The long-awaited NAS synthesis report on surface temperature reconstructions over the last few millennia is being released today. It’s a long (155 page) report and will take a while to digest, but we applaud the committee for having tried to master a dense thicket of publications and materials on the subject over a relatively short time.
It is probably expecting too much for one report might to put to rest all the outstanding issues in a still-developing field. And given the considerable length of the report, we have little doubt that keen contrarians will be able to mine the report for skeptical-sounding sentences and cherry-pick the findings. However, it is the big picture conclusions that have the most relevance for the lay public and policymakers, and it is re-assuring (and unsurprising) to see that the panel has found reason to support the key mainstream findings of past research, including points that we have highlighted previously:
1) The authors of the report accurately report the considerable uncertainties that were acknowledged by seminal earlier studies. In particular, Mann et al 1999, which was entitled (emphasis added) “Northern Hemisphere Temperatures During the Past Millennium: Inferences, Uncertainties, and Limitations, emphasized the uncertainties and caveats, particularly with regard to reconstructing large-scale surface temperature patterns prior to about AD 1600
The report makes due note of this (pg. 119 of the report):
The Mann et al. large-scale surface temperature reconstructions were the first to include explicit statistical error bars, which provide an indication of the confidence that can be placed in the results. In the Mann et al. work, the error bars were relatively small back to about A.D. 1600, but much larger for A.D. 1000–1600. The lower precision during earlier times is caused primarily by the limited availability of annually resolved paleoclimate data: That is, the farther back in time, the harder it is to find evidence that provides reliable annual information. For the period before about A.D. 900, annual data series are very few in number, and the non-annually resolved data used in reconstructions introduce additional uncertainties.
2) The authors accurately note that, despite those uncertainties, the key conclusions reached by those studies (i.e., that hemispheric-scale warmth in recent decades is likely unprecedented over at last the past millennium) have been substantiated by many other studies, and the confidence in those conclusions appears greater, not lesser, after nearly an additional decade of research (pg. 109 of the report):
The basic conclusion of Mann et al. (1998, 1999) was that the late 20th century warmth in the Northern Hemisphere was unprecedented during at least the last 1,000 years. This conclusion has subsequently been supported by an array of evidence that includes the additional large-scale surface temperature reconstructions and documentation of the spatial coherence of recent warming described above (Cook et al. 2004, Moberg et al. 2005, Rutherford et al. 2005, D’Arrigo et al. 2006, Osborn and Briffa 2006, Wahl and Ammann in press), and also the pronounced changes in a variety of local proxy indicators described in previous chapters (e.g., Thompson et al. in press). Based on the analyses presented in the original papers by Mann et al. and this newer supporting evidence, the committee finds it plausible that the Northern Hemisphere was warmer during the last few decades of the 20th century than during any comparable period over the preceding millennium.
3) Despite the attempts of some commentators to attempt conflate the evidence for the existence of human influences on climate with the validity of a single reconstruction (e.g. that of Mann et al) it is quite clear that the evidence for anthropogenic impacts on climate is quite strong irrespective of whether or not the original “hockey stick” is correct. The report makes repeated note of this key point, for example on page 4 of the report:
Surface temperature reconstructions for periods prior to the industrial era are only one of multiple lines of evidence supporting the conclusion that climatic warming is occurring in response to human activities, and they are not the primary evidence.
and again on page 9 of the report:
The reconstruction produced by Dr. Mann and his colleagues was just one step in a long process of research, and it is not (as sometimes presented) a clinching argument for anthropogenic global warming, but rather one of many independent lines of research on global climate change.
4) That it is time for the paleoclimate research community to move ahead of the now tired debates about the ‘Hockey Stick”. The authors, as most scientists currently working in the field, recognize the need to work to reduce current uncertainties by aiming to:
- obtain additional, improved, and updated paleoclimate proxies of past climate that can aid in decreasing the existing uncertainties,
- focus greater attention on the relative strengths and weaknesses of alternative types of proxy information such as tree-ring, corals, and ice cores (see e.g. here and here) so that more robust climate reconstructions can be formed making use of complementary information available from “multi-proxy” networks,
- pay attention to legitimate (rather than specious, e.g. here and here) issues with regard to the strengths and weaknesses of alternative paleoclimate proxy reconstruction methods [it was encouraging, for example, that the authors of the report favor the use of some combination of the standard measures of the fidelity or "skill" of paleoclimate reconstructions ("RE" and "CE") generally used by paleoclimate researchers, and dismiss as without merit the use of simple correlation coefficients], and
- move beyond the often inappropriate focus given to hemispheric mean temperature, and give greater future attention to the detailed spatial patterns of past temperature changes, as well as reconstructions of precipitation and atmospheric circulation variables. These can provide greater insight into the underlying dynamics of the climate system and the key role that dynamical modes such as the El Nino/Southern Oscillation may play in climate change.
