C.3 Observed and Modelled Changes in Aerosols
Aerosols (very small airborne particles and droplets) are known to influence significantly
the radiative budget of the Earth/atmosphere. Aerosol radiative effects occur
in two distinct ways: (i) the direct effect, whereby aerosols themselves scatter
and absorb solar and thermal infrared radiation, and (ii) the indirect effect,
whereby aerosols modify the microphysical and hence the radiative properties and
amount of clouds. Aerosols are produced by a variety of processes, both natural
(including dust storms and volcanic activity) and anthropogenic (including fossil
fuel and biomass burning). The atmospheric concentrations of tropospheric aerosols
are thought to have increased over recent years due to increased anthropogenic
emissions of particles and their precursor gases, hence giving rise to radiative
forcing. Most aerosols are found in the lower troposphere (below a few kilometres),
but the radiative effect of many aerosols is sensitive to the vertical distribution.
Aerosols undergo chemical and physical changes while in the atmosphere, notably
within clouds, and are removed largely and relatively rapidly by precipitation
(typically within a week). Because of this short residence time and the inhomogeneity
of sources, aerosols are distributed inhomogeneously in the troposphere, with
maxima near the sources. The radiative forcing due to aerosols depends not only
on these spatial distributions, but also on the size, shape, and chemical composition
of the particles and various aspects (e.g., cloud formation) of the hydrological
cycle as well. As a result of all of these factors, obtaining accurate estimates
of this forcing has been very challenging, from both the observational and theoretical
standpoints.
Nevertheless, substantial progress has been achieved in better defining
the direct effect of a wider set of different aerosols. The SAR considered
the direct effects of only three anthropogenic aerosol species: sulphate aerosols,
biomass-burning aerosols, and fossil fuel black carbon (or soot). Observations
have now shown the importance of organic materials in both fossil fuel carbon
aerosols and biomass-burning carbon aerosols. Since the SAR, the inclusion of
estimates for the abundance of fossil fuel organic carbon aerosols has led to
an increase in the predicted total optical depth (and consequent negative forcing)
associated with industrial aerosols. Advances in observations and in aerosol
and radiative models have allowed quantitative estimates of these separate components,
as well as an estimate for the range of radiative forcing associated with mineral
dust, as shown in Figure 9. Direct radiative
forcing is estimated to be -0.4 Wm-2 for sulphate, -0.2 Wm-2
for biomass-burning aerosols, -0.1 Wm-2 for fossil fuel organic carbon,
and +0.2 Wm-2 for fossil fuel black carbon aerosols. Uncertainties
remain relatively large, however. These arise from difficulties in determining
the concentration and radiative characteristics of atmospheric aerosols and
the fraction of the aerosols that are of anthropogenic origin, particularly
the knowledge of the sources of carbonaceous aerosols. This leads to considerable
differences (i.e., factor of two to three range) in the burden and substantial
differences in the vertical distribution (factor of ten). Anthropogenic dust
aerosol is also poorly quantified. Satellite observations, combined with model
calculations, are enabling the identification of the spatial signature of the
total aerosol radiative effect in clear skies; however, the quantitative amount
is still uncertain.
Estimates of the indirect radiative forcing by anthropogenic aerosols
remain problematic, although observational evidence points to a negative aerosol-induced
indirect forcing in warm clouds. Two different approaches exist for estimating
the indirect effect of aerosols: empirical methods and mechanistic methods.
The former have been applied to estimate the effects of industrial aerosols,
while the latter have been applied to estimate the effects of sulphate, fossil
fuel carbonaceous aerosols, and biomass aerosols. In addition, models for the
indirect effect have been used to estimate the effects of the initial change
in droplet size and concentrations (a first indirect effect), as well as the
effects of the subsequent change in precipitation efficiency (a second indirect
effect). The studies represented in Figure 9
provide an expert judgement for the range of the first of these; the range is
now slightly wider than in the SAR; the radiative perturbation associated with
the second indirect effect is of the same sign and could be of similar magnitude
compared to the first effect.
The indirect radiative effect of aerosols is now understood to also encompass
effects on ice and mixed-phase clouds, but the magnitude of any such indirect
effect is not known, although it is likely to be positive. It is not possible
to estimate the number of anthropogenic ice nuclei at the present time. Except
at cold temperatures (below -45°C) where homogeneous nucleation is expected
to dominate, the mechanisms of ice formation in these clouds are not yet known.
C.4 Observed Changes in Other Anthropogenic Forcing Agents Land-use (albedo) change
Changes in land use, deforestation being the major factor, appear to have produced
a negative radiative forcing of -0.2 ± 0.2 Wm-2 (Figure
8). The largest effect is estimated to be at the high latitudes. This
is because deforestation has caused snow-covered forests with relatively low albedo
to be replaced with open, snow-covered areas with higher albedo. The estimate
given above is based on simulations in which pre-industrial vegetation is replaced
by current land-use patterns. However, the level of understanding is very low
for this forcing, and there have been far fewer investigations of this forcing
compared to investigations of other factors considered in this report.
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