Global warming could melt summer Arctic sea ice by the end of the century, scientists say, making for an ice-free North Pole for the first time in more than a million years.
With no apparent natural mechanisms to maintain the summer sea ice, the question is no longer whether such melting could happen, but when and with what impacts, say researchers led by University of Arizona professor Jonathan Overpeck.
The group published its findings in last week's Eos, the weekly newspaper of the American Geophysical Union.
The Arctic plays a major role in the Earth's climate system, and sea-ice cover drives much of its current behavior. When highly reflective ice melts, darker oceans and land absorb more heat, creating a positive feedback loop of melting.
"If we lose the sea ice in the summer, there will be strong downstream effects," said Mark Serreze, a senior research scientist with the Cooperative Institute for Research in Environmental Sciences and one of the paper's co-authors.
"The Arctic is the heat sink of the Northern Hemisphere, and the equator the heat source. If we change the nature of that Arctic heat sink, we radically alter that system."
CIRES is a collaboration between the University of Colorado and the National Oceanic and Atmospheric Administration.
Serreze said that some climate models predict complete summer sea-ice melting by 2070 but that the recent trend toward extreme melting could push the date up to 2040. And changes already are apparent.
Sens. Hillary Clinton, D-N.Y., and John McCain, R-Ariz., toured Alaska recently to view firsthand such warming effects as melting permafrost and beetle-infested forests. Researchers have found that polar bears and seals are losing weight from shorter feeding seasons because of receding sea ice.
Future effects could have broader reach. Flushing sea ice into the Arctic Ocean could slow thermohaline circulation, the ocean conveyor belt that moves warm equatorial water into the North Atlantic and saves Europe from Canadian winters.
But the summer melt also could strengthen the conveyer belt, Serreze said.
"Which wins, we don't know," Serreze said. "That's one of the big mysteries."
Arctic sea ice appears to be in rapid retreat already. Julienne Stroeve, a scientist with the University of Colorado's National Snow and Ice Data Center, said 2005 could surpass 2002 as the lowest sea-ice summer in a century.
