First the warming, then the cooling

The curious Gothamist reader may have noticed a seeming contradiction in the recent entry about the forthcoming movie "The Day After Tomorrow". No, not allegations that NASA may have wanted to silence its scientists, but that one consequence of global warming will be a sudden deep freeze. The earth gets cold when it warms? WTF?

There is a bit of truth to that contradiction, though not so fast as in the movie. The reason the atmosphere circulates and the oceans have currents is because the tropics are warmer than the poles. Being good citizens, the atmosphere and ocean obey the laws of thermodynamics and move the excess energy of the tropics toward the energy deficit near the poles. The oceans do this via the Great Ocean Conveyor Belt, which circulates energy, salt, and nutrients around the earth. The key location of the conveyor belt is the North Atlantic, where warm surface water carried north by the Gulf Stream cools, becomes denser, sinks to the ocean floor, and puts the conveyor belt in motion. If the North Atlantic surface water heats up as the climate warms, or if the surface water gets fresher, say from Greenland's ice sheets melting at a more rapid rate, then there isn't as much deep water being formed. Less deep water formation, less ocean conveyor belt action. Less conveyor belt action and eastern North American and Europe are likely to get colder very quickly. Maybe not as quickly as the fews days depicted in the movie, but quickly as in tens of years.