The Final Analysis   Page 4

Rosenfeld’s research by no means closes the book on aerosols. Now that he has proven that aerosols from human activity can dramatically affect the properties of clouds, he and the rest of the scientific community are puzzling over what this means for our climate and our day-to-day weather.

Rosenfeld believes that these aerosols may be impacting local weather systems in a number of ways. In particular, they may be suppressing rain in areas that need moisture the most. Aerosols cause clouds to retain their water until the wind currents blow them into another weather system sometimes thousands of miles away. "So you may actually have a shift in the precipitation from one area where you have less precipitation to somewhere where you have more," he said. In normally unpolluted tropical regions such as Brazil, where large amounts of biomass burning takes place through the clearing of rain forests, Rosenfeld fears that the resulting aerosols may be hurting local farmers who depend on their rainfall for their livelihoods. For regions such as the East Coast, where the pollution is always present, it’s a little more difficult to tell exactly how the aerosols are affecting precipitation.

Smokestack Tracks in Canada

In terms of global climate change, Rosenfeld’s work as well as the ship track studies could go a long way towards explaining why the Southern Hemisphere of the Earth has warmed up more than the Northern Hemisphere in recent decades. When fossil fuels are burned, carbon dioxide, sulfur dioxide, and other pollutants are released. Most Earth scientists believe that carbon dioxide is causing heat from solar radiation to become trapped in the Earth’s atmosphere (King et al., 1993). As Rosenfeld’s research established, aerosols from these same smokestacks create brighter clouds, which reflect sunlight. Any light that is reflected cannot reach the ground and heat the surface of the Earth. "Of course, reflecting more solar energy back to space means less warming of the system," said Rosenfeld. Over the short term, many scientists think these two sources of pollution have the ability to balance each other out in areas where there is a lot of pollution such as the industrial nations of the Northern Hemisphere.

This is not to say we should increase our burning of fossil fuels to try to stop global warming. As many reading this article have probably guessed, acid rain is the result of sulfate aerosols. Also, carbon dioxide remains in the atmosphere for a long time and can circulate around the globe, while aerosols fall to the Earth after a relatively short time when the clouds precipitate. So we can only receive the "benefits" of aerosols if we create a lot of pollution. Over the long term, most scientists believe that the warming effect of human-produced greenhouse gases will be greater than the cooling effect of aerosols (King et al., 1993).

But before these hypotheses can be accepted or denied, more tests and more experiments will have to be run. Scientists are still a ways off from being able to use satellites to discern exactly how many different types of pollutants a given cloud contains or how great an effect a given amount of pollution has on clouds. "Our research is not the end of the story. Much work is left to be done to understand the exact impact we are having on our climate," said Rosenfeld.


Graham, S., 1999: Tropical Rainfall Measuring Mission Fact Sheet, Earth Observatory, NASA’s Goddard Space Flight Center, Greenbelt, MD.

Hastings, D., 1998: Advanced Very High Resolution Radiometer (AVHRR) Overview, NOAA National Data Centers Web site, Washington, DC.

King, M.D., L.F. Radke, and P. V. Hobbs, 1993: Optical Properties of Marine Stratocumulus Clouds Modified by Ships, Journal of Geophysical Research, 98(D2), pp. 2729-2739.

King, M.D., S. Tsay, and S. Platnick, 1995: In Situ Observations of the Indirect Effects of Aerosols on Clouds. Aerosol Forcing of Climate, ed. by R. J. Charlson and J. Heintzenberg, John Wiley & Sons, New York, pp. 228-246.

Rosenfeld, D., 2000: Suppression of Rain and Snow by Urban and Industrial Pollution, Science, 287, pp. 1793-1796.

Rosenfeld, D., 1999: TRMM Observed First Direct Evidence of Smoke from Forest Fires Inhibiting Rainfall, Geophysical Research Letters, 26(20), pp. 3105-3107.

Rosenfeld, D., and Lensky, I. M., 1998: Satellite-Based Insights into Precipitation Formation Processes in Continental and Maritime Convective Clouds, Bulletin of the American Meteorological Society, 79(11), pp. 2457-2476.

Toon, O. B., 2000: How Pollution Suppresses Rain, Science, 287, pp. 1763-1764.

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The long term effects of the suppression of rain by pollution are unknown. Unanswered questions include: to what extent does pollution shift rainfall patterns? How do aerosols affect rainfall in heavily polluted areas? and what are the varying effects of different types of aerosols? (Image by Daniel Rosenfeld, Hebrew University of Israel)