August 25, 2005
Thursday

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, says a team of researchers led by University of Arizona professor Jonathan Overpeck. The group, which included two Boulder scientists, published its findings in this week's Eos, the weekly newspaper of the American Geophysical Union.

The impacts will go well beyond Santa's workshop needing pontoons. 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 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.

Changes already are apparent.

Sens. Hillary Clinton, D-N.Y., and John McCain, R-Ariz., toured Alaska last week 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 due to receding sea ice.

Future impacts 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 says.

"Which wins, we don't know," Serreze said. "That's one of the big mysteries."

Marika Holland, a National Center for Atmospheric Research scientist and co-author of the new paper, uses climate models to understand the polar climate. She said some model runs are showing abrupt changes in sea ice that would reverberate throughout the system.

"I think it's coming out of numerous studies that the Arctic is changing rapidly and in ways we haven't seen for a long time," Holland said.

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.

Stroeve uses satellite data to monitor Arctic sea-ice cover, which reaches its annual minimum in September.

"So far, the numbers are pretty disturbing," Stroeve said. "The rate of change has taken us by surprise a little bit. I think it's changing a lot quicker than we expected."

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