February 16, 2007
No Comfort Here: We Can't Confidently Predict Ice Sheet Behavior
The significance of ice sheet behavior to policy has resulted in substantial research efforts that have been bearing fruit in the last year or two. But recent studies suggest that the IPCC's uncertainty about ice sheet behavior is justified -- and may not be resolved quickly enough to allow us to make policy based on a narrow range of estimated sea level change. So what are we to do? It's the traditional problem of scientific uncertainty rearing its ugly head -- just at the time when we are convinced that something needs to be done, but now we have to decide what to do and how quickly it must be done.
The temptation is to say as much as feasible. But that begs the cost question, which defines our sense of what is feasible. So, we need to set "technology-forcing" or what I'd prefer to call "technology-facilitating" goals and let the genius of the market find ways to meet them. And the goals need to be set not based on what we guess is the central tendency of estimated climate impacts, but based on the higher end of estimated climate impacts --not based on a worst case scenario, but based on a moderately worse case scenario. To me, this is James Hansen's recommendation to hold SST increase to 1 degree by the end of the century. Now, what does that require? I don't know....but I'd like the answer to that question.
Here's the latest from the AAAS meeting:
Clues to Sea Rise May Lie Beneath Antarctic Glaciers
Images courtesy of NASA
A network of rapidly filling and emptying lakes lies beneath at least two of West Antarctica’s ice streams, according to new research published online today by the journal Science, at the Science Express website.
More than 100 subglacial lakes have already been discovered, but the new ones are particularly interesting because they occur below fast-moving ice. Though it’s too early to say exactly how this liquid water is affecting the rates of ice flow above, understanding the behavior of these fast-moving ice streams is essential for predicting how Antarctica may contribute to sea level rise.
Helen Fricker of the University of California San Diego’s Scripps Institution of Oceanography and colleagues analyzed elevation data recorded by NASA’s Ice Cloud and land Elevation Satellite (ICESat) collected over the lower parts of the Whillans and Mercer Ice Streams. These are two of the major, fast-moving glaciers that are carrying ice from the interior of the West Antarctic Ice Sheet to the floating Ross Ice Shelf.
“We’ve found that there are substantial subglacial lakes under ice that’s moving a couple of meters per day. It’s really ripping along. It’s the fast-moving ice that determines how the ice sheet responds to climate change on a short timescale,” said Robert Bindschadler of NASA Goddard Space Flight Center, one of the study’s coauthors.
“We aren’t yet able to predict what these ice streams are going to do. We’re still learning about the controlling processes. Water is critical, because it’s essentially the grease on the wheel. But we don’t know the details yet,” he said.
Bindschadler presented the findings at a news briefing for reporters on Thursday, 15 February, at the AAAS Annual Meeting in San Francisco, California. In coordination with the briefing, NASA released satellite images of West Antarctica.
Glaciologists have known that water exists under ice streams, but the observation of a system of water storage reservoirs is unprecedented. The surprising thing about this discovery is the amount of water involved, and the pace at which it moves from one reservoir to another, according to Fricker, the lead author.
“We didn’t realize that the water under these ice streams was moving in such large quantities, and on such short time scales,” Fricker said. “We thought these changes took place over years and decades, but we are seeing large changes over months.”
The authors identified numerous spots that either rose or deflated from 2003 to 2006, likely because water flowed into or out of them. Water would be capable of this because it is highly pressurized under the weight of the overlying ice.
The three largest regions are between approximately 120 and 500 square kilometers, while the others are widely scattered and smaller. One of the large regions, referred to as Subglacial Lake Engelhardt, drained during the first 2.7 years of the ICESat mission, while another, Subglacial Lake Conway, steadily filled during the same period.
“I’m quite astonished that with this combination of satellite sensors we could sense the movement of large amounts of water like this. From 600 kilometers up in space, we were able to see small portions of the ice sheet rise and sink,” Bindschadler said.
Studies of the subglacial environment are rare, being expensive, risky and labor-intensive. Bindschadler explained that before the ICESat mission, researchers would typically have to drill holes in the ice streams in order to study what was occurring beneath them. These holes, generally just about 4 inches in diameter, provided a much more limited view of the entire ice stream than the satellite images do.
“Until now, we’ve had just a few glimpses into what’s going on down there. This is the most complete picture to date what’s going on beneath fast flowing ice,” Bindschadler said.
