December
12, 2007-Record-breaking amounts of ice-free water have deprived the Arctic
of more of its natural "sunscreen" than ever in recent summers. The
effect is so pronounced that sea surface temperatures rose to 5 C above average
in one place this year, a high never before observed, says the oceanographer who
has compiled the first-ever look at average sea surface temperatures for the
region.
Such superwarming of surface
waters can affect how thick ice grows back in the winter, as well as its ability
to withstand melting the next summer, according to Michael Steele, an
oceanographer with the University of Washington's Applied Physics Laboratory.
Indeed, since September, the end of summer in the Arctic, winter freeze-up in
some areas is two months later than usual.
The extra ocean warming also
might be contributing to some changes on land, such as previously unseen plant
growth in the coastal Arctic tundra, if heat coming off the ocean during
freeze-up is making its way over land, says Steele, who is speaking Wednesday at
the American Geophysical Union meeting in San Francisco.
He is lead author of "Arctic
Ocean surface warming trends over the past 100 years," accepted for
publication in AGU's Geophysical Research Letters. Co-authors are physicist
Wendy Ermold and research scientist Jinlun Zhang, both of the UW Applied Physics
Laboratory. The work is funded by the National Science Foundation.
"Warming is particularly
pronounced since 1995, and especially since 2000," the authors write. The
spot where waters were 5 C above average was in the region just north of the
Chakchi Sea. The historical average temperature there is -1 C -- remember that
the salt in ocean water keeps it liquid at temperatures that would cause fresh
water to freeze. This year water in that area warmed to 4 C, for a 5-degree
change from the average.
That general area, the part of
the ocean north of Alaska and Eastern Siberia that includes the Bering Strait
and Chukchi Sea, experienced the greatest summer warming. Temperatures for that
region were generally 3.5 C warmer than historical averages and 1.5 C warmer
than the historical maximum.
Such widespread warming in those
areas and elsewhere in the Arctic is probably the result of having increasing
amounts of open water in the summer that readily absorb the sun's rays, Steele
says. Hard, white ice, on the other hand, can work as a kind of sunscreen for
the waters below, reflecting rather than absorbing sunlight. The warming also
may be partly caused by increasing amounts of warmer water coming from the
Pacific Ocean, something scientists have noted in recent years.
The Arctic was primed for more
open water since the early 1990s as the sea-ice cover has thinned, due to a
warming atmosphere and more frequent strong winds sweeping ice out of the Arctic
Ocean via Fram Strait into the Atlantic Ocean where the ice melts. The wind
effect was particularly strong in the summer of 2007.
Now the situation could be
self-perpetuating, Steele says. For example, he calculates that having more heat
in surface waters in recent years means 23 to 30 inches less ice will grow in
the winter than formed in 1965. Since sea ice typically grows about 80 inches in
a winter, that is a significant fraction of ice that's going missing, he says.
Then too, higher sea surface
temperatures can delay the start of freeze-up because the extra heat must be
discharged from the upper ocean before ice can form.
"The effect on net winter
growth would probably be negligible for a delay of several weeks, but could be
substantial for delays of several months," the authors write.
