Antarctic Ice Cap Has Shrunk, Researcher Says
In what commonly is thought to be a portent of global warming, the vast canopy of sea ice that forms each year around Antarctica may have declined abruptly by as much as 25% in a 20-year period, according to research made public Wednesday.
Every winter, the crushing shield of ice that caps Antarctica is swelled by 7.7 million square miles of seasonal sea ice--an expanse twice the size of the United States that can affect weather around the world. Since the 1970s researchers have monitored its annual ebb and flow via satellite--watching vast ice shelves shatter and jettison icebergs the size of Rhode Island into open seas. But they have been unable to detect any conclusive evidence of the changes predicted by computer models of a warming world climate.
Now, however, an Australian researcher who analyzed the meticulous records kept by whaling vessels operating around Antarctica before satellite observations began has determined that the average extent of the annual sea ice shrank dramatically between 1950 and 1970 by almost three degrees of latitude.
“The decline occurred relatively quickly, beginning in the mid-1950s, and was largely complete by 1973,” said Australian Antarctic expert William K. de la Mare, whose research is published today in the journal Nature. “Frustratingly, the change predates reliable satellite observations.”
Several experts Wednesday said the reported decline in the sea ice, if true, is “astonishing.”
Kenneth C. Jezek, director of the Byrd Polar Research Center at Ohio State University, said, “The [study’s] results are quite surprising and very intriguing. A 25 percent decline would be quite remarkable.”
But Jezek and other experts all cautioned that there has been no significant change in the pack ice in the 20 years during which it has been the subject of sustained scientific scrutiny. Nor is there any way to be certain that the decline documented by the whaling records stems from global warming. The natural forces involved are so complex that it is too soon to know whether the decline in the sea ice is due to normal variability of the weather or some more lasting climate change.
Cornelius W. Sullivan, a former director of the National Science Foundation’s Office of Polar Programs who is now vice provost for research at USC, was skeptical. “From my point of view, I am not sure how much confidence I would have in the basic premise of the data.”
Nonetheless, it is the latest in a series of provocative examples of what appears to be climate in flux, from earlier springs and longer growing seasons in the Northern Hemisphere to greater extremes of summer heat and winter cold. Indeed, temperatures at the South Pole this July were the lowest since record-keeping began about 40 years ago, dipping to 108 degrees Fahrenheit below zero.
Many climate experts believe the sea ice is crucial to regulating the planet’s climate, and a change of such magnitude could have global significance.
David Bromwich, a meteorologist at the Byrd Center, said that contrary to previous belief, new computerized models of the planet’s atmosphere show there is an intimate connection between the state of the sea ice around Antarctica and weather patterns as far away as China.
But long-term data on the behavior of the sea ice has been hard to come by.
For his study, De la Mare turned to a previously overlooked source: the International Whaling Commission and the computerized records it maintains of 1.5 million whaling catches--every whale caught by factory ships in Antarctic waters since 1931. The whalers’ logbooks recorded the precise longitude, latitude, date and species of every leviathan they landed.
The whales, especially highly prized blue whales, tended to congregate along the edge of the sea ice and, consequently, the catch records turned out to be a reasonable record of the extent of the sea ice every summer from 1931 until 1987, when commercial whaling there stopped.
De la Mare determined that the seasonal sea ice was constant until about 1954, when it began to shrink. It stabilized again beginning in 1973 and has remain roughly constant since.
“The sea ice is an indicator of climatic variations and I think the study shows that up through the 1950s or so, things were different,” said Bernhard Lettau, head of the ocean and climate program in the National Science Foundation polar programs office, which runs the U.S. Antarctic research effort. “It clearly was colder, for whatever reasons, in the early part of this century than in the later part of this century.”
