* Warming Arctic affects world weather
* Greenhouse emissions reverse natural cooling trend
By Deborah Zabarenko, Environment Correspondent
WASHINGTON, Sept 3 (Reuters) - Climate-warming greenhouse gas emissions pushed Arctic temperatures in the last decade to the highest levels in at least 2,000 years, reversing a natural cooling trend that should have lasted four more millennia.
Carbon dioxide and other gases generated by human activities overwhelmed a 21,000-year cycle linked to gradual changes in Earth's orbit around the Sun, an international team of researchers reported on Thursday in the journal Science.
"I think it really underscores how sensitive the Arctic is to climate change ... and it's really the place where you can see first what's happening to the (climate) system and how the rest of the Earth will or might follow," David Schneider, a co-author and a scientist with the U.S. National Center for Atmospheric Research said in a telephone interview.
The big cool-down started about 7,000 years ago, and Arctic temperatures bottomed out during the so-called "Little Ice Age" that lasted from the 16th to the mid-19th centuries, dove-tailing with the start of the Industrial Revolution.
This cooling trend was caused by a characteristic wobble in Earth's orbit that very gradually pushed the Arctic away from the Sun during the northern summer. Earth is now about 620,000 miles (1 million km) farther from the Sun in the Arctic summer than it was 2000 years ago, said Darrell Kaufmann of Northern Arizona University.
This cooling should have continued through the 20th and 21st centuries and beyond as the 21,000-year cycle played out. This latest research confirms that it hasn't.
EARTH'S AIR CONDITIONER
"If it hadn't been for the increase in human-produced greenhouse gases, summer temperatures in the Arctic should have cooled gradually over the last century," Bette Otto-Bliesner, a co-author from the National Center for Atmospheric Research, said in a statement.
What happens in the Arctic doesn't stay there, since it is among the world's biggest weathermakers, sometimes called Earth's air-conditioner. As Arctic sea ice melts in summer, it exposes the darker-colored ocean water, which absorbs sunlight instead of reflecting it, accelerating the warming effect.
Arctic warming also affects land-based glaciers; if these melt, they would contribute to a global rise in sea levels.
Warming in this area could also thaw frozen ground called permafrost, sending methane, a powerful greenhouse gas, into the atmosphere.
Climate scientists have long known that Earth wobbles in its orbit, which affects how much sunlight reaches the Arctic in the summer. This is the first time a large-scale study has tracked decade-by-decade changes in Arctic summer temperatures this far back in time.
To figure this out, researchers looked at natural archives of temperature -- tree rings, ice cores and lake sediments -- along with computer models, which tallied closely with the natural record.
Average summer temperatures in the Arctic have increased by about 3 degrees F (1.66 degrees C) from what they would have been had the long-term cooling trend remained intact. (Editing by Todd Eastham)