BioEd Online

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.

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.

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.