deniers will jump on this”.  One hopes that this concern did not contribute to the overlong publication delay, but one fears that it might have.

Blog Reaction to Zwally et al 2015

Anthony Watts alertly noticed NASA’s subdued announcement of Zwally et al 2015 at WUWT here and, entirely reasonably, pointed back to his earlier cover of Zwally’s 2012 workshop presentation.

Predictably, Zwally’s results were repudiated by warmists.  Gavin Schmidt told VICE news that he would “pin more weight” on gravimetric data:

“data from a pair of satellites called GRACE, which measure gravity, actually points towards a net loss of ice on the Antarctic continent in more recent years. Schmidt said that there are two methods for measuring the mass of an ice sheet. The first measures gravity, and the second measures the elevation of the top of the ice sheet. Both methods need to take different variables into account to be accurate. The method used in this most recent study measured the ice sheet’s elevation, and the most recent time period it considered ended in 2008. “I would pin more weight to the GRACE data than to this latest paper,” Schmidt told VICE News

Since the impact of problematic GIA adjustments is nearly six times larger on GRACE estimates than altimetry estimates, it is not clear why Schmidt believed that it was appropriate to “pin more weight” on GRACE estimates, other than he liked the answer better.

Michael Mann, whose own comparisons of observations to models notoriously ended in 2005 (see ^), told the Guardian that  Zwally’s results were already obsolete and did not deal with the “acceleration” observed since 2008 (on this issue, see above analysis on the “acceleration” ):

According to climatologist Michael Mann, who was not involved in either study, the use of older satellite data could be the cause for the disconnect.  “It sounds to me as if the key issue here is that the claims are based on seven-year-old data, and so cannot address the finding that Antarctic ice loss has accelerated in more recent years,” he told AFP.

While one can regret Zwally’s failure to publish his laser altimetry results more promptly,  Mann’s comment is merely a talking point and does not refute the large difference between Zwally and IPCC results in the overlapping period.

SKS (e.g. here) has long asserted that claims that Antarctica was gaining ice as a skeptic “myth”.  They conceded that there had been gains in sea ice, but argued that satellite data showed that Antarctica is “losing land ice at an accelerating rate”, and that it was “cherry picking” to focus on one index, rather than the other. This argument was reiterated in their 2015 online lecture.  They stated that “skeptic arguments that Antarctica is gaining ice frequently hinge on an error of omission, namely ignoring the difference between land ice and sea ice [SKS bold].”  They have posted up a caveat saying that “that until 2008 there might have been a bigger increase in ice on East Antarctica than there is a decrease in the west, meaning that total Antarctic land ice is increasing”, adding that “the study disagrees with many other techniques”. They promise to return to the issue when further technical analyses become available, in the mean time pointing to one-sided comments at other warmist blogs.

Media Matters focused on Zwally’s concern that his results would (in Mann’s phraseology) give “fodder to the skeptics” and headlined its article: “NASA Scientist Warned Deniers Would Distort His Antarctic Ice Study — That’s Exactly What They Did”.  While Anthony Watts had certainly taken satisfaction in Zwally’s results,  Media Matters did not identify any actual distortion: it was Zwally’s results that undermined the narrative, not the supposed distortion of the results.

Like Mann, Phil Plait criticized criticized the study for ending in 2008 (when IceSat failed) and claimed that we “know” that Antarctica is losing “more than 130 billion tons of ice per year” and that mass loss “accelerated” after 2008 :

here’s more. They looked at data going from 1992–2008. Starting right around that time, mass loss due to melting ice in Antarctica (mostly in the west) has accelerated. It’s actually been speeding up for some time, but in recent years it’s really kicked in. Every year, about 6 billion more tons of ice are lost than the year before. In the past two decades, the loss rate has doubled. This is enough to easily outpace the mass gained by snowfall over East Antarctica. Using data taken by the Grace satellites (which measure how mass underneath them changes over time), we know that overall, Antarctica is currently losing more than 130 billion tons of ice per year, and again, that number is increasing every year. Since 2002 it’s lost about 2 trillion tons of ice.

As noted above, GRACE proponents have dramatically walked back their earlier estimates of ice mass loss, though these obsolete high estimates continue in use by warmists.  We don’t “know” that Antarctica is losing more than 130 Gt/year; on the contrary, current GRACE specialist estimates are much lower than that and, as Zwally observed, only slight changes in GIA adjustments would make even GRACE estimates positive.

Plait also fretted that the conversion of height into mass was “a little bit problematic” and a “little worrisome”:

There’s one step in the new study I found interesting, and a little worrisome: They measure how the height of snow increased in East Antarctica, but it’s a little bit problematic converting that to how much mass of ice was gained. As the snow falls, it gets compressed over time and turns into ice. That process isn’t completely understood, as Robin Bell (who studies Antarctic ice) points out in a quote in an article about this on Vice

While the process isn’t “completely understood”,  there are dozens of ice cores in which density has been measured downhole, so the process is well understood.  There is no evidence that the uncertainties in this process are anywhere close to the uncertainties in GIA adjustment, where specialists have already made dramatic changes to the adjustments.

Greg Laden agreed with Mann’s talking point about the 2008 ending:

The fact that the study being reported uses older data could explain why it conflicts with everything else the science is telling us. Michael Mann, quoted in The Guardian, notes, “…the claims are based on seven-year-old data, and so cannot address the finding that Antarctic ice loss has accelerated in more recent years.”

Laden also introduced a concern that the laser “observations are wrong”.  Laden pointed out that the satellite stopped working in 2008 and posited that there might have an instrumentation problem prior to that time:

If the satellite method is just a little off, this could cause a problem. (By the way, the data end in 2008 because the instrumentation on the satellite stopped working then.) This study’s main contribution may, in the end, to be to point out a problem with the instrumentation prior to that time.

While concerns over the possibility of instrumentation error must be considered, no specialist in the field has challenged Zwally on this point. Laden also worried that errors in calculations of the “response of the bedrock” underneath the ice sheet could also cause problems:

The second possible reason seems more likely. Part of the process of determining that Antarctica has a positive mass balance (more ice over time rather than less) involves assumptions (and some measurements) about the response of the bedrock underneath the very thick ice sheets. If that is wrong, then that is a problem.

Concern about glacial isostatic response is obviously a legitimate concern, but Laden was obviously incorrect in using this as a stick against Zwally: as noted above, the impact of GIA adjustments is six times higher on GRACE gravity estimates than on altimetry estimates.  To the extent that Laden is worried about this factor, he should “pin” that much less weight on the GRACE estimates which he likes.

Laden and other readers even wondered why Zwally would publish an article showing ice mass gain in Antarctica and how it passed peer review. Greg Laden speculated on its present appearance as follows:

Sometimes to close out a grant you need to submit a promised peer review publication. This looks to me like a paper that might have been hanging around a while.

Clearly, the paper had been “hanging around a while” – the reasons for this remain unexplained.

Sou purported to shrug off the difference as an uninteresting dispute among scientists:

So one group of scientists find that ice has been on balance increasing, while others find that ice has been on balance decreasing.

Had IPCC taken the same position, the topic would be of little interest.

Bamber at Realclimate

The most coherent warmist response came from Jonathan Bamber, an opposing specialist, who, writing at realclimate here, posed the question as follows:

why does it contradict a plethora of previous studies that suggest Antarctica has been losing mass over the same time period?

Unfortunately, Bamber did not rebut or even analyze Zwally’s thoughtful and detailed commentary on GRACE gravity estimates or Rignot’s input-output estimates, a commentary which provides a plausible explanation of how Zwally arrived at different answers than others.

Bamber instead challenged Zwally’s conversion of altimetry data on height to estimation of mass. Bamber pointed out that “small biases in the laser and radar data can have a big effect when the signal is only on the order of 1 cm/year”.  This is true enough, but the same criticism is even more valid against highly negative GRACE gravity estimates, on which Bamber had been silent.

As support for this criticism, Bamber pointed out that Zwally’s 2012 IMBIE calculations from the same satellite data were “only about half” of his most recent estimates.  Bamber challenged whether the new calculations were an improvement, editorializing that it “is probably more accurate to say that it is just different, rather than better”.   Bamber didn’t mention that GRACE estimates had changed far more dramatically – as shown above for the change between Velicogna et al 2009 and Velicogna et al 2014. Further, the change in East Antarctica, the main issue, was only ~22%.  Because the overall mass balance is an offset between East Antarctic mass gain and West Antarctic/Peninsula mass loss, the percentage change on the overall balance looks larger than the change in the component.

Bamber also observed that the laser altimetry data used in Zwally et al 2015 came from the IceSat satellite, which had a life span of 2003-2009 and that results from the most recent satellite equipped with laser altimetry (Cryosat 2) “may yield a different picture yet again”,  noting that intercomparisons thus far are “not very encouraging”.

Bamber also stated that there is “another way to measure the changing mass of the ice sheet and that is to weigh it using the Gravity Recovery and Climate Experiment (GRACE) twin satellites”.  As an editorial comment, the term “weigh” in this context is a huge exaggeration.  The GRACE satellites measure gravity as they pass over.  The estimation of the “weight” of the ice sheet results from a very complicated by-difference calculation that appears to have just as many (if not more) problems and assumptions as the competing altimetry calculations.  Bamber conceded that the satellite cannot distinguish changes in gravity due to changes in ice thickness from changes in gravity due to relative change in height of continental mass arising from “changes in ice loading over thousands of years”. Bamber conceded that “some critical variables” are not well known and that there was “very limited information” for the (very large) East Antarctica interior:

Geophysicists have attempted to model how the solid Earth responds to changes in ice loading and, although the physics is fairly well known, some critical variables such as the viscosity of the lower mantle and the history of ice load changes are not…in the interior of East Antarctica we have very limited information on how big the bedrock signal is.

