Climate Sensitivity Estimated From Earth's Climate History

• Main Authors: James E. Hansen, Makiko Sato
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Subjects: Ice Sheet
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.423.6542; http://www.columbia.edu/~jeh1/mailings/2012/20120508_ClimateSensitivity.pdf

Earth's climate history potentially can yield accurate assessment of climate sensitivity. Imprecise knowledge of glacial-to-interglacial global temperature change is the biggest obstacle to accurate assessment of the fast-feedback climate sensitivity, which is the sensitivity that most immediately affects humanity. Our best estimate for the fast-feedback climate sensitivity from Holocene initial conditions is 3 ± 0.5°C for 4 W/m 2 CO2 forcing (68 % probability). Slow feedbacks, including ice sheet disintegration and release of greenhouse gases (GHGs) by the climate system, generally amplify total Earth system climate sensitivity. Slow feedbacks make Earth system climate sensitivity highly dependent on the initial climate state and on the magnitude and sign of the climate forcing, because of thresholds (tipping points) in the slow feedbacks. It is difficult to assess the speed at which slow feedbacks will become important in the future, because of the absence in paleoclimate history of any positive (warming) forcing rivaling the speed at which the human-caused forcing is growing. 1.