Weather clues in ancient bubbles

By STEVE CONNOR in London

Scientists are preparing to study tiny bubbles of air trapped inside the Antarctic ice sheet for more than 800,000 years as part of an ambitious research programme to investigate the history of the Earth's climate using the frozen time capsules of the North and South poles.

The air bubbles will provide information on temperature and atmospheric composition at the time when the ice formed. The findings could shed light on the sudden changes in the global climate that have in the past threatened the planet's ecosystems.

Polar scientists from 10 European nations said at the weekend that they had now drilled the deepest hole into the Antarctic's oldest ice. At 3201m, more than two miles, the ice core is twice as deep and old as the previous record.

Professor Heinz Miller, a researcher at the Alfred Wegener Institute in Bremmerhaven, Germany, and spokesman for the European Project for Ice Coring in Antarctica (Epica), said that the drill had another 100m to go before it hit the bedrock. When completed, the project should be bringing ice more than a million years old to the surface, he said.

"We need to learn from our climate history. Ice cores tell us about temperature, precipitation and atmospheric composition at a time long ago," Professor Miller said.

The ice had shown, for instance, that the world was 20C cooler 11,000 years ago.

Epica, begun in 1995, has cost the European Union about US$10 million ($19.4 million) and involves drilling at two sites in Antarctica so that the analysis of one core can be cross-checked with a second core to calibrate the dating procedure.

The deepest hole is at Dome C, site of one of the thickest layers of ice sheet on the continent. Five years were spent drilling into ice that was 70,000 years old. Two more years' drilling hit ice more than 800,000 years old - at that depth the ice is far more compacted and each metre of drilling penetrates many more seasons of snow fall.

When the ice is brought to the surface, it is stored for months at sub-zero temperatures so that it can "relax", allowing trapped air to move out from the ice crystals to the spaces in between, where it forms minute bubbles.

Similar ice sheet cores from Greenland have shown that the Earth has had rapid periods of warming and cooling, where average global temperatures have changed by as much as 10C in less than 30 years.

Professor Nicholas Shackleton, of Cambridge University, is comparing the ice core data with findings from cores taken from the muddy sediments of the North Atlantic seabed.

He said the North Atlantic cores do not go back as far as those from the Antarctic but they can tell scientists, with greater accuracy, about global temperatures and atmospheric composition, such as the amount of methane given off by the decomposition of organic matter in tropical wetlands.

High levels of methane in the sediment indicate a warm, wet climate and low levels a dry, cold period. Sediment cores drilled off Florida and Greenland both indicated rapid periods of cooling and warming in support of the ice-core data - indicating that such periods were a global rather than a regional phenomenon, Shackleton said.

"Greenland and Florida both experienced periods of warming and cooling in parallel. Occasionally we see material melted out from icebergs, which gives an indication of how significant these events were."

A consensus is emerging that these dramatic climate changes were brought about by sudden shifts in ocean currents. The fear is that such changes could also be triggered by man-made global warming.

- INDEPENDENT