Climate is woven into the fabric of Indian social, economic, and political thought in a way that it is not (or is no longer) elsewhere. In the late 20th century, that claim would have raised hackles among scholars of South Asia; it might still do so today. A fundamental assumption of modernity was that we had mastered nature. The notion of India in thrall to the monsoon would seem to perpetuate a colonial idea of India’s irredeemable backwardness. To emphasize the power of the monsoon would be to portray Indian lives as so many marionettes moved by a climatic puppet master. That is how this story would have been understood a generation ago.

But now, alarmed by the planetary crisis of climate change, a reminder of nature’s power has different implications. This is not a story of geography as destiny. It is a story of how the idea of geography as destiny provoked, from the mid-19th century on, a whole series of social, political, and technological responses within and beyond India.

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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.