* Research offers response to climate skeptics
* Rising temperatures followed release of greenhouse gas
By Deborah Zabarenko
WASHINGTON, April 4 Planet-warming carbon
dioxide emissions - similar to those caused by burning fossil
fuels and other human activities now - helped heat the planet
and end the last ice age some 11,700 years ago, scientists
reported on Wednesday.
In a finding that offers a response to those skeptical about
human-caused global warming, researchers from Harvard
University, Oregon State University and other institutions
reported in the journal Nature that rising temperatures followed
increases in carbon dioxide.
Climate scientists had long suspected this was the case, but
the geologic record was murky. Earlier studies looked at air
bubbles trapped in ancient ice in Antarctica that revealed
carbon dioxide levels in the late Pleistocene Epoch, some 20,000
to 10,000 years ago, the period when the ice age tapered off.
In these previous studies, it appeared that carbon dioxide
levels rose after temperatures did, leading climate change
skeptics to question whether carbon dioxide was a driver of
global warming, then or now.
This latest study managed to look at ice cores and samples
of undersea sediments - the deeper the dig, the older the
sediment, with biochemical information that indicates
temperature variation through time - at 80 sites around the
In Antarctica alone, the earlier studies were confirmed:
temperatures there increased before carbon dioxide rose. But
globally, a rising amount of carbon dioxide in the air preceded
temperature change, according to this new report.
The rise in carbon dioxide over the end of the ice age was
significant, from about 180 carbon dioxide molecules for every
million in the atmosphere to 260, a measurement called parts per
million or ppm, according to study author Jeremy Shakun.
THE BIG MELT
This increase took place over about 7,000 years, Shakun told
a telephone news briefing. By contrast, the current level of
atmospheric carbon dioxide is 392 ppm, a rise of about 100 ppm
in the last century or two, he said.
"In this century, we're probably going to be going up about
100 (ppm) more," Shakun said, but added that Earth probably
won't feel the total impact from this carbon dioxide rise for
"The system has a lot of inertia to it," he said. "To warm
up the oceans takes quite a while, and we've also got ice sheets
and you can't melt an ice sheet in 100 years."
But while carbon dioxide pushed temperatures up to
accelerate the ice age's end, that was not the initial cause.
Instead, the big melt was first prompted by a periodic wobble in
the Earth's axis, the scientists said.
At some points in the wobble, the Northern Hemisphere leans
slightly closer to the sun and this occurred at the beginning of
the end of the Pleistocene, when ice sheets covered much of what
is now North America and Europe.
That slight sun-ward tilt melted those northern ice sheets
within a few hundred years, pushing global sea levels up by
about 33 feet (10 metres), or by more than the total melting of
the ice covering Greenland now would do, said co-author Peter
Clark of Oregon State University.
Greenland's ice sheet covers most of the island, over
656,000 square miles (1.7 million square kilometres), three
times the size of Texas. Summer melt of this ice sheet increased
by 30 percent from 1979 through 2006, and reached a new record
in 2007, according to the U.S. National Snow and Ice Data Center
Because ice sheets are made of compacted snow, they produce
fresh water when they melt, and the gush of fresh water into the
salty North Atlantic altered ocean chemistry enough to shut down
the Atlantic Merdional Overturning Circulation, sometimes called
the conveyor belt, which typically sends heat from the tropics
northward, moderating northern Europe's climate.
When the conveyor belt slowed or stopped, cool temperatures
stayed in the north and warmth stayed in the south, letting the
Antarctic get warmer. That warming trend may also have shifted
the winds and melted sea ice, drawing carbon dioxide out of the
deep ocean, where quantities of it are stored, Shakun said.
(Editing by Philip Barbara)