By Robinson Shaw

The USS Pogy surfaces through an Arctic ice flow at sunrise, Nov. 5, 1996, during a 45-day research mission to the North Pole. A portion of the submarine's torpedo room was converted into laboratory space, but the ship remained a front-line warship.

Scientists analyzing decades of data from Arctic Sea ice recently reported a significant reduction in the thickness of the ice during the last decade. The scientists found a decrease in sea ice all across the Arctic Ocean and that corresponds to previously reported evidence that the Arctic climate is warming, according to Dr. D. Andrew Rothrock of the University of Washington and colleagues. 

A report on the data, Thinning of the Arctic Sea-Ice Cover, will be published in the Dec. 1 issue of Geophysical Research Letters. 

The Scientific Ice Expeditions program, which consisted of six extended voyages, acquired the data using nuclear submarines. This study analyzed data from three autumn cruises: USS Pargo in 1993, USS Pogy in 1996 and USS Archerfish in 1997. 

The average draft of the sea ice (its thickness from the ocean surface to the bottom of the ice pack) has declined by 4.3 feet, or 40 percent, since the first measurements were made in 1958, said the scientists. 

The SCICEX cruises covered most of the deep Arctic Ocean basin. Measurements of the sea ice thickness showed a perennial ice cover of three to nine feet in mean draft, which was considerably thinner than previous estimates. The earlier data, used for comparison, began with the first nuclear submarine, USS Nautilus, in 1958 and continued through a cruise of HMS Sovereign in 1976. 

All of the 29 sites compared between the earlier cruises and those of the 1990s showed a decline in ice thickness, according to Rothrock. In certain areas, such as the Nansen Basin and the eastern Arctic, the thinning is more than five and a half feet. In areas known as the Beaufort Sea and Chukchi Cap, thinning is around three feet and at the North Pole and in the Canada Basin the measured decrease is between those extremes. 

This is not an instance of ice thinning in one area while thickening in another, which could be induced by a change in surface wind patterns, the scientists point out. 

Ice sheets in the Beaufort Sea thinned about three feet since 1958, nine years after this photo was taken.

The thinning of Arctic ice that has already occurred is "a major climatic signal that needs to be accounted for in a successful theory of climate variability," according to the scientists. To help fill the gaps between the earlier and more recent submarine observations, they call for the public release of other ice thickness data gathered by submarines over the past 40 years, which they believe would be "of immense help" in understanding the cause of thinning. 

The available data are insufficient to provide answers about the cause of the ice loss, said the researchers. They suggest several hypotheses about the flow of heat from the ocean itself, the flow of heat from the atmosphere as well as from short-wave radiation. Other possible avenues to explore include the amount of precipitation and snow cover in the region and ice movement. 

A related and important topic for future research is whether ice volume has reached a minimum in the past few decades or whether the decline will continue into the future. 

Data from the earlier cruises were adjusted for the time of year they took place to correspond with the autumn data acquired in the 1990s. There is little data available from the 1976-1993 period. The researchers estimate the overall error in measurement is less than one foot. 

The study was funded by the National Science Foundation, the Office of Naval Research and NASA.