I have followed with some apprehension the often repeated claim in the media that the Gulf Stream is driven by the thermohaline circulation and that the addition of great masses of fresh water due to the melting of ice sheets may lead to its shutdown. I live in Finland and scenarios forecasting the average temperature in this country falling dramatically due to a shutdown of the Gulf Stream really worried me.
In a letter to the Economist, where that claim had once again appeared, Carl Wunsch, Cecil and Ida Green Professor of Physical Oceanography at
MIT, assuaged my apprehension.
“Your statement that “The Gulf Stream is driven both by the rotation of the Earth and by a deep-water current called the Thermohaline Circulation” is false. The Gulf Stream is a wind-driven phenomenon (as explained in a famous 1948 paper by Henry Stommel). It is part of a current system forced by the torque exerted on the ocean by the wind field. Heating and cooling affect its temperature and other properties, but not its basic existence or structure. As long as the sun heats the Earth and the Earth spins, so that we have winds, there will be a Gulf Stream (and a Kuroshio in the Pacific, an Agulhas in the Indian Ocean, etc)…The primary mechanism of heat transport in the ocean is the wind-forcing of currents that tend to push warm water toward the poles, cold water toward the equator. Widely disseminated and grossly oversimplified pictures showing the ocean as a “conveyor belt” have misled people into thinking ocean circulation is driven by a sinking motion at high latitudes…If the sinking were stopped by adding fresh water (a deus ex machina often invoked to change the climate), the Gulf Stream would hardly care except in so far as the wind system changed too. The amount of heat transported by the system would shift, but could not become zero…Many writers, including scientists, toss around the words “Thermohaline Circulation” as though they constituted an explanation. In the ocean, most of the movement of heat and salt, the real Thermohaline Circulation, is driven directly and indirectly by the wind field. Thus the Gulf Stream, and hence the wind, rather than being minor features of oceanic climate are best regarded as the primary elements. Many real climate change effects exist and require urgent attention; focusing on near-impossible Gulf Stream failure is an unproductive distraction.”