Pollution in India Could Reshape Monsoons
More than 400 large dams will be built on the Himalayan rivers—by India, China, Nepal, Bhutan, and Pakistan—to feed the region’s hunger for electricity and its need for irrigation during the next decade. New ports and thermal power plants line the coastal arc that runs from India, through Southeast Asia, to China.
India and China have embarked on schemes to divert rivers to bring water to their driest lands: Costing tens or hundreds of billions of dollars, they are the largest and most expensive construction projects the world has ever seen. At stake in how these plans unfold is the welfare of a significant portion of humanity. At stake is the future shape of Asia, the relations among its nations.
Despite a vast expansion in irrigation since 1947, 60 percent of Indian agriculture remains rain-fed, and agriculture employs about half of India’s population. Unlike China, unlike most large countries in the world, India’s population will continue to be predominantly rural until the mid-21st century. No comparably large number of human beings anywhere in the world is so dependent on such intensely seasonal rainfall.
The future behavior of the South Asian monsoon has implications for the whole world. Arguably no other part of the global climate system affects more people, more directly.
The most fundamental forces driving the monsoon are the thermal contrast between the land and the ocean and the availability of moisture. Climate change affects both of these drivers of wind and rain. The warming of the ocean’s surface is likely to augment the amount of moisture the monsoon winds pick up on their journey toward the Indian subcontinent.
But if the ocean surface warms more rapidly than the land, which appears to be happening in equatorial waters, this would narrow the temperature gradient that drives the winds, and so weaken circulation. Put simply, many climate models predict that the first of these processes will predominate: “Wet gets wetter” as a result of greenhouse-gas emissions.
A further driver of regional climate change is rapid changes in land use. Over the past 150 years, forest cover over most parts of Asia has declined dramatically. The intensification of agricultural production in India, and the use of more water for irrigation, has affected the moisture of the soil, its capacity to absorb or reflect heat. Crops reflect more solar radiation than forests, which tend to absorb it; the greater reflexivity of land planted with crops makes it cooler, once again weakening the temperature differentials that drive circulation and rainfall.