by Judith Curry

Our new paper on climate sensitivity is now published.

Nicholas Lewis and Judith Curry: The implications for climate sensitivity of AR5 forcing and heat uptake estimates. Climate Dynamics, September 2014 [link]

Nic Lewis has posted a link to the full paper [here], along with data and code.

Context

Some context is provided, excerpted from my recent Congressional testimony:

The sensitivity of our climate to increasing concentrations of carbon dioxide is at the heart of the scientific debate on anthropogenic climate change, and also the public debate on the appropriate policy response to increasing carbon dioxide in the atmosphere. Climate sensitivity and estimates of its uncertainty is a key input into the economic models that drive cost-benefit analyses and estimates of the social cost of carbon.

One of the most significant outcomes of the recent IPCC Fifth Assessment Report is the change in conclusions relative to the Fourth Assessment Report. The equilibrium climate sensitivity (ECS) is defined as the change in global mean surface temperature at equilibrium that is caused by a doubling of the atmospheric CO2 concentration. The IPCC AR4 conclusion on climate sensitivity is stated as:

“The equilibrium climate sensitivity. . . is likely to be in the range 2oC to 4.5C with a best estimate of about 3oC and is very unlikely to be less than 1.5C. Values higher than 4.5oC cannot be excluded.” (AR4 SPM)

The IPCC AR5 conclusion on climate sensitivity is stated as:

Equilibrium climate sensitivity is likely in the range 1.5°C to 4.5°C (high confidence), extremely unlikely less than 1°C (high confidence), and very unlikely greater than 6°C (medium confidence) (AR5 SPM)

The bottom of the ‘likely’ range has been lowered from 2 to 1.5C in the AR5, whereas the AR4 stated that ECS is very unlikely to be less than 1.5C. It is also significant that the AR5 does not cite a best estimate, whereas the AR4 cites a best estimate of 3C. Further the AR5 finds values of ECS exceeding 6C to be very unlikely, whereas the AR4 did not have sufficient confidence to identify an upper bound at this confidence level. The stated reason for not citing a best estimate in the AR5 is the substantial discrepancy between observation-based estimates of ECS (lower), versus estimates from climate models (higher).

Nic Lewis’ blog post

Here are excerpts from Nic Lewis’ blog post at ClimateAudit on the paper:

When the Lewis & Crok report “A Sensitive Matter” about how the IPCC Fifth Assessment Working Group 1 report (AR5) was published by the GWPF in March, various people criticised it for not being peer-reviewed. But peer review is for research papers, not for lengthy, wide-ranging review reports. The Lewis & Crok report placed considerable weight on energy budget sensitivity estimates based on the carefully considered AR5 forcing and heat uptake data, but those had been published too recently for any peer reviewed sensitivity estimates based on them to exist.

I am very pleased to say that the position has now changed. Lewis N and Curry J A: The implications for climate sensitivity of AR5 forcing and heat uptake estimates, Climate Dynamics (2014), has just been published. The paper’s results show the best (median) estimates and ‘likely’ (17–83% probability) ranges for equilibrium/effective climate sensitivity (ECS) and transient climate response (TCR) given in the Lewis & Crok report to have been conservative.

Our paper derives ECS and TCR estimates using the AR5 forcing and heat uptake estimates and uncertainty ranges. The analysis uses a global energy budget model that links ECS and TCR to changes in global mean surface temperature (GMST), radiative forcing and the rate of ocean heat uptake between a base and a final period. The resulting estimates are less dependent on global climate models and allow more realistically for forcing uncertainties than similar estimates such as those from the Otto et al (2013) paper.

Base and final periods were selected that have well matched volcanic activity and influence from internal variability, and reasonable agreement between ocean heat content datasets. The preferred pairing is 1859–1882 with 1995–2011, the longest early and late periods free of significant volcanic activity, which provide the largest change in forcing and hence the narrowest uncertainty ranges.

Table 1 gives the ECS and TCR estimates for the four base period – final period combinations used.

