by Judith Curry

Humanity has a second chance to stop dangerous climate change. Temperature data from the last decade offers an unexpected opportunity to stay below the agreed international target of 2 °C of global warming. – New Scientist

A new article is in press at Nature Geoscience [link]:

Energy budget constraints on climate response

Alexander Otto, Friederike E. L. Otto, Olivier Boucher, John Church, Gabi Hegerl, Piers M. Forster, Nathan P. Gillett, Jonathan Gregory, Gregory C. Johnson, Reto Knutti, Nicholas Lewis, Ulrike Lohmann, Jochem Marotzke, Gunnar Myhre, Drew Shindell, Bjorn Stevens & Myles R. Allen

Read the list of authors, read it carefully. Note the presence of a number of names that are well known as IPCC lead authors and otherwise. Also note the presence of Nicholas Lewis.

The punchline is this:

The most likely value of equilibrium climate sensitivity based on the energy budget of the most recent decade is 2.0 °C, with a 5–95% confidence interval of 1.2– 3.9 °C , compared with the 1970–2009 estimate of 1.9 °C (0.9–5.0 °C; grey, Fig. 1a). Including the period from 2000 to 2009 into the 40-year 1970–2009 period delivers a finite upper boundary, in contrast with earlier estimates calculated using the same method. The range derived from the 2000s overlaps with estimates from earlier decades and with the range of ECS values from current climate models (ECS values in the CMIP5 ensemble are 2.2–4.7 °C), although it is moved slightly towards lower values. Observations of the energy budget alone do not rule out an ECS value below 2 °C, but they do rule out an ECS below 1.2 °C with 95% confidence.

The best estimate of TCR based on observations of the most recent decade is 1.3 °C (0.9–2.0 °C). This is lower than estimates derived from data of the 1990s (1.6 °C (0.9–3.1 °C) or for the 1970–2009 period as a whole (1.4 °C (0.7–2.5 °C); grey, Fig. 1b). Our results match those of other observation-based studies and suggest that the TCRs of some of the models in the CMIP5 ensemble with the strongest climate response to increases in atmospheric CO2 levels may be inconsistent with recent observations — even though their ECS values are consistent and they agree well with the observed climatology. Most of the climate models of the CMIP5 ensemble are, however, consistent with the observations used here in terms of both ECS and TCR.

Nic Lewis has a post on this at Bishop Hill, some excerpts:

The Nature Geoscience paper, although short, is in my view significant for two particular reasons. First, using what is probably the most robust method available, it establishes a well-constrained best estimate for TCR that is nearly 30% below the CMIP5 multimodel mean TCR of 1.8°C (per Forster et al. (2013), here). The 95% confidence bound for the Nature Geoscience paper’s 1.3°C TCR best estimate indicates some of the highest-response general circulation models (GCMs) have TCRs that are inconsistent with recent observed changes. Some two-thirds of the CMIP5 models analysed in Forster et. al (2013) have TCRs that lie above the top of the ‘likely’ range for that best estimate, and all the CMIP5 models analysed have an ECS that exceeds the Nature Geoscience paper’s 2.0°C best estimate of ECS. The CMIP5 GCM with the highest TCR, per the Forster et. al (2013) analysis, is the UK Met. Office’s flagship HadGEM2-ES model – see their webpage “Advanced climate modelling for policymakers” and their document “Advance: Improved advice for science mitigation advice”. The uncertainty distribution for the Nature Geoscience paper’s best TCR estimate of 1.3°C indicates that it is extremely unlikely that real-world TCR is as high as that of the HadGEM2-ES model. It has a TCR of 2.5°C, nearly double 1.3°C and 0.5°C beyond the top of the 5–95% uncertainty range. The paper obtains similar, albeit less well constrained, best estimates using data for earlier periods than 2000–09.

Secondly, the authors include fourteen climate scientists, well known in their fields, who are lead or coordinating lead authors of IPCC AR5 WG1 chapters that are relevant to estimating climate sensitivity. Two of them, professors Myles Allen and Gabi Hegerl, are lead authors for Chapter 10, which deals with estimates of ECS and TCR constrained by observational evidence. The study was principally carried out by a researcher, Alex Otto, who works in Myles Allen’s group.

The New Scientist has an article on this paper that is worth reading.

James Annan has a post that says what really struck me also:

The analysis itself is not particularly novel or exciting: what makes it newsworthy in my view is the list of authors, which includes some who had previously been trying to talk down these recent estimates. Even though this paper is too late for the IPCC AR5, I hope it reflects a change in thinking from the IPCC authors involved.

They argue that the new result for sensitivity “is in agreement with earlier estimates, within the limits of uncertainty”. The contrasting claim that the analysis of transient response gives a qualitatively different outcome (being somewhat lower than both the previous IPCC assessment, and the range obtained from GCMs) is just weird, since both their ECS and TCR results are markedly lower than the IPCC and GCM ranges.

This looks like a pretty unreasonable attempt to spin the result as nothing new for sensitivity, when it is clearly something very new indeed from these authors, and implies a marked lowering of the IPCC “likely” range. So although the analysis does depend on a few approximations and simplifications, it’s hard to see how they could continue to defend the 2-4.5C range.

JC comments: James Annan’s blog post starts with this sentence: “At last the great and the good have spoken.” I.e., some IPCC lead authors are paying attention to the lower sensitivity estimates. It will be very interesting to see how the IPCC AR5 plays this. I suspect that the uncertainty monster will become their good friend, ‘not inconsistent with.’ It will be very interesting indeed to see if the IPCC budges from the 2-4.5 C range that has remained unchanged since the 1979 Charney report.