Sea Ice & Glaciers

Figures - Global Warming

Arctic Sea Ice Extent 1979-2005

Glacier Mass Balance 1980-2008

During the summer, Arctic sea ice melts at the edges and the polar ice cap decreases in extent, then during the winter there is freezing and growth at the edges. In recent years the polar ice cap has been shrinking, meaning that the summer melting rate exceeds the winter freezing rate. This figure compares the summer Arctic sea ice minimum in 1979 (red) with the summer sea ice minimum in 2005. Since 1979, more than 20% of the Polar Ice Cap has melted in response to increasing air and ocean temperatures. This melting does not contribute to a rise in sea level because it is ocean ice that was already contributing to the sea level before it melted. However, melting of ice on land contributes to sea level rise because it is new water added to the oceans. Land-based sources whose melting contributes to sea level rise include the Antarctic and Greenland ice sheets, and mountain glaciers. Disappearing sea ice is leading to the extinction of the polar bears because they rely on having sea ice within swimming distance for hunting. Disappearing sea ice is also causing the loss of coastal villages to erosion that didn't previously occur when there was sea ice rather than open ocean on the coast.

Source: NASA and the Natural Resources Defense Council (NRDC). Satellite data of Arctic sea ice minimums from 1979 and 2005 are from NASA and the National Snow and Ice Data Center.

The mass of glaciers, which is the total amount of water they contain, is a combination of the area of the glacier and its average thickness. The World Glacier Monitoring Service oversees annual reporting on the "mass balance" of over 90 glaciers worldwide. Mass balance is the net gain or loss of mass in units of "mm w.e." (millimeters of water equivalent), which is the average thickness of ice that was gained or lost if it was melted. The figure shows the average mass balance for all reported glaciers (black) and for a subset of 30 reference glaciers (red) for 1980-2008. Over this time period the glaciers on average lost an amount of mass equivalent to a thickness of 12,000 mm (12 meters, or 39 feet) of water, and the loss rate (slope) has been increasing since the mid 1990s. This water ends up in the oceans and contributes to a rising sea level. A particularly distressing aspect of disappearing mountain glaciers is that they are the source of water for billions of people in various parts of the world.

The loss of ice is uneven across a glacier, and the lower portion of glaciers has been disappearing, called "glacial retreat". This is captured visually with repeat glacier photography, such as these examples at the National Snow and Ice Data Center and at Global Warming Art. The Extreme Ice Survey has documented the changes and fascinating behavior of glaciers around the world with time-lapse video and photography. The PBS/Nova Extreme Ice documentary is very interesting and cool and can be downloaded or purchased on DVD, and James Balog's TED talk is a good overview of the project and its conclusions.

Figure is from published data at the World Glacier Monitoring Service.