Bamber then said that Zwally had “argue[d]” that GRACE GIA adjustments in East Antarctica were “wrong because they do not know the full ice loading history”:

Zwally argues that the GIA estimates that have been used for correcting GRACE data are wrong in East Antarctica because they do not know the full ice loading history over the last ~22,000 years.  Maybe. But GIA in the EAIS is poorly constrained and there is, in general, poor agreement among different estimates of GIA over the EAIS. [my bold]

This summary is both tendentious and incoherent.  GIA specialists (Ivins, Peltier, Whitehouse and others) have themselves taken the position that their earlier GIA estimates were incorrect in light of improved knowledge and additional data.  Zwally et al 2015 used up-to-date GIA estimates from Ivins et al 2013 – estimates that have also been widely used in more recent GRACE articles (e.g. Velicogna et al 2014).

Further, to the extent that Bamber is correct that there is “poor agreement among different estimates of GIA” over East Antarctica, this is a much larger problem for the GRACE gravity estimates, which are approximately six times more sensitive (as Gt/year relative to mm/year) than altimetry estimates.  If this is an issue for Bamber, he ought not to have been silent on the GRACE studies.

Bamber concluded that the difference between Zwally and other studies ultimately rested in Zwally making a “different set of assumptions” about East Antarctica and that these assumptions were “by their nature, subjective and difficult, without additional evidence, to corroborate”:

So what is really happening? One thing that Zwally’s study does highlight is how difficult it is to nail what is happening in East Antarctica because the signal is small and contaminated by unwanted effects that are as large or even larger. Zwally et al get a different result from previous studies because they make a different set of assumptions. Those assumptions are, by their nature, subjective and difficult, without additional evidence, to corroborate.

This is hardly an argument for disregarding Zwally’s conclusions.  The problem of the East Antarctic “signal” being “contaminated by unwanted effects that are as large or even larger” as the “signal” itself is much larger for the GRACE survey data than for Zwally’s laser altimetry data.  Bamber ought to have spoken out earlier.

Despite seeming to concede that it was impossible to get past a “subjective” interpretation of the Antarctic  satellite data, Bamber himself remained undeterred in his conviction of Antarctic mass loss, falling back on the difficulty of explaining observed sea level rise without an Antarctic contribution:

There are, however, other lines of evidence that suggest that Antarctica is unlikely to have been gaining mass in the last few decades. That would, for example, make closing the sea level budget a whole lot harder (that is, making the sum of the sinks and sources match the observed rate of sea level rise)

However, this is a very indirect argument. IPCC Table 13.1 attributed only 0.27 of 3.2 mm/year sea level rise from 1993-2010 to the Antarctic ice sheet (equivalent to about 90 Gt/year), rating multiple other factors (thermal expansion, non-icesheet glaciers, Greenland ice sheet, changes in water storage on land) as greater contributors.  Uncertainties for thermal expansion and glaciers were each greater than the contribution attributed to Antarctica. In addition, a recent study argued that groundwater extraction could have contributed up 0.8 mm/year, far higher than values contemplated by IPCC.  While Bamber is correct that mass gain in Antarctica would make closure of the sea level budget more difficult, there is clearly considerable play in the numbers.

Nor was this line of argument relied upon in the IPCC chapter on the cryosphere in which the IPCC reported “high confidence” in its estimates of large Antarctic mass loss.


While it is obviously up to specialists to try to ultimately figure out whether Antarctic ice mass was increasing in the periods 1992-2001 and 2003-2008 (per Zwally) or whether it was decreasing (as IPCC and others had previously asserted),  there does not appear to be any objective basis by which, for example, Gavin Schmidt could reasonably “pin more weight” to highly negative estimates from GRACE gravity data than to Zwally’s positive estimates from laser altimetry.

The size of the GIA adjustment for GRACE gravity estimates is the same order of magnitude as the estimate of ice mass loss and, in many cases, is larger. These GIA adjustments have been dramatically reduced by specialists over the past decade and have concurrently reduced estimates of ice mass loss.

Many popular (warmist) discussions of Antarctic ice mass loss continue to use obsolete (overly high) estimates of ice mass loss e.g. NASA’s estimate of “134 billion tons” per year.   Such estimates rely on GRACE estimates using obsolete GIA adjustments.

The estimates of mass loss in IPCC AR5 were highly questionable. They were much higher (nearly double) than contemporary specialist (IMBIE) estimates.  They appear to have been based on studies using GIA adjustments, already known to be obsolete.  It was separately highly questionable to attribute “high confidence” (and relatively narrow confidence intervals) to these very high estimates of mass loss.

Most of the Antarctic continent (especially East Antarctica) appears to be experiencing ice mass gain, with ice mass loss being localized to less than 5% of the continent:  parts of the Antarctic Peninsula and est Antarctica (especially Pine Island and Thwaites glaciers).  This peculiar localization requires its own explanation. Recent specialist literature has concluded that West Antarctica was up to 3 km higher in the LGM, while the height of East Antarctica has changed little and might even have increased slightly through the Holocene. West Antarctica has experienced dramatic ice mass loss through the Holocene, attenuating to the present.

AR4 had pointed out the possibility that localized ice mass loss in Antarctica was continued Holocene ice mass loss.   This possibility vanished in AR5 without discussion. In an inline comment to Bamber’s realclimate article, Eric Steig said that his opinion, and that of “50% of experts”, was that the connection of Antarctic glacier retreat to “anthropogenic global warming” was “weak” and that the localization of the glacier retreat to West Antarctica was “well understood” and something that he had written about “extensively”:

I think the evidence that the current retreat of Antarctic glaciers is owing to anthropogenic global warming is weak. The literature is mixed on this, about 50% of experts agree with me on this. So you’ll get no argument from me there.  Second, the localization in West Antarctica is well understood, and I’ve written about it extensively.

Elsewhere, Steig has attributed the West Antarctic glacier retreat to erosion of the grounding line of the glaciers by relatively warm Circumpolar Water, rather than to very slight warming of air temperatures above West Antarctica.  Given the continuous retreat of West Antarctic grounding lines over the Holocene, it seems implausible to attribute present grounding line erosion to a different cause than past grounding line erosion that has taken place over the Holocene.  Steig’s position on this point seems entirely reasonable.

However, it still seems like one of those too typical situations where the less alarming explanation is presented in specialist literature, but left unmentioned or unconfronted when retreat of West Antarctic glaciers is presented as a cause of alarm.


  1. milodonharlani
    Posted Dec 2, 2015 at 2:44 PM | Permalink

    Discovery of a subglacial volcano under the West Antarctic Ice Sheet was announced in 2013:

    Steve: this is not material to climate. There are enough real issues in this thread, that I do not wish to have this coatracked.

  2. Klapper
    Posted Dec 2, 2015 at 3:10 PM | Permalink

    I’m at work so only have time to skim this now but it looks to be a most excellent source for discerning the truth about continental ice sheet melting. No doubt some of what was discerned here is applicable to Greenland too.

  3. Posted Dec 2, 2015 at 3:31 PM | Permalink

    Thanks, Steve. Lots to digest here.


  4. DayHay
    Posted Dec 2, 2015 at 3:53 PM | Permalink

    What are the odds that warmists like Mann, etc, 100%, ALWAYS come out for the CAGW side of the narrative?
    Have they EVER thought even one supposed “fact” they dribble out could be wrong??

  5. Posted Dec 2, 2015 at 5:27 PM | Permalink

    Gavin Schmidt told VICE news that he would “pin more weight” on gavinmetric data…

    Well that’s how it read to me initially.

    Superb explanation of the various issues Steve. This almost reads like an assessment of the available scholarly literature.

    • Posted Dec 2, 2015 at 11:45 PM | Permalink

      Pin the tail on his own donkey, but seriously West Antarctica and the peninsula are the Greenland of the southern hemisphere. In the several senses of propensity to accumulate/lose massive volumes of ice, experience dizzyingly rapid temperature fluctuations, and having low mantle viscosity=hot thin crust. An intriguing combination. The ice accumulation in both cases might be explained by hoisting a salient into the hurricanes of planetary winds. The rapid variations (perhaps?) to the complete wildcard of mantle heat flux.

      BTW. Totally know you are going to delete the first phrase. Just for you.

  6. Mike Jonas
    Posted Dec 2, 2015 at 5:48 PM | Permalink

    Force as long a delay on the paper as you can, then when it comes out say it can be ignored because it is out of date. Neat.
    “By the time that it was presented, it was criticized for being out of date – raising an obvious question of why there was such a delay between availability of data and its technical presentation.”

  7. mpainter
    Posted Dec 2, 2015 at 7:34 PM | Permalink

    Steve, possibly you will want to can this comment.
    Are you aware of recent studies that attribute ice loss in west Antarctica to sub-ice geothermal activity (volcanism, etc)? These are convincing and resolve the climate issues regarding ice mass loss there.
    I would imagine such studies are ignored by “mainstream” types. I only mention this because I thought that you might be unaware of them.

  8. Ralph Reiter
    Posted Dec 2, 2015 at 8:59 PM | Permalink

    Question word “firn” at end of sentence that starts paragraph: “Zwally converted changes in altitude of the ice sheet to mass through a model of changes from snow to firn.”

    • eloris
      Posted Dec 3, 2015 at 10:22 AM | Permalink

      Googling that, it means “granular snow, especially on the upper part of a glacier, where it has not yet been compressed into ice”.

  9. Posted Dec 2, 2015 at 9:48 PM | Permalink

    The whole issue seems like a discussion of whether one drop exited the tap and fell into the bathtub or not, days after the fact. There are what–150 million Gt in the Antarctic Ice Sheets?

    The discussion on WAIS has been ongoing since the Twenties, IIRC, and they specifically mentioned the threat to ice there due to ‘mechanical’ reasons, long before there was talk of climate change.

