in the thermohaline circulation (e.g., Broecker, 1997; Lemke et al., 2007; Levermann et al., 2007), an impact with global consequences for climate variability. The reduced latitudinal temperature gradients that result from the Arctic warming will modify the atmospheric circulation dynamics in the Northern Hemisphere. Mid-latitude storm tracks may shift (e.g., Deser and Teng., 2008), the westerlies may weaken, and storm intensities may decrease poleward of 45 N (e.g., Royer et al., 1990; Honda et al., 1999). Large-scale pressure systems such as the Azores High (Raymo et al., 1990) as well as the Asian monsoon and the Hadley Cell circulation systems may be affected (Liu et al., 2007).

Along with these impacts on the atmospheric and oceanic circulation, loss of Arctic sea ice has the potential to enhance the rates of surface melt of Greenland’s glaciers. Present-day enhanced melting of Greenland’s ice sheet is associated with increased advection of ocean heat onto the ice sheet from a warmer ocean, resulting in enhanced melt (e.g., Rennermalm et al., 2009). The warmer ocean surface temperatures that will occur in the absence of sea ice can be expected to enhance the rates of warming. The increased melt will contribute to sea level rise.

Sea ice in the Arctic is of major ecological importance; it is a habitat for a variety of species. An ice-free Arctic will promote large scale changes in Arctic marine ecosystems. Already in the Arctic, loss of sea ice has been associated with polar bear population decrease (e.g., DeWeaver, 2007); seasonal or perennial loss of sea ice will only exacerbate this situation. Sea ice protects the shorelines from erosion and helps maintain continuous permafrost. Lawrence et al. (2008b) show that loss of Arctic sea ice speeds the degradation of permafrost. Warming of the permafrost has already led to the destabilization of infrastructure in the Arctic, and removal of the protective cover of ice has already led to increased shoreline erosion (IPCC, 2007a,b); this can only worsen as sea ice cover is lost. Additionally, warming of the permafrost may lead to the emission of methane to the atmosphere, which has the potential to enhance greenhouse gas-related warming (Macdonald, 1990).



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