The bay’s turbulent climate has played an outsize role in the region’s history. Sailors crossed its waters from the earliest times; their trading routes linked India, China and Southeast Asia for centuries. The bay’s natural bounty attracted the European powers in the early modern era, making it an arena for imperial competition and economic vitality. But the monsoons and their rainfall have always been volatile: periodic droughts and dangerous storms have posed a recurrent threat and shaped the region.

In the second half of the 19th century, land-hungry investors in an expanding British Empire created tighter connections across the bay. Migration reached huge proportions in the age of the steamship. More than 25 million people crossed the bay between the 1870s and the 1930s; most of them were young men from southern and eastern India destined for the tea estates of Sri Lanka, the rubber plantations of Malaysia and the docks and rice mills of Myanmar. Combined with the concurrent movement of Chinese to Southeast Asia, this was one of the world’s great migrations, though much of it was circular rather than permanent.

This surge in migration coincided with two of the worst cases in a millennium of the failure of monsoons to bring needed rains. Especially intensive episodes of the phenomenon known as El Niño — the periodic warming of surface waters in the equatorial Pacific — brought drought to large sections of Asia in the 1870s and again in the 1890s. In India, millions died in the famines that ensued. Thousands sought survival overseas; many more moved locally. The people became more interdependent.

Only families with access to credit and wide enough social networks could take advantage of opportunities overseas. Colonial law distinguished between groups who could migrate and those who could not. For those who could not, the price of leaving was often the servitude of indentured labor across the bay. Poverty was as likely as sudden disaster to propel people’s journeys. Once patterns of migration were established, they outlasted particular climatic or economic conditions.

The global economic depression of the 1930s, followed by the Second World War, stemmed migration and trade. After winning independence from colonial rule in the 1940s, Asia’s new states policed their contested borders and controlled migration. Like many leaders of his generation, India’s first prime minister, Jawaharlal Nehru, believed that modern science had “curbed to a large extent the tyranny and the vagaries of nature.”