Climate Change and the European Water Dimension - Agri ...

The direct radiative forcing caused by anthropogenic aerosol has been estimated
using models that simulate the global distribution of particles as well as their optical
properties. There are large uncertainties involved in such models, both related to
their concentrations and to their optical properties. Validation of model simulations
has been performed through comparison with observations, e.g. surface
measurements of particle concentrations and composition as well as estimates of
atmospheric optical depths and total aerosol reflectivity based on satellite
observations. Direct forcing by anthropogenic aerosols is estimated to be –0.4 Wm -2
for sulphate, -0.2 Wm -2 for aerosols from biomass burning, –0.1 Wm -2 for fossil fuel
organic carbon and +0.2 Wm -2 for fossil fuel black carbon aerosols. The uncertainties
on these numbers are illustrated in Figure I.4. Other estimates have recently been
presented by NASA scientists (Hansen, 2002, Hansen and Sato, 2001) who
calculates a net forcing (including indirect effects) of black carbon of 1 ±0.5 W m 2
and of -0.6 to -1.0 W m -2 for sulphate aerosol.
Figure I.5. Aerosol optical thickness (AOT) derived from satellite observations
(Kaufman et al., 2002).
(a). Distribution of fine AOT, showing fine particles from pollution (a, c
and e) and vegetation (b and d).
(b). Distribution of coarse AOT from dust (a), salt particles (b) and desert
dust (c).
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