    In any event, I read on the Internet (so it must be true) that it takes 361 Gt to raise ocean sea level by one millimeter, 9,137 Gt to raise sea level by one inch.

    I think NASA wrote a few years back that ‘nothing much is happening in Antarctica.’ That seems to be true. Why they felt they needed to promote warming seems unclear, given climate change theory, which divorced melt in the NH from events in the SH. Another unforced error.

    I hope you enjoyed unraveling the tangled skein here. Otherwise I would lament that you had to expend so much effort.

    • Howard
      Posted Dec 3, 2015 at 5:04 PM | Permalink

      Nice order of magnitude points, Tom. However, I don’t agree that SLR is the only important side effect. Getting the ice balance right is a very important factor for figuring out what is going on. Especially since some researchers believe that the LGM was ended by a shift in Antarctica.

  10. Posted Dec 3, 2015 at 12:19 AM | Permalink

    Reblogged this on I Didn't Ask To Be a Blog.

  11. AntonyIndia
    Posted Dec 3, 2015 at 12:41 AM | Permalink

    The crafting of an (Antarctic) ice hockey stick during
    IPCC AR5? An up hill struggle, but they kind of delivered what was expected from them.
    Hopefully they have good protective gear.

  12. maxberan
    Posted Dec 3, 2015 at 8:50 AM | Permalink

    The conclusion section should perhaps also draw out your two findings (1) the understatement by IPCC of confidence interval through the absence of the error term due to the GIA uncertainty, and (2) the flaw in the justification of a significant quadratic term leading to acceptance of acceleration.

    Steve: I am aware that I haven’t pulled the conclusions together particularly well. Often the conclusions emerge as you write and get articulated better in a re-draft or after discussion. After a while, I had too much time tied up in this and just wanted to get finished. The acceleration issue is an interesting distinct issue which seems incontrovertible. It would be nice to have actual unadjusted data without having to reverse engineer laborious digitizations.

  13. Toml
    Posted Dec 3, 2015 at 10:10 AM | Permalink

    Has anybody addressed the question of spatial resolution of the GRACE data? My understanding is that it is around 300 km, implying that the shortest wavelength that can be meaningfully measured is around 600 km. Is it possible to say anything meaningful at all about a feature the size of e.g. Thwaits Glacier at that resolution?

  14. Jeff Norman
    Posted Dec 3, 2015 at 1:44 PM | Permalink

    Christmas comes early. Thank you Steve. Your posts are always educational.

    GIA = Glacial Isostatic Adjustment, had to look it up. (I did look)

    ” Zwally et al attributed the differences to different assumptions on the density of the snow/firn/ice as the altitude changed, noting that the Zwally estimates were based on density of 900 kg/m^3 (the density of ice) whereas the Davis and Wingham estimates were based on density of 350 kg/m^3 (the density of snow).”

    I would have thought the field would have standardized this sort of thing.

    “Like Mann, Phil Plait… and several butchers’ aprons.”

  15. Willis Eschenbach
    Posted Dec 3, 2015 at 2:09 PM | Permalink

    Dang, you continue to amaze.

    I did so much appreciate this line (emphasis mine) …

    They observed that the inconsistency could result from Late Holocene (recent millenia) accumulations and attempted ad hoc patches.

    I also noted you saying that the IMBIE comparison …

    … does not incorporate uncertainty/error from changing GIA adjustments, about which IMBIE noted uncertainty of “up to 130 Gt/year” (though this uncertainty was not carried forward into the mass loss uncertainties.)

    For me, the takeaway message confirms my general rule of thumb, which is that the error range of the final result of any data “adjustment” I make needs to incorporate the estimated error in my adjustments themselves … and in turn, the estimated error of the adjustments needs to consider the possibility that my adjustment is totally incorrect. One hopes that that possibility is small, but it is usually not zero …

    And that doesn’t even touch the problems with autocorrelation.

    Finally, is the calculation of the mean in the Shepherd et al 2008 Antarctic Figure 8 correct? It looks to me like they have simply averaged the errors, rather than summing them in quadrature etc. … what am I missing here?

    Best regards, and thanks for a most interesting overview of the uncertainty in calculating the uncertainty …


    Steve: there are numerous peculiar technical details and I could not write up every rabbit hole. I too scratched my head at the averaging in Shepherd et al. Also in their Table S2. If you get inconsistent answers from different methods, it seems more logical to me that this shows very wide uncertainties though I can’t point to a statistical authority for this. Also the errors in Table S2 are very different in nature: they calculated standard deviations of estimates from different groups for the GRACE series, but they only had one IOM example and I coulnd’t figure out how its “error” was calculated. Zwally’s persuasive criticism indicated that its “error” wasnt worth the powder to blow it to hell anyway.

  16. Posted Dec 3, 2015 at 2:28 PM | Permalink

    Nice to see you reappearing on radar. I was getting worried.


    • Tom Wiita
      Posted Dec 3, 2015 at 3:11 PM | Permalink

      From me too, welcome back. You had me worried that you took a squash racket to the face or something, which has happened to me more than once.

    • jorgekafkazar
      Posted Dec 4, 2015 at 12:16 AM | Permalink

      I’ve been checking in often. Glad to see this post appear.

  17. Posted Dec 3, 2015 at 2:58 PM | Permalink

    The Velicogna 2009 figures are interesting. Adjust by -176 Gt/yr and then report on the significance of the mass balance over time. This is like using y/x on the y-axis and plotting it against x, and then remarking that the y-variable (i.e. y/x) is negatively dependent on x.

    This used to happen in population biology, where population change was plotted against population size, which was used to diagnose density dependence (i.e. that high populations tend to decline, and v.v.).

    If I have misunderstood this, please someone enlighten me.

    Also, if the GIA is large and negative, is it ever possible to arrive at a +ve mass balance?

    Steve: I don’t entirely understand your analogy, however you’ve clearly understood the peculiar feature of the “acceleration” calculations. In my own thinking, I viewed the data in two parts: (1) you have data that goes up a smidge in the first half; and down a smidge in the second half, neither “significant” relative to the autocorrelation; (2) you then subtract a steeply increasing amount. And voila, you have “acceleration” of mass loss. And if Zwally’s critique of East Antarctica are right, we don’t even know the sign of the GIA adjustment in East Antarctica.

    • maxberan
      Posted Dec 4, 2015 at 4:37 PM | Permalink

      I would presume the alleged high significance of the quadratic (acceleration) term is conditional on the slope and intercept of the subtracted trend line being treated as known quantities. If they were to be fed into a combined statistical model as parameters subject to sampling error then one may expect that significance to disappear along with the degrees of freedom, doubly so if allowance were to be made for autocorrelation. One could even end up with more parameters than data points!

      Of course this tendency to ignore the error contribution of pre-assumed quantities pervades climate science (and indeed many other branches of science). In a sense it pervades the entire IPCC enterprise which is explicitly predicated on the truth of the AGW hypothesis so all that is left is “how much”, not “if”. This allows all those opportunities for the data to be explained by other hypotheses and all those other observations that don’t accord with the hypothesis may safely be bundled into the paradigm and not be allowed to contribute to a final error.

      • Steve McIntyre
        Posted Dec 4, 2015 at 5:37 PM | Permalink

        I would presume the alleged high significance of the quadratic (acceleration) term is conditional on the slope and intercept of the subtracted trend line being treated as known quantities

        I am unable to replicate the claimed significance. It looks like they might have calculated the “significance” based on smoothed versions of the data, but I haven’t parsed this yet.

  18. vangelv
    Posted Dec 3, 2015 at 3:50 PM | Permalink

    Great posting. I have to say that I am envious of both the intellect and the patience that is required to put it together. Nice job Steve.

    The melting in Western Antarctica can be attributed to volcanic activity under the ice sheet. As has been made clear many times before by many commentators we are looking at a complex system in which there are many factors in play. Human emissions of CO2 are being ignored as time passes and the literature points to natural factors that play a bigger role.

    Steve: There is geothermal heat, but it is much too low to account for the melt. There is convincing evidence that the West Antarctica ice sheet is being eroded at the grounding line by “warm” Circumpolar Water – as it has throughout the Holocene. I like geology and find it interesting, but this issue is a distraction and I dont want to divert any discussion onto this topic. Some commenters at WUWT are interested in the topic and you’d be better off discussing it there.

    • DB2
      Posted Dec 4, 2015 at 7:18 AM | Permalink

      Geophysics could slow Antarctic ice retreat
      Gomez and co-authors David Pollard of Pennsylvania State University and David Holland of New York University also factor another important variable into their simulations. When an ice sheet retreats, the solid Earth beneath it, freed from the load of the ice, rebounds upward. This rebound occurs in two parts: an elastic component that happens right away, and a viscous component that happens over hundreds to thousands of years….

      “Our simulations show that when we assume a structure for the Earth’s interior that resembles the structure underneath the West Antarctic, the Earth’s surface rebounds higher and more quickly near the edge of the retreating ice sheet,” says co-author Holland of NYU. “This makes the water along that edge shallower, which slows the retreat of the ice sheet.”

      • Les Johnson
        Posted Dec 4, 2015 at 2:21 PM | Permalink

        While geothermal heat does contribute to some local melting, it is still insignificant to the ice cap as a whole.

        This paper gives an average of about 0.14 W/m2 heat flux, which is quite small. And again, it is a quite small geographical area.

        I agree with Steve. There is little to add to this discussion, using geothermal.

    • Howard
      Posted Dec 4, 2015 at 8:48 AM | Permalink

      Look at the shape of Antarctica. West Antarctica and the peninsula are one appendage sticking out into the ocean from East Antarctica, the main portion of the continent. The surrounding ocean water is way (way!) warmer than the air, it circulates from warmer climes, it has 1,000-times the thermal mass than air, is relatively incompressible and has essentially zero shear strength.

