Massive pollution cloud observed over Indian Ocean

A cloud of pollution covering about 100 million square kilometres of the tropical Indian Ocean - roughly the same area as the continental United States - has been observed by scientists.

This finding by an international group of scientists participating in the Indian Ocean Experiment (INDOEX) raises serious questions about what impact the extensive pollution is having on climate processes and on marine life in the ocean below.

INDOEX, a $25 million project, sponsored in part by the National Science Foundation (NSF), is investigating how tiny pollutant particles called aerosols are transported through the atmosphere, and their resulting effect on climate.

“Aerosols affect the amount of solar radiation that is absorbed and reflected back to space by the atmosphere,” explained Jay Fein, program director in NSF’s division of atmospheric sciences. “They can also change the composition of clouds, thereby altering the amount of solar radiation they absorb and reflect back to space. Aerosol – cloud – radiation processes are highly complex and not well understood. In fact, the effect of aerosols on our atmosphere’s radiation balance is one of the largest sources of uncertainty in predicting future climate. INDOEX was designed to reduce this uncertainty.”

V. Ramanathan, director of C4 at the Scripps Institution of Oceanography, 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.

The INDOEX scientists reported finding a dense, brown haze of pollution extending from the ocean surface to altitudes of one to three kilometres. 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.

The haze is caused by high concentrations of aerosols. Comprised primarily of soot, sulphates, nitrates, organic particles, fly ash and mineral dust, the particles often reduced visibility over the open ocean to less than 10 km, 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 sulphur dioxide, providing conclusive evidence that the haze layer is caused by pollution.

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 north-east. 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. 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 kilometres 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.

The project is coordinated by the Centre for Clouds, Chemistry and Climate (C4) at the Scripps Institution of Oceanography, a National Science Foundation (NSF) Science and Technology Centre at the University of California, San Diego.

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