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02/12/2010

 

The underside of the Spanish island of Mallorca is as perforated as Swiss cheese. The rising and falling ocean has worn hollow caves into a soft layer of calcium, and stalactites and stalagmites in these caves bear evidence of prehistoric sea levels. Now a team of scientists around the geochemist Jeffrey Dorale, from the University of Iowa, claims the Mediterranean some 81,000 years ago stood a full meter higher than it does today.

The results were published in the journal Science on Friday. Dorale and his team won't speculate why sea levels were so high back then -- or why, in fact, they seemed to surge all of a sudden -- but they believe their findings have "major implications for future concerns with sea-level change," according to the Science Web site.

Mallorca is a good place to study these changes because the island barely moves, the scientists say. It's tectonically stable, and the buildup or melting of glaciers hasn't raised or lowered the island. The stalactites and stalagmites, moreover, have have collected deposits of calcite from the ocean, and these deposits give up secrets like rings in a tree. Dorale's team dated the deposits by measuring the radioactive decay of uranium traces. "We've reconstructed sea levels with a high degree of precision," Dorale told SPIEGEL ONLINE.

Dorale and his co-author, Bogdan Onac at the University of South Florida, realize their work may be controversial. If the scientists are right, and the sea was really one meter higher 81,000 years ago than it is today, a number of questions present themselves. In those days the atmosphere would not have contained so much carbon dioxide. So how important is CO2 in global warming?

"Our work does not say anything directly about global temperature," says Dorale.

A Spike in Sea Levels

The planet, as a rule, swings between frigid glacial periods and warmer "interglacial" periods. One cycle takes about 100,000 years. So 125,000 years ago the planet was extremely warm; 20,000 years ago, extremely cold. Between those two extremes -- as the glaciers grew and the polar caps gathered up seawater -- global sea levels receded by about 130 meters.

But the fall may not have been steady. Dorale says his results show a sudden spike in sea levels 81,000 years ago, while the glaciers should have been growing and the seas receding. The seas around Mallorco rose perhaps by as much as two meters in 100 years, according to Dorale's team. Even half that rate, Dorale says, would be "a major finding."

There could be a number of reasons for the spike, including the so-called Milankovic cycle, related to a wobble in earth's orbit around the sun. But is it good or bad news? Is it a hint that CO2 is less important for global warming than scientists currently believe -- or a suggestion that current sea levels may rise faster than anyone has predicted?

Dorale and Onac have no doubt that humans have influenced global warming, but they warn against facile comparisons to previous interglacial periods. "What was happening 80,000 years ago," said Onac to SPIEGEL ONLINE, "is not the same as what's happening now."