Sea level could rise in South, fall in North

Gerald Traufetter -

REUTERS

Climate change is expected to cause sea levels to rise -- at least in some parts of the world. Elsewhere, the level of the ocean will actually fall. Scientists are trying to get a better picture of the complex phenomenon, which also depends on a host of natural factors.

When presented as a globe, the Earth looks as round and smooth as a billiard ball. To anyone standing on a beach, the ocean looks as flat as a pancake.

 

But perception is deceptive. "In reality, the water in the oceans wobbles all over the place," says oceanographer Detlef Stammer. He isn't talking about waves, but large-scale bulges and bumps in the sea level.

Stammer, who is the director of the Center for Marine and Climate Research at the University of Hamburg, is familiar with the incorrect notions that lay people have, which is why he likes to present them with two numbers to shatter their illusions. "In the Indian Ocean, the sea level is about 100 meters (330 feet) below the average, while the waters around Iceland are 60 meters above the average."

The incorrect belief that ocean water is evenly distributed lives on in the debate over climate change, says Stammer. The rising sea level is widely viewed as the most threatening consequence of global warming. Images of Bangladeshis wading through floodwaters are a favorite horror scenario used by some environmentalists. "But people act as if the water from melting glaciers were distributed as uniformly in the oceans as the water in our bathtubs at home," says Stammer.

But the reality is counter-intuitive. According to the most recent estimates, the sea level is expected to rise by about 1 meter (3.28 feet) -- on average -- in the next 100 years. This is the number that will be mentioned again and again during negotiations at the United Nations Climate Change Conference in Cancun, Mexico over the next two weeks. "But this average value doesn't really help coastal planners," says Stammer.

It is certainly correct that the total amount of liquid in the oceans is increasing. But the way water expands in ocean basins differs widely. There will be regions of the world where nothing much will change, while the sea level will rise by well over the 1-meter average in others. "The sea level could even fall along some coasts," says Stammer.

Scientists still don't know exactly the degree to which glaciers will melt as a result of rising temperatures. The most important factor in this equation will be the rate at which the Greenland and Antarctic ice sheets shrink. At the moment, it appears that the net amount of melt water is still rising in Greenland. It is currently at 237 cubic kilometers (57 cubic miles) per year. But the volume of ice at the South Pole seems to be generally stable. "The ice is melting in western Antarctica, but in the larger eastern part, snow is actually building up," says Stammer.

The sea level currently rises by about 3 millimeters (0.1 inches) a year on average. A number of factors contribute to this rise, including water from melting glaciers and the constant increase in the amount of ground water used in agriculture. It is also partly due to a simple thermal effect: Because water expands as it gets warmer, rising temperatures cause the sea level to rise. All of this will accelerate even further by the end of the century, leading to a total increase of 1 meter, according to the current consensus among oceanographers.

In reality, the simple message of rising waters is greatly oversimplified. The process behind it is highly complex, and one that will produce winners and losers. Scientists are only gradually beginning to understand the phenomenon and its processes, some of which work in opposing directions. "It's just in the last few years that science has taken a more in-depth look at regional prognoses for rising sea levels," Stammer says.

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