Summer monsoons provide much of the water for agriculture, forestry, wetlands, and fisheries on the Indian subcontinent. But the pattern of rain in the region has shifted dramatically during the last half of the 20th century: The fertile Ganges Valley in north-central India became drier, while northwestern India, southern India, and Pakistan got wetter. In a new study, researchers pin the blame on sulfate, soot, and other aerosol particles from human activities.
The rains that have long drenched South Asia from June to September originate as moisture from the Indian Ocean and the Arabian Sea, carried by monsoon winds blowing from southwest to northeast. When the humid air reaches the Indian subcontinent, it rises and begins to cool, and the water it carries falls out as torrential rains.
The monsoon winds themselves are driven by what is essentially a large heat engine, says climate scientist Yi Ming of the Geophysical Fluid Dynamics Laboratory/NOAA in Princeton, New Jersey. In the summer, the Northern Hemisphere receives significantly more energy from the sun than the Southern Hemisphere. The atmosphere over landmasses such as the Indian subcontinent also heats up more quickly than it does over water. Because air flows from regions of higher pressure (denser, colder air) to lower pressure (warmer air), these heat imbalances create winds that blow strongly onto the Indian subcontinent. And those winds carry a lot of moisture.
So what might alter that precipitation pattern? In addition to natural variations in solar radiation, greenhouse gases such as carbon dioxide and aerosols such as soot can have an impact on the summer monsoon. Ming and colleagues compared the history of rainfall from 1951 to 1999 with multiple climate simulations that showed how precipitation during the South Asian summer monsoon would change based on aerosols alone, greenhouse gases alone, natural forces alone, and all of these factors working at the same time.
Aerosols turn out to be the culprit, the researchers report online today in Science. Different aerosols—such as sulfates from the burning of fossil fuels and soot from the burning of local cook fires or large industrial fires—can have varying impacts on climate, so the researchers looked only at the net effect of all of the aerosols in the region's atmosphere.
Ming says that the aerosols clouding in the atmosphere over the Indian subcontinent act like an umbrella, cooling the region and reducing the difference in heat between Northern and Southern hemispheres. Without that strong heat contrast, the winds slow, and the rain begins to fall over the ocean and southern India rather than pushing forward into the north-central region. Those changes to the water cycle could also increase the incidence of water-borne diseases such as cholera and hepatitis, as well as mosquito-borne malaria.
"I think that they have helped solve a quandary," says Peter Webster, a climate scientist at the Georgia Institute of Technology in Atlanta. But, he cautions, there is still a lot of uncertainty in aerosol distributions, particularly before about 1970, when satellites began collecting better data.
Although the team's model predictions show "convincingly" that aerosols are the major contributor to rainfall changes in the South Asian monsoon, notes Andrew Turner, a climate scientist at the University of Reading in the United Kingdom, it fails to explain recent changes in the East Asian monsoon. The simulations predict that aerosols would promote drying over southern China, but the opposite has actually happened.
There are still many uncertainties in the study, Ming agrees. The South Asian monsoon study is part of a longer-term effort to include precipitation as well as temperature in models of climate and to gradually zoom in from the global mean models to more continental, or regional-scale, models. "For now," Ming says, "we're focusing on the tropics, the Sahel, the Amazon, the southwestern United States. [India] is our test case to see how we're doing."