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 more than a factor of 2. 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. 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 8.5 ± 2.5 yr. The resultant equilibrium climate sensitivity, 0.51 ± 0.26 K/(W m-2), corresponds to an equilibrium temperature increase for doubled CO2 of 1.9 ± 1.0 K, somewhat lower than the central estimate of the sensitivity given in the 2007 Assessment Report of the Intergovernmental Panel on Climate Change, but consistent within the uncertainties of both estimates. The short climate system time constant implies that global mean surface temperature is in near equilibrium with the applied forcing. Forcing over the twentieth century other than that due to greenhouse gases, ascribed mainly to tropospheric aerosols, is estimated as -1.1 ± 0.7 W m-2.
Schwartz S. E., Heat capacity, time constant, and sensitivity of Earth's climate system. J. Geophys. Res., 112D24S05 (2007). doi:10.1029/2007JD008746
Schwartz S. E., Reply to comments by G. Foster et al., R. Knutti et al., and N. Scafetta on "Heat capacity, time constant, and sensitivity of Earth's climate system". J. Geophys. Res. 113, D15105 (2008), doi:10.1029/2008JD009872.
Schwartz S. E., Uncertainty in Climate Sensitivity: Causes, Consequences, Challenges. Energy Environ. Sci. 1, 430-453, doi:10.1039/b810350j
This page was last updated 2008-11-17.
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