Climate sensitivity: should the climate tail wag the policy dog?
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The small but stubbornly unyielding possibility of a very large long-term response of global temperature to increases in atmospheric carbon dioxide can be termed the fat tail of high climate sensitivity. Recent economic analyses suggest that the fat tail should dominate a rational policy strategy if the damages associated with such high temperatures are large enough. The conclusions of such analyses, however, depend on how economic growth, temperature changes, and climate damages unfold and interact over time. In this paper we focus on the role of two robust physical properties of the climate system: the enormous thermal inertia of the ocean, and the long timescales associated with high climate sensitivity. Economic models that include a climate component, and particularly those that focus on the tails of the probability distributions, should properly represent the physics of this slow response to high climate sensitivity, including the correlated uncertainty between present forcing and climate sensitivity, and the global energetics of the present climate state. If climate sensitivity in fact proves to be high, these considerations prevent the high temperatures in the fat tail from being reached for many centuries. A failure to include these factors risks distorting the resulting economic analyses. For example, we conclude that fat-tail considerations will not strongly influence economic analyses when these analyses follow the common—albeit controversial—practices of assigning large damages only to outcomes with very high temperature changes and of assuming a significant baseline level of economic growth.
KeywordsClimate Sensitivity Deep Ocean Damage Function Climate Response Climate Feedback
We are very grateful to Marcia Baker and Marshall Baker for conversations and comments that are central to this work; and without implying agreement, to Marty Weitzman and Antony Millner for guidance and advice, to Simon Dietz, Charles Mason, Steve Newbold, and Alex Marten for instructive feedback; finally we are grateful to three reviewers for thoughtful and constructive reviews that substantially improved the manuscript, and to Gary Yohe, the editor.
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