In a recent RC post entitled “Ice Hockey” and a recent Nature article, Steig and coauthors have introduced a novel and very baroque “hockey stick”, one without a blade. A true Halloween of horrors: in addition to Gergis’ zombie hockey stick, the bladeless Hockey Stick of Sleepy Hollow is now at large.
The appearance of Steig’s bladeless hockey stick was apparently so horrifying that he dared not show it in the RC post. However, I believe that CA readers are made of sterner stuff and will be able to withstand the sight of even a bladeless hockey stick, which is shown below.
Figure 1. Steig et al 2013 Figure 3. Original caption: Decade-average d18O from the WAIS Divide ice core for the past 2,000 years. Grey shading shows 2 s.d. about the decadal mean, based on the upper 100 years of the multi-core d18O composite, providing an estimate of the 95% confidence range. The dashed line shows the 97.5 percentile value relative to the average linear trend.
Steig described d18O values in “recent decades” as “highly unusual”:
Our results thus show that, indeed, recent decades in West Antarctica, which have been characterized by very rapid warming, and very rapid loss of ice from the West Antarctic Ice Sheet, are highly unusual.
Steig also asserted that there was a “strong trend” in O18 values in the past 50 years, which was, according to Steig, “largely driven” by high values in the closing portion of the series. Here’s his exact language:
Our results show that the strong trend in δ18O in West Antarctica in the last 50 years is largely driven by anomalously high δ18O in the most recent two decades, particularly in the 1990s (less so the 2000s).
It seems odd to say that the supposed trend was “largely driven” by higher values in the closest portion: how would one get a trend without higher closing values. For comparison, here is a detail of the WAIS d18O record (plotted from PAGES2K data) for the past century. Values in the 1990s were locally elevated, but values in the 1970s were the lowest in the entire record, contradicting Steig’s claim about “recent decades“. Nor is the “trend” since 1950 even statistically significant. Indeed, the values in the 1990s appear more like a fluctuation, as opposed to a “trend” (let alone a “strong trend”), particularly given the subsequent downtick in the 2000s. Nor is this data set is one that any reasonable person would compare to a Hockey Stick.
Figure 2. Modern portion of WAIS d18O.
Even with the most liberal allowance for imprecise language and lack of statistical acumen on the part of “real climate” scientists,
Considering decadal averages (as Steig did), the 1990s were far from being “highly unusual”. They were at the 77th percentile within the data set: slightly elevated but not “highly unusual”. The average of values to date in the 2000s were at the 15th percentile!
Although Steig conceded at RC that the recent results can’t be considered “unprecedented”, he purported to deduce from this data that similar results occurred only once per century and were “probably” a harbinger of ice sheet collapse in west Antarctica:
What we’ve observed is unusual, but it is also dominated by decadal climate variability, and can’t be considered “unprecendeted”…
Looking at the very long term record from the WAIS Divide ice core, it appears that similar conditions could have occurred about once per century over the last 2000 years. Hence our answer to the question, “are the observations of the last few decades a harbinger of continued ice sheet collapse in West Antarctica?”, is tentative: “Probably”.
While the 1990s did indeed have the highest value in their century, as noted above, they were far from being “highly unusual” in the context of the data itself. Even within the past three centuries, the slight downward trend persisted and the 1990s appear more as a fluctuation than a “trend”.
From this very unpromising bladeless hockey stick, Steig was hard-pressed to support alarm, but did his best. Steig even claimed that the long record showed that recent values were “anomalous” over the past two millennia:
Analysis of the long record at WAIS Divide shows that d18O in West Antarctica is anomalous not only with respect to the past two centuries, but also with respect to the past two millennia.
… Decadal-average 18O values comparable to the 1990s in the WAIS Divide record are reached on only four occasions in the past 1,000 years (Fig. 3). Before 1,000 years ago, modern decadal-average 18O values are reached more frequently, but these are superimposed on a declining trend attributable to the influence of Milankovitch orbital forcing and ice flow. Assuming that the decadal variability is independent of orbital forcing, we calculate d18O anomalies relative to the long-term trend (dashed line in Fig. 3). Anomalies in d18O similar to those of the 1990s occur just twice in the past 2,000 years; assuming sampling error estimated from the multiple shorter records, comparably elevated 18O values were reached about 1% of the time.
Watch the pea here.
Steig is not talking about the d18O values that people are actually interested in, but in values after subtraction of the long-term trend. These are plotted below. The residual for the 1990s is indeed relatively large, but there is nothing to suggest that it is “significant” – particularly allowing for the unexceptional nature of the residual for the 2000s.
Steig’s RC post glossed over the long-term decline in d18O values. A commenter challenged him as follows:
1. Would it be correct to say your δ18O data indicates a decline over the past 2,000 years, thus the WAIS has cooled over the past 2,000 years?
2. The abstract says, “However, δ18O anomalies comparable to those of recent decades occur about 1% of the time over the past 2,000 years.” This appears to be only in relation to the declining δ18O trendline. Would it be correct to say that if compared to the mean δ18O of the past 2,000 years, the anomalies of recent decades would actually still be negative in comparison to the mean?
Steig responded testily that pointing out this relevant point was “tiresome” when the answers were given in the (paywalled) paper that few readers would have purchased:
[Response: Yes and yes. It is a bit tiresome answering questions whose answers are given unambiguously in the paper. Read it, please! The mean cooling is consistent with Milankovitch forcing, and is not particularly relevant to the question of atmospheric circulation and glacier anomalies.–eric]
The article itself made an armwaving reference to Milankovitch forcing as follows:
Before 1,000 years ago, modern decadal-average 18O values are reached more frequently, but these are superimposed on a declining trend attributable to the influence of Milankovitch orbital forcing and ice flow [13, Neumann et al 2008 JGR].
Unfortunately the citation did not establish that the declining trend was “attributable” to Milankovitch forcing and ice flow. Indeed, it contained no reference whatever to Milankovitch forcing.
Steig’s RC article also discussed a dataset from the Antarctic Peninsula which showed increased melt in the 20th century. Steig rhetorically asked
Why the difference between the Peninsula and the WAIS?
After all, both locations are warming at about the same rate.[linking to Steig et al 2009]
Of course, Steig et al 2009 is no authority at all for the proposition that the Peninsula and West Antarctica are warming at about the same rate. As discussed in blog posts at the time and in O’Donnell et al 2010, Steig et al 2009 had simply spread Peninsula warming to West Antarctica through faulty math. But even when confronted by his own adverse d18O data, Steig refused to raise the possibility of the correctness of the criticisms of his flawed math.
There are a few points on which I agree with Steig. Steig argued that there was merit in single proxy rather than multiproxy studies:
Amidst the continuous chatter in the blogosphere about the strengths and limitations about “multiproxy” studies, these studies may be a refreshing return to simpler methods relying on just one type of “proxy”: data from ice cores. While ice core data aren’t perfect proxies of climate, they come pretty close, and aren’t subject to the same kinds of uncertainties that are unavoidable in biological proxies like tree rings.
I substantially agree with Steig that there would be great benefit of focusing on single classes of proxy. Steig also pointed out the potential benefit of focusing on d18O data without translating it into temperature- a tactic that seems eminently sensible to me and which I’ve employed from time to time at Climate Audit. Reconstructions of past d18O seems like a very worthy enterprise to me.
Steig claimed that his paper placed recent changes in a “longer-term” context:
Both our paper and that of Abram et al. add to our understanding of recent climate, glacier, and ice sheet changes in Antarctica by placing them into a longer-term context.
Again, I agree that Steig et al 2013 places recent changes in a longer-term context though not necessarily with the conclusion that Steig desires.