Earlier this month, the Intergovernmental Panel on Climate Change (IPCC) declined to extrapolate the recent accelerated loss of glacial ice far into the future (ScienceNOW, 2 February). Too poorly understood, the IPCC authors said. Overly cautious, some scientists responded in very public complaints (Science, 9 February, p. 754). The accelerated ice loss--apparently driven by global warming--could raise sea level much faster than the IPCC was predicting, they said. Yet almost immediately, new findings have emerged to support the IPCC's conservative stance.
In a surprise development, glaciologists reported online last week in Science (10.1126/science.1138478) that two major outlet glaciers draining the Greenland ice sheet--Kangerdlugssuaq and Helheim--did a lively two-step in the first part of the decade. By gauging the elevation and flow speed of the glaciers using satellite data, Ian Howat of the University of Washington's Applied Physics Laboratory in Seattle and his colleagues found that Kangerdlugssuaq sped up abruptly in 2005, no doubt accelerating sea level rise just a bit. But then it fell back to near its earlier flow speed by the next year. Helheim gradually accelerated over several years, also sped up sharply in 2005, and then slowed abruptly to its original flow speed. Apparently, these glaciers were temporarily responding to the loss of some restraining ice at their lower ends, much as a river's flow would temporarily increase with the lowering of a dam.
Helen Fricker of Scripps Institution of Oceanography in San Diego, California, and her colleagues report another glaciological surprise in a paper published online today in Science. Fricker also presented the study this morning at the annual meeting of the American Association for the Advancement of Science (which publishes ScienceNOW) in San Francisco, California. Using a new satellite-based laser technique, the team discovered an unexpectedly active network of linked lakes beneath two ice streams--Whillans and Mercer--draining the West Antarctic Ice Sheet. Researchers knew of pools of meltwater at the base of Antarctic ice, but Fricker and her colleagues recorded the rising and falling of the surface by up to 9 meters over 14 patches of ice, the largest three spanning 120 to 500 square kilometers. Water that could lubricate the base of the ice and perhaps accelerate its flow was seeping from one subglacial lake to another in a matter of months, and in one case escaping to the sea. "We didn't know as much about the Antarctic ice sheet as we thought we did," says Fricker.
Glaciologist Richard Alley of Pennsylvania State University in State College agrees. "Lots of people were saying we [IPCC authors] should extrapolate into the future," he says, but "we dug our heels in at the IPCC and said we don't know enough to give an answer." Researchers will have to understand how and why glacier speeds can vary so much, he adds, before they can trust their models to forecast the fate of the ice sheets, much less sea level.
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