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GEOPHYSICAL RESEARCH LETTERS, VOL. 33, L06704, doi:10.1029/2005GL025259, 2006

Using multiple observationally-based constraints to estimate climate sensitivity

J. D. Annan

Frontier Research Center for Global Change, JAMSTEC, Yokohama, Japan


J. C. Hargreaves

Frontier Research Center for Global Change, JAMSTEC, Yokohama, Japan


Abstract

Climate sensitivity has been subjectively estimated to be likely to lie in the range of 1.5–4.5°C, and this uncertainty contributes a substantial part of the total uncertainty in climate change projections over the coming century. Objective observationally-based estimates have so far failed to improve on this upper bound, with many estimates even suggesting a significant probability of climate sensitivity exceeding 6°C. In this paper, we show how it is possible to greatly reduce this uncertainty by using Bayes' Theorem to combine several independent lines of evidence. Based on some conservative assumptions regarding the value of independent estimates, we conclude that climate sensitivity is very unlikely (<5% probability) to exceed 4.5°C. We cannot assign a significant probability to climate sensitivity exceeding 6°C without making what appear to be wholly unrealistic exaggerations about the uncertainties involved. This represents a significant lowering of the previously-estimated bound.

Received 18 November 2005; accepted 6 February 2006; published 18 March 2006.

Index Terms: 3309 Atmospheric Processes: Climatology (1616, 1620, 3305, 4215, 8408); 3315 Atmospheric Processes: Data assimilation; 3275 Mathematical Geophysics: Uncertainty quantification (1873); 3245 Mathematical Geophysics: Probabilistic forecasting (3238).

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Citation: Annan, J. D., and J. C. Hargreaves (2006), Using multiple observationally-based constraints to estimate climate sensitivity, Geophys. Res. Lett., 33, L06704, doi:10.1029/2005GL025259.