Not long ago, researchers drilled a hole deep into the Greenland Icecap, one of the deepest and oldest glaciers in the northern hemisphere.

A cylinder of ice more than three kilometres long was pulled from the hole in segments and distributed among a gaggle of scientists, all eager to get their hands on ice formed tens of thousands of years ago.

Ice cores from the ancient depths of glaciers contain a wealth of information, says Erik Blake. The Whitehorse geophysicist's company, Icefield Instruments, builds specialized drills and other equipment for investigating icefields, in co-operation with the Geological Survey of Canada.

Blake's ice-coring drills have been used on icefields from the Canadian Arctic to Antarctica. One of his drills, designed for portability, is only three metres long and weighs about 200 kilograms.

"That drill was used in the Himalayas last summer, at an altitude of 6,500 metres on Mount Everest," says Blake.

Glacial ice is a frozen record of the atmosphere's past, often dating back hundreds of thousands of years. Properly retrieved and tested, the ice can tell scientists about ancient climates, atmospheric conditions, and even catastrophic events like volcanic eruptions, Blake says.

In fact, the traces of volcanic eruptions are so easy to detect that they can be used in dating the ice cores.

Glaciers are formed by hundreds or thousands of years of successive snowfalls. The snow settles, compacts, and spreads under later snowfalls. Along the way, it turns into ice. The further down you go in the glacier, the older the ice.

A common way of establishing the chronology of the ice core is to look for layers of ash deposited after volcanic eruptions, Blake says. The ash makes the ice more acidic and enables it to conduct electricity more easily, so it can be detected by instruments that measure electrical conductivity.

"There's a really good record for about a thousand years of volcanic eruptions in Iceland," says Blake. "The Vikings kept accurate records."

Records from elsewhere extend the volcanic history of the northern hemisphere to about two thousand years before the present.

Other techniques are required for older ice. One approach that works well in Canada is to look for traces of salt, or sodium chloride, in the ice. Winter storms over the oceans kick up salt spray that drifts into the atmosphere, Blake says.

"So the winter snow has more chlorine than the summer snow, and that makes a nice seasonal pattern."

Another approach is to look for the balance between two naturally occurring forms of oxygen in the ice, Blake says. Relating that balance to the standard for ocean water gives a clear indication of the temperature of the snow when it fell on the icefield long ago.

The pattern of changing snow temperature reveals both seasonal changes and larger-scale climate change, says Blake. A couple of recently-sampled cores from the Greenland icefields indicate that, in the past, radical shifts in climate have sometimes happened very quickly, within the space of a few decades.

Blake says some scientists are analyzing the gases trapped in bubbles in the ice to track past carbon dioxide levels in the atmosphere. Others look for ions of sulphate, nitrate, or sodium.

"All these things give you different bits of information."

For example, a high level of calcium carbonate in ice layers indicates a dry period, Blake says. Calcium carbonate is part of natural dust, so its presence in the ice means the climate was dry enough to contribute plenty of dust to the atmosphere.

Using ice cores to build a picture climate in the past is also helping scientists develop ways of predicting future climate change, says Blake. By testing computer-based models of climate change against the evidence of the past, they can determine how accurate the models' predictions are.

"That's one of the big pushes for doing ice cores right now," says Blake.

For more information, contact Environment Canada, Whitehorse.