Local emissions over the subcontinent make summer storms more erratic, and may have global consequences.

Over the next decade, 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. 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.

The Indian subcontinent is the crucible of the monsoon. In its simplest definition, the monsoon is “a seasonal prevailing wind.” There are other monsoons, in northern Australia and in North America; none is as pronounced, as marked in its reversal between wet and dry seasons, as the South Asian monsoon. More than 70 percent of total rainfall in South Asia occurs during just three months each year, between June and September. Even within that period, rainfall is not consistent: It is compressed into just 100 hours of torrential rain across the summer months.

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. In the first decade of the 20th century, the finance minister in the imperial government declared that “every budget is a gamble on the rains”; more than a century later, the leading environmental activist Sunita Narain reversed the terms but retained the substance of the observation: “India’s finance minister is the monsoon,” she declared.

The South Asian monsoon has effects far beyond South Asia. We know this, at least in part, because of climate research undertaken in India in the 20th century. Sir Gilbert Walker, a pioneer of global climate science, wrote in 1927 that “the climate of India is of special interest, not merely as that of the greatest tropical region in the British Empire, but also because it seems to have been designed by nature with the object of demonstrating physical processes on a huge scale.” That sense of scientific opportunity, combined with the pressing material need to understand the monsoon, inspired a century of study in India. Charles Normand, Walker’s successor as head of the Indian weather service, insisted that the monsoon is “an active, not a passive, feature in world weather.”

Subsequent research has confirmed his view—the Asian monsoon is entwined with many aspects of the global climate. It has an important influence on global atmospheric circulation. 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 breakthroughs in tropical meteorology of the late 20th century shed new light on the scale and complexity of internal variability in the monsoon on multiple timescales—from the quasiperiodic impact of the El Niño–Southern Oscillation system to the tropical-weather fluctuation pattern known as the Madden-Julian Oscillation. In recent years, the focus of scientific research has been on how the effects of anthropogenic climate change interact with the monsoon’s natural variability in dangerous and unpredictable ways.

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. They predict, that is to say, that the moist monsoon lands will see an increase in rainfall.