Climatic changes appear to be destabilizing vast ice sheets of western Antarctica that had previously seemed relatively protected from global warming, researchers reported yesterday, raising the prospect of faster sea-level rise than current estimates.
While the overall loss is a tiny fraction of the miles-deep ice that covers much of Antarctica, scientists said the new finding is important because the continent holds about 90 percent of Earth's ice, and until now, large-scale ice loss there had been limited to the peninsula that juts out toward the tip of South America. In addition, researchers found that the rate of ice loss in the affected areas has accelerated over the past 10 years -- as it has on most glaciers and ice sheets around the world.
"Without doubt, Antarctica as a whole is now losing ice yearly, and each year it's losing more," said Eric Rignot, lead author of a paper published online in the journal Nature Geoscience.
The Antarctic ice sheet is shrinking despite land temperatures for the continent remaining essentially unchanged, except for the fast-warming peninsula.
The cause, Rignot said, may be changes in the flow of the warmer water of the Antarctic Circumpolar Current that circles much of the continent. Because of changed wind patterns and less-well-understood dynamics of the submerged current, its water is coming closer to land in some sectors and melting the edges of glaciers deep underwater.
"Something must be changing the ocean to trigger such changes," said Rignot, a senior scientist with NASA's Jet Propulsion Laboratory. "We believe it is related to global climate forcing."
Rignot said the tonnage of yearly ice loss in Antarctica is approaching that of Greenland, where ice sheets are known to be melting rapidly in some parts and where ancient glaciers have been in retreat. He said the change in Antarctica could become considerably more dramatic because the continent's western shelf, an expanse of ice and snow roughly the size of Texas, is largely below sea level and has broad and flat expanses of ice that could move quickly. Much of Greenland's ice flows through relatively narrow valleys in mountainous terrain, which slows its motion.
The new finding comes days after the head of the Intergovernmental Panel on Climate Change said the group's next report should look at the "frightening" possibility that ice sheets in Greenland and Antarctica could melt rapidly at the same time.
"Both Greenland and the West Antarctic ice sheet are huge bodies of ice and snow, which are sitting on land," said Rajendra Pachauri, chief of the IPCC, the United Nations' scientific advisory group. "If, through a process of melting, they collapse and are submerged in the sea, then we really are talking about sea-level rises of several meters." (A meter is about a yard.) Last year, the IPCC tentatively estimated that sea levels would rise by eight inches to two feet by the end of the century, assuming no melting in West Antarctica.
The new Antarctic ice findings are based on mapping of 85 percent of the continent over the past decade using radar data from European, Japanese and Canadian weather satellites. Previous studies had detected the beginning of ice loss in West Antarctica and substantial loss along the peninsula, but the current research found significantly greater changes.
Rignot and his team found that East Antarctica, which holds a majority of the continent's ice, has not experienced the same kind of loss -- probably because most of the ice sits atop land rather than below sea level, as in the west. In several coastal areas of East Antarctica, however, small but similar losses have been detected, he said.
In all, snowfall and ice loss in East Antarctica have about equaled out over the past 10 years, leaving that part of the continent unchanged in terms of total ice. But in West Antarctica, the ice loss has increased by 59 percent over the past decade to about 132 billion metric tons a year, while the yearly loss along the peninsula has increased by 140 percent to 60 billion metric tons. Because the ice being lost is generally near the bottom of glaciers, the glacier moves faster into the water and thins further, as a result. Rignot said there has been evidence of ice loss going back as far as 40 years.
The new findings come as the Arctic is losing ice at a dramatic rate and glaciers are in retreat across the planet. At a recent annual meeting of the American Geophysical Union, Ohio State University professor Lonnie Thompson delivered a keynote lecture that described a significant speed-up in the melting of high-altitude glaciers in tropical regions, including Peru, Tibet and Mount Kilimanjaro in Kenya.
Thompson, who has studied the Quelccaya glacier in the Peruvian Andes for 30 years, said that for the first half of that period, it retreated on average 20 feet per year. For the past 15 years, he said, it has retreated an average of nearly 200 feet per year.
"The information from Antarctica is consistent with what we are seeing in all other areas with glaciers -- a melting or retreat that is occurring faster than predicted," he said. "Glaciers, and especially the high-elevation tropical glaciers, are a real canary in the coal mine. They're telling us that major climatic changes are occurring."
While the phenomenon of ice loss worldwide is well documented, the dynamics in the Antarctic are probably the least understood. Glaciers and ice sheets are sometimes miles deep, and researchers do not know what might be happening at the bottom of the ice -- but it clearly is being lost along the peninsula and West Antarctic coast.
Rignot theorizes that the warmer water of the Antarctic Circumpolar Current is the cause. Douglas Martinson, a senior research scientist fellow at the Lamont-Doherty Earth Observatory of Columbia University, has studied the issue and agrees.
Martinson said the current, which flows about 200 yards below the frigid surface water, began to warm significantly in the 1980s, and that warming in turn caused wind patterns to change in ways that ultimately brought more warm water to shore. The result has been an increased erosion of the glaciers and ice sheets.
Martinson said researchers do not have enough data to say for certain that the process was set in motion by global warming, but "that is clearly the most logical answer."
Pachauri, the IPCC's chief of climate science, will visit Antarctica this week with Norwegian Prime Minister Jens Stoltenberg to get a firsthand view of the situation.
"You can read as much as you want on these subjects, but it doesn't really enter your system. You don't really appreciate the enormity of what you have," Pachauri said.