Recent eyewitness reports of open water from melting ice at the North Pole have prompted climatologists and other scientists to make a closer study of satellite imagery and other observations of northern sea ice, past and present.
They have found substantial evidence that, on average, Arctic temperatures in the winter have risen 11 degrees over the past 30 years, and in the late 20th century were the warmest in four centuries. Data also show that the ice pack over the entire Arctic Ocean has in recent decades been shrinking in area and thickness.
But climatologists said they were still not sure whether diminishing polar ice reflected some short-term natural cycle or was a wake-up call of possibly drastic climatic consequences of an industrial civilization's release of heat-trapping gases.
Specialists in Arctic climate and satellite observations agreed that the existence of patches of open water at the North Pole was not as surprising as suggested in a news article on Aug. 12 in the New York Times, which was based on the descriptions and interpretations of two scientists who had just visited there.
"There seems to be a pretty coherent picture of change going on now in the Arctic," said Dr. Mark Serreze, a climatologist at the National Snow and Ice Data Center in Boulder, Colo. "But there's nothing to be necessarily alarmed about. There's been open water at the pole before. We have no clear evidence at this point that this is related to global climate change."
The ice covering most of the Arctic Ocean, several researchers said, is broken by long, wide cracks and gaping holes in many places, sometimes even at the pole, and especially in the summer. During a typical summer, 90 percent of the high Arctic region is covered with ice, with the remaining 10 percent open water. This has probably been true for centuries, they said, the result of motions in the ice sheet caused by winds and the ocean currents, as well as higher temperatures.
Serreze said that an examination of satellite images from July 15 showed what looked like a large body of ice-free water about 10 miles long and 3 miles wide near the pole. On Friday, the snow and ice data center made available two images taken by a NASA satellite named Terra on July 26, about the time visitors on the Russian icebreaker Yamal were in the polar region. Although the pole was obscured by clouds, the images revealed how fractured the ice was near the pole and the many large patches of open water over much of the area.
"The fact of having no ice at the pole is not so stunning," said Dr. Claire L. Parkinson, a climatologist at the Goddard Space Flight Center in Greenbelt, Md. "But the report said the ship encountered an unusual amount of open water all the way up. That is reason for concern."
Parkinson said that her examination of satellite data since the 1970s revealed that the Arctic ice cover had been retreating on an average of one-quarter of a percent a year. But there are fluctuations up and down; the retreat was striking in the 1980s, then rebounded somewhat in the '90s. "So we are very reluctant to make projections into the future" based on only two or three decades of observations, she said.
In a report in February, Parkinson said that "if trends toward shortened sea ice seasons and lesser sea ice coverage continue, this could entail major consequences to the polar and perhaps global climate, and to the lifestyles and survivability of selected Arctic plant and animal species."
Dr. James J. McCarthy, a Harvard oceanographer who was one of the sources of the New York Times article, said he would not argue with critics who said that open water at the pole was not unprecedented.
"What was really unusual was that over a period of two weeks we never had a day of what would be considered normal ice," McCarthy said. "When we reached the pole and found open water, that simply punctuated what we were seeing everywhere. These were conditions that did not seem representative of a transient phenomenon."