© Eugene S. Takle, 2002
Aerosol (liquid and solid particles, usually other than natural water droplets) are an important part of the earth?s atmosphere because they serve as condensation nuclei for formation of cloud droplets, they scatter and absorb solar radiation, and they can make clouds look brighter. Much , but not all, of the aerosol content of the earth?s atmosphere arises from human activity. Ramanathan et al (2001) examine the implications of this increase in atmospheric aerosol on climate. Increases in aerosol increase the droplet number concentration in clouds, which increases cloud extent and leads to global cooling. If the amount of water available for condensation in the cloud is not changed, this means that there will be more smaller drops, which are less likely to grow to sufficient size to fall out as precipitation and making clouds last longer, again contributing to cooling. For two reasons, therefore, - cooling of the surface because of increased reflection of solar energy, and reduced efficiency of clouds to produce rain ? increases in aerosol will weaken the hydrological cycle. Because most aerosol can be washed from the atmosphere by natural processes, their lifetimes in the atmosphere generally are short (perhaps two weeks). This means they can have regional as well as global impact on climate. Therefore the impact (cooling)of excess atmospheric aerosol will be felt predominantly in the Northern Hemisphere, most notably in regions of high emission such as southeast Asia. These regional changes could lead to weakening of the hydrological cycle and reduce the availability and quality of fresh water.
Reference
Ramanathan, V., P. J. Crutzen, J. T. Kiehl, and D. Rosenfeld, 2001: Aerosols, climate, and the hydrological cycle. Science, 294, 2119-2124.