Why Is Climate Sensitivity So Unpredictable?
Abstract
Uncertainties in projections of future climate change have not lessened substantially in past decades. Both models and observations yield broad probability distributions for long-term increases in global mean temperature expected from the doubling of atmospheric carbon dioxide, with small but finite probabilities of very large increases. We show that the shape of these probability distributions is an inevitable and general consequence of the nature of the climate system, and we derive a simple analytic form for the shape that fits recent published distributions very well. We show that the breadth of the distribution and, in particular, the probability of large temperature increases are relatively insensitive to decreases in uncertainties associated with the underlying climate processes.
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Supplementary Material
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References and Notes
1
S. Solomonet al., Eds. Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (Cambridge Univ. Press, 2007).
2
Climate sensitivity is a measure of equilibrium climate change. During the adjustment to the new equilibrium, time-dependent factors, in particular ocean heat uptake, are also important.
3
M. R. Allenet al., in Avoiding Dangerous Climate Change, H. J. Schellnhuber, W. Cramer, N. Nakicenovic, T. Wigley, G. Yohe, Eds. (Cambridge Univ. Press, 2006), pp. 281–289.
4
N. G. Andronova, M. E. Schlesinger, J. Geophys. Res.106, 22605 (2001).
5
J. M. Gregory, R. J. Stouffer, S. C. B. Raper, P. A. Stott, N. A. Rayner, J. Clim.15, 3117 (2002).
6
C. E. Forest, P. H. Stone, A. P. Sokolov, M. R. Allen, M. D. Webster, Science295, 113 (2002).
7
D. J. Frameet al., Geophys. Res. Lett.32, L09702 (2005).
8
J. M. Murphyet al., Nature430, 768 (2004).
9
D. A. Stainforthet al., Nature433, 403 (2005).
10
B. M. Sanderson, C. Piani, W. J. Ingram, D. A. Stone, M. R. Allen, Clim. Dyn. published online 3July2007;
11
G. C. Hegerl, T. J. Crowley, W. T. Hyde, D. J. Frame, Nature440, 1029 (2006).
12
D. L. Royer, R. A. Berner, J. Park, Nature446, 530 (2007).
13
C. Piani, D. J. Frame, D. A. Stainforth, M. R. Allen, Geophys. Res. Lett.32, L23825 (2005).
14
S. Bonyet al., J. Clim.19, 3445 (2006).
15
There are ambiguities surrounding feedback terminology in the climate literature. Hansen et al. (22) reversed the definition of feedback factor and gain from what is conventional in electronics (23–25). The convention adopted by Hansen et al. (22) has permeated much, but not all, of the climate literature [compare with (26–28)]. For the standard electronics definitions, it can be shown that the feedback factor is proportional to the fraction of the system response fed back into the system input and that the gain is the proportion by which the output has gained (i.e., been amplified) by the inclusion of the feedbacks. In this study, we retain the traditional electronics definitions. See also the SOM.
16
17
R. Colman, Clim. Dyn.20, 865 (2003).
18
B. J. Soden, I. M. Held, J. Clim.19, 3354 (2006).
19
Researchers (17, 18) estimated mean and SD of feedback factors calculated from two different suites of climate models. First, Colman (17) found a mean and SD of (0.11, 0.06) for the albedo feedback factor; (0.17, 0.11) for the cloud feedback factor; and (0.42, 0.06) for the water vapor and lapse rate feedbacks combined. Second, Soden and Held (18) found a mean and SD of (0.09, 0.02) for the albedo feedback factor; (0.22, 0.12) for the cloud feedback factor; and (0.31, 0.04) for the water vapor and lapse rate feedbacks combined. The water vapor and lapse rate feedbacks are typically combined because models show a strong negative correlation between the two. Although the combined feedback for water vapor and lapse rate has the largest magnitude, the greatest contributor to uncertainty is the cloud feedback.
20
P. F. Hoffman, D. P. Schrag, Terra Nova14, 129 (2002).
21
L. C. Sloan, D. K. Rea, Palaeogeogr. Palaeoclimatol. Palaeoecol.119, 275 (1996).
22
J. E. Hansenet al., in Climate Processes and Climate Sensitivity, J. E. Hansen, T. Takahashi, Eds. (Geophysical Monograph 29, American Geophysical Union, Washington, DC, 1984), pp. 130–163.
23
H. W. Bode, Network Analysis and Feedback Amplifier Design (Van Nostrand, New York, 1945).
24
R. Kories, H. Schmidt-Walter, Electrical Engineering: A Pocket Reference (Springer, Berlin, 2003).
25
J. G. Graeme, Optimizing Op-Amp Performance (McGraw-Hill, New York, 1997).
26
National Research Council, Understanding Climate Change Feedbacks (National Academies Press, Washington, DC, 2003).
27
R. S. Lindzen, M.-D. Chou, A. Y. Hou, Bull. Am. Meteorol. Soc.82, 417 (2001).
28
J. M. Wallace, P. V. Hobbs, Atmospheric Science: An Introductory Survey (Academic Press, San Diego, ed. 2, 2006).
29
The curve from (5) is a numerical calculation from climate observations and uses an equation isomorphic to Eq. 3.
30
We thank P. Guttorp, J. M. Wallace, D. Hartmann, D. Battisti, C. Bretherton, E. Steig, D. Stolar, and S. Warren for insightful comments on drafts of the manuscript; M. R. Allen, M. Cane, and one anonymous reviewer for constructive suggestions that substantially improved the manuscript; and H. J. Smith, the editor. G.H.R. thanks Yale University for support as a Flint Visiting Professor.
Information & Authors
Information
Published In
Science
Volume 318 • Issue 5850 • 26 October 2007
Pages: 629 - 632
History
Received: 7 May 2007
Accepted: 14 September 2007
Copyright
American Association for the Advancement of Science.
