Gibraltar Dam Unnecessary
In July last year, a call for a massive dam across the Straits
of Gibraltar made international headlines. American geologist R. Johnson
had claimed that without the dam, a new Ice Age would be triggered, reasoning
that after the building of the Aswan dam on the river Nile the Mediterranean
salinity would rise, increasing the outflow of salty water into the Atlantic
and disrupting ocean currents there. Computer simulations by Stefan Rahmstorf,
an oceanographer at the Potsdam Institute for Climate Impact Research in
Germany, have now shown that this fear is unfounded. His results appear
in a front-page story in the latest issue of Eos, the magazine of the American
Geophysical Union (Eos, Transactions, American Geophysical Union, Volume
79 (24), June 16, 1998).
The salty outflow from the Mediterranean, caused by the high evaporation rates there, is a prominent feature of the present ocean circulation. It creates a tongue of exceptionally salty water stretching westward across the Atlantic at around 1,000 m below the surface. Rahmstorf used a global ocean circulation model to investigate the effects of this salty water on the Atlantic currents and surface climate. His computer simulations show that the present outflow from the Mediterranean enhances the Gulf Stream, heating the surface temperatures in the northern North Atlantic by a few tenths of a degree.
However, the change in the salty outflow that could arise from the diversion of fresh water by the Aswan dam is very small. Rahmstorf's simulations showed that this change is too small to have a noticeable effect on the surface climate in the Atlantic. "There's no way you could trigger an Ice Age in this way", he said. "Building a dam across the Strait of Gibraltar to prevent climatic change would be a complete waste of time, quite apart from the negative side-effects that such a massive project would undoubtedly have".
Rahmstorf stressed that there is serious reason for concern that global warming could disrupt Atlantic ocean currents. But this would be caused by temperature and rainfall changes over the Atlantic, and has nothing to do with the Mediterranean outflow.
The full article ("Influence of Mediterranean Outflow on Climate") with illustrations is available at https://www.pik-potsdam.de/~stefan
For further information, contact Dr. Stefan Rahmstorf, +49-331-2882688 (office) or 331-715429 (home), email mailto:Stefan.Rahmstorf@pik-potsdam.de
