Climate change, impacts and vulnerability in Europe ... - MemoFin.fr

Changes in the climate system2.3.3 Greenland ice sheetRelevanceThe fate of the Greenland ice sheet highlightspotentially major consequences of climate changeas it is directly linked to global sea-level rise. Thespeed of ice loss, known as the ice sheet 'massbalance', is the most important indicator of ice sheetchange. An increased rate of mass loss results in afaster rise in sea level. In addition, melt water fromGreenland reduces the salinity of the surroundingocean. An upper layer of fresher water may reducethe formation of dense deep water, one of themechanisms driving global ocean circulation.Past trendsThe mass balance of the Greenland ice sheet isdetermined by snow fall, summer melting ofsnow, and the icebergs breaking off the glaciers.Several different methods are used to monitor thechanges of the Greenland ice sheet (Krabill et al.,2002; Shepherd and Wingham, 2007; Chen et al.,2011; Rignot et al., 2011; Zwally et al., 2011). Theoverall conclusion is that Greenland is losing massat an accelerating rate (Figure 2.12). The yearlycumulated area where melting occurs has alsoincreased significantly (Figure 2.13). Since 2006, highsummer melt rates have led to a Greenland ice sheetmass loss of 273 billion tonnes a year (Rignot et al.,2011). This ice loss corresponds to a sea-level riseof approximately 0.7 mm per year (about a quarterof the total sea‐level rise of 3.1 mm a year) (seeSection 3.2.2 for further information on global andEuropean sea‐level rise).Exceptional melting was recorded on the Greenlandice sheet in the summer of 2012. On 12 July 2012nearly the entire ice cover experienced some degreeof surface melting (NASA, 2012). The extreme meltevent coincided with an unusually strong ridgeof warm air over Greenland. The ridge was oneof a series that dominated Greenland's weatherin the summer of 2012. Ice core data suggest thatlarge‐scale melting events of this type have occurredabout once every 150 years on average, the mostrecent one in 1889. It is not currently possible to tellwhether the frequency of these rare extensive meltevents has changed.Figure 2.12 Mass balance of the Greenlandice sheet from mass budgetcalculations (1989–2009)Mass balance (Gt/year)2001000– 100– 200– 300– 4001985 1990 1995 2000 2005 2010Source: van den Broeke, 2011.Key messages: 2.3.3 Greenland ice sheet• The Greenland ice sheet is the largest body of ice in the northern hemisphere and plays an importantrole in the cryosphere. It changed in the 1990s from being in near mass balance to losing about100 billion tonnes of ice per year. Ice losses have since then more than doubled to 250 billion tonnes ayear averaged over 2005 to 2009.• The contribution of ice loss from the Greenland ice sheet to global sea-level rise is estimated at0.14–0.28 mm/year for the period 1993–2003 and has since increased. The recent melting of theGreenland ice sheet is estimated to have contributed up to 0.7 mm a year to sea-level rise, which isapproximately one quarter of the total sea-level rise of about 3.1 mm/year.• Model projections suggest further declines of the Greenland ice sheet in the future but the processesdetermining the rate of change are still poorly understood.78 Climate change, impacts and vulnerability in Europe 2012