Added Fricker: “The approach used for this work provides glaciologists with a new tool to survey and monitor the nature of the subglacial water system and to link these observations to the motion of the ice sheet. We still don’t know how the subglacial water system varies on longer time-scales from decades to centuries. To do this, we need to continue monitoring the ice streams with ICESat and future follow-on missions.”
15 February 2007 3:24 pm
On ice sheets, U.W. reports:
Two of Greenland's largest glaciers shrank dramatically and dumped twice as much ice into the sea during a period of less than a year between 2004 and 2005. And then, less than two years later, they returned to near their previous rates of discharge.
The variability over such a short time, reported online Feb. 9 on Science magazine's Science Express, underlines the problem in assuming that glacial melting and sea level rise will necessarily occur at a steady upward trajectory, according to lead author Ian Howat, a post-doctoral researcher with the University of Washington's Applied Physics Laboratory and the University of Colorado's National Snow and Ice Data Center. The paper comes a year after a study in the journal Science revealed that discharge from Greenland's glaciers had doubled between 2000 and 2005, leading some scientists to speculate such changes were on a steady, upward climb.
"While the rates of shrinking of these two glaciers have stabilized, we don't know whether they will remain stable, grow or continue to collapse in the near future," Howat says. That's because the glaciers' shape changed greatly, becoming stretched and thinned.
"Our main point is that the behavior of these glaciers can change a lot from year to year, so we can't assume to know the future behavior from short records of recent changes," he says. "Future warming may lead to rapid pulses of retreat and increased discharge rather than a long, steady drawdown."
The findings come on the heels of the widely publicized Intergovernmental Panel of Climate Change's report issued Feb. 2. Some scientists criticized the report for disregarding the surprisingly high discharges of ice from Greenland's glaciers since 2000 when the panel estimated the amount of future sea level rise that will be caused by melting glaciers.
In the summary for policy makers (http://www.ipcc.ch/SPM2feb07.pdf), the Intergovernmental Panel on Climate Change explained its position saying, "Dynamical processes related to ice flow and not included in current models but suggested by recent observations could increase the vulnerability of the ice sheets to warming, increasing sea level rise. Understanding of these processes is limited and there is no consensus on their magnitude."
"I think the IPCC authors made a responsible decision in producing their estimates while noting the recent discharges are a real concern that we do not yet understand well enough to make accurate predictions," says Ian Joughin, a glaciologist with the UW's Applied Physics Laboratory and co-author on the Science Express article.
Getting accurate computer models of Greenland and Antarctic glaciers is important because 99 percent of the Earth's glacial ice is found in those two places. Glacial ice is second only to the oceans as the largest reservoir of water on the planet.
Previous findings published a year ago showed that Greenland's glaciers had doubled their discharge between 2000 and 2005, but these results were based on "snapshots" of discharge taken five years apart, Howat says.
"Did an equal amount of discharge occur every year? Did it happen all in one year? Is there a steady upward trajectory? We didn't know," he says.
Last week's Science Express article adds details from Greenland's second and third largest glaciers, Kangerdlugssuaq and Helheim, in the southwest part of Greenland. The two are known as "outlet" glaciers because their front edges reach all the way to the sea, unlike other glaciers that are landlocked. Together the two glaciers represent 35 percent of East Greenland's total discharge. The scientists examined the glaciers' speed, geometry and discharge between 2000 and 2006.
At Kangerdlugssuaq, roughly 80 percent of the total increase in discharge occurred in less than one year in 2005, followed by a 25 percent drop the following years, the authors say. At Helheim, discharge increased between 2000 and 2003, and then by an even greater amount between 2004 and 2005. It then dropped in 2006 to its near 2000 value.
The scientists say what they've learned is that the shape of these two glaciers changed as they surged toward the sea, changes that put the brakes on. The glaciers lost ice as their front edges began calving, became lighter and floated off the bottom, which led to more ice breaking off as the ice was buoyed up by water. The fronts stablized once the ice had retreated to shallower parts of the fjords and again rested on the bottom.
They also found the pace toward the sea was faster at the front edge of the glaciers than farther up the mountain. For example Kangerdlugssuaq's front edge increased in speed by 80 in 2005 percent while 19 miles inland the speed increased 20 percent. This caused the glaciers to thin, stretch and weigh less overall, which also slowed them down.
"All this in a matter of a few short years for these two glaciers is not the way glaciologists are used to thinking," Howat says. "We're used to thinking of the ice sheets in terms of millennia or centuries."
Predicting Fate of Glaciers Proves Slippery Task
By Richard A. Kerr
ScienceNOW Daily News
15 February 2007
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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