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

There are two indirect effects of aerosols on clouds that can lead to radiative forcing.
The first indirect effect is due to an increase in droplet number in clouds caused by
an increase in the concentration of cloud condensation nuclei leading to an enhanced
reflectivity of these clouds and thus to a negative radiative forcing. The hypothesis of
this effect is supported by experimental evidence, e.g. satellite observations of
reflectivity and cloud drop concentrations in polluted vs. clean air. The second
indirect effect is related to the hypothesis that the influence of aerosols also will
reduce the precipitation efficiency of clouds and this, in turn, will lead to more
persistent clouds and thus increase cloud albedo. There is clear experimental
evidence for the influence of aerosols on the precipitation efficiency of clouds, e. g.
based on satellite observations of the impact of forest fires (Rosenfeld, 1999), urban
and industrial air pollution (Rosenfeld, 2000) and desert dust (Rosenfeld et al., 2001)
on rainfall. All in all, the indirect effect of aerosols on clouds is potentially very
important but at the moment their magnitude is highly uncertain.
A semi-direct effect of absorbing aerosols on clouds, suggested by model studies, is
due to the fact that a warming of the atmosphere may cause evaporation of clouds
(e.g. Hansen et al. 1999).
Several important studies on the effect of aerosols on climate and, particularly, on the
hydrological cycle, have been published after the TAR. A major finding was that of
the so-called “Asian Brown Cloud” during the INDOEX experiment, which is a brown
haze covering most of the Arabian Sea, Bay of Bengal and the South Asian region in
the period between November and April (and possibly longer) (Ramanathan et al.
2001). Anthropogenic sources were found to contribute about 80% to the aerosols in
this ‘cloud’, which has a high content of absorbing, black carbon aerosol. This leads
to a large negative forcing at the surface (-20 ± 4 W m -2 ), about an order of
magnitude larger than the positive forcings by greenhouse gases, and to a strong
heating of the atmosphere. The resulting reduced ocean heating is likely to lead to a
reduction in the precipitation. In fact, as pointed out by Ramanathan et al. (Current
Science 2002), a pronounced decrease in precipitation in the tropics has taken place
since the 1950s (Hulme et al., 1998), contrary to predictions by models that do not
include aerosol effects. Model calculations indicated that the perturbations caused
by the ‘cloud’ have an important influence on tropical rainfall patterns.
Another recent example of the importance of black carbon aerosol in regional climate
change comes from the model simulation study by Menon et al. (2002) of the
influence of aerosols on precipitation and temperature changes in China. It was
found, that the observed trend towards increased summer floods in South China,
increased drought in North China and moderate cooling in China and India (contrary
to the global trend) were comparable to the effects of aerosols simulated by a global
climate model.
Andreae et al. (2004) show that the influence of smoke from Amazon forest fires
suppresses the onset of precipitation from 1.5 kilometres above cloud base in pristine
clouds to more than 5 kilometres in polluted clouds. This means invigorating of
updrafts which causes intense thunderstorms, large hail and increased likelihood that
cloud tops may penetrate into the stratosphere, which may give profound impacts on
the climate system
Satellite observations of aerosol optical depths (Kaufman et al., 2002) show that
phenomena like the “Asian Brown Cloud” are found in and downwind of all inhabited
areas of the world; thus emphasizing the importance of improving the understanding
23