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of the realm of possibility, however, that future clusters of large volcanic
eruptions would have a major effect on decadal to centennial variability.
Lindzen (1994) has argued that the sequence of large volcanic eruptions,
the 1883 eruption of Krakatoa and the 1912 eruption of Katmai, produced
more lasting effects than would a single volcanic eruption. Since the aerosol
output of these volcanic eruptions was not measured, the argument must
be taken as indicative rather than definitive. The possibility of clusters of
volcanic eruptions is, according to our current capabilities, unpredictable.
2.3 Anthropogenic Aerosols
In contrast to volcanic aerosols which are injected into the stratosphere
sporadically but reside there for a long time, anthropogenic aerosols are
constantly injected into the troposphere and last only a few days. They
are a major component of the surface radiation balance, and can affect
temperature in regions close to the sources of emission (Charlson et aI.,
1991). These sulfate aerosols have a lifetime of only 5 days or so and
therefore do not reach laterally far from their origin or vertically into the
stratosphere. The net effect of these aerosols is to considerably decrease
the solar radiation reaching the ground (by as much as 5 W/m 2 locally
near large sources of emissions) and leads to trends of net surface cooling
in these regions, even when the warming effect of greenhouse gas emission
tends to warm the rest of the earth (Karl et aI., 1995).
Anthropogenic aerosols are now estimated to have a global cooling impact of order 0.5 W/m 2 (at the top of the atmosphere) compared to the
warming impact of about 2.3 W/m 2 due to the increases of radiatively
active greenhouse gases (e.g. IPCC, 1995). Decadal variations in emissions, as in the oil stoppages of the early 1970's, could have significant
impact on local decadal variations and cannot be neglected as a decadal
climate variability mechanism. Recent calculations of anthropogenic climate change have included direct aerosol effects, i.e. the direct effect on
the radiative budget, but cannot yet estimate the indirect effect, i.e. the
effects of aerosols on clouds through their actions as cloud condensation
nuclei. A complete state- of-the-art review is given in Chapter 3 of the
IPCC 95 report. We must conclude that decadal variations of emissions of
anthropogenic aerosols are a possible mechanism for local decadal climate
change, but one that is predictable only if the social and economic factors
that determine their emissions are themselves predicted.
of the realm of possibility, however, that future clusters of large volcanic
eruptions would have a major effect on decadal to centennial variability.
Lindzen (1994) has argued that the sequence of large volcanic eruptions,
the 1883 eruption of Krakatoa and the 1912 eruption of Katmai, produced
more lasting effects than would a single volcanic eruption. Since the aerosol
output of these volcanic eruptions was not measured, the argument must
be taken as indicative rather than definitive. The possibility of clusters of
volcanic eruptions is, according to our current capabilities, unpredictable.
2.3 Anthropogenic Aerosols
In contrast to volcanic aerosols which are injected into the stratosphere
sporadically but reside there for a long time, anthropogenic aerosols are
constantly injected into the troposphere and last only a few days. They
are a major component of the surface radiation balance, and can affect
temperature in regions close to the sources of emission (Charlson et aI.,
1991). These sulfate aerosols have a lifetime of only 5 days or so and
therefore do not reach laterally far from their origin or vertically into the
stratosphere. The net effect of these aerosols is to considerably decrease
the solar radiation reaching the ground (by as much as 5 W/m 2 locally
near large sources of emissions) and leads to trends of net surface cooling
in these regions, even when the warming effect of greenhouse gas emission
tends to warm the rest of the earth (Karl et aI., 1995).
Anthropogenic aerosols are now estimated to have a global cooling impact of order 0.5 W/m 2 (at the top of the atmosphere) compared to the
warming impact of about 2.3 W/m 2 due to the increases of radiatively
active greenhouse gases (e.g. IPCC, 1995). Decadal variations in emissions, as in the oil stoppages of the early 1970's, could have significant
impact on local decadal variations and cannot be neglected as a decadal
climate variability mechanism. Recent calculations of anthropogenic climate change have included direct aerosol effects, i.e. the direct effect on
the radiative budget, but cannot yet estimate the indirect effect, i.e. the
effects of aerosols on clouds through their actions as cloud condensation
nuclei. A complete state- of-the-art review is given in Chapter 3 of the
IPCC 95 report. We must conclude that decadal variations of emissions of
anthropogenic aerosols are a possible mechanism for local decadal climate
change, but one that is predictable only if the social and economic factors
that determine their emissions are themselves predicted.