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Higher Watermarks

Ice-age sea-level rise brings up questions about glacier melting



Courtesy of Bogdan P. Onac University of South Florida

Mallorca, Spain’s largest island, is not just a desirable place for a Mediterranean vacation; it is also a treasure trove of data on the earth’s past. Thanks to the island’s long-term geologic stability, the caves record sea levels over the past tens to hundreds of thousands of years in the form of stone structures. In examining such rock formations, a team of geologists conclude that, compared with today, sea levels were roughly one meter higher 81,000 years ago, when the world was thought to be experiencing an ice age that should have locked up water in glacial ice, which should have lowered sea level as much as 30 meters.

More disturbing, the record suggests that sea level can go up or down as quickly as two meters a century—nearly 12 times faster than sea-level rise in the past 100 years—an indication of the potential for a meter of sea-level increase within one human lifetime. “This has major implications for future concerns with sea-level change,” says geoscientist Jeffrey Dorale of the University of Iowa, one of the authors of the new research published in the February 12 Science. “Our study indicates rapid rates of ice melting and ice formation. The mechanisms underlying these dramatic changes need further consideration as we look to a future of impending climate changes.”

The researchers drew their conclusions after collecting rock samples from five different caves from stalagmitelike formations known as speleothems. These structures are scattered above and below present-day sea level and thereby build up a carbonate crust left on them by the lapping waters of the sea over time. The decay of radioactive uranium and thorium in the encrusted speleothems served to date the layers.

Speleothems provide a more precise measure of historic sea level than those left in coral reefs or ancient shorelines, which had been used in the past to estimate primordial sea levels. In fact, the speleothem data back findings from Caribbean islands as well as the U.S. East and West coasts that sea level 81,000 years ago was at least as high as it is today and probably higher.

What might have caused the sea-level climb remains unclear, although the rise coincides with an increase in the amount of sunlight hitting the Northern Hemisphere resulting from slight variations in the earth’s orbit, known as Milankovitch cycles. “Maybe unstable ice was involved in some of the rapid rise,” Dorale speculates. “But we don’t even know for sure what the ice configuration was at this time.” Yet the record in rock also suggests that Milankovitch cycles cannot entirely explain ice growth or melt during the past 100,000 years, notes geologist Bogdan P. Onac of the University of South Florida, who collaborated in the analysis. “It must have been a dramatic melting event. What exactly caused it is hard to tackle with just this set of data.”

Glaciologist Richard Alley of Pennsylvania State University, who was not involved in the project, calls the findings solid and careful and notes that this study confirms that ice-sheet changes can happen quickly. “It points to rather rapid shrinkage and growth of ice,” he says, while cautioning that more research is needed to confirm the results. “The growth rates are surprising, but not impossible,” he observes.

One thing is certain, however: the finding points to how complex the earth’s climate is. “Greenhouse gases are clearly important to climate,” Dorale says, “but just as clearly they are not the only major factor at work.”

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