By Dr. Pat Michaels at World Climate Report
A new, lower estimate of climate sensitivity
There is word circulating that a paper soon to appear in Science magazine concludes that the climate sensitivity—how much the earth’s average temperature will rise as a result of a doubling of the atmospheric concentration of carbon dioxide—likely (that is, with a 66% probability) lies in the range 1.7°C to 2.6°C, with a median value of 2.3°C. This is a sizeable contraction and reduction from the estimates of the climate sensitivity given by the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4), in which the likely range is given as 2.0°C to 4.5°C, with a best estimate of 3.0°C.
Further, the results from the new analysis largely eliminate the “fat tail” of the distribution of possible values of the climate sensitivity (that the IPCC AR4 report was fond of) which included the possibility that very large climate sensitivities are a realistic possibility. In the new paper, the authors find only “vanishing probabilities” for a climate sensitivity value greater than 3.2°C and that values greater than 6.0°C are “implausible.”
Contrast that with the IPCC assessment of the literature (summarized in our Figure 1) which routinely includes studies concluding there is a greater than a 10% possibility that the true climate sensitivity exceeds 6°C and some which find that there is a greater than 5% possibility that it exceeds 10°C.
Figure 1. Climate sensitivity distributions retained (and in some cases recast) by the IPCC from their assessment of the literature. Note the “fat tail” towards the right which indicates the possibilities of the climate sensitivity having a very large positive value (that is, a huge degree of global temperature rise for a doubling of the atmospheric carbon dioxide concentration) (source: IPCC AR4).
The new paper, from a team of researchers led by Andreas Schmittner of Oregon State University, throws cold water on the IPCC’s tails. Here is its rather provocative abstract:
Assessing impacts of future anthropogenic carbon emissions is currently impeded by uncertainties in our knowledge of equilibrium climate sensitivity to atmospheric carbon dioxide doubling. Previous studies suggest 3 K as best estimate, 2–4.5 K as the 66% probability range, and non-zero probabilities for much higher values, the latter implying a small but significant chance of high-impact climate changes that would be difficult to avoid. Here, combining extensive sea and land surface temperature reconstructions from the Last Glacial Maximum with climate model simulations we estimate a lower median (2.3 K) and reduced uncertainty (1.7–2.6 K 66% probability). Assuming paleoclimatic constraints apply to the future as predicted by our model, these results imply lower probability of imminent extreme climatic change than previously thought.
Figure 2 shows the distribution of the range of the earth’s probable climate sensitivity as determined by Schmittner et al. Note the rapid drop-off in the probability that the climate sensitivity is much greater than 3°C (the IPCC “best estimate” for the sensitivity), and that the distribution falls off less slowly towards the left (towards lower sensitivity) than towards the right (higher sensitivities). The “fat right-hand tail” of the distribution is gone and the possibility that the climate sensitivity is in the 1°C to 2°C range is not minimal.
Figure 2. Distribution of the land/ocean climate sensitivity as determined by Schmittner et al. (adapted from Schmittner et al., 2011).
The Schmittner et al. results join a growing number of papers published in recent years which, by employing investigations of the earth’s paleoclimate behavior (that is, how the earth’s temperature changes in the past when subject to changing climate forcings) have come to somewhat similar conclusions, especially regarding the (lack of) evidence to support the existence of the fat right-hand tail.
For example, researchers James Annan and Julia Hargreaves published a paper in 2009 that concluded many of the assumptions underlying the possibilities of very high climate sensitivities were unjustified. They wrote:
When instead reasonable assumptions are made, much greater confidence in a moderate value for [the climate sensitivity] is easily justified, with an upper 95% probability limit for [the sensitivity] easily shown to lie close to 4°C, and certainly well below 6°C. These results also impact strongly on projected economic losses due to climate change.
Annan made repeated comments during the IPCC AR4 review process that the IPCC’s handling of climate sensitivity and its probability distributions were incorrect. His complaints largely fell upon deaf ears.
However, as there are appearing more and more examples in the literature, of which Schmittner et al. is one of them, making a convincing case that the very high climate sensitivities are not defendable, there will be growing pressure on the IPCC in its Fifth Assessment Report to greatly shrink the fat tail of the probability distribution for the true climate sensitivity. However, the climate “realists” very bad experience with the last IPCC process makes them chary. James Annan, writing at his blog in reference to the new Schmittner et al. paper had this to say as to what may result from it:
That said, [the Schmittner et al. paper] is a useful antidote to the exaggerated uncertainty estimates that have been prevalent over recent years, and I certainly applaud the intentions and effort underlying this substantial piece of work. In any case, I expect the merchants of doubt to do their worst on it when they cite it in the IPCC report.
But, as the evidence mounts against a high value for the climate sensitivity, and evidence grows for a low value (recall that the observed rate of global warming for the past several decades has fallen well below IPCC best estimates), the IPCC is going to be hard-pressed to retain the status quo in its Fifth Assessment Report, especially in light of the enhanced scrutiny that its AR4 misdeeds brought upon the process.
