February 17, 2002 March 5, 2002

Most remarkable of the landscape changes underway in both the Arctic and Antarctic involves the extent and thickness of sea ice. Sea ice is exceedingly important as a climate variable, as it can function as either as an indicator or, given its impact on everything from surface reflectivity to heat and moisture exchange at the ocean surface, agent of climate change (2) .

Over the past 30 years, the Arctic has experienced a decrease in annual sea ice extent of approximately 8% (roughly 1 million square kilometers).  Similarly, sea ice thickness has dropped, on average, 10-15% across the region, and as much as 40% in some areas. As would be expected, this loss is most pronounced in the summer (15-20%), and the trend is accelerating (2). In the Antarctic, by contrast, the extent of sea ice is actually increasing overall. However, around the Antarctic Peninsula, which is experiencing warming 2 or 3 times faster than the global mean, sea ice is increasingly at risk. Though it represents only 4% of the entire continental area, it is losing sea ice in the form of ice shelves at an alarming rate.  A significant portion of the Larsen B ice sheet (see left) broke away over a period of approximately 2 weeks in 2002. What now remains represents only 40% of the ice that was present in 1995.  In keeping with this trend, the Antarctic Peninsula has lost 13,500 square km of ice, across 7 shelves, since 1974 (12).

Sea ice is of particular concern at both poles for a number of reasons, not the least of which is its importance to the albedo effect. Ice and snow, which are highly reflective, play a very important role in the reflection of incoming solar radiation. When ice and snow give way to less reflective water, soil, or vegetative cover (as occurs when forest encroaches on tundra), this reflectivity is compromised. Less reflected solar radiation leads to warmer temperatures here on earth, which leads to more melting and additional loss of reflective cover. This continues in a self-sustaining "positive feedback" loop that further inflates the effects of climate change both on the poles and the earth as a whole (6).

The same warming that leads to these changes in sea ice and the loss of tundra as ecosystem ranges move north is responsible for the melting of permafrost. Permafrost, or the frozen soil that spans most of the Arctic, contains a great deal of frozen organic matter, which represents a significant amount of trapped carbon. When permafrost thaws in the spring and summer, this organic matter decays, releasing carbon dioxide and methane. Higher temperatures mean more extensive melting, which should result in larger amounts of these two greenhouse gases being released. Though it is possible that the more extensive vegetative cover warming may also generate would act as a sink in this case, it is also possible that melting permafrost could create another positive feedback loop as higher GHG concentrations lead to higher temperatures and more melting (2).