Perspectives on the Arctic's

Linear trends in arctic sea-ice extent over the period 1979 to 2006 are negative in every month. This ice loss is best viewed as a combination of strong natural variability in the coupled ice-ocean-atmosphere system and a growing radioactive forcing associated with rising concentrations of atmospheric greenhouse gases, the latter supported by evidence of qualitative consistency between observed trends and those simulated by climate models over the same period. Although the large scatter between individual model simulations leads to much uncertainty as to when a seasonally ice-free Arctic Ocean might be realized, this transition to a new Arctic state may be rapid once the ice thins to a more vulnerable state. Loss of the ice cover is expected to affect the Arctic's freshwater system and surface energy budget and could be manifested in middle latitudes as altered patterns of atmospheric circulation and precipitation. The most defining feature of the Arctic Ocean is its floating sea-ice cover, which has traditionally ranged from a maximum extent of about 16 x rob km in March to a mini-mum extent of 7 x 106 km 2 at the end of the summer melt season in September (Fig. r). Consistent satellite-derived monthly time series of sea-ice extent are provided by the Nimbus-7 Scanning  Multiplicand  Microwave Radiometer 

(October 1978 to August 1987) and the Defense Meteorological Satellite Program Special Sensor Microwave Image (1987 to present). Based on regression analysis of the combined record over the period 1979 to 2006, ice extent has declined for every month (Fig. 2), most rapidly for September. for which the trend is –8.6 2.9% per decade or about loo,000 km per year. Ice extent is defined as the area of the ocean with a fractional ice cover (i.e., an ice concentration) of at least 15% ft –3). Every year since atom has yielded pronounced September minims, the most extreme of which was in 2005 (5.56 X 10 G km). When compared with the mean ice extent over the period 5979 to zoo*, this represents a spatial reduction of 25% (1.6 x to km 2), an area roughly the size of Alaska (Fig. Comparisons with earlier records, which combine visible•band satellite imagery and aircraft and ship reports, suggest that the September zoos ice extent was the low-est in at least the past 50 years. Data for the past few years suggest an accelerating decline in winter sea-ice extent (4). Evidence for accompanying reductions in ice thickness (5) is inconclusive. Upward-looking sonar aboard submarines provides informs. tron on ice draft—the component of the total thickness (about 90%) that projects below the water surface. Comparisons between early Mar records (5958 to 1976) and those for 1993 to 1997 indicate reductions of I.) m in mean late summer ice draft over much of the central Arctic Ocean (6). but sparse sampling complicates interpretation. Further analysis of the