Sea Ice Reduction May Be Another Climate Change Clue

As I was looking over the past few Science Forums, I noticed each had something to do with climate change. I didn't consciously decide to go on a global warming binge; it just seems we have unique indicators here---such as stressed upland spruce trees and receding permafrost ---that hint our planet may indeed be getting warmer.

Another far northern (and southern) heat gauge is sea ice that forms in the Arctic Ocean and adjacent seas and around Antarctica. According to a report in Nature, Norwegian scientists recently found what they believe to be a significant reduction in the amount of sea ice in both the Arctic and the Antarctic.

By analyzing satellite photos of sea ice at the top and bottom of the globe, researchers saw the area of water covered by sea ice in the Arctic dropped by 2.5 percent from 1978 to 1987, and by 4.3 percent from 1987 to 1994. The scientists, Ola M. Johannessen, Martin Miles and Einar Bjorgo, studied satellite images at the Nansen Environmental and Remote Sensing Center in Bergen, Norway.

Sea ice forms when salty ocean water freezes. Because salt water needs temperatures slightly colder than 32 degrees Fahrenheit to freeze, sea ice tends to congregate around the colder regions of the planet. The amount of sea ice floating about the poles varies with the season. In the arctic late winter, sea ice covers about 10 million square miles on top of the globe, while in summer the ice pack shrinks to about 6 million square miles, according to Martin Jeffries, an associate research professor of geophysics at the Geophysical Institute.

The sea ice surrounding Antarctica, where Jeffries often studies, varies much more drastically according to season because the sea ice isn't surrounded by land. At the end of the antarctic winter in September, sea ice surrounds the continent in a thick belt covering about 12 million square miles. At summer's end, sea ice disappears from much of Antarctica's coast and concentrates in pockets that total about 2 million square miles.

The arctic sea ice cover isn't a solid, unmoving shelf of ice. Jeffries said if you plant a flag on the sea ice at the North Pole, that same flag will be miles away days later. Large cracks in the ice that sometimes evolve into huge tracts of open water are acted upon by high winds, transforming sea ice into a large, moving, jigsaw puzzle with a couple of pieces missing.

It's the dynamic nature of sea ice that makes it both a thermostat that helps regulate the earth's temperature and a thermometer that shows when the planet is getting warmer.

Sea ice cools the planet by acting as an insulating lid and a solar energy reflector, Jeffries said. Sea ice forms a cap over polar oceans that prevents heat escaping the ocean from warming the atmosphere. Although polar seas are cold enough to discourage most humans (except maybe those in the Polar Bear Club) from jumping in, they contain quite a bit of heat when compared to polar air in winter, which may be 60 degrees F colder.

Sea ice covered with snow makes the atmosphere even colder because it reflects much of the meager solar energy that reaches the polar regions.

If sea ice at the poles shrinks significantly, the earth could respond by warming because of a positive feedback loop, Jeffries said. With less sea ice, there's more open water. The open water collects energy from the sun in summer, which hampers the formation of sea ice when temperatures drop in the winter.

With less sea ice at the poles, less sunlight is reflected. Sunlight that would have been reflected is absorbed by the water, which heats up and repeats the warming cycle.

The Norwegian researchers say they can't determine the cause of the sea ice reduction, but it does match a pattern expected from greenhouse warming, where gases such as the carbon dioxide (exhaled by our vehicles and power plants) warm the planet by trapping heat.

Jeffries says modeling studies show that if carbon dioxide levels were significantly greater, the effects could be felt first in polar regions, mostly in the Arctic. With sea ice and other indicators, Alaska and the northern polar region act as an early warning center for global warming.