While we agree with the bottom-line conclusions in the report, this is not to say that we don’t also have some criticisms:
The report provides an unbalanced discussion of some significant technical details. For example, there is quite a bit of discussion of possible biases involved in centering conventions used in Principal Component Analysis (PCA). Yet the report only vaguely alludes to the fact that published work [see both Wahl and Ammann (2006) and Rutherford et al (2005) cited in the report] clearly demonstrates that this doesn’t introduce any significant bias as long as statistically significant patterns in the data are not discarded.
The report calls into question the confidence in certain fairly specific previous conclusions, e.g. the tentative conclusion in Mann et al (1999) that the 1990s and 1998 were the warmest decade and year, respectively, of the past 1000 years. There are two important points here left unmentioned in the report: (1) Mann et al (1999) attached the qualifier “likely” to these conclusions, which in standard (e.g. IPCC) parlance corresponds to a roughly 2/3 probability, i.e., implies slightly better than even odds of being true, a fairly conservative conclusion. The conclusion was followed by the statement “More widespread high-resolution data which can resolve millennial-scale variability are needed before more confident conclusions can be reached…”. (2) The conclusions regarding the decade of the 1990s and the year 1998 follow from reasonable assumptions. The late 20th century Northern Hemisphere average warmth, according to Mann et al (1999) and all subsequent studies, appears anomalous in at least the past 1000 years. So the base state about which higher-frequency (e.g. interannual) fluctuations occur was substantially higher at the end of the late 20th century then during any earlier comparable period. Unless the interannual fluctuations in hemispheric mean temperature during earlier centuries were significantly greater in amplitude than during the 20th century (and there is no obvious evidence that they were), then it reasonably follows that the thresholds reached during the 1990s or during 1998–an anomalously warm decade and year respectively from the perspective of the instrumental record–are unlikely to have been breached in earlier centuries.
The committee does not seem to have grasped fully the significance of some very recently published results that are cited in the report, notably the papers by Rutherford et al (2005) and Wahl and Amman (2006) that further demonstrate the robustness of the Mann et al (1998;1999) conclusions and subject some published criticisms of those conclusions to rigorous scrutiny. The authors cite somewhat uncritically the Von Storch et al (2004) study arguing that climate field reconstruction techniques can significantly underestimate long-term trends, despite the fact that errors have now been acknowledged in that study (see here and here), and that an independent study not cited, but published well before the report was drafted, comes to very different conclusions. This reflects one of a number of inevitable minor holes in this quickly prepared report.
One of our main criticisms though doesn’t involve the report itself, but the press release that accompanied it. We’ve noted before the importance of making sure that the press will be able to correctly contextualise a release and the bad consequences of that not happening. Well, in this case the press release annoucing the publication of the report was often inconsistent with what was actually stated in the report. It was titled: ‘High Confidence’ That Planet Is Warmest in 400 Years; Less Confidence in Temperature Reconstructions Prior to 1600 which is not news at all and almost trivially true. However, it is likely to be mis-interpreted to imply that there is no confidence in reconstructions prior to 1600, which is the opposite of the conclusion of the report. Additionally, the text appears to have confused the key distinction between our knowledge of global mean temperature in past centuries (which is very limited owing to the sparseness of long available proxy data in the Southern Hemisphere, and for which a reconstruction was not attempted by Mann et al or most other researchers), with our knowledge of Northern Hemisphere mean temperature (which is considerably better; hence the emphasis of this quantity in past work).
Finally, it is worth pointing out and emphasising that the report provides absolutely no support for the oft-heard claims that the original hockey stick was the result of ‘programming errors’, or was ‘not reproducible’, or there was some scientific misconduct involved. These claims were always spurious and should now finally be laid to rest. Hopefully, we can all start to move forward with the science again.