4.7
SEA ICE, SNOW, AND RELATED FACTORS

Current State of Sea Ice

Sea ice plays a critically important role in our climate system. It controls the rate of heat exchange between the polar ocean and atmosphere, reflects much of the solar energy incident on it, helps to maintain the equator-pole temperature gradient, influences ocean circulation, and is of great ecological importance. Typically, sea-ice area is about 14 to 16 million Km2 in late winter in the Arctic and 17 to 20 million Km2 in the Antarctic Southern Ocean. In late summer, on average, only about 3 to 4 million Km2 remain in the Southern Ocean while in the Arctic there are approximately 7 million Km2 (NSIDC; http://nsidc.org/sotc/sea_ice.html). Sea-ice extent, defined here as the area of ocean with at least 15% sea-ice concentration, is negatively correlated to global average surface temperature so that as globally averaged surface temperature increases, sea-ice extent decreases (e.g., Gregory et al., 2002). This sea-ice response to the increasing global surface temperatures occurs directly through thermally driven flux exchanges with the atmosphere and their impact on sea-ice extent and thickness, and indirectly, through the additional impact of increasing temperatures on dynamic mechanisms such as ENSO (e.g., Timmermann et al., 1999) and SAM (e.g., Arblaster and Meehl, 2006). Satellite observations show that there has been a decrease in globally averaged sea-ice extent since 1979. This decrease has occurred in the Arctic, while the Antarctic sea ice has increased slightly (Figure 4.11).

Since the 1950s Arctic sea-ice extent has exhibited a statistically significant decrease (Vinnikov et al., 1999), and the rate of decrease has been faster in summer (–7.8% per decade) than in winter (–1.8% per decade) (Stroeve et al., 2007). Satellite observations beginning in 1978 show that the annual average Arctic sea-ice extent has shrunk by 2.7% per decade (IPCC, 2007a) with larger decreases at the end of summer (9.1% per decade) than at the end of winter (2.9% per decade) (Stroeve et al., 2007). By some measures, from 1979 to 2006, September (late summer) sea-ice extent decreased by almost 25% or about 100,000 km2 per year (Serreze et al., 2007). Comparison of observed Arctic sea-ice decline to IPCC AR4 projections show that the observed rate of ice loss is faster than that predicted by any of the IPCC AR4 models (Stroeve et al., 2007).

The decrease in sea-ice extent has been accompanied by thinning perennial and seasonal ice (Kwok and Rothrock, 2009), a decrease in multiyear ice (Maslanik et al., 2007), and record minima in September sea-ice cover (Serreze et al., 2007; Stroeve et al., 2007; Stroeve et al., 2008). Spring



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