【雙語閱讀】Sea level rise overflowing estimates

Sea levels may swell much higher than previously predicted, thanks to feedback mechanisms that are speeding up ice melt in Greenland and Antarctica.

Climate simulations need to take such feedbacks into account, William Hay, a geologist at the University of Colorado Boulder, told the Geological Society of America meeting in Charlotte, N.C., on November 4. So far the models haven’t incorporated such information because “it just makes them much more complicated, ” he says.

Many scientists share Hay’s concerns, says geologist Harold Wanless of the University of Miami. “The rate at which ice melt and sea level rise is happening is far faster than anything predicted, ” he says.

Global sea levels rose an average of about 15 centimeters over the past century. Current data suggest they will rise another 1 meter by the year 2100, and some scientists predict far more. But the 2007 report of the Intergovernmental Panel on Climate Change projected a rise of just 0.2–0.6 meters over the same time period. “The data weren’t available in 2007 to say Greenland and Antarctica were melting, ” says earth scientist Benjamin Horton of the University of Pennsylvania in Philadelphia. “Sea levels are going to be greater than the upper estimate of the 2007 IPCC, but the big question is, when?”

To help answer that question, Hay is looking at underappreciated feedbacks. For one thing, big infusions of freshwater in the Arctic — from melting sea ice and from northern rivers — are driving cold ocean currents away from the North Pole and bringing up warm ones. This process is helping melt Greenland’s ice sheet, which could cause sea level to change very quickly, Hay says. Also, Arctic melt is exposing large areas of dark water, which absorbs the sun’s heat instead of reflecting it like ice does. As a result, warmer temperatures are disrupting local weather. After a record melt in August, for example, the high-pressure weather system that keeps the Arctic in a deep freeze was seasonally replaced by a low-pressure system that sucked in warm air, causing even more melting. To top it all off, the Arctic Ocean and thawing permafrost are releasing methane, a powerful greenhouse gas that traps atmospheric heat and raises temperatures still more.

The last time Earth’s climate was as balmy as today was about 120, 000 years ago, when the planet was 2–3° Celsius warmer and sea levels were 4–6 meters higher. Much of the Greenland ice sheet was melted then. “Those sea level change rates are very, very large, and that was under natural conditions, not the human perturbation that’s going on now, ” says Hay.

And Greenland isn’t the only concern — Antarctica contains a vast amount of ice that, if emptied entirely into the ocean, would cause 80 meters of sea level rise. For years scientists suspected the Antarctic ice was frozen to the ground, but evidence now suggests there is liquid water under many regions, lubricating the ice base like a skating rink. The only things stopping that ice from sliding into the sea are ice shelves, which act like corks in a bottle, Hay says. As these ice shelves break up — as some are already doing — “it’s like taking the cork out of the bottle.”

Even modest sea level rise can have far-reaching impacts. Higher sea levels make it easier for storm surges — like those produced by Hurricane Sandy — to reach further inland and inflict damage, Wanless says. “The future of our coastal cities is at stake.”

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