Scientists document dense pollution layer over Indian Ocean

Extensive air pollution covers 10 million square miles of the tropical Indian Ocean, researchers say

June 9, 1999
Web posted at: 11:40

An international group of scientists participating in the Indian Ocean Experiment has documented extensive air pollution covering the Indian Ocean. The findings raise serious questions about the impact that widespread pollution is having on climate processes and on marine life.

Costing $25 million, the Indian Ocean Experiment is investigating how tiny pollutant particles called aerosols are transported through the atmosphere and how they impact climate.

Director of C4 at the Scripps Institution of Oceanography, Dr. V.Ramanathan said the team of scientists was shocked by the extent of the pollution they encountered during the six-week field experiment that began in early February and continued through the end of March. The dense pollution layer was caused by sources thousand or more kilometers away

The INDOEX scientists reported finding a dense, brown haze of pollution extending from the ocean surface to altitudes of one to three kilometers. The haze layer covered much of the research area almost continually during the six-week experiment. The affected area, roughly the same area as the continental United States, includes most of the northern Indian Ocean, including the Arabian Sea, much of the Bay of Bengal, and spills over into the equatorial Indian Ocean to about 5 degrees south of the equator.

The haze is caused by high concentrations of small particles known as aerosols that are usually less than a few micrometers in diameter. Comprised primarily of soot, sulfates, nitrates, organic particles, fly ash and mineral dust, the particles often reduced visibility over the open ocean to less than 10 kilometers, a range typically found near polluted regions of the United States and Europe. The haze layer also contains relatively high concentrations of gases, including carbon monoxide, various organic compounds and sulfur dioxide, providing conclusive evidence that the haze layer is caused by pollution.

Asia and the Indian subcontinent, which together have a growing population of more than 2 billion people, emit large quantities of pollutants that can be carried to the Indian Ocean during the northern hemisphere winter by monsoon winds from the northeast.

The dark airborne particles over the Indian Ocean appear to be markedly different from those over North America and Europe, where advanced pollution control technologies remove much of the dark material and yield particles that are relatively brighter. Thus, the impact on climate processes by the particles originating from Asia appears to be fundamentally different from those originating in the United States and Europe.

The measurements taken in the Indian Ocean are also important because they characterize emissions from the rapidly emerging economies in this region. Emissions of pollutants are expected to increase over the Indian Ocean and in other parts of the globe as additional economies grow.

Preliminary results indicate that aerosols in the polluted region scatter the incoming solar radiation and reduce the amount of energy absorbed by the ocean surface by as much as 10 percent.

"If you cut the amount of sunlight going into the ocean, you will also impact the amount of moisture evaporating from the sea surface either regionally or globally and, consequently, the amount of rainfall that will be generated," Ramanathan said. "So the entire hydrological cycle is being perturbed."

Ramanathan added that reduction in the amount of sunlight reaching the ocean surface can also have a detrimental effect on plant life that depends on photosynthesis, including plankton, which provides a key link to the entire marine food chain.

Early results indicate that the aerosol pollutants play a dual role in that they have both warming and cooling effects. The tiny particles produce a cooling effect in that they scatter sunlight back to space. By acting as seeds for cloud condensation, they also produce an indirect cooling effect by increasing both the longevity and reflectivity of clouds.

Nevertheless, the pollutants can also have a warming effect because they absorb a large amount of sunlight. The airborne particles over the northern Indian Ocean are dark because they contain large amounts of soot and other materials from incompletely burned fuels and wastes. Dark aerosols lead to the increased absorption of solar radiation

"It is just too early to say at this point whether the net effect is one of cooling or warming," Ramanathan said.

The next stage of the experiment will seek the rate at which air pollution is getting worse over the years. Scientists will continue an observation site in the Maldives Islands to monitor the air quality. "The Maldives government has been very interested in the experiment," Ramanathan said.

Coordinating the Indian Ocean Experiment are the Center for Clouds, Chemistry and Climate at the Scripps Institution of Oceanography and a National Science Foundation Science and Technology Center at the University of California, San Diego. The project has received funding from the U.S. Department of Energy, the National Oceanic and Atmospheric Administration and NASA.

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