Uncertainty requirements in radiative forcing of climate change. Schwartz S. E., J. Air Waste Management Assoc. 54, 1351-1359 (2004).

The continuing increase in atmospheric carbon dioxide makes it essential that climate sensitivity, the change in global mean temperature that would result from a given radiative forcing, be quantified with known uncertainty. Present estimates are quite uncertain, 3 +/- 1.5 K for doubling of CO2. Model studies examining climate response to forcing by greenhouse gases and aerosols exhibit large differences in sensitivities and imposed aerosol forcings that raise questions regarding claims of their having reproduced observed large scale changes in surface temperature over the twentieth century. Present uncertainty in forcing, due largely to uncertainty in forcing by aerosols, precludes meaningful model evaluation by comparison with observed global temperature change or empirical determination of climate sensitivity. Uncertainty in aerosol forcing must be reduced at least three-fold for uncertainty in climate sensitivity to be meaningfully reduced and bounded.

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