Climate Audit

Mar 2, 2021. This post was written in 2015 but, for some reason, I didn’t publish it at the time.  Seems just as valid today as when it was written.

Esper et al 2012, Orbital Forcing of Tree Ring Data pdf SI, is one of the few paleoclimate articles in past decade which really made me stop and think. It connected two obvious points:

• high-latitude tree ring proxies are sensitive to summer (JJA, even JJ) temperature, not annual temperature.
• high-latitude NH summer insolation, which has long had special interest as the “prime forcing” of Milankovitch theory of ice ages, had declined by ~6 w m-2 over the past 2000 years, the period covered by many popular IPCC temperature reconstructions. An amount that is approximately four times larger than anthropogenic forcing from CO2 since 1750 AD (~1.5 w m-2).

From these two points, they made plausible and compelling observation that the very large changes in high-latitude Holocene summer insolation should be visible in long high-latitude tree ring chronologies, especially those chronologies reaching back to the Roman period and earlier.

But it isn’t, as they demonstrated in an important graphic, which, unfortunately, was buried in the SI where it passed unnoticed. (One of the authors drew my attention to it several years ago or I too would have missed it.) Long tree ring chronologies have negligible millennial-scale variance – one more reason to distrust the temperature reconstructions of PAGES2K and IPCC.

Esper Figure S1

Here is the interesting figure S1 from Esper et al 2012. It compared three long tree ring width chronologies (grey-black) to Norwegian glacier equilibrium line (blue) and Yamal treeline (km north of present treeline.)  While the glacier equilibrium line (and Yamal tree line) have both migrated lower (southerly) with

However, as shown in Figure S1 of Esper et al 2012 (shown below), these long chronologies have more of less zero millennium-scale variability over the past 7000 years i.e. in addition to other defects in tree ring chronologies as temperature proxies, they only show high-frequency variability.  In contrast, Holocene-scale changes were visible in equilibrium lines of Norwegian glaciers and the treeline in Yamal.

Figure 2. Esper et al 2012 Figure S1. Original Caption: Showing multi-millennial TRW records from Sweden, Finland, and Russia (all in grey)
together with reconstructions of the glacier equilibrium line in Norway(blue), northern treeline in Russia (green), and JJA temperatures in the 60-70°N European/Siberian sector from orbitally forced ECHO-G7,8 (red) and ECHAM5/MPIOM9 (orange) CGCM runs10. All records, except for the treeline data (in km) were normalized relative to the AD 1500-2000 period. Resolution of model and TRW data were reduced (to ~ 30 years) to match the glacier data. Comment: Tree ring width chronologies (grey) are Grudd et al, 2002 (Tornetrask, northern Sweden); Helama et al 2010 (Finland); and Hantemirov and Shiyatov 2002 (Yamal); Norwegian glacier equilibrium lines (blue) are Aspvatnet (Bakke et al 2005a) and Lyngen (Bakke et al 2005b); treeline northing (in km) is from Yamal (Hantemirov and Shiyatov, 2002). 

In their important diagram in the Supplementary Information, Esper et al showed proxies back to 7000 BP, but neglected to show or discuss insolation changes earlier in the Holocene. These are even more dramatic as shown in figure below. Summer insolation at 50N has decreased by more than 35 w m-2 (!!!) since the early Holocene (10000 BP) and is presently at levels characteristic of the Last Glacial Maximum (19000 BP). The disintegration of the Laurentide ice sheet, previously covering Canada, took place primarily in the period of maximum summer insolation (12000-8000 BP). I’ve shown the scale of modern anthropogenic forcing in red for reference.

In older paleoclimate texts, paleoclimatologists reported signs of “neo-glaciation” during the past 4000 years, consistent with Milankowitch factors.  In the 19th century, ice-rafted debris (IRD) was observed at Hvitarvatn lake for the first time since the LGM due to expanded local glaciers. (This proxy has been repeatedly discussed at Climate Audit.) Varve thicknesses at Hvitarvatn also increased dramatically in the Little Ice Age, reaching their maximum in the 19th and early 20th century. BSi productivity at Hvitarvatn was at its maximum from 9000-5000 BP, slightly after the insolation maximum – presumably delayed until LGM glacier had sufficiently receded.

