Book 2.indb - US Climate Change Science Program

The U.S. Climate Change Science Program Chapter 2Box 2.1 Figure 2. The ice cover in Greenlandand Antarctica has two components—thick, grounded, inland ice that rests on amore or less solid bed, and thinner floatingice shelves and glacier tongues. An ice sheetis actually a giant glacier, and like most glaciersit is nourished by the continual accumulationof snow on its surface. As successive layers ofsnow build up, the layers beneath are graduallycompressed into solid ice. Snow input isbalanced by glacial outflow, so the height ofthe ice sheet stays approximately constantthrough time. The ice is driven by gravity toslide and to flow downhill from the highestpoints of the interior to the coast. There iteither melts or is carried away as icebergswhich also eventually melt, thus returning thewater to the ocean whence it came. Outflowfrom the inland ice is organized into a seriesof drainage basins separated by ice dividesthat concentrate the flow of ice into eithernarrow mountain-bounded outlet glaciers orfast-moving ice streams surrounded by slowmovingice rather than rock walls. In Antarctica,much of this flowing ice has reached thecoast and has spread over the surface of theocean to form ice shelves that are floatingon the sea but are attached to ice on land.There are ice shelves along more than half ofAntarctica’s coast, but very few in Greenland(UNEP Maps and Graphs; K. Steffen, CIRES,University of Colorado at Boulder).Box 2.1 Figure 3. An ice shelf is a thick, floatingplatform of ice that forms where a glacier or icesheet flows down to a coastline and onto the oceansurface. Ice shelves are found in Antarctica, Greenland,and Canada. The boundary between the floatingice shelf and the grounded (resting on bedrock) icethat feeds it is called the grounding line. The thicknessof modern-day ice shelves ranges from about100 to 1,000 meters. The density contrast betweensolid ice and liquid water means that only about 1/9of the floating ice is above the ocean surface. Thepicture shows the ice shelf of Petermann Glacier innorthwestern Greenland (right side of picture) witha floating ice tongue of 60 km in length and 20 kmwide. Glaciers from the left are merging with theice shelf (Petermann Glacier, northwest Greenland,photograph courtesy of K. Steffen, CIRES, Universityof Colorado at Boulder).40