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Title: Potential Problems Measuring Climate Sensitivity from the Historical Record

This study investigates potential biases between equilibrium climate sensitivity inferred from warming over the historical period (ECShist) and the climate system’s true ECS (ECStrue). This paper focuses on two factors that could contribute to differences between these quantities. First is the impact of internal variability over the historical period: our historical climate record is just one of an infinity of possible trajectories, and these different trajectories can generate ECShistvalues 0.3 K below to 0.5 K above (5%–95% confidence interval) the average ECShist. Because this spread is due to unforced variability, I refer to this as the unforced pattern effect. This unforced pattern effect in the model analyzed here is traced to unforced variability in loss of sea ice, which affects the albedo feedback, and to unforced variability in warming of the troposphere, which affects the shortwave cloud feedback. There is also a forced pattern effect that causes ECShistto depart from ECStruedue to differences between today’s transient pattern of warming and the pattern of warming at 2×CO2equilibrium. Changes in the pattern of warming lead to a strengthening low-cloud feedback as equilibrium is approached in regions where surface warming is delayed: the Southern Ocean, eastern Pacific, and North Atlantic near Greenland. This more » forced pattern effect causes ECShistto be on average 0.2 K lower than ECStrue(~8%). The net effect of these two pattern effects together can produce an estimate of ECShistas much as 0.5 K below ECStrue.

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Award ID(s):
Publication Date:
Journal Name:
Journal of Climate
Page Range or eLocation-ID:
p. 2237-2248
American Meteorological Society
Sponsoring Org:
National Science Foundation
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