This is a guest post by Bernie Lewin.

I have noticed around the blogs some patient and kindly folks attempting to summarise my overly verbose account of investigations into the background of the Chapter 8 controversy. I welcome these. And I welcome various interpretations. I am purposely inconclusive. However, I may have confused some folks as to what I am suggesting it was that went wrong in Madrid. Therefore, I thought it might be useful at this stage to explicitly clarify my current understanding of how the Scientific Assessment of the IPCC was first corrupted in its first positive human attribution claim.

What was the purpose of the Madrid Plenary?

According to IPCC Procedures set for the Second Assessment, the purpose of the Working Group 1 Inter-governmental Plenary meeting in Madrid was two-fold. Firstly, it was to 'accept' the 'final draft Report' as a thoroughly peer reviewed assessment that presented a 'comprehensive, objective and balanced view' of the science. Secondly, it was to 'approve' every line of a Summary for PolicyMakers (SPM) as 'consistent' with the underlying Report. The 'acceptance' is not about accepting or contesting the scientific grounds of the Report, but about whether the process by which its findings are established is thorough and proper according to the best practice of science. In theory an Inter-governmental Plenary could change a Report to make it acceptable in this sense. However, the Procedures point out that the extent to which it could agree to such changes is limited by practical constraints.

What has consensus got to do with it?

As for the vexed issue of consensus, there is a common misconception that the IPCC enforces an oxymoron, namely, consensual science. In fact, there is no obligation upon the Report authors to find or present a consensus. On the contrary, the Procedures makes a point of asking Lead Authors 'to clearly identify disparities of view,' saying that 'it is important that the reports describe different (possibly controversial) scientific or technical views on a subject, particularly if they are relevant to the political debate.' Where the consensus is required is among the government delegations at their Plenary. Full inter-governmental agreement is required for the Plenary to acceptance the scientific assessment and to approve the Summary in fidelity with it.

What went wrong at Madrid?

At Madrid it appears that the SPM was already inconsistent with the Report and that further inconsistencies were introduced. There were also inconsistencies between the Report and the underlying evidence-base to which it referred -- inconsistent in such a way as to affect a conclusive and consistent positive human attribution in place of the Report's scepticism.

How does the justification for changing the original assessment stand up?

The explicit justification offered in Madrid for re-drafting the SPM out of alignment with the Report was to reflect important new evidence. This justification is without foundation. In the first place, this 'new' evidence had been incorporated into the 'final draft' of the Report after it had been extensively discussed during the final Lead Author Plenary drafting of the SPM in Asheville. Secondly, the new evidence does not even address, let alone over-turn, the grounds for the Report's original scepticism (thus explaining why Chapter 8's sceptical conclusion remained after Asheville). Finally, some of these distortions of the SPM out of alignment with the underlying Report were 'approved' without the full agreement of the Plenary. In fact, some were made in the face of explicit, sustained and entirely legitimate objections by governmental delegations on the very grounds of inconsistency with the Report.

What is wrong with the defence of the post-Madrid chapter changes?

When the controversy over the post-Madrid changes to Chapter 8 broke in the spring of 1996, the scientists involved offered three post-facto justifications: Chapter 8 contained inconsistencies; the US Government had made (extraordinary, belated and unsolicited) requests for changes to the underlying Report in its (solicited) comments on the SPM; and the Plenary had agreed to these changes in advance -- not specifically, but at least consistent with what had been agreed. There is truth in each of these claims. But it is difficult to see how any of them justify the post-Madrid changes in accord with the IPCC Procedures -- which had been designed expressly to achieve an independent scientific assessment.

In what way was the authority of science abused?

What sent the Madrid Plenary off the rails was the way the representative expert scientists permitted and promoted the re-opening of the attribution question for assessment at a meeting of governmental delegations. Not only did the scientists present a distorted view of the evidence, but their audience had no time nor means for evaluating whether their conclusions upon this evidence were 'comprehensive, objective and balanced.' The recent positive results promoted in Madrid (as also the sceptical results there obscured) lacked adequate scientific scrutiny due to the simple fact that they had not been published. Perhaps this course of action would be acceptable at a conference of scientific experts (as at Asheville) and the matter might have been resolved there. (It was not.) But to re-open the controversy in Madrid was an abuse of process: it removed the IPCC assessment on the human attribution question further from its already dubious grounding in peer-review science.