Requirements for empirical determination of Earth's climate sensitivity. S. E. Schwartz. AAAS Annual Meeting, Denver CO, February 14-18, 2003.

Atmospheric carbon dioxide has increased ~30% over the industrial period. Radiative forcing of climate change by increased concentrations of CO2 and other long-lived greenhouse gases is 2.4 0.2 W m-2 relative to the preindustrial era; doubling of CO2 would result in a forcing of about 4 W m-2. Over a hundred years ago Arrhenius calculated the sensitivity of Earth's mean temperature to increased atmospheric CO2 considering direct effects and feedbacks, importantly those of increased water vapor and decreased snow cover, obtaining a sensitivity for CO2 doubling of 5.5 K or 1.38 K/(W m-2). This sensitivity is presciently close to the upper end of the range given by the most recent report by the Intergovernmental Panel on Climate Change, 1.5 4.5 K or 0.38 1.13 K/(W m-2). The factor of three uncertainty in the present estimate is certainly unacceptably large for planning for mitigation or adaptation. Should Earth experience the forcing of a doubled CO2, which may be expected around the middle of this century, a sensitivity at the high end of this range would result in dramatic and severe consequences. The increase in global mean temperature to date, relative to preindustrial, is 0.6 0.2 K, suggesting a sensitivity of 0.25 0.09 K/(W m-2), well below the low end of the IPCC range, but such an empirical estimate assumes that climate response is near equilibrium and that forcing is due entirely to long-lived greenhouse gases. The equilibrium assumption is shown here to be valid, suggesting the feasibility of the empirical approach. However forcings other than by greenhouse gases, particularly the cooling influence of anthropogenic aerosols due to their scattering of solar radiation, are thought to offset much of the greenhouse gas forcing on a global basis, resulting in a much lower total forcing and consequently much greater sensitivity. Present uncertainty in total forcing is so great as to preclude a meaningful empirical estimate of climate sensitivity from the temperature record and forcing over the industrial period. This paper suggests target uncertainties for forcing and response (20%) required to obtain a useful empirical determination of climate sensitivity. Such target uncertainties provide a basis for specification of research required for meaningful empirical determination of Earth's climate sensitivity and/or for evaluation of climate models by their performance over the industrial period.

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