Those sold on global warming have argued that the earth's ice caps are steadily melting, and, as a result, sea levels are rising--threatening to wash away coastal communities. Disbelievers, however, have countered that water levels are not increasing across the planet: while the seas in some regions have risen during the past century, they have dropped in others. Now a team led by two geophysicists at the University of Toronto says they have settled the matter--and can explain the geographic variations in sea level changes as well. They describe their work in today's issue of Nature.

Using computer models, Jerry Mitrovica and his colleagues created what they call fingerprints of sea level changes. They found that different scenarios--melting from Greenland, say, or from the Antarctic--produced quite different patterns of rising and falling. Next they compared the fingerprints to the observed record of sea level changes--and discovered it was a near-perfect fit. In conclusion, "sea level is rising," Mitrovica says, "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."

Indeed, the fingerprints overthrow some basic ideas about ice cap melting. "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," Mitrovica explains. But this is not at all what happens. The top image at right shows sea level changes--both up (red) and down (blue)--associated with melting in the Antarctic. The bottom image reveals the effect on sea level when the polar ice sheets of Greenland are melted.

"If the entire Greenland ice cap melted," Mitrovica says, "then places relatively close by, such as 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."