The Secret of Sea Level Rise: It Will Vary Greatly by Region

22 Mar 2010: Analysis

The Secret of Sea Level Rise:
It Will Vary Greatly by Region

As the world warms, sea levels could easily rise three to six feet this century. But increases will vary widely by region, with prevailing winds, powerful ocean currents, and even the gravitational pull of the polar ice sheets determining whether some coastal areas will be inundated while others stay dry.

by michael d. lemonick

For at least two decades now, climate scientists have been telling us that CO2 and other human-generated greenhouse gases are warming the planet, and that if we keep burning fossil fuels the trend will continue. Recent projections suggest a global average warming of perhaps 3 to 4 degrees C, or 5.4 to 7 degrees F, by the end of this century.

But those same scientists have also been reminding us consistently that this is just an average. Thanks to all sorts of regional factors — changes in vegetation, for example, or ice cover, or prevailing winds — some areas are likely to warm more than that, while others should warm less.

What’s true for temperature, it turns out, is also true for another frequently invoked consequence of global warming. Sea level, according to the best current projections, could rise by about a meter by 2100, in large part due to melting of the Greenland and West Antarctic ice sheets. But that figure, too, is just a global average. In some places — Scotland, Iceland, and Alaska for example — it could be significantly less in the centuries to come. In others, like much of the eastern United States, it could be significantly more.

And among the most powerful influences on regional sea level is a surprising force: the massive polar ice sheets and their gravitational pull, which will lessen as the ice caps melt and shrink, with profoundly different effects on sea level in various parts of the globe.

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Acceleration of melting of ice-sheets, glaciers and ice-caps: A wide array of satellite and ice measurements now demonstrate beyond doubt that both the Greenland and Antarctic ice-sheets are losing mass at an increasing rate. Melting of glaciers and ice-caps in other parts of the world has also accelerated since 1990.

Rapid Arctic sea-ice decline: Summer-time melting of Arctic sea-ice has accelerated far beyond the expectations of climate models. The area of summertime sea-ice during 2007-2009 was about 40% less than the average prediction from IPCC AR4 climate models.

Current sea-level rise underestimates: Satellites show great global average sea-level rise (3.4 mm/yr over the past 15 years) to be 80% above past IPCC predictions. This acceleration in sea-level rise is consistent with a doubling in contribution from melting of glaciers, ice caps and the Greenland and West-Antarctic ice-sheets.

Sea-level prediction revised: By 2100, global sea-level is likely to rise at least twice as much as projected by Working Group 1 of the IPCC AR4, for unmitigated emissions it may well exceed 1 meter. The upper limit has been estimated as – 2 meters sea-level rise by 2100. Sea-level will continue to rise for centuries after global temperature have been stabilized and several meters of sea level rise must be expected over the next few centuries.

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