Читать книгу Green Earth - Kim Stanley Robinson - Страница 13

Water flows through the oceans in steady recycling patterns, determined by the Coriolis force and the particular positions of the continents in our time. Surface currents can move in the opposite direction to bottom currents below them, and often do, forming systems like giant conveyor belts of water. The largest one is already famous, at least in part: the Gulf Stream is a segment of a warm surface current that flows north up the entire length of the Atlantic, all the way to Norway and Greenland. There the water cools and sinks, and begins a long journey south on the Atlantic Ocean floor, to the Cape of Good Hope and then east toward Australia, and even into the Pacific, where the water upwells and rejoins the surface flow, west to the Atlantic for the long haul north again. The round trip for any given water molecule takes about a thousand years.

Cooling salty water sinks more easily than fresh water. Trade winds sweep clouds generated in the Gulf of Mexico west over Central America to dump their rain in the Pacific, leaving the remaining water in the Atlantic that much saltier. So the cooling water in the North Atlantic sinks well, aiding the power of the Gulf Stream. If the surface of the North Atlantic were to become rapidly fresher, it would not sink so well when it cooled, and that could stall the conveyor belt. The Gulf Stream would have nowhere to go, and would slow down, and sink farther south. Weather everywhere would change, becoming windier and drier in the Northern Hemisphere, and colder in places, especially in Europe.

The sudden desalination of the North Atlantic might seem an unlikely occurrence, but it has happened before. At the end of the last Ice Age, for instance, vast shallow lakes were created by the melting of the polar ice cap. Eventually these lakes broke through their ice dams and poured off into the oceans. North America still sports scars from three or four of these cataclysmic floods; one flowed down the Mississippi, one the Hudson, one the St. Lawrence. These flows stalled the world ocean conveyor belt current, and the climate of the whole world changed as a result, sometimes in as little as three years.

Now, with Greenland’s ice cap melting fast, and the Arctic sea ice breaking into bergs, would enough fresh water flow into the North Atlantic to stall the Gulf Stream again?