Arctic ice cap: It's thinner than we thought

Scientists mining old data have discovered "a major climatic signal." The Arctic Ocean's ice cap has shrunk more dramatically than anyone realized.

Comparing ice thicknesses measured by submarines between 1958 and 1976 with measurements made this decade, scientists found that, on average, the cap is 4.3 feet thinner. It has lost some 40 percent of its volume.

Several earlier studies had indicated widespread thinning. But they didn't give a comprehensive picture. Now a team of scientists at the University of Washington in Seattle has sketched that picture.

The results, to be published in the Dec. 1 edition of Geophysical Research Letters, are another indicator of substantial changes in the Arctic that could impact global weather. And they also present an intriguing mystery.

Because the data essentially cover the entire ice cap, they indicate "there is something fundamental going on," says Gary Maykut, who collaborated with D. Andrew Rothrocka and Yanling Yu on the study. This is not just melting around the edges. It's not a balanced situation where ice lost in one region is made up by ice formation in another area.

Yet "there's no smoking gun here," says Dr. Maykut. There's nothing to tie the ice loss to any man-made or natural global warming. Climatologists can only speculate as to whether it's part of a natural Arctic climatic cycle that some researchers have identified.

For these reasons, Maykut says a more detailed study of the data is vital. Several dozen Arctic scientists made that plea two years ago to the US National Science Foundation, citing factors such as a 2 degree F or more warming in large parts of the Arctic Ocean. That includes incursions of relatively warm Atlantic water well beyond its normal seasonal range.

Indeed, the behavior of Atlantic currents is of critical importance, scientists say. Climatologists worry about changes in current partly because of the way the Arctic Ocean interacts with the North Atlantic, a region of huge climatic significance.

In the normal seasonal cycle, cold Arctic water pushing into the Atlantic sinks and flows southward. Other currents flowing nearer the surface bring warmer water northward. It's a key part of the large-scale ocean circulation that drives our present climate.

If the Arctic were to lose substantial parts of its ice cover, though, the cold, dense water flow might be cut off or replaced by less-dense water that would not sink, upsetting the climate throughout much of the Northern Hemisphere.

Arctic research has picked up in this decade through ship-based, ice-camp-based, and undersea explorations. The US Navy's Scientific Ice Expeditions (SCICEX), which sent research submarines under the ice cap, have played a major role.

The Seattle team used SCICEX data gathered between September 1993 and September 1997 in their study.

Release of formerly secret US and Russian data also boosts Arctic research. This gave the Seattle team the crucial data from 1958 through 1976.

Meanwhile, Maykut warns against making too much of the findings just yet. The researchers lumped the early data together to get an average picture of the ice cap. They did the same with the SCICEX data. He says: "Basically, what we have are two data points [with nothing in between]. And now we say, 'Wow! Something's happening.' "

That's not enough to uncover trends or causes. It's just enough to justify further research. What's needed for that research, the team says, is for the US Navy to release the detailed ice-thickness measurements its submarines have made over the past 40 years.

What's been available so far are anonymous averages for submarine runs. These hide the identity of which submarine took which measurements at specific points and at specific times. If Russian technicians had that information, they might be able to identify the acoustic signature of specific American submarines, some of which are still in service.

That's the kind of security issue that has kept the detailed ice-thickness data locked up. Maykut says he thinks the issue is becoming obsolete, especially as older submarines leave service, and he expects more of the crucial data to become available over the next few years.

(c) Copyright 1999. The Christian Science Publishing Society