Widespread pollution in Indian Ocean

An international group of scientists participating in a climate field experiment has documented widespread pollution covering about 10 million square kilometers of the tropical Indian Ocean -- roughly the same area as the continental United States. The finding raises serious questions about what impact the extensive pollution is having on climate processes and on marine life in the ocean below.

The scientists are participating in the Indian Ocean Experiment (INDOEX), a $25 million project, sponsored in part by the National Science Foundation, to investigate how tiny pollutant particles called aerosols are transported through the atmosphere and their effect on climate. The project is coordinated by the Center for Clouds, Chemistry and Climate (C4) at Scripps Institution of Oceanography, a National Science Foundation Science and Technology Center at the University of California, San Diego. Paul J. Crutzen, director of the Max Planck Institute for Chemistry and a 1995 Nobel Laureate in Chemistry and V. Ramanathan, director of C4 at the Scripps Institution of Oceanography, serve as co-chief scientists.

Ramanathan said the team of scientists was shocked by the extent of pollution they encountered during the six-week field experiment that began in early February and continued through the end of March 1999. "There was a brownish haze layer all over the Indian Ocean almost 1,000 miles off the coast," he said. "That was what really stunned us -- how pervasive these aerosols were and how they could survive at such long distances from where they originated." 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 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.

"We have conducted ship operations in various parts of the Indian Ocean, Arabian Sea, and Bay of Bengal in the past, but we never experienced such high concentrations of pollutants," said Joseph Prospero, a professor of marine and atmospheric chemistry at the University of Miami who participated in INDOEX. "What was also unusual was the fact that the pollution was so persistent."

"It appeared as if the whole Indian subcontinent was surrounded by a mountain of pollution," agreed Ramanathan. "At times, we couldn't even see the low clouds because the haze layer was so thick."

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.

"There is little doubt that future levels of pollution from India and other nations bordering on the Indian Ocean region are going to grow substantially in the future. Therefore, the INDOEX campaign has been of very great value in identifying an important pollution problem with potentially major and growing consequences for the energy budget of a large part of the Indian Ocean region and beyond," said Crutzen.

Asia and the Indian subcontinent, which together have a 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. As part of INDOEX, more than 150 scientists from around the globe are investigating how these pollutants are transported through the atmosphere and how they affect the atmospheric composition and solar radiation processes over the ocean. A major objective is to estimate the climate effects of manmade airborne particles. 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."

A 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 in the marine food chain.

One of the primary goals of INDOEX is to determine the role that aerosols play in global climate change. Early results indicate that the 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, or albedo, of clouds. The pollutants have a warming effect, however, in that they absorb a large amount of sunlight. The airborne particles over the northern Indian Ocean are unusually 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.

"The soot contributes a substantial amount of heating of the atmosphere, but it also reduces the amount of sunlight reaching the ocean," Ramanathan said. "So, it is just too early to say at this point whether the net effect is one of cooling or warming."

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 of pollution particles stemming from Asia appears to be fundamentally different from those originating in the United States and Europe. INDOEX has provided a wealth of important and unique data to further assess how these two classes of aerosols affect climate processes differently.

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 similar economies grow. The INDOEX scientists were surprised to find that such a dense pollution layer in the Indian Ocean was caused by sources at least a thousand or more kilometers away. They suggest that the pollution events observed in INDOEX may be symptomatic of large-scale pollution transport that may be occurring in other regions of Earth.

"What we are seeing here is an example of extremely high concentrations of pollutant aerosols being transported many thousands of kilometers without a substantial reduction in their concentrations," Prospero said. "What INDOEX has pointed out very dramatically is that the long-range transport of aerosols can be extremely important and that we should be looking more closely at what impact this is having on global climate."

"There will thus be a need to follow up periodically with additional INDOEX missions to determine future trends in pollution loadings and their consequences for regional and maybe global climate," Crutzen said.

In contrast to the situation over the northern Indian Ocean, the lower atmosphere over the southern Indian Ocean remains remarkably clean, thanks to the InterTropical Convergence Zone (ITCZ), which is found around 5-10 degrees south of the equator at this time of year. A narrow zone of deep and towering thunderstorms that form over the warmest part of the equatorial ocean, the ITCZ intercepts polluted air masses and removes much of the pollution as rainfall. But the ITCZ clouds can also move substantial amounts of pollutants into the upper atmosphere where they can be spread over large areas.

 

INDOEX is a cooperative program involving scientists from the United States, Europe, India, and the Maldives. The experiment includes the use of four research aircraft, two oceanographic ships, several surface stations, balloons, and a wide range of satellites. The European Space Agency moved their geostationary satellite to the Indian Ocean to support INDOEX. The center of operations was on the island of Male, where the aircraft are based. The American component of INDOEX was funded by the National Science Foundation, the U.S. Department of Energy, the National Oceanic and Atmospheric Administration, and NASA.