Table 1: Best estimates are medians (50% probability points). Ranges are to the nearest 0.05°C

AR5 does not give a 95% bound for ECS, but its 90% bound of 6°C is double that of 3.0°C for our study, based on the preferred 1859–1882 and 1995–2011 periods.

Considerable care was taken to allow for all relevant uncertainties. One reviewer applauded “the very thorough analysis that has been done and the attempt at clearly and carefully accounting for uncertainties”, whilst another commented that the paper provides “a state of the art update of the energy balance estimates including a comprehensive treatment of the AR5 data and assessments”.

There is thus now solid peer-reviewed evidence showing that the underlying forcing and heat uptake estimates in AR5 support narrower ‘likely’ ranges for ECS and TCR with far lower upper limits than per the AR5 observationally-based ‘likely’ ranges of: 2.45°C vs 4.5°C for ECS and 1.8°C vs 2.5°C for TCR. The new energy budget estimates incorporate the extremely wide AR5 aerosol forcing uncertainty range – the dominant contribution to uncertainty in the ECS and TCR estimates – as well as thorough allowance for uncertainty in other forcing components, in heat uptake and surface temperature, and for internal variability. The ‘likely’ ranges they give for ECS and TCR can properly be compared with the AR5 Chapter 10 ‘likely’ ranges that reflect only observationally-based studies, shown in Table 1. The AR5 overall assessment ranges are the same.

The CMP5 GCMs used for AR5 all have ECS values exceeding 2°C, whereas 70% of our preferred main results ECS the probability lies below that level, and over 90% lies below the 3.2°C mean ECS of CMIP5 models. The 33 CMIP5 models with suitable archived data show TCR values exceeding our preferred best estimate of 1.33°C in all but one case, with an average TCR exceeding the top of our 1.8°C ‘likely’ range.

The study does not assume any possible contribution to the increase in GMST from indirect solar influences not allowed for in the AR5 forcing estimates, or from natural internal climate variability affecting ocean heat uptake and/or forcing.

JC remarks

Nic Lewis and I have been discussing the issue of climate sensitivity for some time, and I wrote the foreword to the Lewis and Crok essay published by the GWPF. Nic invited me to coauthor this paper, and I was delighted to given my concerns about ignoring uncertainties in external forcing in attribution arguments and climate sensitivity estimates (which I discussed in the Uncertainty Monster paper).

One of major reasons for submitting the paper to Climate Dynamics, apart from its relatively high impact factor, is the excellence and fairness of its editorial staff. The paper did not encounter any major problems in the review process, and one of the reviewers made suggestions that substantially improved the readability of the paper. I would like to commend the editor, Edwin Schneider, for his excellent service to the community in his position as editor of Climate Dynamics.

One of the most important contributions of this paper is the assessment of uncertainties in external forcing on estimates of climate sensitivity, which is something that has hitherto been only partially allowed for in most climate sensitivity estimates. As pointed out by the AR5 and this paper, the uncertainties in external forcing are substantial, particularly for aerosols.

Is this paper the last word on climate sensitivity estimates? No. The uncertainty analysis in the Lewis and Curry paper relates only to the uncertainty in external forcing, surface temperature and ocean heat uptake. There remains considerable meta uncertainty in the determination of climate sensitivity, including how the problem is even framed.

In particular, the energy balance approach does not account for factors that do not directly relate to the energy balance, e.g. solar indirect effects and natural internal variability that affects forcing (although an attempt has been made in the Lewis and Curry paper to make some allowance for uncertainty associated with these factors) . Further, there was ‘something else’ going on in the latter 19th and early-mid 20th century that was causing warming, that does not seem to relate directly to external forcing. The paper does attempt to factor out the impact of the Atlantic Multidecadal Oscillation through the selection of base and final periods, but this is by no means a complete account for the effects of multi-decadal and century scale internal variability, and how this confounds the energy balance estimate of climate sensitivity.

Resolving the reasons for differences between observational/energy balance estimates and GCM estimates of climate sensitivity is an issue of substantial importance. At this point, I find the estimates in the Lewis and Curry paper to be the most convincing estimates available to date.

Moderation note: This is a technical thread, comments will be moderated for relevance.