Damming Gibraltar to avert an ice age.

While most climate experts worry about global warming, one physicist warns that Earth will soon tumble into an ice age unless nations construct a dam across the Strait of Gibraltar.

This controversial idea comes from Robert G. Johnson of the University of Minneapolis, who describes his proposal in the July 8 Eos. Johnson opposes the conventional explanation of ice ages, which holds that changes in Earth's orbit cool the far northern latitudes, spurring the growth of ice sheets there. He argues that the Mediterranean is the key to starting ice ages, both past and future.

When orbital changes reduce rainfall in Africa, the Nile River carries less water into the Mediterranean and the sea gets saltier. The denser, saltier water flows into the Atlantic and then moves north, where it diverts the Gulf Stream toward the Labrador Sea. The warm gulf waters trigger increased evaporation and more snow, which builds an ice sheet, he suggests.

Dams on the Nile are currently preventing freshwater from reaching the Mediterranean, thus increasing its salinity. Global warming is adding to the problem by accelerating evaporation in the Mediterranean. The result, says Johnson, is that "we're just perilously close to triggering [the ice age] in Canada today." A specially constructed dam could prevent much of this salty Mediterranean water from reaching the Atlantic, he says.

Wolfgang H. Berger, director of the Scripps Institution of Oceanography in La Jolla, Calif., says that many elements of Johnson's climate theory are speculative and run counter to accepted ideas. Other researchers are even harsher, saying that Eos was irresponsible in publishing this article.