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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 per-
mafrost. 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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