Scientists conducting the Indian Ocean Experiment (INDOEX) studied a dense brownish haze of pollution that covered much of the nothern Indian Ocean. The project was the first major international attempt to understand how tiny pollutants called aerosols are transported through the atmosphere and affect regional and global climate.

INDOEX was led by Scripps Institution and co-chaired by Veerabhadran Ramanathan,CAS researcher and director of Scripps's Center for Clouds, Chemistry and Climate (C4). Professor Paul Crutzen of the Max Planck Institute of Chemistry in Germany and adjunct professor at Scripps was the other co-chair. INDOEX involved over 150 researchers from the U.S., Europe, India, and the Maldives, a chain of islands in the northern Indian Ocean.

Various methods were employed to collect data from the ocean's surface through the lower stratosphere. Scientists aboard aircraft laden with instruments sampled aerosols for chemical analysis. From research vessels, the amount of solar energy reaching the ocean was measured. Satellites provided a continuous stream of meteorological and radiation data over a broad area. A climate observatory established by Scripps scientists on the Maldivian island Kaashidhoo documented seasonal changes in air pollution, allowing researchers to relate results of the brief field experiment to the long term.

"We had six weeks to conduct the experiment. At a cost of 25 million dollars, if you make a bad decision and lose a day, you've lost about a half million dollars," said Ramanathan. "My role [as co-chief scientist] was to coordinate and integrate all these different aspects of the experiment and maximize the scientific output."

During winter, the northeast monsoon sweeps polluted air from the Indian subcontinent, China, and Southeast Asia to the ocean. Typically, aerosols are diffused--either removed by rain or blown away by winds--within days. However, the climatology of this region causes aerosols to linger for several months.

"There was a brownish haze layer over the Indian Ocean almost 1,000 miles off the coast," Ramanathan said. "It appeared as if the whole Indian subcontinent were surrounded by a mountain of pollution."

Preliminary results indicate aerosol pollutants scatter incoming solar radiation and reduce the amount of energy reaching the ocean's surface by as much as 10 percent. This finding raises serious questions concerning the impact pollution may have on climate processes and marine life.

The scientists were surprised to find that the tiny pollutants called aerosols covered an area of ten million square kilometers (3.861 million mi2) and extended to altitudes as high as three kilometers (1.9 mi).

"Imagine the smog in Los Angeles spread over most of the U.S., persisting for two to three months," said Ramanathan. "You can imagine how oppressive it is. Just the existence of this haze was a major finding."

Most aerosols are ultimately removed by rain when they reach the Intertropical Convergence Zone (ITCZ), a narrow band of clouds and thunderstorms that exists where air from the southern and northern hemispheres meet. But some aerosols, said Ramanathan, may be exported to clouds in the upper atmosphere, where they could be transported around the globe in a matter of weeks.

Comprised primarily of soot, sulfates, nitrates, fly ash, and organic particles, aerosols both absorb significant amounts of light and scatter sunlight back into the atmosphere. The result is a reduction--by as much as ten percent--in the amount of solar energy reaching the ocean, raising new questions as to how such regional cooling might affect climate and ocean organisms.

To complicate matters, the absorption of solar energy by dark aerosol particles heats the atmosphere.

"There is a possibility that this heat may in turn burn off low clouds, which reflect sunlight and regulate climate," explained Ramanathan. "Low clouds have a large cooling effect and prevent our climate from warming excessively."

Ramanathan and fellow INDOEX scientists are eager to learn which influence dominates. The potentially large effect of aerosols is currently the greatest source of uncertainty in climate change prediction.