Air pollution changes rainfall, say scientists who implicate it in killer drought of last century

Sunday, July 21st 2002, 12:00 am

By: News On 6

Nearly two decades after one of the world's most devastating famines in Africa, scientists are pointing a finger at pollution from industrial nations as one of the possible causes.

The starvation brought on by the 1970-85 drought that stretched from Senegal to Ethiopia captured the world's attention with searing images: skeletal mothers staring vacantly, children with bloated bellies lying in the sand, vultures lurking nearby. Before rains finally returned, 1.2 million people had died.

Now, a group of scientists in Australia and Canada say that drought may have been triggered by tiny particles of sulfur dioxide spewed by factories and power plants thousands of miles away in North America, Europe and Asia.

The short-lived pollution particles, known as aerosols, didn't have to travel to Africa to do their dirty work. Instead, they were able to alter the physics of cloud formation miles away and reduce rainfall in Africa as much as 50 percent, say the researchers, who used a computer to simulate the atmospheric conditions.

The process, known as teleconnection, continues in the atmosphere today. Some scientists suspect it might help explain the drought gripping parts of the United States, although that question has not been specifically examined.

And while pollution may affect the behavior of rain clouds, scientists stopped short of solely blaming industry's effluent for the famine and starvation that wracked the region of Africa called the Sahel.

``It's more subtle than that,'' said atmospheric scientist Leon Rotstayn, lead author of the study on the subject.

``The Sahelian drought may be due to a combination of natural variability and atmospheric aerosols,'' said Rotstayn, of the Commonwealth Scientific and Industrial Research Organization, a government research agency in Australia. The CSIRO study will be published in the August Journal of Climate.

Over the years, the disastrous lack of rainfall over the Sahel has been blamed on everything from overgrazing to El Nino. Many scientists still argue those are chief culprits.

One interesting clue: In the 1990s, rain returned to the Sahel. During the same period, emissions laws in the industrialized West reduced aerosol pollution. A coincidence? Scientists don't think so.

``Cleaner air in the future will mean greater rainfall in the region,'' Rotstayn said.

Some researchers say the CSIRO study is intriguing, but that the computer simulation is too simple to solve the mystery by itself.

``It is quite a plausible argument,'' said atmospheric scientist V. Ramanathan of Scripps Institute of Oceanography in La Jolla, Calif.

Last year, Ramanathan co-authored a global pollution study examining an industrial haze that covered nearly 4 million square miles and upset the water cycle over the Asian subcontinent.

He said similar processes appeared to be at work over the Sahel, but the CSIRO model must be sharpened to prove it.

Until then, ``I would be cautious about overextending these conclusions,'' Ramanathan said.

Other scientists were even more guarded. Teleconnection is a reasonable, but complicated, explanation, they said.

``Rotstayn focuses on an indirect effect of aerosols that is really hard to quantify,'' said Philip Rasch, senior scientist at the National Center for Atmospheric Research in Boulder, Colo.

Some scientists complained that the global rainfall pattern simulated by the computer model does not match up with actual rainfall observed at weather stations around the world during the drought. This lack of a neat correlation makes the study's Sahel conclusions ``highly speculative,'' they said.

For example, the real weather observations and those generated by the computer model correspond for the Sahel, Senegal and parts of Brazil, said Yogesh Sud, senior research meteorologist at NASA's Goddard Space Flight Center in Greenbelt, Md.

``But in India and Australia, there is absolutely no match'' between recorded rainfall and the simulated conditions, Sud said.

Nations share the same atmosphere and, increasingly, the same pollution.

Pollution is known to alter temperature and precipitation patterns near its source. Recent studies suggest that one country's pollution can become a problem for other countries, too.

Over the desolate North African Sahel, the influence of global pollution is less direct. Normally, this harsh land receives patchy summer rainfall. Soil studies show that milder droughts came in the 1680s, the 1750s, the mid-1800s and the early 20th century.

Rotstayn believes industrial smokestacks are the smoking guns for the more recent, more intense drought.

The sulfur dioxide pollution particles, which can remain in the air 5 to 20 days, probably drifted over the North Atlantic where they created more condensation nuclei for cloud formation, the scientists theorize.

The additional nuclei remained suspended in clouds rather than growing into fewer, larger droplets and falling as rain.

In addition, these clouds were brighter than normal, in part because of the added nuclei, and they reflected more of the Sun's energy into space.

This cooled the surface of the North Atlantic, which reduced the normal evaporation rate from the ocean and further hampered the moisture cycle.

South of the Sahel, the sea surface remained warm and evaporation increased so more rain fell to the south, Rotstayn said.