The World in 2050_ Four Forces Shaping Civilization's Northern Future - Laurence C. Smith [156]
286 L. C. Smith et al., “Rising Minimum Daily Flows in Northern Eurasian Rivers: A Growing Influence of Groundwater in the High-Latitude Hydrologic Cycle,” Journal of Geophysical Research 112, G4, (2007): G04S47.
287 Ice caps are large glacier masses on land. Unlike Antarctica, a continent buried beneath mile-thick glaciers and surrounded by oceans, the Arctic is an ocean surrounded by continents. It is thinly covered with just one to two meters of seasonally frozen ocean water called “sea ice.”
288 The Fall Meeting of the American Geophysical Union, which convenes each December in San Francisco, California.
289 The Arctic Ocean freezes over completely in winter but partially opens in summer. The annual sea-ice minimum occurs in September.
290 By September 2009 sea-ice cover was nearing recovery to its old trajectory of linear decline. However, the extreme reductions of 2007-2009 were a major excursion from the long-term trend and clearly demonstrate the surprising rapidity with which the Arctic’s summer sea-ice cover can disappear.
291 Unlike land-based glaciers, the formation or melting of sea ice does not significantly raise sea level because the volume of buoyant ice is compensated by the volume of water displaced (Archimedes’ Principle). A slight exception (about 4%) to this does arise because sea ice is fresher than the ocean water it is displacing (thus taking up slightly more volume than the equivalent mass of sea water).
292 This albedo feedback works in the opposite direction, too, by amplifying global cooling trends. If global climate cools, then Arctic sea ice expands, reflecting more sunlight, thus causing more local cooling and more sea-ice formation, and so on.
293 Sea ice does form around the edge of the Antarctic continent, but its areal extent is much less than in the Arctic Ocean and it does not survive the summer. Other reasons for the warming contrast between the Arctic and Antarctica include the strong circumpolar vortex around the southern oceans, which divorce Antarctica somewhat from the global atmospheric circulation, and the cold high elevations of interior Antarctica, where air temperatures will never reach the melting point, unlike the Arctic Ocean, which is at sea level.
294 The sea-ice albedo feedback is the most important factor causing the global climate warming signal to be amplified in the northern high latitudes, but there are also others. Reduced albedo over land (from less snow), a thinner atmosphere, and low evaporation in cold Arctic air are some of the other positive warming feedbacks operating in the region. The transition to a new summertime ice-free state is likely to happen rapidly once the ice pack thins to a vulnerable state. M. C. Serreze, M. M. Holland, J. Stroeve, “Perspectives on the Arctic’s Shrinking Sea-Ice Cover,” Science 315, no. 5815 (2007): 1533-1536. Not all northern albedo feedbacks are positive—for example, more forest fires, an expected consequence of rising temperatures, actually raise albedo over the long term. E. A. Lyons, Y. Jin, J. T. Randerson, “Changes in Surface Albedo after Fire in Boreal Forest Ecosystems of Interior Alaska Assessed Using MODIS Satellite Observations,” Journal of Geophysical Research 113: (2008) G02012.
295 Based on projections of the NCAR CCSM3 climate model. You can view