Sunday, May 19, 2013

A chink of light at the end of the tunnel?

At last the great and the good have spoken. There's a new article in Nature Geoscience (here, but the bulk of the details are in the SI which seems to not be paywalled) concerning the energy balance of the climate system, which basically confirms what had already been presented in the slew of recent papers pointing to a lowering of climate sensitivity estimates.

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 (e.g. Knutti: "my personal view is that the overall assessment hasn’t changed much"). Even though this paper is too late for the IPCC AR5, I hope it reflects a change in thinking from the IPCC authors involved. (Notable also that Nic Lewis is involved.)

The results are described in rather strange terms, considering what they have actually presented. They argue that the new result for sensitivity "is in agreement with earlier estimates, within the limits of uncertainty". But of course none of the published estimates are inconsistent with each other in the sense of having non-overlapping uncertainty ranges - no-one credible has excluded a value of about 2.5C, that I am aware of. 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. Although the paper does not explicitly mention it, the "likely" range for equilibrium climate sensitivity using the full 40y of data seems to be about 1.3-3C (reading off the graph by eye, the lower end may be off a bit due to the nonlinear scale). 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.

Update: post by Nic Lewis here, also coverage in NewScientist.


Carrick said...

Do you have a link?

James Annan said...

I don't think the paper is up yet, though the embargo has passed (according to several sources). I'll edit the post when it appears.

Carrick said...

Thanks for the response James.... I just found it:

Rattus Norvegicus said...

Doesn't seem to be paywalled, either.

Carrick said...

Rattus, I understand it's free for a short time, then it goes behind a paywall.

Unknown said...

Table S2 in the supplementary information gives 1.4-4C more or less as the 95% confidence range.

Are these the conclusions of the paper (which I have not seen yet).

Paul S said...

They don't actually cite which earlier estimates they think are in agreement, which makes that judgement pretty tough. I'm not sure a direct comparison with the IPCC 2-4.5ºC range is entirely fair since that is an overall assessment which takes into account a lot more than just energy balance estimates. AR4 does, in several places, give a separate assessment for observationally-constrained estimates: 'Estimates based on observational constraints indicate that it is very likely that the equilibrium climate sensitivity is larger than 1.5°C with a most likely value between 2°C and 3°C.' This paper would (just about) fit in that range.

However, where they give explicit comparisons to the CMIP5 ensemble on ECS and TCR it's a bit confusing. The CMIP5 ECS ensemble range is given as 2.2-4.7ºC and compared to study range 1.2-3.9º, with the judgement that the two are consistent though slightly offset towards lower values. They don't give a CMIP5 TCR range but, according to the AR5 draft, it's 1.1-2.5ºC, compared to 0.9-2.0ºC. Like James, I'm not sure why one of these comparisons is considered to be in agreement while the other isn't. It seems like they deferred their TCR conclusion to Gillett et al. (2012) - a study looking at a single high sensitivity model - rather than analysing their own results.

Paul S said...

Looking at the SI there are a few other things which don't seem to add up.

They define a forcing time series from the CMIP5 ensemble, using results from Forster et al. (2013), and find a mean 2010 value of 1.9W/m2, compared to 1850. They then decide to scale the forcing time series by reference to observational constraints for aerosol forcing, adding 0.3W/m2 onto 2010 in the process. On the face of it this is potentially entirely reasonable, but the way it's done here doesn't work for me:

1) They compare ACCMIP ensemble mean for aerosol forcing to selected observation-based studies, then apply the difference to their CMIP5 mean forcing. The problem is that the ACCMIP ensemble is not the same as the CMIP5 ensemble. They seem to assume the comparison can be transposed onto the CMIP5 ensemble but give no basis for this assumption.

2) The two references they use for observational studies are not apples-to-apples comparisons with ACCMIP results because they only refer to direct+1st indirect forcing, rather than total aerosol forcing including all indirect effects. There is a relevant well-known recent satellite obs-based total aerosol forcing paper - Quaas et al. (2009) - and that finds a best estimate of -1.2W/m2, seemingly in agreement with the ACCMIP mean (though see point 1).

Paul S said...

I can't make sense of the next paragraph in the SI.

All other forcing estimates mentioned were related to a reference (zero) year of 1850. Here they talk about defining the LLGHG component of their forcing time series using the value 2.83 from Skeie et al. (2011) but that relates to 1750 as reference.

Paul Matthews said...

I said much the same thing at Bishop Hill-

There's a nice misleading bit of spin right at the start of the paper:

[quote first 3 sentences of paper]

If you only read this far, you'd get the impression that the paper supports the IPCC 2-4.5 value and refutes lower values. To describe 1.2-3.9 as 'in agreement' with 2-4.5 seems somewhat misleading. I'm guessing that Nic didn't write that sentence.

James Annan said...

Well it has certainly become very clear to me what this is all about. I'll put up another post soonish.

Rattus Norvegicus said...

Carrick, it turned out I had my VPN connection to work on and that gives me free access to most journals.

Paul S said...


It isn't paywalled (at the moment, don't know if Carrick is right about a time limit), but it is registration-walled. Anyone can register for free though.

Carrick said...

I definitely wasn't using VPN and could read that article without going through my university account.

Nic says on his follow-on comment on BH that the editors have agreed to provide this paper free of charge for a short period.