Radiative forcing of climate change by anthropogenic aerosols is now recognized as the largest uncertainty in climate forcing over the industrial period. This uncertainty limits inference of Earth's climate sensitivity either empirically or by comparison of observed temperature change over the industrial period with modeled temperature change obtained by imposing a time-dependent forcing in a climate model. Either way, for a desired uncertainty in sensitivity of, say, 30% (e.g., temperature increase resulting from doubling atmospheric CO2, the required uncertainty in forcing is about 20%. The resultant required uncertainty in aerosol forcing depends on the magnitude of this forcing. If total aerosol forcing is small, the requisite uncertainty can be quite large, e.g., a factor of 2 for aerosol forcing -0.4 W m-2. However as aerosol forcing magnitude increases the requirement is much more stringent, e.g., for aerosol forcing -1.2 W m-2, 10%, comparable to present uncertainty in greenhouse gas forcing. This talk examines quantifiable uncertainties in aerosol forcing and apportions them between contributions from atmospheric chemistry, atmospheric radiation, and cloud microphysics. Unless and until present uncertainties are greatly reduced it will not be possible to place confident limits on Earth's climate sensitivity, limiting society's ability to confidently plan to adapt to or mitigate future climate change arising from increasing atmospheric concentrations of greenhouse gases.
This page was last updated 2003-12-10.
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