Despite decades of intense research the equilibrium sensitivity of Earth's climate system, the amount by which the global mean surface temperature would change in response to a sustained change in a radiative flux component, remains uncertain to roughly a factor of three. Earth's climate system is a balance between incoming shortwave (solar) radiation and outgoing longwave (thermal infrared) radiation. Consequently any changes in Earth's temperature or heat content must be due to an imbalance between these two energy fluxes, and vice versa. From energy balance considerations the equilibrium sensitivity of such an isolated system is equal to the quotient of the relaxation time constant of the system and the pertinent heat capacity. These considerations are applied to Earth's climate system to provide an independent empirical determination of Earth's climate sensitivity. Observational data are used to determine the heat capacity of the global ocean from regression of ocean heat content vs. global mean surface temperature, GMST, is 14 ± 6 W yr m-2 K-1, equivalent to 110 m of ocean water; other sinks raise the effective planetary heat capacity to 17 ± 7 W yr m-2 K-1 (all uncertainties are 1-sigma estimates). The time constant pertinent to changes in GMST is determined from autocorrelation of that quantity over 1880-2004 to be 5 ± 1 yr. The resultant equilibrium climate sensitivity, 0.30 ± 0.14 K/(W m-2), corresponds to an equilibrium temperature increase for doubled CO2 of 1.1 ± 0.5 K, well less than the generally accepted sensitivity of 3 (-1, +1.5) K. This situation invites a scrutiny of the each of these findings for possible sources of error of interpretation.
This page was last updated 2007-11-21.
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