      Coupled with the rift-valley structure, the narrow isthmus connection to the main portion of the continent, the abundant basal ice streams and water saturated sediments, the WAIC is much more mobile than the ice sheets of the stable East Antarctica craton. This geographically determined ice sheet instability is clearly evident in the Mackintosh figures in the OP.

      • mpainter
        Posted Dec 4, 2015 at 10:22 PM | Permalink

        “the abundant basal ice streams and water saturated sediments,”
        No permafrost beneath that ice cap? That 12 million year-old ice cap? Surely there is.
        “Water saturated sediments” does not seem right.

        • Howard
          Posted Dec 5, 2015 at 9:58 AM | Permalink

          Google is more informative than the voices in your head and/or the mutterings of like-minded fantasists.

          Click to access LanoilEtAlBacteriaWAIS.pdf

          The abundance of subglacial water is a key factor in the dynamics of ice sheets and for isolated life in an environment beneath hundreds to thousands of meters of ice (Vogel et al., 2005; Fricker et al., 2007). Seismic and borehole evidence indicate that regions of fast-moving ice, known as ice streams, move over a layer of uncon- solidated, water-saturated sediments. These ice streams are the primary route for mass loss from the West
          Antarctic Ice Sheet (WAIS), with about half of the ice mass flow feeding into the Ross Ice Shelf (Oppenheimer, 1998). Water flows along the hydrological potential gradient towards the coastal ocean forming a hydrological system with lakes, freshwater-saturated sediments (‘wetlands’) and subglacial drainage pathways (Fricker et al., 2007; Priscu et al., 2008). While studies of accretion ice indicate that life may be present in subglacial lakes like Lake Vostok (Karl et al., 1999; Priscu et al., 1999; Christner et al., 2001; Bulat et al., 2004), no direct evidence has been described for the presence of microbial communities in other parts of the Antarctic subglacial
          hydrological system. Here we present such evidence from subglacial sediments recovered from beneath the Kamb Ice Stream (KIS), one of the six major West Antarctic ice streams draining into the Ross Ice Shelf.

      • mpainter
        Posted Dec 5, 2015 at 1:51 PM | Permalink

        One source gives that half of the WAIS is underlain by meltwater, even lakes. Lakes!
        And no permafrost, only water saturated sediments. No permafrost!
        Hmmm. Have any of the “specialists” offered an explanation for this extraordinary phenomenon?

        I can think of only one.

        • Howard
          Posted Dec 5, 2015 at 3:12 PM | Permalink

          Only one? Don’t quit your day job.

        • kuhnkat
          Posted Dec 5, 2015 at 5:02 PM | Permalink

          The lakes are associated with hydrothermal features, but, our host doesn’t want to side track the discussion.

          Steve: exactly right. NO more coatrack.

    • James at 48
      Posted Jan 15, 2016 at 4:20 PM | Permalink

      @Steve – The Antarctic Peninsula and perhaps by extension West Antarctica might be thought of as poleward extensions of The South Pacific Littoral and hence more “Polar” neighbors of the Marine West Coast regime of Chile. Similarly I would deem East Antarctica an extension of The South Atlantic Littoral and hence a more “Polar” neighbor of the Humid Continental grading into Continental Subarctic regime of Patagonia. YMMV.

  19. Michael Jankowski
    Posted Dec 3, 2015 at 7:35 PM | Permalink

    My fave Zwallyism (albeit about Greenland)

    “…Zwally joined his colleagues there on May 8 in the regular spring migration of scientists to the Arctic.

    He has been coming to Swiss Camp every year since 1994 and has been studying the polar regions since 1972, monitoring the polar ice through satellite sensors.

    Eventually he realized he had to study the ice firsthand…”

    Appears that Zwally “studied” polar ice/regions for over 2 decades before he actually went too take a look at it for himself. Unbelievable.

    • maxberan
      Posted Dec 4, 2015 at 6:20 AM | Permalink

      Bit of a cheap jibe if you don’t mind me saying. Many roles in research, including in the environmental sciences, are basically deskbound. While knowledge of the rigours and problems of fieldwork are necessary they don’t have of necessity to be gained at first hand. His seat in the chopper and use of resources out there may well have been put to better use by a real field scientist.

      • mpainter
        Posted Dec 4, 2015 at 11:28 AM | Permalink

        There is seldom any satisfactory substitute for actual observation of the subject when one is studying natural processes.
        True, such firsthand observation might not be practicable or even possible for the researcher, but this is a disadvantage, imo.

        • mpainter
          Posted Dec 4, 2015 at 11:34 AM | Permalink

          There is seldom any satisfactory substitute for actual observation of the subject when one is studying natural processes.
          True, such firsthand observation might not be practicable or even possible for the researcher, but this is a disadvantage, imo

          Allow me to rephrase: there is seldom any substitute as satisfactory as actual observation of the subject when studying natural processes.

        • maxberan
          Posted Dec 4, 2015 at 1:00 PM | Permalink

          I do not know of course what hands-on experience you and Michael Jankowski have of the practicalities of environmental research but you must surely recognise that (1) different people have different skills and (2) there are so many hours in a day. For myself, I was one of those decried deskbound scientists. There is doubtless a long list of reasons why my researches into floods and droughts might have been deficient but I do not judge the fact that I was a user of field data collected by others rather than a participant in its collection to rank high on that list. I have no reason to think that Zwally would be less aware of the deficiencies of the data he was analysing than I was of those in the data I was supplied with.

        • Howard
          Posted Dec 5, 2015 at 10:03 AM | Permalink

          Yes Mpainter, please extol us with your vast experience as a field geologist. Do you prefer the Brunton or Silva. Do you play pool with the rig hands or do they dope your hardhat?

        • mpainter
          Posted Dec 5, 2015 at 1:53 PM | Permalink

          Interesting comment by Howard.

      • Michael Jankowski
        Posted Dec 4, 2015 at 1:16 PM | Permalink

        Great point…except that Zwally himself obviously realized the importance of seeing it himself and doing so regularly. So it seems that you are taking a “cheap jibe” at him for making annual visits? Can’t have it both ways.

  20. John F. Hultquist
    Posted Dec 3, 2015 at 8:22 PM | Permalink

    So far — I’ve gotten to the chart with the 2 blinks (first and second).
    Am I the only person with the blink rate so fast my eyes water and my head hurts?

    I’ll take 2 aspirin and maybe I’ll be fine in the morning.

    (Is it possible for me to change the rate on my computer?)

    Steve: I was experimenting with a new-to-me program when I produced this. I’ll fix it in a day or two.

    • Bob Koss
      Posted Dec 4, 2015 at 1:40 AM | Permalink

      I made one with a 5 second blink rate.

      • Posted Dec 6, 2015 at 4:41 AM | Permalink

        Thanks Bob!

      • John F. Hultquist
        Posted Dec 8, 2015 at 8:51 PM | Permalink


  21. Geoff Sherrington
    Posted Dec 3, 2015 at 8:40 PM | Permalink

    Hi Steve, it is great to see at last your essay on Climate Audit.
    Here is a shortened version of a view on the same topic. It uses mass as a fundamental parameter, starting with reference to BIPM, the International Bureau of weights and measures, HQ in France.
    “…activity focuses on the unit of mass, the kilogram. Since 1889, this unit has been defined as the mass of a unique object known as the ‘international prototype of the kilogram’. It is simply a cylinder of platinum alloy stored at the BIPM and, to conserve its mass, used only rarely. Research shows that it is now practical to define the kilogram in terms of the constants of physics … “
    In professional metrology, all significant measurements of mass are to be traced back to the Pt standard or its approved successor. Each step that moves away from the Pt standard will typically introduce more errors. Almost by definition, the present Pt standard is the lowest-error mass measurement and all others are less accurate.
    Accuracy is in two parts. One is bias, whereby the error, irrespective of its reproducibility, is shifted with respect to the best known value. A bias error might for example be demonstrated by a result of “1.007 +/- 0.002 kilogram” for a nominal 1 kg mass. The second is precision error, that might be expressed as “1.000 kilogram with a 2 sigma value of 0.05 kg.” The first error is a deviation from the best known result. The second is more a statistical expression of the spread of multiple observations, hopefully ceteris paribus, hopefully derived from a distribution whose shape is known and whose subsequent math is compatible with that shape.
    These are kindergarten words for professionals. The aim here is not to nitpick the ways I have expressed them, but to encourage pursuit of examples in which these basic principles are violated. Such violations appear to populate much of Steve’s essay.
    GRACE was invented to measure mass. One can devise thought experiments in which the calibration of GRACE was traced back the Pt standard kilogram. One way might involve the making of a portable mass of 1 kg, putting it in various Antarctic locations and detecting it using GRACE. Of course, this is not plausible because GRACE cannot resolve 1 kg mass on the ground. In a thought experiment, one can see more difficulties. One is sensitivity and its distribution. GRACE sees all mass around it until distance diminishes its effect into the noise envelope. A critical part of GRACE design is to attribute position to anomalous mass in 3D. This is an exceedingly complex problem. The question here is whether that problem has been solved in concept, then tested to show the size of each form of error. Final testing must be able to be traced back to the Pt standard.
    After reading Steve’s essay, my dominant thought experiment was not of placing a known mass in a known location, then detecting and quantifying it using GRACE. The problem is more like trying to measure the distance between a mobile measuring device like a satellite laser and a target object on a waterbed in active use.
    This troubled thought arises mainly because the expression of errors is commonly vague and ill-defined, even outright wrong. One suggested remedy is not to publish papers that use untried, uncalibrated, untraced-to-standards methods as sources of policy decisions. Until they are shown to be adequately accurate, such papers belong in the category of advances in methods, not in policy areas. Another suggested remedy is to rid the relevant scientific process of belief, in favour of reproducible data, and to follow the excellent BIPM guidelines for the measurement and expression of errors. Although BIPM is a central authority, I have never seen a reference to it in any climate paper. I must have missed those authors who have been schooled on the formal requirements of error analysis and include it properly in their papers.
    There is abundant relevant material on errors at, for example,

  22. TAC
    Posted Dec 4, 2015 at 7:36 AM | Permalink

    Thank you providing your characteristically superb analysis of this topic. Antarctic ice seems to present a conundrum: Why are East and West Antarctica behaving so differently? Why are earth’s two poles exhibiting such different responses? I used to have confidence in the uniform concept of “polar amplification,” but, as in so many areas, the data suggest it’s not that simple. Mother Nature has her own plans.

