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Tracking the Fingerprints of the Rising Seas

The figures are computer simulations of what happens to sea levels in the event of ice sheet melts near the Northern or Southern poles. The blue colors show a drop in sea level while the red and orange colors show a rise.

The top image shows the impact on sea level when polar ice sheets in the Antarctic are melted. The blue color indicates a decrease in sea level near the Antarctic while it also shows a corresponding rise in sea level in the Northern Hemisphere.

The bottom image shows the effect on sea level when the polar ice sheets of Greenland are melted. The sea level decreases around Greenland but rises in the Southern Hemisphere.

Images courtesy of Jerry Mitrovica, University of Toronto

by Janet Wong
Toronto - Feb. 21, 2001
Global climate change is having a direct impact on the Earth's sea level and a group of scientists led by two U of T geophysicists is providing the sea level "fingerprints" of polar ice sheet melting to prove it.

Rates of sea level change over the last century vary widely from one geographic location to another even after these rates have been corrected for known effects. The question has always been, why? What is causing these significant variations?

Jerry Mitrovica, U of T geophysics professor, is lead author of a paper to appear in the Feb. 22 issue of Nature that claims to have discovered the answer. And it is an answer that has an important impact on the debate over global climate change.

Mitrovica and his colleagues argue that scientists have not widely appreciated that melting from the Antarctic, for example, will have a distinctly different pattern or fingerprint in how it affects sea level than melting from Greenland or small mountain glaciers. It is these patterns that are causing the variation in the global sea level rise.

"We calculated these fingerprints using computer models and then showed that the observed record of sea level change displays the fingerprints," says Mitrovica. "Sea level is rising, and based on our work and the analysis of sea level data, not only can we assess the total amount melting from the ice caps, but we can also tell where that meltwater is coming from."

Mitrovica conducted this research with Mark Tamisiea, a U of T post-doctoral fellow and second author on the paper, James Davis of the Harvard-Smithsonian Center for Astrophysics, and Glenn Milne of the University of Durham.

"In the past, people have been puzzled by the significant variations in sea levels in different parts of the world," says Mitrovica. "Like throwing water in a bathtub, many scientists assumed that if polar ice melting were contributing to sea level rise, it would present itself evenly and uniformly across the Earth's oceans."

And that assumption, he says, is simply wrong. Mitrovica uses Greenland as an example. It was assumed that if the ice caps on Greenland were melting, all coastal locations would flood evenly.

"In fact," he continues, "if the entire Greenland ice cap melted, then places relatively close by, like Britain and Newfoundland, would actually see sea levels fall. The reason is fairly simple: despite its small size, the Greenland ice sheet exerts a strong gravitational pull on the seas.

"As the polar sheet melts, it will exert less pull, resulting in lower - not higher - sea levels around Greenland. Of course, sea levels will rise on average, and as the meltwater moves away from Greenland it will create problems for countries in the Southern Hemisphere.

"In the same way, melting from the Antarctic will raise sea levels in the Northern Hemisphere, but not in places like Australia," explains Mitrovica.

To look for evidence of their ideas, the scientists re-examined the data from tide gauges that measure sea levels. The results startled even them.

They found that they could fit nearly all the geographic variations in sea level that they saw in these tide gauges using the distinct sea level patterns they predicted for the melting of polar ice sheets. It is estimated that sea levels are rising, on average, by about 1.8 millimetres per year.

"We've really strengthened the link between today's sea level changes and ice melting and we've found a way of unraveling the details of this link.

"By doing that, we've also strengthened extrapolations being made for the future effect of climate warming. And these extrapolations show continued acceleration of sea level rise late into the present century, leading to more flooding of coastal communities," added Mitrovica.

Janet Wong is a news services officer with the Department of Public Affairs.

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