But, as James alludes to, perhaps we ought not be holding our breath.
And, for those keeping score out there, about 10 years ago, a couple of us here at WCR were part of a team which published a paper in the journal Climate Research in which we employed a variety of techniques to derive empirical estimates of the amount of temperature rise that we could expect by the end of this century—a rise that could pretty well be considered to be in-line with the climate sensitivity. We concluded that the expected temperature rise between 1990 and 2100 would be in the range 1.0°C to 3.0°C with our best guess being 1.8°C (in contrast to the IPCC estimates, which, at the time, were for a rise of between 1.4°C and 5.8°C).
References:
Annan, J.D., and J.C. Hargreaves, 2009. On the generation and interpretation of probabilistic estimates of climate sensitivity. Climate Change, 104, 423-436, doi:10.1007/s10584-009-9715-y, http://www.jamstec.go.jp/frsgc/research/d5/jdannan/probrevised.pdf
Michaels, P.J., P.C. Knappenberger, O.W. Frauenfeld, and R.E. Davis, 2002. Revised 21st century temperature projections. Climate Research, 23, 1-9.
Schmittner, A., et al., 2011. Climate sensitivity estimated from temperature reconstructions of the Last Glacial Maximum, Science, in press*, http://www.princeton.edu/~nurban/pubs/lgm-cs-uvic.pdf
*According to the authors
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Joel Shore (Nov. 20, 2011 at 12:43 pm):
In an outstanding display of the ad hominem approach to argumentation you’ve failed to address the issue originally under discussion by changing it to me. Coming from an experienced debater such as yourself, the issuance of an ad hominem argument amounts to a tacit admission of defeat on the original issue.
Joel Shore says:
November 21, 2011 at 4:37 pm
Joel, you mentioned the Trenberth-Kiehl diagram, I suggest you take another good look at it. That diagram makes no distinction between ocean and land with respect to the absorption of back radiated (backscattered) LWIR. I again urge you to conduct the empirical experiment I have described. You will learn that the cooling rate of water that is free to evaporatively cool is not effected to the same degree by incident LWIR as other materials.
No matter how much hand waving you wish to indulge in about poorly quantified radiation budgets, the fact remains that the ability of back radiated LWIR to slow the cooling of the Earth’s surface is fundamental to the original global warming claims. The “absorbed by the atmosphere” part of the Trenberth-Kiehl diagram is rubbish with respect to CO2. CO2 molecules almost instantly re-radiate any LWIR photons they intercept. The only way that CO2 is going to warm our atmosphere is by slowing the cooling of the Earth’s surface. I have empirically demonstrated that 71% of the Earth’s surface does not respond to LWIR in the manner shown in the Trenberth-Kiehl diagram.
Kevin Trenberth, as evidenced by the very diagram you mentioned, believes back radiated LWIR has an equal effect over water as it does over land. Further, he believes it has trapped heat in the oceans that strangly we can’t detect. I claim that this is not the case. If you wish to dispute my claims further, please do so through links to lab experiments involving LWIR and liquid water.
Terry: This http://wattsupwiththat.com/2011/11/09/climate-sensitivity-lowering-the-ipcc-fat-tail/#comment-802864 is not ad hominem…Explaining where you are confused about the science is not ad hominem. I would say that making claims that an argument is ad hominem as a way to avoid addressing the scientific points “amounts to a tacit admission of defeat on the original issue”.
Konrad: I don’t think it is useful to continue this discussion further. Your understanding of the greenhouse effect is so poor and you refuse to try to learn more about it. I am sure arguments so silly that they can’t even gain traction amongst skeptical scientists like Willis Eschenbach will be very impressive to the larger scientific community.
Joel Shore (Nov. 26 at 2:16 pm):
Your ad hominem arguments are: 1) “Your post shows that you are suffering under a too simplistic view of the greenhouse effect.” (Nov. 17 at 4:22 pm) 2) “you have a very black-and-white, two-valued view of the world” (Nov. 20 at 6:16 am) and 3) “Terry: As near I can tell, it is you who are running yourself in circles” (Nov. 20 at 6:16 am).
Each of these arguments makes me the issue while avoiding the issue of ambiguous reference by terms in the vernacular of climatology such as “greenhouse” and “heat.” As a portion of an argument, a term that makes ambiguous reference has the logical shortcoming of facilitating proofs of various falsehoods via its violation of the law of non-contradiction. For person of honest intent, to facilitate proofs of falsehoods has no merit. In this sense, terminology does matter.
Joel Shore says:
November 26, 2011 at 2:20 pm
Joel,
If you read the Willis post on the impact of LWIR on the oceans he claimed that LWIR impacting on the surface of water would slow the cooling of liquid water in the same manner as other materials. I proved this wrong through empirical experiment. If this is irrelevant to AGW theory, why did Willis spend so long arguing it? He ended up running away from the discussion. You appear to have done the same.