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. Last Updated: 07/27/2016

Polar Ice Cap 40% Thinner After 4 Decades

The great ice cover that stretches across the top of the globe has become about 40 percent thinner than it was two to four decades ago, scientists have found after analyzing data collected by nuclear submarines plying the Arctic Ocean.

Since the ice is already floating in the water, no rise in sea level has accompanied its melting, as happens when land-based glaciers shrink. But the finding raises anew the question of whether climatic changes observed in recent decades are the result of the atmosphere's natural behavior, or a global warming caused by human activity, or some combination of the two.

The scientists found in a new study that during the period 1958 through 1976, the average thickness of the Arctic sea ice was about 3 meters. Over the period 1993 through 1997, it was about 2 1/2 meters. In the 1990s, according to the researchers, the thinning appeared to be continuing at a rate of about 10 centimeters a year.

"It's a startling result," said Dr. D.A. Rothrock, an oceanographer at the University of Washington in Seattle who is the leader of the study, which is being published in the December issue of the journal Geophysical Research Letters. The research involved measurements of sea ice thickness made by upward-looking sonar aboard naval submarines operating under the ice sheet. The first period of data began in 1958 with the first nuclear submarine, the USS Nautilus, and concluded with a cruise by HMS Sovereign in 1976. The second data set was collected by American vessels from 1993 through 1997. Rothrock and two colleagues, Y. Yu and G.A. Maykut of the University of Washington, compared data from the two periods at 29 points where the courses of submarines in the 1990s intersected with the courses of those in the earlier period.

Substantial evidence has emerged that the climate of the Arctic and sub-Arctic region is warming, at least in some seasons. The area covered by sea ice has diminished, and the duration of the cover has shortened in many places. Mountain glaciers in Alaska have shrunk, as has the Greenland ice cap.

The average surface temperature of the earth has risen by more than one half a degree Celsius during the last century, and by several times that amount in northern regions. Mainstream scientists expect that the global average will rise by about another two degrees by the year 2100 if emissions of heat-trapping waste industrial gases like carbon dioxide continue at present levels. By comparison, the earth's surface temperature has risen by three to five degrees since the depths of the last ice age, 18,000 to 20,000 years ago.

Rothrock believes that a shift in prevailing natural patterns of atmospheric circulation in the Arctic may be responsible for the warmer north and the thinning sea ice. Other scientists say that the shift in natural patterns may have been touched off or enhanced by global warming, and Rothrock agrees that it is possible a warming climate could be related in some way.

The key is a seesaw sort of winter circulation pattern in which prevailing westerly winds in the Arctic, sub-Arctic and North Atlantic vary between two basic states, one stronger and one weaker. In this Arctic oscillation, as it is called, one pattern or another has been dominant for decades at a time before switching to the opposite mode.

In one mode, which has been dominant since about the mid-1970s, stronger westerly winds blow surface water away from the Arctic and toward the southeast. This, said Rothrock, could carry cold surface water away from the Arctic, bearing chunks of sea ice with it. Other observations have established this.

As the chunks depart, areas of open water enlarge, presenting more ice edges to melt should a source of warming appear. One source, said Dr. Drew Shindell, an atmospheric physicist at the NASA Goddard Center for Space Studies in New York City, is warm, deep water that flows upward to replace the departing surface water that has been chilled by the winter. In addition, said Rothrock, the stronger winds may also bring warmer air to the Arctic, furthering the melting.

Computer modeling studies, said Shindell, indicate that the present situation is "not a natural thing." According to the models, the buildup of heat-trapping greenhouse gases is largely responsible. The reason, he said, is apparently that one effect of global warming is to increase the temperature contrast between the tropics and polar regions at critical altitudes, thereby strengthening the westerly winds. He suggests that this pattern will prevail for the next 30 or 40 years. Whether the present state will persist or the Arctic oscillation will swing back to its alternate condition is going to be "extremely interesting," Rothrock said.