Judith Curry: The New Climate Sensitivity Debate

  • Date: 05/02/13
  • Judith Curry, Climate Etc.

Since the release of the AR5 SOD, there has been a flurry of blog posts on the topic of climate sensitivity.  The triggers seem to have been Nic Lewis’ analysis (discussed here), plus a press release from the Norwegians (discussed here).  Both find values of sensitivity to be significantly lower than the consensus values.

In response to this, we have been ‘reassured’ by RealClimate  and SkepticalScience that nothing has really changed with regards to the consensus on sensitivity.  Gavin Schmidt states:

In the meantime, the ‘meta-uncertainty’ across the methods remains stubbornly high with support for both relatively low numbers around 2ºC and higher ones around 4ºC, so that is likely to remain the consensus range.

The ‘consensus’ range has been 1.5 – 4.5C (centered on 3C) since the 1979 Charney Report.  With all the many different ways of calculating these numbers (empirically and from simple models and general circulation models), and different results that have been obtained from these analyses, why hasn’t this range and central value budged in over 3 decades?  Here are some reasons:

1.  The ‘experts’ are convinced.. Zickfeld et al. (2010)  conducted face-to-face interviews with 14 leading climate scientists, using formal methods of expert elicitation.  The results were not surprising.  Apparently the results of this expert elicitation have a substantial influence in the AR5 report.  Before you start criticizing the formal expert elicitation process, it is WAY better (less biased) than a consensus building process (see my paper no consensus on consensus).   You are of course allowed to criticize all of this in the context of how many, and which, experts were included in this process.

2.  Anchoring devices.  Pielke Jr reminded me of this paper by van der Sluijs et al. Anchoring devices in climate for policy: the case of consensus around climate sensitivity.   Excerpt:

We show how the maintained consensus about the quantitative estimate of a central scientific concept in the anthropogenic climate-change field – namely, climate sensitivity – operates as an `anchoring device’ in `science for policy’. In international assessments of the climate issue, the consensus-estimate of 1.5°C to 4.5°C for climate sensitivity has remained unchanged for two decades. Nevertheless, during these years climate scientific knowledge and analysis have changed dramatically. We propose that the remarkable quantitative stability of the climate sensitivity range has helped to hold together a variety of different social worlds relating to climate change, by continually translating and adapting the meaning of the `stable’ range. But this emergent stability also reflects an implicit social contract among the various scientists and policy specialists involved, which allows `the same’ concept to accommodate tacitly different local meanings.

3.  The ‘experts’ have taken into account the latest knowledge on external forcing and uncertainties, model uncertainties, methodological uncertainties, etc. in preparing their estimates.  Oops, looks like they forgot to do this (see James Annan’s comments below)

Andy Revkin

Revkin has two recent posts on this topic:

Revkin has elicited some remarkable statements from IPCC authors regard climate sensitivity, as well as some startling comments from climate bloggers.

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