The IAC made "some incorrect assumptions and conclusions" in its report, according to this letter from IPCC Working Group II (fourth assessment), responding to criticism about the IPCC's process, comment review policies and methodology (see InterAcademy Council Report Urges 'Fundamental Reform' Of IPCC).

Here is their letter:

To: Dr Harold Shapiro, (Chair, IAC Committee to Review the IPCC)

From: The Co-Chairs, all Convening Lead Authors and Head of Technical Support Unit of Working Group II of IPCC Fourth Assessment

4th October 2010

Dear Dr Shapiro:

The InterAcademy Council report Climate Change Assessments, Review of the Processes and Procedures of the IPCC, 2010 is a valuable review of the IPCC and we welcome its recommendations to improve the way in which the IPCC conducts its assessments. However, the IAC makes some incorrect assumptions and conclusions about AR4 IPCC Working Group II (WGII), and we write to correct these.

IAC is not correct in stating that in, seeking high confidence, the WGII Summary for Policymakers (SPM) sometimes makes obvious statements.

IAC presented the following as the single example of this (page 33): “Nearly all European regions are anticipated to be negatively affected by some future impacts of climate change, and these will pose challenges to many economic sectors (Very high confidence)”. But this is misquoted and its meaning is altered by being taken out of context: This is an introductory sentence (without no specific confidence level attached) to a longer paragraph which immediately continues: “Climate change is expected to magnify regional differences in Europe’s natural resources and assets.” In other words, all regions are expected to have some negative effects but climate change will tend to exaggerate the differences between regions, which is an important geo-political point. And the paragraph continues to provide the detail: “Negative impacts will include increased risk of inland flash floods, and more frequent coastal flooding and increased erosion (due to storminess and sea-level rise). The great majority of organisms and ecosystems will have difficulty adapting to climate change. Mountainous areas will face glacier retreat, reduced snow cover and winter tourism, and extensive species losses (in some areas up to 60% under high emission scenarios by 2080).Very high confidence.” The confidence statement clearly applies to the full paragraph, not the first sentence alone, which is intended as a general introduction to the more specific information which follows.

WGII does not make un-necessarily imprecise statements in its SPM.

 IAC says (page 33) that the list of main projected impacts in the WG II SPM includes imprecise statements made without reference to the time period under consideration or to a climate scenario under which the conclusions would be true; and does not consider adaptation. But this fails to recognise the introductory paragraph to the list of impacts (SPM page 11) which defines timing (‘over this century’), scenarios (those ‘projected by the IPCC’ and described elsewhere) and adaptation (which is ‘discussed further in later sections of the Summary’). It is thus made clear that each of the impacts listed has this context.

WGII does not ascribe higher confidence levels than appropriate, according to the definitions used by WGII.

The IAC states (page 33) there are instances where the WGII SPM lists impacts to which authors ascribe ‘high confidence’, but where (the IAC says) there is not “high agreement, much evidence” which IAC quotes as the criteria for high confidence given in the IPCC Guidelines. However, this fails to recognise that, after extensive discussion at the time, each Working Group in IPCC adopted slightly differing methods of treating uncertainty. WG II clearly defined level of confidence for its readers as follows (see End Box 2 in SPM and Box TS2 in TS): “Authors have assigned a confidence level…on the basis of their assessment of current knowledge …based on a comprehensive reading of the literature and their expert judgement [the latter is added in the Technical Summary]”. The WGII confidence levels reflect the authors’ judgement of current knowledge. 

WGII authors have responded openly and willingly to external enquiries.

IAC states (page 27):  “An unwillingness to share data with critics and enquirers and poor procedures to respond to freedom-of-information requests were the main problems uncovered in some of the controversies surrounding the IPCC (Muir Russell et al., 2010; PBL, 2010). However, the Director of PBL has written to you (30 September) to confirm that “In our report ‘Assessing an IPCC assessment’ of 5 July 2010 there is no mention of any of such unwillingness. We would like to stress that the previous co-chair of Working Group II Dr. Martin Parry and the previous coordinating lead authors of the regional chapters of Working Group II AR 4 report have given us full cooperation with regard to the many questions we had and we have been in an intensive dialogue over many weeks in preparation of our report”.