The last ice age began with gradual
cooling rather than a dramatic climate crash, according to initial
studies of the oldest Greenland ice core ever extracted. The core
arms climate modellers with robust data to predict our own
future.
The 3-kilometre-long cylinder of ice records, in
year-by-year detail, how Earth has warmed and cooled over the past
123,000 years. It also provides a unique snapshot of conditions
about 115,000 years ago, when Earth slid into the most recent ice
age. The international team of scientists on the North Greenland Ice
Core Project (NGRIP) drilled the ice core between 2000 and 2003.
Previous ice cores taken from central and southern
Greenland had shown that during the ice age, the North Atlantic area
experienced sudden temperature changes of 10 °C up or down in just a
few decades. But the cores were unreliable beyond 105,000 years ago,
and did not record how the ice age began.
My suspicion is they've got
something that's been there since ice first formed
on Greenland
Robert
Mulvaney British
Antarctic Survey in Cambridge,
UK
Scientists had speculated that the whole ice age might have
been triggered by an abrupt temperature change, perhaps when a rapid
warming period melted a substantial part of the Greenland ice sheet.
This in turn would have diluted the North Atlantic with fresh water,
damping down the ocean currents that bring warmer water to that area
and plunging the area into a deep freeze.
The NGRIP team now
suggests that the ice age started much more gradually and was not
linked to a fast glacial melt, even though the ice shows a hitherto
unrecognized warm spell 115,000 years ago.
The core also reveals that
the North Atlantic climate was stable during the last interglacial
period, despite being about 5 °C warmer than today. "This provides
an analogue for our possible future climate warmed by atmospheric
pollution," says Kurt Cuffey, a climatologist at the University of
California, Berkeley.
The study, published today in
Nature1,
was led by Dorthe Dahl-Jensen, a glaciologist at the University of
Copenhagen, Denmark. The team is now analysing the levels of
greenhouse gases such as carbon dioxide and methane in the ice to
see what effect they had on the climate cycles through the last
glacial period. Although ice ages are driven by our planet's
proximity to the Sun, greenhouse gases magnify the warming effect to
produce interglacial periods, such as the one we are experiencing
now.
"We also have visions of drilling a core even farther north
in 2007, to get even further back in time," says Dahl-Jensen, who
thinks it may be possible to extract ice more than 130,000 years
old.
North and south
The NGRIP ice is
positively young compared with the EPICA core, recently taken from
the Antarctic2, which stretches back 740,000 years. But
because it is so detailed, the Greenland core gives a fresh
perspective on our climate's history, explains Robert Mulvaney, a
glaciologist with the British Antarctic Survey based in Cambridge,
UK.
This fragment of plant matter is
thought to have lain underneath the Greenland ice sheet
for millions of years.
Rapid changes in climate don't occur to
the same extent in the Southern Hemisphere, and the ice in
Antarctica accumulates so slowly that it is impossible to make out
the bands formed in a single year in the EPICA core.
In contrast,
the bands of light and dark in the NGRIP ice are like a barcode.
White bands represent a spring season where dust is blown over the
ice sheet. Small air bubbles form around individual dust grains,
turning the ice milky. Each annual band is about 1 cm wide, enough
to tease apart seasonal temperature changes.
The NGRIP scientists
calculated these temperatures by measuring the amount of a heavy
form of oxygen (18O) in the ice. In a warmer climate,
more of this heavy-oxygen water evaporates from the seas before
being delivered to the ice sheet as snow.
Life at the
bottom
When the NGRIP team reached the bottom of the glacier last
year, they got a surprise - liquid water, formed by Earth's heat
melting the bottom of the glacier. "We were totally awed," says
Dahl-Jensen. Although the melting had removed a few thousand years
of climate record, it has also given the project an exciting new
twist.
The team has found a fragment of plant matter - possibly
bark - in the water, which may have lain under the ice for millions
of years, since it was warm enough for trees to grow.
"It is
unlikely to be contamination," says Mulvaney. "My suspicion is that
they've got something that's been there since ice first formed on
Greenland." Precise dating of the object could reveal when Greenland
was first covered by ice, he adds, giving a glimpse even further
back into our planet's climate history.
The researchers also found
DNA in the water, although they have not been able to rule out
contamination from the drilling equipment. They are now studying a
fresh sample of this water taken with sterilized equipment for signs
of ancient life - or perhaps even a community of microbes that is
still living there today.
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My suspicion is they've got
something that's been there since ice first formed
on Greenland 
