James Hansen et al., Climate Sensitivity, Sea Level, and Atmospheric CO2 [in review]

Climate Sensitivity, Sea Level, and Atmospheric CO₂

James Hansen, Makiko Sato, Gary Russell and Pushker Kharecha

NASA Goddard Institute for Space Studies and Columbia University Earth Institute, New York, NY, U.S.A.

Abstract

Cenozoic temperature, sea level and CO₂ co-variations provide insights into climate sensitivity to external forcings and sea level sensitivity to climate change. Pleistocene climate oscillations imply a fast-feedback climate sensitivity 3 ± 1 °C for 4 W/m² CO₂ forcing for the average of climate states between the Holocene and Last Glacial Maximum (LGM), the error estimate being large and partly subjective because of continuing uncertainty about LGM global surface climate. Slow feedbacks, especially change of ice sheet size and atmospheric CO₂, amplify total Earth system sensitivity. Ice sheet response time is poorly defined, but we suggest that hysteresis and slow response in current ice sheet models are exaggerated. We use a global model, simplified to essential processes, to investigate state-dependence of climate sensitivity, finding a strong increase in sensitivity when global temperature reaches early Cenozoic and higher levels, as increased water vapor eliminates the tropopause. It follows that burning all fossil fuels would create a different planet, one on which humans would find it difficult to survive.

Full paper [in review] here: http://arxiv.org/ftp/arxiv/papers/1211/1211.4846.pdf