Along the Bay of Bengal, where nearly 1.4 billion people live, water has become perilously unpredictable. On the coast of India’s Odisha state, repeated floods swallow villages. In Sri Lanka, a scarcity of water is carving cracks where ponds once formed and drying out paddy fields. In the mangrove swamps of the Sundarbans forest that straddles the border between India and Bangladesh, rising seas and cyclones are driving people inland, to congested cities like Kolkata for work.

Climate change is warming waters, shifting ocean patterns and transforming the region’s yearly monsoon from a reliable lifeline into a menace.

The water in the bay is rising faster than in other major bodies of water. The challenges confronting nations adjacent to it, densely populated along their coasts, probably foreshadow the struggles ahead elsewhere on Earth.