Just over 90 years after the first humans set foot at the South Pole, a team of US scientists has completed a rare cross-country trek to Antarctica's southernmost point as part of an international effort to uncover the history of climate change on Earth's most inhospitable continent.
The 775-mile route they followed would take 11 hours to cover on an interstate highway. Under polar conditions, however, the 10-member team took 27 days to make the trip. Along the route, the expedition's 14-ton sled-tugging tractors crept through deep snowdrifts, hunkered down during howling blizzards, dodged deep crevasses, and stopped to extract hundreds of core samples from the top 160 to 400 feet of the two-mile-thick mantle of ice that covers the continent.
Researchers are keenly interested in that ice cover and the climate regimes that govern its growth and shrinkage. The West Antarctic Ice Sheet is seen as a possible source of the water that raised sea levels by as much as 25 meters in a period of less than 1,000 years - probably between 14,000 and 11,000 years ago. During the past 10,000 years, the sheet has thinned by more than 2,300 feet at the coast, a process that continues today.
Understanding the links and tracking the ice's history are vital to forecasting its future behavior, particularly if the global climate's current warming trend continues, according to Paul Mayewski, a geologist at the University of Maine and the expedition's leader.
Research reported last week in the journal Science indicates that the sheet's long-term decline is expected to continue, part of a natural process that began with the end of the last ice age.
"In the short-term the West Antarctic Ice Sheet is generally stable," Dr. Mayewski says. The new ice cores and other data the group collected track changes that have occurred over the past 200 to 1,000 years.
By comparing these data with those from deeper core samples from elsewhere on the continent, scientists hope to find out how much of this deglaciation is part of a long-term natural process and whether the trend may be accelerating due to global warming.
"We'll be able to answer these questions in the next year or two," after all the data have been analyzed, he says.
The expedition, which reached the South Pole Jan. 2, got off to an inauspicious start.
The team initially attempted the trip Nov. 23. But "within 48 hours, we realized we weren't going to make it," Mayewski told a press conference conducted via phone from the South Pole.
The snow was much heavier than predicted, bogging down the team's two 27,000 pound tractors and their 50,000 pound sleds in snowdrifts up to the tractors' axles. In two days the team had moved a mere 25 miles from Byrd Station, its starting point. By Dec. 7, however, mechanics had replaced the sleds' skis with runners that looked more like sea-plane pontoons, giving the sleds greater buoyancy.
The heavy snow and milder-than-expected temperatures (between minus 4 and minus 18 degrees Fahrenheit instead of minus 40 degrees F.) were consistent with climate-model predictions of El Niño's effect on the continent, still something of a surprise since its effect is much less evident in the Arctic, researchers say.
"It really shows the strong connection between conditions in the tropical Pacific and the South Pole," Mayewski says.
Along the way, members of the three-woman, seven-man research team lived in insulated plywood bunkhouses and ate in a special mess tent on the sleds the tractors pulled. The tent doubled as the women's dorm, while one of the bunk houses did double duty as a lab. The mini tractor train also carried the fuel for transportation, heat, and electric generators.
The expedition was conducted under the aegis of the International Trans-Antarctic Scientific Expedition (ITASE), a multi-year program of Antarctic research involving 19 countries.
Where some polar field projects are interested in pinning down the physical processes determining how the climate system works at high latitudes, ITASE is trying to gather climate records from across as much of the frozen continent as possible.
"Antarctica is one of the most data-sparse areas on the planet" when it comes to climate, notes David Bromwich, an associate professor of atmospheric science at Ohio State University.
The ice-core samples provide tree-ring-like information on precipitation levels. They also trap air samples and particles from past events such as volcanic eruptions or major storms. By taking cores from regions where the layers are relatively flat, and calibrating them using the past 50 years of instrument measurements, the cores can provide a more comprehensive window on climate than information from a handful of locations on the continent.
In addition, researchers are using radar to map the bedrock far below the ice sheet's surface. The radar also is proving useful for exploring ice layers without the need to drill or dig trenches, and for spotting crevasses ahead of the tiny convoy.
The deep radar aims to see how the bedrock influences the surface shape of the ice sheet, according to expedition scientist Steve Arcone, a geophysicist with the US Army Corps of Engineers Cold Regions Research and Engineering Laboratory in Hanover, N.H.
That information is critical to selecting sites for coring because ice layers can get compressed and jumbled as the sheet glides over bedrock features, making interpretation of the layers more difficult.
One solution was to use a shallower radar scan. That resulted in layers of ice showing up in the radar return, yielding a way to gauge year-to-year snowfall rates - a key to understanding the degree to which the ice sheet is losing or gaining mass, and an indicator of historic weather patterns.
The radar returns seem to vary in strength every six miles or so, Dr. Arcone says. "It looks like the radar picked up different climate regimes" as it moved from region to region.
Between the radar data and weather balloons the team regularly launched, the researchers took measurements "from bedrock to 23 kilometers above the ice surface," Mayewski says.