Essential directions for climate change research: Atmospheric composition and radiative forcing. S. E. Schwartz. U. S. Climate Change Science Program Planning Workshop for Scientists & Stakeholders, Washington DC, December 3-5, 2002.

The factor of three uncertainty in present best estimates of the sensitivity of the mean global temperature change to radiative forcing is 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 the range of present estimates 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 current estimates, but such an empirical estimate assumes that climate response is near equilibrium and that forcing is due entirely to long-lived greenhouse gases. 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. Obtaining a useful empirical determination of climate sensitivity requires uncertainties in forcing and response of about 20%. 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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