  23. Klapper
    Posted Dec 4, 2015 at 9:53 AM | Permalink

    There was a clear warning in 2009 in the peer-reviewed literature that the ever increasing estimates (then) of Antarctic ice loss from GRACE were out to lunch, since GPS measurements were at odds with then current PGR models. From the abstract of Bevis et al 2009:

    “The spatial pattern of these velocities is not consistent with any postglacial rebound (PGR) model known to us. …….. Therefore, our initial geodetic results suggest that most GRACE ice mass rate estimates, which are critically dependent on a PGR correction, are systematically biased and are overpredicting ice loss for the continent as a whole.”

    (Bevis, M., et al. (2009), Geodetic measurements of vertical crustal velocity in West Antarctica and the implications for ice mass balance,Geochem. Geophys. Geosyst.,10, Q10005,doi:10.1029/2009GC002642)

    But the story doesn’t stop there. This was published in October of 2009. In November of 2009 Chen et al was published (Chen et al 2009, Accelerated Antarctic ice loss from satellite gravity measurements, Nature Geoscience), and said this in the “Method” description:

    “Nevertheless, the true PGR model error over the Antarctica is unknown, owing to the lack of in situ
    uplift measurements and other data. Here we use the difference between IJ05 and ICE5G model estimates to approximate PGR model error.”

    I know what you’re thinking: how could an Antarctica scientist working in Texas know what an Antarctica scientist working in Ohio was working on? Think of how far Texas is from Ohio. But Chen et al did know about the GPS networks since they referenced GPS measurements on a peripheral matter to the PGR models.

    • Steve McIntyre
      Posted Dec 4, 2015 at 10:40 AM | Permalink

      A very relevant reference – online here. Unsurprisingly, it was not cited or discussed in AR5.

      Its approach appears to have influenced the subsequent PGR studies (Whitehouse et al 2102; Ivins et al 2013) which reduced GIA estimates, as both of these took note of GPS uplift measurements. This re-appraisal is continuing e.g. some citations in post, as some of the Whitehouse coauthors have continued to examine regions in detail through this method. An interesting issue – one which I didn’t mention in my post – is that uplift and GIA can be impacted by Little Ice Age loading, especially in low-viscosity mantle areas e.g. Peninsula and West Antarctica. This is mentioned in the most recent (2015) articles and looks to be an area of developing interest.

      • Jeff Id
        Posted Dec 5, 2015 at 10:21 AM | Permalink

        First, thanks for doing all of this work. I found myself reading climate papers on purpose again and it has been a while.

        This particular paper is extremely interesting and seems to indicate quite clearly a large source of the problem in ice loss estimates. I need to read about these point measurement instruments but the wide variation in solutions to ice loss doesn’t appear to be as ambiguous a problem as some would have us believe. This appears to be a significant fraction of the discrepancy in the results.

    • Posted Dec 4, 2015 at 2:01 PM | Permalink

      When I was at university, failure to cite such relevant research papers would have resulted in a failing grade.

      I guess it’s a kinder, gentler academia today.

  24. EdeF
    Posted Dec 4, 2015 at 11:45 AM | Permalink

    Steve, probably the most interesting article in a long while. A complex problem, you need to measure ice sheet altitude with either laser, radar, whatever, and then somehow measure the rise or fall in the continental surface due to the accumulation or decrease in snow/ice plus the density of the snow/ice pack. Rife for large estimation errors. I need to read more of the articles with respect to measurements of the thickness of the ice sheet itself. I think you could do this with a pulsed radar or sonar at multiple locations; possibly many passes from a low flying aircraft. Fascinating stuff, thanks for the time you have spent on this subject.

    • milodonharlani
      Posted Dec 4, 2015 at 12:58 PM | Permalink

      Studies of radionuclides in the soil around the EAIS show that it quit retreating about 3000 years ago. Whatever its mass and altitude are, it appears to have stopped melting after the Minoan Warm Period.

    • EdeF
      Posted Dec 10, 2015 at 1:14 AM | Permalink

      I found several articles on the use of low frequency, coherent air-borne radar use in determining the thickness of ice flows/glaciers, the article below was using such a radar to map large portions of Greenland. Low frequency (150 MHZ) signals have the capability of penetrating the 3-4000 m thick ice sheets that you find in Greenland or Antarctica. The radar is tied to a GPS receiver to get very good spatial location data.
      Add either a KU-band radar or LADAR to get good altimeter data to the top of the ice sheet and transform the various points into the WGS84 coordinate system for visualization. Radar uses pulse-compression techniques to get more energy on target and
      increase the S/N ratio, and improve the range accuracy of the radar, thought to be about
      +/- 10 meters. I am good with what I have seen in several of these reports.

  25. Posted Dec 4, 2015 at 12:28 PM | Permalink

    A very detailed excellent summary that I greatly appreciate. In researching my essay on this topic at WUWT, I know how much work and effort is required to wade through the various perspectives and time series. As you rightly conclude “it still seems like one of those too typical situations where the less alarming explanation is presented in specialist literature, but left unmentioned or unconfronted when retreat of West Antarctic glaciers is presented as a cause of alarm.”

    For people like you who truly survey what the experts are saying in the literature, the real consensus is not one of alarm.

  26. Jeff Norman
    Posted Dec 4, 2015 at 12:57 PM | Permalink

    It seems to me that a lot of the uncertainty around ice mass loss revolves around the GIA which is derived from the isostatic rebound after the deglaciation of the last ice age.

    Wikipedia (yes I know) has an interesting graph entitled “Paulson 2007 Rate of Lithospheric Uplift due to PGR” (Post Glacial Rebound).

    This suggests West Antarctica PGR peaks out at ~12 mm/yr while East Antarctica is ~3 mm/yr.

    The only place higher than West Antarctica is around Whitehorse which tops the chart at ~18 mm/yr. I can understand this rebound. Whitehorse is sort of in the middle of the North American Plate and as the snow and ice piled up it had nowhere to go but down, so it just piled up pushing the crust down until the climate changed.

    The Transantarctic Mountains divide the vast reaches of East Antarctic from the relatively smaller West Antarctic. These mountains are snow and ice covered but not generally buried under glaciers. They seem to run down either side of the range. It seems to me that if glaciers were going to pile up higher anywhere, it would be in the East Antarctic, where it is a long way to the sea. The West Antarctic ice could easily flow out to the Ross Sea, the Amundsen Sea and the Weddell Sea.

    If anything I would have thought the rebound in West Antarctic would have been more like the rebound from the Cordilleran ice sheet in British Columbia and the Yukon as described in “History and isostatic effects of the last ice sheet in southern
    British Columbia” Quaternary Science Reviews 21 (2002) 71–87.

    According to the Paulson chart the rebound in British Columbia is maybe 6 mm/yr.

    So, has the isostatic rebound in West Antarctica actually been measured, or just inferred or modelled?

    Is it actually that big or are they mixing in some tectonic uplift that is sugaring the pudding?

    If is is that big why did the glaciers pile up so high there?

    Cool stuff and I am a neophyte.

    • Jeff Norman
      Posted Dec 4, 2015 at 1:02 PM | Permalink

      Busted link:


    • Jeff Norman
      Posted Dec 4, 2015 at 1:45 PM | Permalink

      I just read Steve’s 10:40 link to “Geodetic measurements of vertical crustal velocity in West Antarctica and the implications for ice mass balance”

      Thank you. Some questions answered.

      • Howard
        Posted Dec 4, 2015 at 11:50 PM | Permalink

        Powerpoint overview of GIA and other issues.

        Click to access Wiens.pdf

        • Howard
          Posted Dec 5, 2015 at 12:20 AM | Permalink

          2012 M.Sc thesis showing big problems with previous mantle viscosity’s used for GIA.

          Video talk with Powerpoint showing the GIA problems with the early Grace studies

        • Jeff Norman
          Posted Dec 6, 2015 at 8:23 AM | Permalink


          Thank you.

          In the ppt link, slide 16, Pratt et al seem to be reporting that they measured a Whillans ice sheet slip of ~30 cm (0.006 m/sec for ~500 sec) using a GPS. Is that even possible or am I reading it wrong?

          In the Concerns section it says, “We need to here from the GIA community about what kinds of seismic models you need!”

          There’s a GIA community?

          Based upon some of the criticisms presented it seems they want a model that will give them the answers they want.

        • Howard
          Posted Dec 6, 2015 at 9:49 AM | Permalink


          The seismic picks up the high frequency pulses to determine slipe timing, location, direction, etc. and the gps is used to measure the low frequency surface displacement. The plot is combined gps/seismic. The gps records displacements on the order of 15-m/day, which is well within the accuracy limits.

          Click to access pratt_etal_jgr_2014.pdf

          “We need to here from the GIA community about what kinds of seismic models you need!”

          The GIA community is the client and the seismologists are the consultants with a huge tool box. I imagine the seismologists are not GIA experts and are looking to the client to communicate their what sort of tools might best suit their specialized needs.

