One of the most striking ongoing changes in the Arctic is the rapid melting of sea ice. Some climate models predict that, sometime during the first half of the 21st century, summer sea ice will vanish from the Arctic Ocean. An absence of summer ice would amplify the existing warming trend in Arctic tundra regions as well as in regions beyond the tundra, because sea ice reflects sunlight much more readily than the open ocean and, thus, has a cooling effect on the atmosphere. In addition, research indicates that the retreat of sea ice would enhance the productivity of tundra vegetation, and the resulting buildup of plant biomass might lead to more extreme events such as large tundra fires. Finally, an ice-free Arctic Ocean would improve access to high northern latitudes for recreational and industrial activities; this would likely place additional stress on tundra plants and animals as well as compromise the resilience of the tundra ecosystem itself. In alpine tundras too, climate warming could encourage more human activity and increase damage to plant and animal populations there.
The fate of permafrost in a warmer world is a particularly important issue. Together, tundra and taiga account for approximately one-third of global carbon storage in soil, and a large portion of this carbon is tied up in permafrost in the form of dead organic matter. Some of this organic matter has been preserved for many thousands of years, not because it is inherently difficult to break down but because the land has remained frozen. Thawing of the permafrost would expose the organic material to microbial decomposition, which would release carbon into the atmosphere in the form of CO2 and methane (CH4). Rates of microbial decomposition are much lower under anaerobic conditions, which release CH4, than under aerobic conditions, which produce CO2; however, CH4 has roughly 25 times the greenhouse warming potential of CO2. The Arctic has been a net sink (or repository) of atmospheric CO2 since the end of the last ice age. At the same time, however, the region has been a net source of atmospheric CH4, primarily because of the abundance of wetlands in the region.
