Warming Oceans, Shrinking Ice

The fate of low-lying coastal cities and island nations around the globe could depend on the future of the massive ice sheets covering Greenland and Antarctica. The water locked up in Greenland's roughly 700,000-cubic-mile ice sheet is enough to raise the global sea level by 23 feet. Western Antarctica's ice sheet could contribute another 15–20 feet. Scientists thought these ice sheets would melt over millennia, if at all, until recent observations showed that both have been losing ice mass at an increasing rate since the early to mid-1990s. Is this trend likely to continue as the planet warms, or are these changes due to normal variations in climate?

The observational answers won't be in for another decade or more, and the best computer models for climate prediction can't give realistic answers either—not yet. Traditional climate system models compute ocean circulation, sea ice formation and melting, and atmospheric circulation, and they predict how those processes would change in response to rising temperatures. But until recently, ice sheets have been treated as stationary features of the planet.

That's about to change. Scientists working on Los Alamos's Climate, Ocean, and Sea Ice Modeling project, who have contributed both the ocean and the sea ice components of the widely used Community Climate System Model, are now building a new component that will track the dynamics and melting of ice sheets.

The new model component will account for the two main mechanisms of ice loss to the oceans. One is summertime surface melting, a fraction of which flows into large vertical shafts, called moulins, to the base of the ice sheet. This process lubricates the base of the ice sheet and facilitates faster sliding toward the ocean. The other main mechanism of ice loss is through outlet glaciers and ice streams, rivers of fast-moving ice that flow directly into the ocean. Until the early 1990s, Greenland's outlet glaciers appeared to be flowing at fairly constant speeds, but during the last decade, they mysteriously sped up, some more than doubling their speed. Surface melting increased over the same period. Was an increase in meltwater lubrication causing these glaciers to slide more easily over the bedrock on their way to the ocean?

Surprisingly, the majority of the evidence points not to the lubricating effects of meltwater but to warm ocean water. Over the past decade, warm ocean water has been reaching the fronts of the outlet glaciers, which protrude into the ocean and float above the bedrock. Warm water has triggered the melting and disintegration of these floating ice tongues, as well as melting near the grounding line, the boundary between grounded and floating ice. Both have the effect of "uncorking" the outlet glacier and allowing it to speed up. Some, but not all, of these glaciers have since slowed down again as ocean circulation patterns have shifted.

Los Alamos researchers William Lipscomb, Stephen Price, and Xylar Asay-Davis are beginning to model these effects. They aim to simulate the dynamic interplay between outlet glaciers and ocean circulation and to understand how that interplay affects the loss of land ice to the oceans. If all goes well, this work will contribute to more realistic predictions of sea level rise by 2013, when the United Nations' Intergovernmental Panel on Climate Change tells the world what to expect for the next century.

—Necia Grant Cooper

Iceberg broken off from Greenland's Ilulissat Glacier

Iceberg broken off from Greenland's Ilulissat Glacier.
Photo Credit: James Balog / Extreme Ice Survey

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