An Alaskan Polar bear and cub. Photographer: Steven Kazlowski/Getty Images

When Rachel Obbard, a Dartmouth engineering professor, goes into the field in February to collect samples for her latest project on sea ice and climate, she will have to take an armed guard.

The gun-toting guide is required to keep polar bears at bay. Obbard said her team is going to Barrow, Alaska, to get samples from newly developed sea ice and become the first researchers to store and transport the complex material in its original form.

The ice, carefully preserved, will help climate scientists develop better models to find out what is happening to the weather at the Earth’s poles, Obbard said in a telephone interview this week.

Preservation is a problem that may be trickier than avoiding a hungry, 1,000-pound bear. You can’t just throw the ice in a freezer without ruining its intricate structure.

Obbard, 52, who’s on the faculty of Dartmouth’s Ice Research Laboratory, where research is sponsored by private sources and federal agencies, said she knows how she’s going to do it.

This requires a quick explanation of what makes sea ice so different from the cubes floating in your drink.

The temperature of a meter-long piece of sea ice can vary by as much as 40 degrees Celsius from one end to the other, Obbard said. The bottom of the sample that was in contact with the ocean is usually about minus 2 degrees, while the top, exposed to the air, can be whatever the ambient temperature is.

Salinity Levels

Ice also has a complex architecture from top to bottom. Near the surface, it’s mostly made up of freshwater with only 5 percent salinity, Obbard said.

“The old explorers used to melt it to make their tea,” she said.

New ice is quite porous, more so than old ice, and has a web of “brine channels” that resemble tubes, Obbard said. At the base these pathways get bigger and more interconnected, with a high concentration of sea life.

Transporting the ice all at one temperature or letting it warm and refreeze destroys that structure.

What Obbard and her crew have done is build a set of boxes they call ice mitts -- for “ice core extraction while maintaining in situ temperature transitions” -- that replicate the spread of temperatures. They’re designed to preserve the ice in the same condition as when it came out of the polar cap, until they get it back to the lab on Dartmouth’s campus in Hanover, New Hampshire...

...Obbard said her team, which will consist of a graduate student and one or two undergraduates, will travel the 3,275 miles to Barrow in February to collect the samples because that’s when the first-year ice will be nearing its thickest.

The sun will be rising for a few hours by that point, so they won’t be working in the dark, she said. The goal is to bring back 20 meters of ice cores.