Climate Sensitivity in the Geologic Past

The response of temperature to CO2 change (climate sensitivity) in the geologic past may help inform future climate predictions. Proxies for CO2 and temperature generally imply high climate sensitivities: ≥3 K per CO2 doubling during ice-free times (fast-feedback sensitivity) and ≥6 K during times with land ice (Earth-system sensitivity). Climate models commonly underpredict the magnitude of climate change and have fast-feedback sensitivities close to 3 K. A better characterization of feedbacks in warm worlds raises climate sensitivity to values more in line with proxies and produces climate simulations that better fit geologic evidence. As CO2 builds in our atmosphere, we should expect both slow (e.g., land ice) and fast (e.g., vegetation, clouds) feedbacks to elevate the long-term temperature response over that predicted from the canonical fast-feedback value of 3 K. Because temperatures will not decline for centuries to millennia, climate sensitivities that integrate slower processes have relevance for current climate policy. 277 Click here to view this article's online features: • Download figures as PPT slides • Navigate linked references • Download citations • Explore related articles • Search keywords ANNUAL REVIEWS Further A nn u. R ev . E ar th P la ne t. Sc i. 20 16 .4 4: 27 729 3. D ow nl oa de d fr om w w w .a nn ua lr ev ie w s. or g A cc es s pr ov id ed b y C ar le to n C ol le ge ( M in n. ) on 1 1/ 03 /1 9. F or p er so na l u se o nl y. EA44CH11-Royer ARI 14 May 2016 7:0 Pleistocene: the geologic epoch from 2.59 to 0.01 Myr ago Radiative or climate forcing: the difference in solar energy absorbed by Earth and radiated back to space in units of W m−2 Climate sensitivity: an equilibrium change in global mean surface temperature in response to a doubling of atmospheric CO2 above the preindustrial value of ∼280 ppmv Climate feedback: a component of the Earth system that responds to a radiative forcing causing further changes in the radiative forcing; can be positive (amplifying) or negative (dampening) Aerosols: very fine (<10 μm) solid or liquid particles suspended in the atmosphere Climate proxy: a character that can be measured in geologic materials that is known to covary with climate today