Shedding Light on Melting Arctic Ice
Story posted January 03, 2006
Daniel McGrath '06 came to Bowdoin from White Plains, N.Y., with a love of the outdoors and a fascination with snow and ice. That started him on a circuitous route that led last spring to three weeks in Barrow, Alaska - about as far north as you can go and still be in the United States. McGrath, a geology and environmental studies major, worked at the Barrow Arctic Science Consortium with Donald Perovich, a renowned scientist from the U.S. Army's Cold Regions Research Lab in Hanover, N.H., who is studying how sunlight is transmitted through ice.
"When light shines on sea ice, it can either be reflected, absorbed or transmitted to the ocean," McGrath explained. "There's been a lot written about the absorption of light, but very little work done on the transmission."
The work itself was fairly simple: McGrath first measured the visible light at the surface of the ice. He then drilled a hole in the ice and inserted an instrument that measured the amount of light to 75 cm, every centimeter for the first 25 cm, then every 5 cm for the next 50 cm. The critical number is the ratio of light at the surface to light that is transmitted through the ice.
Understanding how and why that happens is important in understanding the balance of the earth's energy.
"I'm really interested in climate change, particularly polar climate change," he said. "More ice is melting in the summer than ever before. The polar ice has shrunk by nearly 30 percent since 1978 and it's not a linear decrease; it's an exponential change."
Light is transmitted at a higher rate when it passes through pure ice. Snow, on the other hand, reflects much of the sun's visible light. That also explains why snow looks white, and ice looks blue: "At the surface, snow appears white because it scatters light so well in the visible spectrum," McGrath wrote in his findings, "while ice and snow at depth have a blue hue because of preferential absorption in the longer wavelengths."
An interesting feature that McGrath pursued was the formation of a "scattering layer" on the surface of melting sea ice. Saltwater does not freeze solidly like fresh water. Instead, it contains pockets of brine within the ice. As spring approaches and temperatures rise, these pockets start to drain, leaving behind tiny air bubbles within the ice. In May, that drained brine layer is covered with snow. When the snow begins to melt in June, it leaves the ice surface exposed to sunlight. Rather than melt evenly, like solid, fresh-water ice, the drained brine layer begins to "decompose," meaning it breaks down into tiny granules. Those granules, which look very much like white snow, once again reflect much of the sun's light, inhibiting further melting. Even when McGrath shoveled the snow off the surface in May, a month earlier than would occur naturally, the ice formed a 3-cm scattering layer within two days.
"This self defense mechanism is essential to helping the sea ice survive the summer months and exist for more than one season," McGrath wrote.
Understanding how light is reflected by the snow and transmitted by ice and seawater also helped McGrath find his way around on the tundra and adjust to life on the edge of the Arctic Ocean. He noticed that the clouds over the ocean always looked gray and foreboding because the sunlight wasn't reflected back on them, while the clouds over the tundra looked white from the light reflecting off the snow. That was useful in a disorienting environment with 24-hour sunlight and virtually no landmarks.
"It was hard to get used to having 24 hours of sunlight," he said. "I never used to wear a watch, but now I never take it off."
McGrath said he also had to be aware that he shared the ice with polar bears.
His training at the research station included a brief lesson about how to use the loaded shotgun they carried at all times when they were out on the ice. He said he never saw a bear, but he and a field researcher once had to make a lengthy detour back to the head camp when they heard a bear was between them and the camp.
"We were in constant communication by walkie-talkie," he said. "The logistics coordinator told us a polar bear was very close to camp, so we headed out onto the tundra and came in through the back."
McGrath is spending his winter break at the University of Washington's Applied Physics Lab, comparing his findings with those of Bonnie Light, a physicist at the University's Polar Science Center.
McGrath, whose trip to Alaska was funded by a Doherty Fellowship, plans to continue his research in Maine this winter, measuring the light transmitted through ice on Sebago Lake, Cobbosseecontee Stream, the Kennebec River, and Merrymeeting Bay.
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