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2 March 2007: Science Magazine
News Focus
CLIMATE CHANGE:

Pollutant Hazes Extend Their Climate-Changing Reach
Richard A. Kerr

New studies show aerosols from burning fuels altering everything from rainfall to great ocean currents, with effects that can girdle the globe
   The microscopic aerosol particle has long been recognized as a mighty agent of climate change. At a micrometer or less in size, this bit of combustion crud from power plant, tailpipe, or farmer's fire can reflect sunlight back to space and cool the polluted eastern United States. Or it could suppress rainfall over smoggy Houston, Texas. But for years, atmospheric scientists generally assumed that pollutant aerosols worked locally or regionally. Most dramatically, the brown haze over Asia weakens both the Indian and Asian monsoons that bring essential rains to the continent.


Hazy climate driver. The cloud of pollutant particles over Asia may shift climate over Australia.
CREDIT: V. RAMANATHAN, FROM NSF C-130 DURING THE INDIAN OCEAN EXPERIMENT, 1999

   Now, scientists are finding that the effects of aerosols can range far from their source region and well beyond the wind-blown travels of the aerosols themselves. The trick lies in the well-known heating and cooling effects of aerosols, which in turn can shift the way the wind blows. For example, "Australians have tended to assume [pollutant aerosols] are a Northern Hemisphere phenomenon, [because] our skies are quite blue here," says climate modeler Leon Rotstayn of Commonwealth Scientific and Industrial Research Organisation (CSIRO) Marine and Atmospheric Research in Aspendale, Australia. Yet Rotstayn sees signs in his model that heavy aerosol pollution over Asia is increasing rainfall over distant Australia. Such aerosol action-at-a-distance is turning up in the Western Hemisphere as well.
   So far, the expanding reach of aerosols is being documented primarily in global climate models, with tantalizing parallels with what's been happening in the real world in recent decades. In the case of Australia, Rotstayn and colleagues ran a global climate model to simulate the changing climate of the 20th century. In the past decade or two, production of aerosols over Asia has soared as developing economies cranked up, especially those of India and China. When Rotstayn and colleagues plugged increasing Asian aerosols into their model along with increasing greenhouse gases, rainfall and cloud cover increased over Australia, especially in the northwest. Yet when they omitted the distant aerosols, rainfall and cloudiness decreased, contrary to observations.
   In the model, at least, the aerosols increase Australian rain and clouds by altering atmospheric and oceanic circulation, as the group will soon report in the Journal of Geophysical Research. Differences in atmospheric pressure can drive winds, and pressure can depend on temperature. So when aerosols produced in Asia blow downwind toward the Pacific and intercept sunlight, they can warm the surrounding air. At the same time, they cool the surface by blocking sunlight. Both those effects, in turn, can change how the winds blow, especially the rising air of atmospheric convection and the horizontal flow of air toward that convection. Around Asia, aerosols' net effect was to move more moisture-laden marine air into Australia, especially the northwest part of the continent, and thus increase cloudiness and rainfall.
   North American aerosols seem to hold sway over a far more massive moisture flow: the great "conveyor belt" of currents that carries heat from the Southern Hemisphere into the far North Atlantic, called the meridional overturning circulation (MOC). That's according to modeling reported in a January 2006 paper in Geophysical Research Letters (GRL) by Thomas Delworth and Keith Dixon of the Geophysical Fluid Dynamics Laboratory in Princeton, New Jersey. Increasing greenhouse gases should be slowing the MOC, according to a raft of models, but in their model, Delworth and Dixon found that aerosols counter the effect of the strengthening greenhouse on the MOC. By counteracting the greenhouse's warming and its enhancement of precipitation at high latitudes, the aerosols have delayed the MOC's slowing by roughly 40 years, they find. Modeler Wenju Cai of CSIRO Aspendale and colleagues found a similar aerosol-induced MOC slowing in their model, as they reported last November in GRL.
   The record for long-range effects may go to natural, dusty aerosols over the Sahara, abetted by sooty aerosols over East Asia, according to a report last September in the Journal of Climate by Maeng-Ki Kim of Kongju National University in South Korea and colleagues. The group found that in their model, dust raised over the springtime Sahara warms in sunlight, inducing air to rise there. That air eventually falls over southern Europe, warming the region. Then, much as an El Niņo's tropical warmth can form an "atmospheric bridge" to change distant weather, this aerosol-induced circulation transmits some of its energy eastward. That shift alters atmospheric circulation to the east, bringing unusually cold air down to the Caspian Sea region.
   The sunlight-absorbing aerosols of East Asia extend this atmospheric bridge as far as the western Pacific, bringing added warmth to central and northeastern Asia. The model's resulting pattern of springtime cooling and warming relative to broader trends bears a strong resemblance to actual trends, say Kim and his colleagues. Perhaps the thickening brew over Asia is also driving temperature changes over Eurasia, they say.
   Untangling the web of aerosol effects will take a while. In the meantime, aerosol emissions are changing. North American and western European hazes have faded as developed countries reduced their emissions for health reasons. When will the developing nations of Asia follow suit? What will be the effects? Researchers will likely still be playing catch-up as the air clears.


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