An international group of scientists participating in a climatefield experiment has documented widespread pollution covering about 10million square kilometersof the tropical Indian Ocean -- roughly the same area as the continentalUnited States. The finding raises serious questions about what impact theextensive pollution is having on climate processes and on marine life inthe ocean below.
The scientists are participating in the Indian Ocean Experiment(INDOEX),a $25 million project, sponsored in part by the National ScienceFoundation, to investigate how tiny pollutant particles called aerosols aretransported through the atmosphere and their effect on climate. The projectis coordinated by the Center for Clouds, Chemistry and Climate (C4) atScripps Institution of Oceanography, a National Science Foundation Scienceand Technology Center at the University of California, San Diego. Paul J.Crutzen, director of the Max Planck Institute for Chemistry and a 1995Nobel Laureate in Chemistry and V. Ramanathan, director of C4 at theScripps Institution of Oceanography, serve as co-chief scientists.
Ramanathan said the team of scientists was shocked by the extent ofpollution they encountered during the six-week field experiment that beganin early February and continued through the end of March 1999."There was a brownish haze layer all over the Indian Ocean almost1,000 miles off the coast," he said. "That was what really stunned us --how pervasive these aerosols were and how they could survive at such longdistances from where they originated."The INDOEX scientists reported finding a dense, brown haze of pollutionextending from the ocean surface to altitudes of one to three kilometers.
The haze layer covered much of the research area almost continually duringthe six-week experiment. The affected area includes most of the northernIndian Ocean, including the Arabian Sea, much of the Bay of Bengal, andspills over into the equatorial Indian Ocean to about5 degrees south of the equator.
"We have conducted ship operations in various parts of the IndianOcean, Arabian Sea, and Bay of Bengal in the past, but we never experiencedsuch high concentrations of pollutants," said Joseph Prospero, a professorof marine and atmospheric chemistry at the University of Miami whoparticipated in INDOEX. "What was also unusual was the fact that thepollution was so persistent."
"It appeared as if the whole Indian subcontinent was surrounded bya mountain of pollution," agreed Ramanathan. "At times, we couldn't evensee the low clouds because the haze layer was so thick."
The haze is caused by high concentrations of small particles knownas aerosols that are usually less than a few micrometers in diameter.Comprised primarily of soot, sulfates, nitrates, organic particles, fly ashand mineral dust, the particles often reduced visibility over the openocean to less than 10 kilometers, a range typically found near pollutedregions of the United States and Europe. The haze layer also containsrelatively high concentrations of gases, including carbon monoxide, variousorganic compounds, and sulfur dioxide, providing conclusive evidence thatthe haze layer is caused by pollution.
"There is little doubt that future levels of pollution from Indiaand other nations bordering on the Indian Ocean region are going to growsubstantially in the future. Therefore, the INDOEX campaign has been ofvery great value in identifying an important pollution problem withpotentially major and growing consequences for the energy budget of a largepart of the Indian Ocean region and beyond," said Crutzen.
Asia and the Indian subcontinent, which together have a populationof more than 2 billion people, emit large quantities of pollutants that can becarried to the Indian Ocean during the northern hemisphere winter bymonsoon winds from the northeast. As part of INDOEX, more than 150scientists from around the globe are investigating how these pollutants aretransported through the atmosphere and how they affect the atmosphericcomposition and solar radiation processes over the ocean. A major objectiveis to estimate the climate effects of manmade airborne particles.Preliminary results indicate that aerosols in the polluted regionscatter the incoming solar radiation and reduce the amount of energyabsorbed by the ocean surface by as much as 10 percent.
"If you cut the amount of sunlight going into the ocean, you willalso impact the amount of moisture evaporating from the sea surface eitherregionally or globally and, consequently, the amount of rainfall that willbe generated," Ramanathan said. "So the entire hydrological cycle is beingperturbed."
A reduction in the amount of sunlight reaching the ocean surfacecan also have a detrimental effect on plant life that depends onphotosynthesis, including plankton, which provides a key link in the marinefood chain.
One of the primary goals of INDOEX is to determine the role thataerosols play in global climate change. Early results indicate that thepollutants play a dual role in that they have both warming and coolingeffects. The tiny particles produce a cooling effect in that they scattersunlight back to space. By acting as seeds for cloud condensation, theyalso produce an indirect cooling effect by increasing both the longevityand reflectivity, or albedo, of clouds.The pollutants have a warming effect, however, in that they absorba large amount of sunlight. The airborne particles over the northern IndianOcean are unusually dark because they contain large amounts of soot andother materials from incompletely burned fuels and wastes. Dark aerosolslead to the increased absorption of solar radiation.
"The soot contributes a substantial amount of heating of theatmosphere, 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 isone of cooling or warming."
The dark airborne particles over the Indian Ocean appear to bemarkedly different from those over North America and Europe, where advancedpollution control technologies remove much of the dark material and yieldparticles that are relatively brighter. Thus, the impact on climateprocesses of pollution particles stemming from Asia appears to befundamentally different from those originating in the United States andEurope. INDOEX has provided a wealth of important and unique data tofurther assess how these two classes of aerosols affect climate processesdifferently.
The measurements taken in the Indian Ocean are also importantbecause they characterize emissions from the rapidly emerging economies inthis region. Emissions of pollutants are expected to increase over theIndian Ocean and in other parts of the globe as similar economies grow.The INDOEX scientists were surprised to find that such a densepollution layer in the Indian Ocean was caused by sources at least athousand or more kilometers away. They suggest that the pollution eventsobserved in INDOEX may be symptomatic of large-scale pollution transportthat may be occurring in other regionsof Earth.
"What we are seeing here is an example of extremely highconcentrations of pollutant aerosols being transported many thousands ofkilometers without a substantial reduction in their concentrations,"Prospero said. "What INDOEX has pointed out very dramatically is that thelong-range transport of aerosols can be extremely important and that weshould be looking more closely at what impact thisis having on global climate."
"There will thus be a need to follow up periodically withadditional INDOEX missions to determine future trends in pollution loadingsand their consequences for regional and maybe global climate," Crutzensaid.
In contrast to the situation over the northern Indian Ocean, the loweratmosphere over the southern Indian Ocean remains remarkably clean, thanksto the InterTropical Convergence Zone (ITCZ), which is found around 5-10degrees south of the equator at this time of year. A narrow zone of deepand towering thunderstorms that form over the warmest part of theequatorial ocean, the ITCZ intercepts polluted air masses and removes muchof the pollution as rainfall. But the ITCZ clouds can also move substantialamounts of pollutants into the upper atmosphere where they can be spreadover large areas.
INDOEX is a cooperative program involving scientists from theUnited States, Europe, India, and the Maldives. The experiment includes theuse of four research aircraft, two oceanographic ships, several surfacestations, balloons, and a wide range of satellites. The European SpaceAgency moved their geostationary satellite to the Indian Ocean to supportINDOEX. The center of operations was on the island of Male, where theaircraft are based. The American component of INDOEX was funded by theNational Science Foundation, the U.S. Department of Energy, the NationalOceanic and Atmospheric Administration, and NASA.
For more information about the Center for Clouds, Chemistry and Climate atScripps and INDOEX, visit the center's web site at: http://www-c4.ucsd.edu/
Scripps Institution of Oceanography on the World Wide Web:http://sio.ucsd.edu
Materials provided by Scripps Institution of Oceanography. Note: Content may be edited for style and length.
Cite This Page: