About 3 billion people live within 100 miles (160km) of the sea, a number that could double in the next decade as humans flock to coastal cities like gulls. The oceans produce $3 trillion of goods and services each year and untold value for the Earth’s ecology. Life could not exist without these vast water reserves-and, if anything, they are becoming even more important to humans than before.

Mining is about to begin under the seabed in the high seas-the regions outside the exclusive economic zones administered by coastal and island nations, which stretch 200 nautical miles (370km) offshore. Nineteen exploratory licences have been issued. New summer shipping lanes are opening across the Arctic Ocean. The genetic resources of marine life promise a pharmaceutical bonanza: the number of patents has been rising at 12% a year. One study found that genetic material from the seas is a hundred times more likely to have anti-cancer properties than that from terrestrial life.

But these developments are minor compared with vaster forces reshaping the Earth, both on land and at sea. It has long been clear that people are damaging the oceans-witness the melting of the Arctic ice in summer, the spread of oxygen-starved dead […]