New study: Climate sensitivity to CO2 more limited than extreme projections

New study: Climate sensitivity to CO2 more limited than extreme projections

11-24-11

CORVALLIS, Ore. – A new study suggests that the rate of global warming from doubling of atmospheric carbon dioxide may be less than the most dire estimates of some previous studies – and, in fact, may be less severe than projected by the Intergovernmental Panel on Climate Change report in 2007.

Authors of the study, which was funded by the National Science Foundation’s Paleoclimate Program and published online this week in the journal Science, say that global warming is real and that increases in atmospheric CO2 will have multiple serious impacts.

However, the most Draconian projections of temperature increases from the doubling of CO2 are unlikely.

“Many previous climate sensitivity studies have looked at the past only from 1850 through today, and not fully integrated paleoclimate date, especially on a global scale,” said Andreas Schmittner, an Oregon State University researcher and lead author on the Science article. “When you reconstruct sea and land surface temperatures from the peak of the last Ice Age 21,000 years ago – which is referred to as the Last Glacial Maximum – and compare it with climate model simulations of that period, you get a much different picture.

…

Published Online November 24 2011
Science DOI: 10.1126/science.1203513
Report

Climate Sensitivity Estimated from Temperature Reconstructions of the Last Glacial Maximum

Andreas Schmittner1,*, Nathan M. Urban2, Jeremy D. Shakun3, Natalie M. Mahowald4, Peter U. Clark5, Patrick J. Bartlein6, Alan C. Mix1, Antoni Rosell-Melé7

+ Author Affiliations

1College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331–5503, USA.
2Woodrow Wilson School of Public and International Affairs, Princeton University, NJ 08544, USA.
3Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138, USA.
4Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, NY 14850, USA.
5Department of Geosciences, Oregon State University, Corvallis, OR 97331, USA.
6Department of Geography, University of Oregon, Eugene, OR 97403, USA.
7ICREA and Institute of Environmental Science and Technology, Universitat Autònoma de Barcelona, Bellaterra, Spain.

↵*To whom correspondence should be addressed. E-mail: aschmitt@coas.oregonstate.edu

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 to 4.5 K as the 66% probability range, and nonzero 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 to 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.

…

Q: What did you find, and what is the significance of your findings?

The scientific community generally believes that the climate sensitivity is likely to lie between 2 and 4.5 degrees Celsius per doubling of atmospheric CO2, with a best estimate of 3 °C. I will call this the “IPCC” or “consensus” estimate, since it is based on a review of the scientific literature found in the latest report of the Intergovernmental Panel on Climate Change (IPCC).

In our LGM study we find that ECS is “likely” (66% probability) to lie between 1.7 and 2.6 degrees, and “very likely” (90% probability) to lie between 1.4 and 2.8 degrees, with a best estimate of around 2.2 or 2.3 °C. Our estimate of the warming effect of CO2 is therefore on the low end of, and less uncertain than, the currently accepted IPCC range.

Our low sensitivity is interesting, but within the range of previous studies. What is probably more significant is the fact that our analysis seemingly rules out the higher sensitivities (above the IPCC “best” estimate of 3 °C) which other studies have been unable to exclude. (Note the word “seemingly”: more on that later.)

--snip--

Q: Does this study overturn the IPCC’s estimate of climate sensitivity?

No, we haven’t disproven the IPCC or high climate sensitivities. At least, not yet. This comes down to what generalizations can be made from a single, limited study. This is why the IPCC bases its conclusions on a synthesis of many studies, not relying on any particular one.

While our statistical analysis calculates that high climate sensitivities have very low probabilities, you can see from the caveats in our paper (discussed further below), and my remarks in this interview, that we have not actually claimed to have disproven high climate sensitivities. We do claim that our results imply “lower probability of imminent extreme climatic change than previously thought”, and that “climate sensitivities larger than 6 K are implausible”, which I stand by. I do not claim we have demonstrated that climate sensitivities larger than 3 K are implausible, even though we calculate a low probability for them, because our study has important limitations.

--snip--

Q: Given all these caveats, how robust are the results of your study?

I think our lower climate sensitivity estimate will hold up, provided the reconstructed LGM temperature data on which it is based hold up. Our finding of a warmer LGM will prove controversial among the scientific community and the data will be subject to much scrutiny. It remains to be seen whether this temperature data is consistent with everything else we know about that period of time (its climate, its vegetation, the size of its ice sheets, etc.).

I am less confident that our narrow uncertainty range really does exclude climate sensitivities above 3 °C. This is something that could be overturned by future work. It certainly would stimulate a lot of rethinking among scientists if the result isn’t overturned. I can’t say I’m rooting strongly for either outcome, though. I’d be pleased to see our findings confirmed, but if they’re disproven, I’ll learn something from the way in which they are disproven, and this will improve my own research. Who knows, maybe I will disprove them myself.


http://newscience.planet3.org/2011/11/24/interview-with-nathan-urban-on-his-new-paper-climate-sensitivity-estimated-from-temperature-reconstructions-of-the-last-glacial-maximum/