thinning of Arctic ice found
Tuesday, November 16, 1999
Courtesy of ENVIRONMENTAL NEWS NETWORK
By Robinson Shaw
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.
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.
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 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.
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.
Ice sheets in
the Beaufort Sea thinned about three feet since 1958, nine years
after this photo was taken.
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.