Just across the Golden Gate from San Francisco is a very large scientific instrument called the Bay Model. It's an exact miniature duplicate of San Francisco Bay's bottom contours made of concrete, with pipes and pumps to mimic the tides. The model is used to study how the waters of the Bay behave—for instance, if you wanted to dam the Golden Gate.
It works pretty well, but it's about as big as such an analog tool can be built. You couldn't build a model like it for the Mediterranean Sea and the whole North Atlantic Ocean, to test a scientist's proposal to dam the Strait of Gibraltar.
A Dire Chain of Events
Robert Johnson, a retired professor at the University of Minnesota, took the front page of the 8 July 1997 Eos to point out that the Mediterranean Sea is being starved of fresh water, as human activities have diverted the flow of rivers, mainly the Nile, away from the sea. He predicts a dire chain of events.
The Mediterranean evaporates and grows saltier (global warming reinforces this too), and more seawater pours in from the Atlantic through the Strait of Gibraltar and spreads across the surface. That in turn pushes more of the very salty deep Mediterranean water out at Gibraltar into the deep waters of the Atlantic, and there's where the trouble begins, Johnson says. The same set of events happened when the last major ice age began 120,000 years ago, he explained in a 1997 issue of Earth and Planetary Science Letters, though what dried up the Nile at that time was weaker African monsoons.
The salty bottom water flowing out at Gibraltar mixes with very cold water on the Atlantic floor and turns rightward, to the north, until it bumps up against the seafloor rise off Ireland, Scotland, and the Faeroe Islands to their north. The obstacle pushes the cold, salty water toward the surface, into the warm surface currents that bring Gulf Stream water to the British Isles and Scandinavia. Normally the warm current isn't much affected, but if the Mediterranean outflow is boosted, so is this effect. The waters that warm northern Europe will be pushed aside by cold upwelling.
That warm flow in turn heads westward, past Iceland and Greenland, and into the cold seas off Canada, and there it creates increased snowfall on the places where the Canadian ice caps form. Imagine all the rains of England, the showers of Ireland, and the mists of Scotland falling as snow up on Baffin Island instead.
A Good Dam Idea
In "the next few decades," Johnson warns, northern Europe will cool and the snowfields of eastern Canada will expand. The high albedo or reflectance of the white snowfields, and the white clouds that hover above them, will keep them cold in a feedback cycle. Global warming will not prevent this but rather, by increasing the movement of moisture in the atmosphere, will make things worse.
Hence Johnson's call for a dam across the Strait of Gibraltar. It would pinch off most of the bottom part of the strait but have wide openings on the surface. The effect would be to slow the volume of deep water pouring out and, over the next few centuries, turn the Mediterranean saltier than today.
Keeping that salt cooped up, says Johnson, would have a happy side effect. Mediterranean salt is a key part of the hydrodynamic mechanism that keeps Antarctica surrounded by sea ice; if the sea ice can be maintained against global warming by reducing the salt balance, then the West Antarctic ice sheet could be kept from collapsing and raising the worldwide sea level by 6 meters.
No Time to Lose
The time to act is now, he says, while world society is fairly stable and oil supplies are strong. But we can't build a concrete model to test the plan first. We have to trust the digital models from—knock on wood—our computers.
Johnson is not the only visionary who wants to dam the Mediterranean. In 1929 Hermann Sorgel proposed using a Gibraltar dam to let the Mediterranean evaporate down 50 meters below its current level. The resulting basin could be tapped for hydroelectric power to make the Sahara bloom. In a recent refinement of this scheme by the irrepressible Richard Cathcart, some of this electric power would be used to modify the Earth's carbon dioxide balance and save us from global warming.
PS: Cathcart's ultimate engineering project is turning the whole planet into a gigantic shell around the Sun. That would be planeteering on the ultimate scale.