Rapid sea ice loss is one of the most prominent indicators of Arctic climate change (Section 4.2). There has been a trend of decreasing Northern Hemisphere sea ice extent since 1978, with the summer of 2012 being the lowest in recorded history (see Section 4.2 for details). The 2012 minimum sea ice extent was 49% below the 1979 to 2000 average and 18% below the previous record from 2007. The amount of multi-year sea ice has been reduced, i.e., the sea ice has been thinning and thus the ice volume is reduced (Haas et al., 2008; Kwok et al., 2009). These changes make the sea ice less resistant to wind forcing.
Sea ice extent has been diminishing significantly faster than projected by most of the AR4 climate models (SWIPA, 2011). While AR4 found no consistent trends in Antarctica sea ice, more recent studies indicate a small increase (Section 4.2). Various studies since AR4 suggest that this has resulted in a deepening of the low-pressure systems in West Antarctica that in turn caused stronger winds and enhanced ice production in the Ross Sea (Goosse et al., 2009; Turner and Overland, 2009).
AR4 concluded that taken together, the ice sheets in Greenland and Antarctica have very likely been contributing to sea level rise. The Greenland Ice Sheet has lost mass since the early 1990s and the rate of loss has increased (see Section 4.4). The interior, high-altitude areas are thickening due to increased snow accumulation, but this is more than counterbalanced by the ice loss due to melt and ice discharge (AMAP, 2009; Ettema et al., 2009). Since 1979, the area experiencing surface melting has increased significantly (Tedesco, 2007; Mernild et al., 2009), with 2010 breaking the record for surface melt area, runoff, and mass loss, and the unprecedented areal extent of surface melt of the Greenland Ice Sheet in 2012 (Nghiem et al., 2012). Overall, the Antarctic continent now experiences a net loss of ice (Section 4.4). Significant mass loss has been occurring in the Amundsen Sea sector of West Antarctica and the northern Antarctic Peninsula. The ice sheet on the rest of the continent is relatively stable or thickening slightly (Lemke et al., 2007; Scott et al., 2009; Turner et al., 2009). Since AR4, there have been improvements in techniques of measurement, such as gravity, altimetry and mass balance, and understanding of the change (Section 4.4).
As discussed in the earlier assessments, most glaciers around the globe have been shrinking since the end of the Little Ice Age, with increasing rates of ice loss since the early 1980s (Section 4.3). The vertical profiles of temperature measured through the entire thickness of mountain glaciers, or through ice sheets, provide clear evidence of a warming climate over recent decades (e.g., Lüthi and Funk, 2001; Hoelzle et al., 2011). As noted in AR4, the greatest mass losses per unit area in the last four decades have been observed in Patagonia, Alaska, northwest USA, southwest Canada, the European Alps, and the Arctic. Alaska and the Arctic are especially important regions as contributors to sea level rise (Zemp et al., 2008, 2009).
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- SWIPA, 2011: Snow, water, ice and permafrost in the Arctic. SWIPA 2011 Executive Summary. AMAP, Oslo, Norway, 16 pp.
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