3.3a. The Greenland Ice Sheet

The Greenland Ice Sheet is the second largest body of ice on the planet, after the Antarctic Ice Sheet. It covers an area of more than 1.7 million km² (a little smaller than Mexico) and is generally more than 2 km thick (more than 3 km at maximum). The total ice sheet volume is 2.85 million km³, which would feed an average global sea-level rise of 7.2 m, should it all melt. The so-called ‘deglaciation threshold’ above which the Greenland Ice Sheet would melt irreversibly is an average local temperature rise of 2.7° C. As the poles are warming faster than the rest of the planet, this could happen for a mean global temperature rise of less than 2° C – virtually certain before 2100, and possibly by 2050. The Greenland Ice Sheet is already, however, showing signs of collapsing. The area affected by summer melting has increased by 16 percent over the past 27 years, and now affects more than a third of the surface area of the ice sheet (Figure 6). It is also steadily climbing and in 2002 and 2005 reached 2,000 m above sea-level. Around the margins of the ice sheet, the situation is even more worrying, with the total annual ice loss – even allowing for snow-fall in the interior – doubling in the last decade from 96 km³ in 1996 to 220 km³ in 2005. A cubic kilometre of ice is equivalent to 264 billion gallons of water, and Greenland is losing this amount every 40 hours. The enormous acceleration in melting seems to be related to the aforementioned rise in the extent of summer melting, and is happening because meltwater is percolating down from the surface via crevasses, and lubricating the base of the glaciers that carry ice from the interior towards the sea. In just the last couple of years, two of Greenland’s biggest glaciers – the Helheim (Figure 7) and the Kangerdlugssuaq – have doubled their speed to around 14 km a year. In 1998, the Jakobshavn glacier showed a similar acceleration, and together the three glaciers drain nearly a fifth of the entire ice sheet. With close to half the discharge from the entire ice sheet occurring via 12 glaciers, there is real concern that the remaining glaciers will follow, leading to the wholesale collapse of the ice sheet. Even if the collapse does not happen this century, it seems highly likely that temperature rises in the next several decades will trigger a tipping-point that will result in a sea-level rise of several metres in the centuries ahead.

Figure 6: Area of summer melting of the Greenland Ice Sheet. The pale red shows the melt area in 1992, while the dark red area shows the extension of this area in 2005. Courtesy: NOAA and CIRES

Figure 7: Changes in the position of the Helheim glacier’s ‘calving front’ from 2001 (a) to 2005 (b). This location of this margin, where the glacier breaks up into icebergs, has changed dramatically over the period, at a time when the glacier has doubled its rate of flow. Courtesy: NASA
(a)

(b)

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