  27. David Eden
    Posted Dec 5, 2015 at 8:35 AM | Permalink

    Very interesting, Steve.

    It’s been a while but we met one time back when I worked for OPG. We had a coffee one time and discussed possibilities for your son who was interested in work at one of the nuclear plants.

    I’m a geological engineer by training and a good opportunity came up in 2012 to go back to mining and I’ve been with Kinross since, very much enjoying it despite where the gold price has gone.

    I work mostly on reserve and resource reporting (and audits) and NI 43-101 TRs, which has deepened my appreciation for the mining business investor perspective you bring to climate questions.

    Anyway I just wanted to say hello and that I’m glad you’re doing these analyses.


    Steve: Hi, Dave. thanks for saying hello.

  28. Eric Steig
    Posted Dec 5, 2015 at 9:51 AM | Permalink


    I found your post pretty interesting and have no quibbles with the it except you’re unfair to Bamber, who indeed *has* “spoken out” about uncertainties in these sorts of data (including his own analyses) on numerous occasions. In his RealClimate post, Bamber’s really not saying “Zwally is wrong”. He’s simply saying it’s premature to conclude we know the answer.

    Measuring the sign of East Antarctic ice volume change is very difficult, and in my view may be unresolvable (unless the trends were to become so big — one way or the other — as to become unambiguous. It’s the classic problem of trying to tell the difference between two very large and very uncertain numbers. Incidentally, there was a big study of ice sheet models done a couple years back, by Bindschadler et al. (J. Glaciology, I think), and the range of projected ice sheet losses over the next 50 (100?) years crosses zero. That is, studies don’t agree with one another for the near term future (though they do for the very long term). This is precisely because while WAIS loss continues, EAIS loss or gain remains uncertain in the face of expected climate change over the next century. This is quite different from Greenland where all the models (and modern observations) show that losses dominate over gains.

    Regarding my point about “50% of scientists” think the evidence for a significant anthropogenic contribution to recent trends in West Antarctica, that comes from an “expert solicitation” study by Jon Bamber. See Figure 4.

    Steve: thank you for stopping by and commenting. The topic deserves a thorough survey by a specialist going through the events in order, as it’s an important one.

    • Sven
      Posted Dec 5, 2015 at 3:24 PM | Permalink

      “… Bamber’s really not saying “Zwally is wrong”. He’s simply saying it’s premature to conclude we know the answer.”

      It would be interesting to know how many “it’s premature to conclude we know the answer” articles by Bamber or anybody else you’ve run on Realclimate as response to research showing decline of Antarctic ice mass.

      • Howard
        Posted Dec 5, 2015 at 9:04 PM | Permalink

        Give Eric a break. He’s in a tight spot trying to run through the raindrops without getting wet.

      • Eric Steig
        Posted Dec 7, 2015 at 10:08 PM | Permalink

        While I’m sure you are simply try to score a point with your comment, the reality is that we try to provide “counter-points” when the media overplays or spins important things in misleading ways, as happened here, regardless of the direction. I’ve written more than one post myself on exactly the subject you raise.

  29. Alberto Zaragoza Comendador
    Posted Dec 5, 2015 at 10:28 AM | Permalink

    Thanks for this long post. Seems like a link is missing here.

    ‘Michael Mann, whose own comparisons of observations to models notoriously ended in 2005 (see ^)’

    The GIA issue seems very similar to aerosols, in which extremely uncertain (but large) estimates that had no observational support were used to justify high sensitivity estimates from models that (i.e. the warming didn’t show up in observations because it was theoretically cancelled by the aerosols). Here too the modellers keep using the old, higher estimates because the new and lower ones throw their numbers out of whack.

  30. Posted Dec 5, 2015 at 12:33 PM | Permalink

    Thanks very much for carrying out such a thorough analysis and making this very detailed and informative post. Most intersting.

  31. human1ty1st
    Posted Dec 5, 2015 at 1:47 PM | Permalink

    Congratulations, a fanastically coherent report based on what must have been a lot of work.

    At the very least the Icesat data and Zwally analysis seem like a good, independant test of whether the ongoing GIA/GRACE work is well grounded. Simply dismissing it as out-of-dated seems to miss alot of the importance this work could have.

    I have a quick question, that you may have already answered in the long post. You mentioned the GIA adjustmests were previously higher and have come down in more recent work. Do those newer adjustments bring the GRACE in line with the Zwally estimates or is there still room for the GIA/GRACE estimates to come into line with Zwally?

    Steve: there is still a big gap between new GIA and Zwally. In the post, I discussed Zwally’s estimate of the amount of difference in East Antarctica in mm/year needed to reconcile.

  32. David L. Hagen
    Posted Dec 5, 2015 at 2:12 PM | Permalink

    Thanks for your effort and highlighting the major “adjustments” comparable to the “effect”. Similar issues are reviewed by the NIPCC in Climate Change Reconsidered II, Physical Science (2013) , 5 The Cyropshere, e.g., Antarctic Ice Cap p 639.

  33. billw1984
    Posted Dec 5, 2015 at 3:51 PM | Permalink

    Steve, (or anyone else)

    I don’t understand in Figure 2 why the isostatic adjustment (PGR)
    would move the data upward in 2003 and downward in 2005.

    What am I missing?

  34. Ted Carmichael
    Posted Dec 6, 2015 at 11:13 AM | Permalink

    Dear Steve,

    This is a thorough and fascinating post. Thanks for providing this detailed summary. I’ve long worried that the GRACE conclusions were based on a new and untested methodology, and as such thought it premature for some to assert that Antarctica is “surprisingly” losing large amounts of ice mass. But that concern was based on intuition; I now understand the issues much better.

    Just before your concluding remarks you said, “IPCC Table 13.1 attributed only 0.27 of 3.2 mm/year sea level rise from 1993-2010 to the Antarctic ice sheet (equivalent to about 90 Gt/year), rating multiple other factors (thermal expansion, non-icesheet glaciers, Greenland ice sheet, changes in water storage on land) as greater contributors. Uncertainties for thermal expansion and glaciers were each greater than the contribution attributed to Antarctica.”

    I thought you might find it interesting to know (if you don’t already) that the measures of sea level change also have a GIA adjustment. And this adjustment is also larger than the Antarctic ice sheet component, about 0.3 mm/year. (“Sea level rise,” as measured at the University of Colorado Sea Level Research Group, is actually a proxy for sea volume, as they explain, rather than the level of the sea surface.) In other words, they use a model that assumes the sea basins are getting deeper to make the GIA adjustment, which adds about 10% to the reported sea level rise.

    The uncertainty of this adjustment – “about 50%” – is more than half the contribution attributed to Antarctica. (

    Thanks again for a great post. Cheers.

    • mpainter
      Posted Dec 6, 2015 at 12:51 PM | Permalink

      ” In other words, they use a model that assumes the sea basins are getting deeper to make the GIA adjustment, which adds about 10% to the reported sea level rise.”
      Ted, its even worse than that. They add the .3 millimeters/year to SL rise. It is a hypothetical figure based on what the SL rise _would_be_ if there were no GIA to the ocean basins. As such, it is spurious addition to SL rise. The U of Colorado sea level rise is full of such trickiness and should be disregarded by serious investigators.

      Also, this type of GIA adjustment is not rebound, but subsidence at the margins of the continents. The hypothetical cause is the flow of mantel toward the interior of the continents, as it rebounds from the weight of the ice mass. The subsidence of the NE US Coast is believed to be due to this GIA subsidence and I imagine the east coast of Canada is also subject to this type of GIA.

    • mpainter
      Posted Dec 6, 2015 at 1:04 PM | Permalink

      I should add that the margins of Anarctica would hypothetically be subject to this “subsidence” type of GIA, according to how much rebound GIA occurs in the interior. Perhaps this is all taken into account by the “specialists”. Seemingly, these two different types of GIA would steepen the gradient of the base that the ice glides on: rebound in the interior, subsidence at the margins. A very complex study, perhaps overly simplified.

  35. Posted Dec 6, 2015 at 11:44 AM | Permalink

    Steve, thanks for your research, which encourages us all to expand our understanding of the climate industry.

    While Anthony Watts had certainly taken satisfaction in Zwally’s results, Media Matters did not identify any actual distortion: it was Zwally’s results that undermined the narrative, not the supposed distortion of the results.

    Once again it seems that in the rush to make conclusions the methods were not sufficiently validated. Steve, do you know if there were any calibration tests like, for example, analyzing GRACE data from a fly-over of easily survey-able volcanic islands in the Pacific or Mt. Denali? Did you find any writing about the anticipated GRACE-FO follow on mission scheduled for 2017? From Wiki I see it will have a laser range finder along with the micro-wave range finder that GRACE has. Perhaps this will reveal some of the precision questions with the system itself. As for the GIA accuracy question I ask the same question about validation experiments; what’s been done?

    • milodonharlani
      Posted Dec 6, 2015 at 11:49 AM | Permalink

      Ground validation of GRACE, linked without comment on its validity:

      Click to access crossley_etal_GRACE_validation_BIM09.pdf

      • Steve McIntyre
        Posted Dec 6, 2015 at 3:51 PM | Permalink

        In GRACE literature, Antarctica is a fairly special case and recognized as such by specialists. It’s one thing to validate the gravity measurements and an entirely different thing to validate the apportionment between underlying rock and overlying ice sheet. That apportionment is obviously a topic of research – that’s what’s caused the changes in GRACE estimates.

        • milodonharlani
          Posted Dec 6, 2015 at 4:10 PM | Permalink


          Thank you for all you have done.

          And for this comment. IMO the issues you raise may never be satisfactorily answered, at least for the EAIS.

          Which makes Gavin’s preference for GRACE even less justifiable, IMO.

