National Academies Synthesis Report

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:

  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.

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