The amount of sea ice in Arctic waters is shrinking, on average, by about 14,000 square miles a year, an area larger than Maryland and Delaware combined, probably because of global warming caused by human activity.
That is the potentially controversial conclusion of a new international study that combined 46 years of data documenting the declining extent of Northern Hemisphere sea ice and analyzed the information using two leading computer programs that simulate world climate.
At issue is one of the most ominous questions in science and environmental policy: Is the disappearance of so much ice the result of ordinary natural variations in Arctic conditions? Or is it the byproduct of global warming caused by civilization's release of "greenhouse" gases into the atmosphere?
The results indicate less than a 2 percent probability that the melting of the past 20 years is due to normal climate variation. That is, a decline that large would be seen only about two out of 100 times in computer models that calculate the long-term interactions of water, air, land, sunlight and the like to simulate the way the world's climate changes naturally over time. The authors further found only a 0.1 percent chance that the whole 46-year trend could have occurred in the course of natural fluctuations.
But when they compared the same data to the output of models incorporating recent greenhouse-gas and aerosol emissions, the computer and observed results were almost identical.
"This strongly suggests that the observed decrease in northern hemisphere sea ice is related to [human-caused] global warming," wrote the team of nine scientists headed by Konstantin Y. Vinnikov of the University of Maryland in today's issue of the journal Science.
Some other researchers have suggested that the shrinking Arctic ice may be related to global warming, but the new study is the first to bring together five independent data sets and show that the trends in each are extremely similar. The computer analysis technique the authors used is familiar in other global warming studies, but has never before been applied to Arctic ice.
The amount of the measured ice decline has not been in dispute. Whether the shrinkage is unnaturally large is a different matter. Even 50 years is a very short time in terms of global climate variation, and there are no measurements going back for thousands of years.
So the only possible comparison is to the results of computer models. The credibility of the result "comes down directly to one question: How good is the model at simulating natural variability?" said veteran ice researcher and co-author Claire L. Parkinson of NASA's Goddard Space Flight Center in Greenbelt. And the team used models that are "very well known and reputable," she said.
The analysis is "as careful and robust a piece of work as you can do," said Jerry Mahlman, director of the National Oceanic and Atmospheric Administration's Geophysical Fluid Dynamics Laboratory in Princeton, N.J. Many climate researchers regard the laboratory's climate model, which was used in the new study, as the world's most sophisticated.
Nonetheless, "I am not convinced that the natural variability of ice extent simulated by the model is realistic," said Richard E. Moritz, director of the multiyear SHEBA (Surface Heat Budget of the Arctic Ocean) Project Office at the University of Washington.
The other computer model, devised by the Hadley Centre research group in England, showed approximately similar results.
The Arctic ice covers an area roughly the size of the United States, or about 4 million square miles, and ranges in thickness from an inch in "seasonal" ice to 10 feet or more in "multiyear" formations that do not melt over the summer. It has a profound effect on heat exchange; open water absorbs as much as 100 times more solar energy than ice. The amount of ice also affects density, salinity and flow of sea water in much of the Atlantic and elsewhere. In theory, a substantial change in ice extent could prompt severe alterations in ocean circulation, and hence in weather.
The new study uses two kinds of data on Arctic sea ice extent. One consists of ground-based measurements beginning in 1953. The other covers the 19.4 years from 1978 to 1998 when comprehensive satellite images became available.
Both sources clearly show the extent of total sea ice decreasing by about 3 percent per decade. Another paper in today's issue of Science, by a joint Norwegian and Russian team, reports that multiyear ice shrank by 14 percent in the 20 years from 1978 to 1998.
Mahlman and many other computer model proponents don't find that surprising, since the Princeton laboratory's model "essentially nails the amount of [global] warming in the 20th century, especially the fast run-up in the past two decades," he said. So naturally, "it also predicts the sea ice" decline.
That is not, however, a universal opinion. Moritz finds it suspicious that the model shows the same amount of sea-ice variation over the 20th century as it does for the last 5,000 years. It is almost always the case, Moritz notes, that any climate trends over a long term show much more variability than in a short term. It is possible, he believes, that the Princeton model "underestimates the natural variability" because relatively little is known about the physics of sea ice and the way heat is transported in the Arctic.
Parkinson said the team is planning to compare the observations with other models.
Her group also used the climate models to extrapolate ice loss into the next century. Both estimates indicated a decline of 20 percent or more from current levels, which are already approximately 8 percent below the measured ice extent early in this century.
If that were to occur, no one knows what impact it might have on the region, or the entire Northern Hemisphere. Certainly it would affect Arctic wildlife, since many marine organisms that live under the ice are the primary energy source at the bottom of the food web.
In addition, "if the Arctic Ocean quit being a heat sink, it would change the balance of heat transfer between the tropics and the pole in ways we can't predict right now," said Michael Ledbetter, director of the Arctic System Science Program at the National Science Foundation. "But I don't know how many highly technological economies would want to go through that experiment."