National Academies Synthesis 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:

  1. obtain additional, improved, and updated paleoclimate proxies of past climate that can aid in decreasing the existing uncertainties,
  2. 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,
  3. 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
  4. 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.

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