Recording the Life Cycle of an Arctic Ice Sheet
BOSTON — For more than a year, people clad in insulated overalls, "bunny boots," and thick parkas have scuttled over an Arctic ice floe, measuring its growth, decay, and the impact the ocean and atmosphere have on its life cycle.
Last Saturday, research teams with the SHEBA (Surface Heat Budget of the Arctic Ocean) project returned home, bringing to a close the most ambitious civilian Arctic Ocean study the United States has ever undertaken.
The project's main purpose is to help improve climate-change forecasts by plugging wide gaps in scientists' understanding of how the ocean, ice, and atmosphere in the Arctic exchange heat.
Although much of this "heat budget" data will take years to analyze, some results already show promise of improving weather forecasts in areas affected by Arctic weather patterns.
The project also appears to be breaking the ice for future large-scale Arctic studies. On Tuesday, Congress approved a 69 percent increase in the National Science Foundation's Arctic-systems science program, from $32 million in fiscal 1998 to $54 million in 1999.
SHEBA "helped focus some attention on the Arctic as a place where there are some real problems that require a big coordinated approach to solve," says Michael Ledbetter, who heads the NSF's Arctic science program.
For the 170 researchers and technicians involved, SHEBA has been a bonanza. Typically, such experiments run for only a month or two, notes Don Perovich, the project's chief scientist. "That's what's really exciting about SHEBA - we got to see the whole movie," he says, from the growth of the ice last winter to its summer melt and subsequent regrowth this fall.
It was a movie with surprising twists. One of the first surprises: The ice was much thinner when the Canadian icebreaker Des Groseilliers arrived Oct. 2, 1997, than either historical data or computer models led the researchers to believe. Instead of the 2.5-to-3-meter-thick floes they expected, the scientists could only find floes 1.8 meters thick within range of the resupply plane.
"It had been a heavy melt season," says Dr. Perovich, a geophysicist at the US Army Corps of Engineers' Cold Regions Research and Engineering Laboratory in Hanover, N.H. "The ice thickness was well below the long-term average, but we figured it would get thicker over the next year. It didn't." Some researchers suggest El Nio may be to blame. However, oceanographer Miles McPhee, who heads his own research firm in Naches, Wash., and led one of SHEBA's research teams, suggests that climate change may be a factor. When sea ice freezes, it squeezes out the water's salt; when it melts, it builds a layer of fresh water beneath itself. Last October, Dr. McPhee's instruments recorded a thicker-than-expected blanket of fresh water beneath SHEBA's location compared with similar measurements from a 1975 Arctic expedition.
Meanwhile, atmospheric scientists David Thompson and John Wallace of the University of Washington in St. Louis have charted temperatures and pressures since the turn of the century in the Arctic Basin and note that the region undergoes cycles of rising and falling average temperatures and surface pressures. At the current stage in the cycle, with its warmer temperatures and lower pressures, "more storms track through, opening the ice more," McPhee says. "This could account for the freshening we saw."
However, he adds, the University of Washington team's work also shows that during the past 30 years, average Arctic winter temperatures have started to rise, even after adjusting for the cycles. McPhee suggests that global warming may be superimposing itself on the Arctic's natural variation, thinning the ice.
Concern over the Arctic's ice centers on the fresh water that long-term melting would release. Significant melting is expected to have little impact on sea levels, since the Arctic's ice floes already ride in the water. Instead, many researchers hold that a growing influx of fresh water into the North Atlantic could change ocean circulation patterns, shifting fisheries and weather worldwide.
SHEBA research also has opened a new window on the impact of clouds on the Arctic's heat budget. Satellites alone can't tell the whole story; they reveal nothing about whether cloud particles are ice crystals or water droplets - a critical difference for handling heat, since liquid water reradiates heat more readily than ice does. Moreover, satellites can miss clouds that are optically thin but still affect heat transfer.
"Even before SHEBA, we knew we weren't getting clouds right," says Judith Curry, a climate modeler at the University of Colorado. Models predicted too few clouds and underestimated the amount of water vapor. What researchers found was a cloudier Arctic than they had anticipated, and a higher level of water droplets in the clouds, even in midwinter.
Insights SHEBA returned on infrared radiation emitted by the atmosphere itself, Dr. Curry adds, have been built on a test basis into weather-prediction models used by the European Center for Medium-Range Weather Forecasts in Reading, England. Using these new insights, Curry says, the test is yielding Arctic temperature forecasts several degrees cooler than before.
Despite the enthusiasm about SHEBA's data, the $19.5 million project has its limits. "We now know a lot about one year in one location," Perovich says. The challenge is to expand the reach of measurements. Discussions are under way, he adds, for a network of buoys in the Arctic Ocean to monitor full time the region's temperature, pressure, and ocean characteristics. In addition, self-guided robotic undersea vehicles hold the promise of extending the ice-related research that SHEBA undertook, notes Dennis Conlon of the Office of Naval Research in Arlington, Va.