302
today the anthropogenic contribution dominates. The annual release of
802, mainly from fossil fuel combustion, amounts to some 80 M tons and
is now larger than the natural emission of sulfate components. Fig. 4
shows the annual mean in the total amount of anthropogenic sulfate in
mg/m 2 (Feichter pers. inf., 1996). Over the most polluted regions of
Europe and North America the sulfate levels have gone up by more than a
factor of 10. Experiments have been undertaken to estimate the effect of
the increase in the sulfate on atmospheric radiation. Since the effect of the
aerosols mainly is to modify the reflectivity of solar radiation directly or
indirectly via increased cloudiness, sulphate aerosols cool the atmosphere.
The present effect is localized to certain areas of the Northern Hemisphere
over and downstream the source region. In these areas it is likely that it
practically can offset the present warming by the greenhouse gases. The
calculations, however, are difficult to do and the result is very preliminary
(Charlston et al. 1991).
Stratospheric aerosols have a more global effect due to their much longer
residence time (several years). They mainly enter the stratosphere in relation to major volcanic eruptions of the explosive type whereby large
amounts of sulphur particles can be emitted high up in the atmosphere. In
recent years there have been two major eruptions of this kind, El Chicon
in 1982 and Pinatubo in 1991. Fig. 5 shows the radiative effects from
these eruptions. It follows that a series of major eruptions occurring over a
longer period of time could create an overall cooling effect. Again, however,
the effect from a single event is limited to at most a few years.
3.4
Internal, natural variations
Meteorological processes are typically chaotic, and infinitesimal errors in
the initial data or in the governing equations are rapidly growing. This is
the main reason why weather forecasts cannot be made very much longer
than a few weeks ahead. Since the errors due to non-linear interaction
are rapidly spreading to the whole spectrum of atmospheric motions it
follows that even the large scale features, which dominate the circulation
over months and seasons, also change. Differences in the initial state, by
say 10- 2 K, can lead to different types of circulation particularly at high
latitudes which can result in a strong westerly flow over Western Europe
or alternatively in a blocking pattern. Even in such a basic quantity as the
global averaged surface temperature such chaotic variations are clearly vis-
today the anthropogenic contribution dominates. The annual release of
802, mainly from fossil fuel combustion, amounts to some 80 M tons and
is now larger than the natural emission of sulfate components. Fig. 4
shows the annual mean in the total amount of anthropogenic sulfate in
mg/m 2 (Feichter pers. inf., 1996). Over the most polluted regions of
Europe and North America the sulfate levels have gone up by more than a
factor of 10. Experiments have been undertaken to estimate the effect of
the increase in the sulfate on atmospheric radiation. Since the effect of the
aerosols mainly is to modify the reflectivity of solar radiation directly or
indirectly via increased cloudiness, sulphate aerosols cool the atmosphere.
The present effect is localized to certain areas of the Northern Hemisphere
over and downstream the source region. In these areas it is likely that it
practically can offset the present warming by the greenhouse gases. The
calculations, however, are difficult to do and the result is very preliminary
(Charlston et al. 1991).
Stratospheric aerosols have a more global effect due to their much longer
residence time (several years). They mainly enter the stratosphere in relation to major volcanic eruptions of the explosive type whereby large
amounts of sulphur particles can be emitted high up in the atmosphere. In
recent years there have been two major eruptions of this kind, El Chicon
in 1982 and Pinatubo in 1991. Fig. 5 shows the radiative effects from
these eruptions. It follows that a series of major eruptions occurring over a
longer period of time could create an overall cooling effect. Again, however,
the effect from a single event is limited to at most a few years.
3.4
Internal, natural variations
Meteorological processes are typically chaotic, and infinitesimal errors in
the initial data or in the governing equations are rapidly growing. This is
the main reason why weather forecasts cannot be made very much longer
than a few weeks ahead. Since the errors due to non-linear interaction
are rapidly spreading to the whole spectrum of atmospheric motions it
follows that even the large scale features, which dominate the circulation
over months and seasons, also change. Differences in the initial state, by
say 10- 2 K, can lead to different types of circulation particularly at high
latitudes which can result in a strong westerly flow over Western Europe
or alternatively in a blocking pattern. Even in such a basic quantity as the
global averaged surface temperature such chaotic variations are clearly vis-