Climate Audit readers will recall that PAGES2K (2013) used Hvitarvatn data upside-down – interpreting wide varves at the height of the Little Ice Age – as evidence of warmth, rather than the opposite, as I noticed almost immediately – see ^. Aside from the use of data upside down being an embarrassing, almost Mannian, gaffe, one has to wonder at the apparent lack of understanding of underlying data on the part of the multiproxy collaters. I have an identical beef in respect to Baffin Island data. Glaciers in Baffin Island – a past center of Laurentide glaciation – similarly expanded in the Little Ice Age through the 19th century. 19th and early 20th century varve thicknesses from Baffin Island, where the time series data pattern is astonishingly similar to Iceland, are nonetheless interpreted by PAGES2K (and IPCC) as evidence of warmth.

Because tree ring width chronologies were unresponsive to large but slow changes in insolation, Esper et al observed that temperature reconstructions relying on long tree ring chronologies (most of the popular IPCC two millennium chronologies) would be similarly unresponsive. Esper stated this conclusion as follows:

an evaluation of long-term temperature reconstructions, even over the past 7,000 years from across northern Eurasia, demonstrates that TRW-based records fail to show orbital signatures found in low-resolution proxy archives and climate model simulations (Supplementary Fig. S1). These discrepancies not only reveal that dendrochronological records are limited in preserving millennial scale variance, but also suggest that hemispheric reconstructions, integrating these data, might underestimate natural climate variability.

This conclusion impacts PAGES2K, Mann et al 2008 and all other temperature reconstructions relied upon by IPCC.

Norwegian Glacier Equilibrium Lines

Details on the two Norwegian glacier equilibrium line series are shown below (slightly different horizontal scale).  At both locations, glaciers are believed to be absent in the early Holocene (during highest summer insolation) and to have formed around 5000-4000 BP (neo-glaciation).  During the late Holocene, the glaciers expanded until ~2000 BP with maximum Holocene extent in the Little Ice Age – a pattern that is characteristic in many locations. During the 20th century, there has been a noticeable retreat of the glaciers – back to levels characteristic of the early first millennium.

Yamal Treeline

The treeline series illustrated in Esper et al 2012 was derived from Hantemirov and Shiyatov Figure 2 (but excluding its Early Holocene portion). It showed mid-Holocene treelines extended approximately 30 km north of present treelines. However, this 30 km figure represented the northern limit of the survey, NOT the actual Holocene treeline. By the time of Hantemirov’s thesis in 2009, the survey – and the mid-Holocene treeline – had been extended nearly 120 km north of the current treeline (see middle panel). It appears that the Holocene treeline may have been even further north: in 1941, Tikhonov reportedly observed sub-fossil Holocene trees at 70N, approximately 275 km north of the present treeline. So, while Esper et al were right to note that Holocene treeline was further north, their diagram dramatically under-estimated the actual distance further north of the Holocene treeline, not just absolutely, but in respect to what was known in Russian literature at the date of their article.

Note that, in the 20th century, the Yamal treeline finally reversed its long march south, though still located far south of its Holocene location. This reversal corresponds to the 20th century reversal of the equilibrium line of Norwegian small glaciers – neither effect being apparent in the Esper et al figure.

Vinther et al 2009

The long decline in Holocene

Vinther et al 2009 (Nature) is a seminal article on the interpretation of Greenland d18O which makes the popular Alley (2000) temperature reconstruction from GISP2 totally obsolete.  Vinther observed that the elevation of the Greenland ice sheet had decreased substantially over the Holocene and that this had a material impact on d18O values at the summit (where GISP2 and GRIP are located): by flattening out the curve through the Holocene. Vinther observed that elevation changes through the Holocene were negligible at Renland (Greenland) and Agassiz (Ellesmere Island) and proposed (convincingly in my opinion) that the d18O records at these locations provided a more accurate record of climate change through the Holocene, and could even be used to estimate elevation changes at the summit of the ice sheet (where GISP2 was located.) Rather than being stable through the Holocene, Renland d18O showed a steady decline through the Holocene.

Conclusion

Over the past two millennia, the Norwegian glacier equilibrium line series in Esper et al Figure S1 (blue in excerpt at right) do have a sort of hockey stick shape that is not derived from ex post screening, stripbark bristlecone ring widths or other Mannian tricks. However, in a longer Holocene context, the reversal is both modest in scale and in a direction that mitigated intensifying neo-glaciation from Milankovitch factors.  One possible interpretation of this data is that anthropogenic CO2 has mitigated and even slightly reverse the Milankovitch forcing into potentially much expanded NH glaciation (compare to Ganopolski’s “near miss” article).