Warming ocean layers will undermine polar ice sheets

Warming of the ocean's subsurface layers will melt underwater portions of the Greenland and Antarctic ice sheets faster than previously thought, increasing the sea level more than already projected, a new study suggests. The subsurface ocean layers surrounding the polar ice sheets will warm substantially as global warming progresses, according to the study led by researchers from the University of Arizona (UA). In addition to being exposed to warming air, underwater portions of the polar ice sheets and glaciers will be bathed in warming seawater, said the study appearing on the website of the American Association for the Advancement of Science (AAAS) on Sunday. The research, based on 19 state-of-the-art climate models, proposes a new mechanism by which global warming will accelerate the melting of the great ice sheets during this century and the next. "To my knowledge, this study is the first to quantify and compare future ocean warming around the Greenland and the Antarctic ice sheets using an ensemble of models," said lead author Jianjun Yin, a UA assistant professor of geosciences. According to the study, the subsurface ocean along the Greenland coast could increase as much as 3.6 F (2 C) by 2100. Most previous research has focused on how increases in atmospheric temperatures would affect the ice sheets, he said. "Ocean warming is very important compared to atmospheric warming because water has a much larger heat capacity than air," Yin said. "If you put an ice cube in a warm room, it will melt in several hours. But if you put an ice cube in a cup of warm water, it will disappear in just minutes." Given a mid-level increase in greenhouse gases, the researchers found the ocean layer about 650 to 1,650 feet (200 to 500 meters) below the surface would warm, on average, about 1.8 F (1 C) by 2100. Along the Greenland coast, that layer would warm twice as much, but along Antarctica would warm less, only 0.9 F (0.5 C). "No one has noticed this discrepancy before -- that the subsurface oceans surrounding Greenland and Antarctica warm very differently," Yin said. Part of the warming in the North comes from the Gulf Stream carrying warm subtropical waters north. By contrast, the Antarctic Circumpolar Current blocks some of the subtropical warmth from entering the Antarctic's coastal waters. Even so, the Antarctic ice sheet will be bathed in warming waters, according to the study. This paper adds to the evidence that sea level would rise by the end of this century by around one meter and a good deal more in succeeding centuries, the study noted. The study, "Different Magnitudes of Projected Subsurface Ocean Warming Around Greenland and Antarctica," is scheduled for the upcoming edition of Nature Geoscience later this month.