  36. Frank
    Posted Dec 6, 2015 at 12:39 PM | Permalink

    Great post, Steve. I have missed your work the past several months.

    If I understand your post correctly and over-simplify, GRACE shows the mass of Antarctica remaining roughly the same. Since glacial isostatic rebound is increasing the mass of Antarctica, ice must have been lost to compensate. A large amount of rebound implies a large loss and a small rebound implies little ice loss or possible even a gain. However, isn’t rebound under Antarctica is accompanied by a drop in the ocean floor around Antarctica? Therefore, whether a large or small amount of ice has been lost from Antarctica is basically irrelevant to SLR. If the mass of Antarctica has remain roughly constant, so has the mass of the ocean.

    Some scientists adjust SLR upward to correct for the falling ocean floor. This appears to create the opportunity to “count” the same phenomena twice: once as ice loss and once as a correction for rebound.

    Steve: since rock is heavier than ice, ocean levels could rise.

    • Posted Dec 6, 2015 at 1:17 PM | Permalink

      Frank, I was thinking the same thing. If the isostatic adjustment effect is now being used to adjust SLR and glacial loss they could be doctoring both sides of the balance, with or without realizing it. I wonder if there has been study of the big island of Hawaii (geologically active) versus the longest geologically dormant, Kauai and Niihau, for evidence of a faster sinking shoreline.

      • Steve McIntyre
        Posted Dec 6, 2015 at 3:58 PM | Permalink

        There has been lots of competent research on glacial isostatic adjustment for decades. I urge readers not to speculate that specialists have failed to consider issues, unless you have specific evidence. Such speculation, if unsupported by any actual reason, tends to debase the coin of other comments.

        • Posted Dec 7, 2015 at 1:56 AM | Permalink

          Steve, although Zwally insists that the gap in GIA estimated for EAIS caused by his research is not to be used by “skeptics,” the logic seems inescapable, as you pointed out, that the appeal to authority is hollow when the authorities themselves have the gap. It is they, after all, who can’t agree on the net sign of EAIS since 1979. And, I willingly admit I know little, but I do surmise that those working on sea floor isostatic adjustments to adjust SLR are different specialists, in a different group, from the GIA specialists, making independent judgments on their side of the ice melt balance sheet while having access to the answer on the other side of the equation from the other group’s published results. So I think Frank and my point is that the potential exists for both groups to be validating with the closure caused by the other sides bias, (not saying they are).

    • Jeff Norman
      Posted Dec 7, 2015 at 9:42 AM | Permalink

      Okay, now I’m confused. Isostatic rebound is a flexing of the crust/mantle. How does this change its mass?

      • Jeff Norman
        Posted Dec 7, 2015 at 9:48 AM | Permalink

        I thought GRACE measured no detectable mass changes (within the margin of error over a short period of time). The altimetry showed the changes in elevation at the surface of the ice. The GIA modified the altimetry for isostatic rebound. The volume of ice is dependent upon the modified elevations of the ice fields.

      • Jeff Norman
        Posted Dec 7, 2015 at 9:55 AM | Permalink

        I thought GRACE measured no changes in mass (within the margin of error of the method over a relatively short period of time). The altimetry data at the surface of the ice shows how the ice volume has changed. The GIA model is used to model changes in the elevation of the underlying bedrock to modify the ice volume to compensate for isostatic rebound.

        How precise is GRACE? If the ice volume changed 10% say what percentage of the overall continental mass would that be, <2%?

        • Jeff Norman
          Posted Dec 7, 2015 at 9:58 AM | Permalink

          Sorry, glitchy browswer.

      • Howard
        Posted Dec 7, 2015 at 7:24 PM | Permalink

        Jeff: Mantle mass is displaced laterally during glacials and then returns during interglacials. There is a lag between the ice mass loss and the mantle mass return. This response in non-linear. I hope you don’t take this the wrong way because I’m not being snotty. I recommend you invest in some geology and oceanography textbooks. Getting a solid foundations in the basics will make these types of studies easy to understand. If you realy get into it, then you will kill yourself on the highways looking at road cuts instead of the road.

    • Frank
      Posted Dec 7, 2015 at 9:58 AM | Permalink

      Steve: Thank you for you reply. If you notice, didn’t draw any speculative conclusions; I asked a serious QUESTION and noted the possibility of double counting:

      “However, isn’t rebound under Antarctica is accompanied by a drop in the ocean floor around Antarctica?”

      From a pragmatic point of view, if rebounding land is “pushing”an equal volume of ice into the ocean, there will be no change in sea level – the most important metric. Your reply correctly notes that the density of rock and ice are very different.

      When the altitude of the ice is measured, a constant altitude/volume represents rising rock replacing lost ice and an increase in the mass of Antarctica. To a first approximation, SLR remains constant. Altimetry has the advantage that the amount of rebound is irrelevant when calculating SLR.

      When the mass measured by GRACE remains constant, dense crust from the ocean floor is being replace by less-dense water. If the density difference is 2.5X (as suggested by Wikipedia), 60% of the estimated ice loss after correcting for rebound will produce SLR.

      If one treats that amount of rebound as a variable, shouldn’t there be some amount of rebound (or subsidence) that will bring the altimetry and GRACE measurements into agreement? (If the mass of East Antarctica is increasing, it could be subsiding while West Antarctica is rebounding.)

      Steve: I don’t understand the argument. In making the sort of argument that you’re presenting here, it’s usually helpful if you refer to original literature rather than making the argument ex cathedra, as such issues usually have been contemplated in original literature. I’m not familiar enough with the sorts of evidence that would be relevant to have an opinion right now.

  37. mpainter
    Posted Dec 7, 2015 at 10:29 AM | Permalink

    A few observations:
    Wikipedia gives that Pine Island Glacier started an accelerated rate of flow in 1974. The article said that the rate of acceleration has been increasing since then. This acceleration does not seem to be related to the present day climate issue, given the 1974 date.
    Also, the article gave that this one glacier accounted for 25% of Anartica’s mass loss. Pine Island Glacier is a grounded glacier with a floating ice shelf.

    I would point out that the melt of a grounded glacier occurs at or below sea level. Hence, the volume of melt does not reflect in sea level measurements because sea level has already adjusted to the volume of the glacial mass which had previously displaced the seawater. The melt is simply a phase change of a mass that had long before made its contribution toward SLR, if any.

    And, by this consideration it seems that calculations of sea level rise based on ablation of grounded glaciers are spurious.

    Consider: if a grounded glacier advanced without melting (very unlikely) sea level would rise according to the volume of sea water displaced by the advance, adjusted for mass gain.
    So, by this we see that volumes of melt do not determine the question of how much, if any, SLR is contributed by a grounded glacier. By the same consideration, the retreat of a grounding line could mean accelerated melt or deceleration of glacier flow. Accelerated melt would have no consequence on sea level, but decelerated flow could, likewise accelerated flow.

    I would put that the correct method of determining the effect of a grounded glacier on sea level would be mass gain adjusted for the volume of glacial mass that is submerged at or below sea level, during the same interval.

    This method requires calculations of rates of flow, given in volumes. I also would put that rates of flow of grounded Anarctic glaciers depend more on conditions at the base of the glacier and the surface which it glides across rather than undetected changes in climate.

    • Steve McIntyre
      Posted Dec 7, 2015 at 1:59 PM | Permalink

      I don’t understand your reasoning. I have seen no reason to dispute specialist “calculations of sea level rise based on ablation of grounded glaciers are spurious”. This sort of calculation is pretty uncontroversial. To make this sort of point, you need to cite original literature and show the actual problem. It’s not an issue that bothers me, based on anything that I’ve read.

      • Frank
        Posted Dec 10, 2015 at 10:55 AM | Permalink

        Steve: Thanks for the reply. Sorry I haven’t been clear. It is an issue I don’t understand as well as I’d like. And I apologize for not citing specialist literature as you requested.

        “We apply a correction for GIA because we want our sea level time series to reflect purely oceanographic phenomena. In essence, we would like our GMSL time series to be a proxy for ocean water volume changes. This is what is needed for comparisons to global climate models, for example, and other oceanographic datasets.”


        OV = TS + AIS + GIS + GW + SW + GIA
        SLR = OV – GIA

        where we are discussing CHANGES in ocean volume (OV), antarctic and greenland ice sheet volumes (AIS, GIS), ground water (GW), surface water on land (SW).

        Society doesn’t care about the volume of the ocean; it cares about SLR damaging coastal areas. Now we have eliminated dependence on GIA and are reporting in more useful units of SLR. (Volume of SLR is converted to mm of SLR.)

        SLR = TS + AIS + GIS + GW + SW

        Your post discusses how assumptions about GIA underneath Antarctica change the calculated volume of water added to the ocean. If I am correct, such assumptions are not needed when the key metric is SLR, not ocean volume.

        The change in volume (vAIS) of the AIS measured by altimetry is the sum of the water (wAIS) ADDED to the AIS plus the volume of glacial rebound under Antarctica (vAGR). If water is lost from the AIS, wAIS will be negative.

        vAIS = wAIS + vAGR
        wAIS = vAIS – vAGR

        However, vAGR is produced by an equal volume of subsidence of the ocean floor near Antarctica. We have to correct SLR for the larger volume of the ocean only near Antarctica.

        SLR = -wAIS – vAGR
        SLR = -vAIS + vAGR – vAGR
        SLR = -vAIS

        Thus SLR is independent of assumptions about Antarctic Glacial Rebound. This isn’t true about ocean volume.

        Now what happens when the change is measured in terms of mass by GRACE. d is density of rebounding crust. Repeating the same steps from above using m for mass measurements, v for volume measurements and wAIS for the volume of water ADDED TO the ice sheet.

        vAIS = mAIS
        vAGR = mAGR/d

        vAIS = wAIS + vAGR (copied from above)
        wAIS = mAIS – mAGR/d

        SLR = -wAIS – vAGR (copied from above)
        SLR = -(mAIS – mAGR/d) – mAGR/d
        SLR = -mAIS

        Again, the calculated SLR is independent of assumptions about the amount of glacial rebound.

        If the key metric is SLR rather than the change in ocean volume (OV), one doesn’t need to worry about the correct value for GIA or for glacial rebound underneath Antarctica.

    • mpainter
      Posted Dec 7, 2015 at 4:40 PM | Permalink

      “cite original literature”
      Steve, it is hardly a matter of specialists or peer review. It is a matter of being able to visualize a few simple principles regarding ice and water. Ice melting in water adds no volume; the melt occupies the volume of the ice. To visualize this principle in regard to a grounded glacier, drop an ice cube into a glass of water. The water level immediately rises in the glass. But there is no rise when the ice melts.
      Similarly, the glacier melt at its grounded front adds no volume to the ocean because the ice is already submerged. You have simply a phase change wherein the melt occupies the volume formerly occupied by the ice, now melted (actually less volume).

      To continue the ice cube into-the-glass analogy, the grounded glacier displaces a volume of sea water as it advances across the sea floor. This displaced sea water is the glacier’s contribution to SLR, after adjustment for mass gain (the displaced water must rise).

      By using ice melt (ice mass loss) at the front of a grounded glacier as a basis for calculating SLR, you obtain spurious results because the melt is simply a phase change, as in the ice cube melting. The water level did not rise in the glass nor does sea level rise.

      The problem is this: Grace data is used to calculate ice mass loss in west Antarctica. This figure is in fact a proxy for ice melt at the front of grounded glaciers because that is where the loss occurs. But, as I have shown, melt at the front of grounded glaciers has no effect on sea level (except theoretically to slightly lower SL through the phase change wherein water occupies about 10% less volume than the ice).

      It is the displacement of seawater by the grounded glacier that should be used to calculate sea level effect- again, the ice cube into the glass of water analogy. This displacement is best calculated by the glacier flow rate, which can by adjusted for mass gain to obtain SLR, if any.

      By a simular consideration, satellite data might show a mass gain, but the real metric is the rate of flow. A mass gain might simply include an advance of the grounding front, hence seawater displacement and SLR, in theory. Measuring flow of the glacier is the only reliable metric for calculating sea level effects, imo.

      • Steve McIntyre
        Posted Dec 7, 2015 at 10:12 PM | Permalink

        I personally have no difficulty accepting that melting of continental ice sheets has increased sea level in the past and could do so in the future. Interesting as this topic may be to you, it has virtually nothing to do with the substantive issues raised in the post.

        Blog policies have long discouraged physics speculations by readers and I’m going to apply this policy to speculations about how glaciers contribute to sea level rise. Sorry about that.

      • mpainter
        Posted Jan 15, 2016 at 7:32 AM | Permalink

        Not so speculative, after all. From the National Snow and Ice Data Center’s Quick Facts on Ice Shelves:


        Because ice shelves already float in the ocean, they do not contribute directly to sea level rise when they break up… Glaciers and ice sheets rest on land, so once they flow into the ocean, they contribute to sea level rise.”

        My comment was ultimately directed at the methods of mass balance calculations. Your figure 1 (Zwally et al fig. 6b) answers that. Note that ice shelves are excluded from the mass balance isopachs shown there. Presumably, the delimitation is the sea level contour and I assume that this is standard with the “specialists”.

        But now a new question arises. Unless the demarcation line (i.e., the sea level contour) for mass balance determinations is precisely adhered to, with constant precision from one study to the next, there is introduced a source of error. The coastline of Antarctica is some 11,000 km. Another reason, imho, to keep the salt handy when reading studies on Antarctic mass balance.

        • mpainter
          Posted Jan 15, 2016 at 7:37 AM | Permalink

          Correction: 18,000 km coastline, according to Wikipedia.

        • milodonharlani
          Posted Jan 15, 2016 at 8:27 AM | Permalink

          Or 11,000 statute miles.

  38. Geoff Sherrington
    Posted Dec 7, 2015 at 5:05 PM | Permalink

    GRACE has 2 ducklings following it at a distance so that gravity features change their relative positions thus leading to a measure of gravity field.
    Gravity features also affect the main platform of GRACE, so presumably its perturbation by gravity needs correction.
    But, it is there to measure gravity, so we have latent circularity of argument.
    One part of me agrees with Steve that a lot of bright people have this sorted. Another part worries about the logic of taking care of circularity especially in papers released before the field has matured agreeably into proper reconciliation of error estimates.
    This premature speculation problem is increasing with for example, quizzical initial releases for satellite estimates of global CO2. In the old days we tended not to publish so soon in the evolution of method.

    • Posted Dec 8, 2015 at 3:43 AM | Permalink

      Grace is made of two identical modules orbiting 230km apart on the same orbital path. Their precise distance is measured by synthetic aperture radar interferometry with its K-band Ranging System (KBR) to within 10 µm. The 2017 follow-on mission will increase that precision by use of laser interferometry. There are accelorometers to detect drag not caused by gravity but I would like to see a third clone duckling to be used for direct error determination from effects like weather flow, etc…

    • Geoff Sherrington
      Posted Dec 11, 2015 at 4:39 AM | Permalink

      Correction in the “what was I thinking?” category.
      Of course, GRACE has 2 very similar satellites, not a pair following a leader as I wrote wrongly.
      However, the points still stand. There is an element of circular logic; and there should be a calibration trail that leads to an accepted standard like the Pt kilogram.
      Obviously, Steve would have had little to write about if errors in the various measurements had been properly calculated and expressed prior to their publication. His essay arises because some of the error estimates are wrong.

  39. Geoff
    Posted Dec 13, 2015 at 12:00 AM | Permalink


    This kind of analytical work on the status of the literature and uncertainties/inconsistencies is very valuable to give a context to the current discussions.

    See also the latest estimate by Pippa Whitehouse which could potentially account for almost half of the 1.7 mm/year Zwally estimates is needed to reconcile.


  40. Posted Dec 13, 2015 at 1:47 AM | Permalink

    See also the recent commentary on ice sheet dynamics by Stokes, ( ) including the GIA review. (Which includes the comment “A challenge for the pure GIA approach is that the space time thickness histories that it delivers will, in general, not be in accord with glaciological first principles”.

  41. TomRude
    Posted Dec 19, 2015 at 12:17 PM | Permalink

    Thank you, a very interesting post. And Steve, having hopefully read Dynamic Analysis of Weather and Climate, cannot not remember Prof. Marcel Leroux explanation for the dynamic warming of the peninsula since obviously Zwally’s work fits like a glove.

  42. hswiseman
    Posted Dec 19, 2015 at 5:15 PM | Permalink

    West Antarctica is relatively easy to get to and photograph glacial calving. Changes in ocean circulation have undoubtedly warmed this easily observable part of the continent, leading to loss of ice mass. Even the tarted up WA temperature numbers generated by Steig cannot explain the loss of mass, otherwise. East Antarctica is a year-round life threatening proposition and no one goes there.

    So, change of ocean circulation increases WA melting and change glacial mass, change of ocean circulation increase global temperature with a massive El Nino. However, there is not a scintilla of evidence that recent changes to atmospheric CO2 can create oceanic forcings with major magnitudes. It is like saying that a human could push an 18 wheeler to highway speed under her own power. Actually the 18 wheeler needs its own powerful engine. So does the ocean-the massive force of naturally variable westerlies over millions of square miles, and even when these forces align, the Pacific’s normal thermocline is disrupted in only a limited area of warm water segregation. There is now an entire generation of scientists who must believe in Santa, Unicorns and the CO2 Fairy to empower this trace molecule with the supernatural capacity to transform the fundemental behavior of the oceans.

  43. Mickey Reno
    Posted Dec 20, 2015 at 4:34 PM | Permalink

    A recent study concluded that deposition on some West Antarctic coastal glaciers had been greatly, vastly increased over the past 30 years and that elevation at the tops of these glaciers was higher. There were some models involved, but if so, the net loss may be in fact, a net gain in the WAIS.

    I understand that actual GRACE and other physical observations cover a brief period compared to the life of the ice itself. But if the West and East Antarctic land ice are growing, then it’s pretty difficult for the IPCC to conclude that Antarctic ice is losing mass at all, and even more difficult to assert that the mass loss is speeding up.

  44. blogollum
    Posted Jan 6, 2016 at 7:31 AM | Permalink

    Fascinating, and I am one of the lay people whom you encourage to look at the E/W “divide”. To state the obvious, the coast of E Antarctica at current sea levels is round (+/- constant latitude), while the coast of W Antartica is anything but. In the inter-glacial, the glaciation seems to be in the late stages of conforming to the coastline. At the LGM, it would seem that the glaciation would have been much rounder. Not a particularly strong use of my MBO, but it helps me understand the Holocene-length trends that might be at work in W Antarctica.

  45. mpainter
    Posted Apr 30, 2016 at 12:59 PM | Permalink

    NASA’s Jet Propulsion Laboratory maintains an alarmist website on polar ice sheet mass balance.

    Note that the last update for this site is given as April 27, 2016, three days ago.

    This site claims a mass loss for Antarctica of 134 gtonnes of ice annually since 2002. Source of data: NASA GRACE satellite. Says nothing about GIA (glacial isostatic adjustments). Says nothing about Zwally, 2015.

    This site also claims a mass loss from Greenland of 247 gtonnes/yr.

    Site editor: Holly Shaftel
    Site manager: Randal Jackson
    Senior Science editor: Laura Tenenbaum

    Thus the Jet Propulsion Laboratory has been shaped into an organ for misinformation on climate issues.

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