Greenland ice sheet (CLIM 009) - Assessment published Sep 2008
Climate change (Primary topic)
Typology: Descriptive indicator (Type A - What is happening to the environment and to humans?)
- CLIM 009
Key policy question: .
- The Greenland ice sheet changed in the 1990s from being in near mass balance to losing about 100 billion tonnes of ice per year. Ice losses may have doubled again by 2005. Accelerated flow of outlet glaciers to the sea accounts for more of the ice loss than melting.
- The contribution of ice loss from the Greenland ice sheet to global sea-level rise is estimated at 0.14-0.28 mm/year for the period 1993-2003 and has since increased. In the long term, melting ice sheets have the largest potential to increase sea level.
- No reliable predictions of the future of the ice sheets can yet be made; the processes causing the faster movement of the glaciers are poorly understood and there is a lack of long-term observations.
Estimated changes of the ice mass in Greenland 1992-2006
Note: Note: The rectangles depict the time period of the observations (horizontal) and the upper and lower estimates of mass balance for that period (vertical), calculated by different techniques as marked with colour codes
Thomas, R.; Davis, C.; Frederick, E.; Krabill, W.; Li, Y.; Manizade, S. and Martin, C., 2008. A comparison of Greenland ice-sheet volume changes derived from altimetry measurements. Journal of glaciology 54 (185): 203212
Area of Greenland ice sheet melting 1979-2007
Note: Note: The maps show the area of the Greenland ice sheet with at least one day of surface melting in summer. The diagram shows the cumulated melt area, which is defined as the annual total sum of every daily ice sheet melt area. For example, if a particular area is melting on 20 days in a given year, it is counted 20 times.
Steffen, K.; Nghiem, S. V.; Huff, R. and Neumann, G., 2004. The melt anomaly of 2002 on the Greenland Ice Sheet from active and passive microwave satellite observations. Geophysical Research Letters 31 (20): L2040210.1029/ 2004GL020444. Witze, A., 2008. Losing Greenland. Nature 452: 798802
The Greenland ice sheet is a huge inland glacier with several glacier tongues calving into the sea. It covers roughly 80 % of Greenland. The average ice thickness is 1 600 m, with the highest summit reaching 3 200 m above sea level. It has a volume of about 3 million km3.
Until recent improvements in remote sensing, it was hard to measure whether the polar ice sheets were growing or shrinking. Most time-series remain short. There is however a general consensus from different approaches that ice loss from the Greenland ice sheet has accelerated. From a near balance in the early 1990s, about 100 billion tonnes were being lost annually at the end of the century. This may have doubled again by 2005 (UNEP, 2007). However, there is still considerable discrepancy between different estimates of ice loss rates (Figure 1).
The IPCC estimated the Greenland ice sheet contribution to sea-level rise during 1993-2003 to be 0.14-0.28 mm/year, based on an annual ice loss in that period of 50 to 100 billion tones (IPCC, 2007a). A study estimating an ice loss of 224 billion tonnes/year in 2005, found a corresponding contribution to sea-level rise of 0.57 mm/year (Rignot and Kanagaratnam, 2006). This is one of the upper estimates in Figure 1 and illustrates the effects that different rates of ice loss can have on sea level.
The ice in the interior of the Greenland ice sheet at high elevations has thickened since it has received more snowfall - on average about 4 cm/year since 2000 (UNEP, 2007). This gain has been more than offset by the loss in lower-lying regions from melting and increased calving of ice bergs. Air temperatures in summers have increased significantly along the coast since the early 1990s, whereas little change or slight cooling has been observed in the high interior (Steffen, 2007 unpubl.).
Ice loss can partly be caused by surface melting. On the low-lying edge of the ice sheet, the surface melts each summer causing evaporation and meltwater runoff from the glacier. If this exceeds the net snow accumulation in winter, the glacier has a negative surface mass balance. Areas with melting can be measured from satellites, and from 1979 to 2007, the cumulative melt area increased by approximately 50 % (Figure 2). Melting has reached higher elevations, and the melting season is lasting longer. However, both snowfall and surface melting has increased. The resulting trend in surface mass balance for the whole Greenland ice sheet between 1958 and 2006 has been modelled to be insignificantly negative (Hanna et al., 2007). Data from recent years extending to 2007 suggest a strong increase in the net loss of surface mass balance (Steffen, 2007 unpubl.).
The other mechanism behind the ice loss since the 1990s is accelerated flow of outlet glaciers towards the sea. Large amounts of meltwater form rivers and melting ponds at the glacier surface and penetrate through crevasses to the bottom. This water probably lubricates the bedrock/ice interface, making the glaciers move faster. Another explanation is that the glacier fronts may be affected by increasing ocean temperatures, reducing their buttress effect. The outlet glaciers are also influenced by the topography of the fjords. Outlet glaciers act as 'bathtub drains' for the inland ice: ice is being transported into the melting zone, and calving into the ocean increases. The speed of the fastest-flowing glacier, Jakobshavn isbrae on the west coast, has nearly doubled, to about 14 km a year (UNEP, 2007). Some glaciers are however reported to be slowing down from the maximum speeds measured, possibly around a new equilibrium position. Acceleration is widespread mostly on the southeast coast and has moved northwards to about 70oN. It is associated with large retreats and thinning of the ice sheet. Near the outlets, glacier surface elevation can subside by tens of metres.
The ice losses caused by accelerated flow of the outlet glaciers (ice dynamics) have exceeded the losses from melting processes (negative surface mass balance) several times during the recent few warmest years. For 2005, it has been estimated that two thirds of the ice loss was caused by ice dynamics (Rignot and Kanagaratnam, 2006).
It is currently not possible to predict the future development of the Greenland ice sheet with confidence. Glacier models account mostly for accumulation of snow during winter and melting in summer (surface mass balance). The accelerated ice flow has been observed for a rather short period of time. Scientists are now trying to understand the processes driving this phenomenon. That should in turn allow better ice models to be developed. But for models to predict the future well, they must be validated by data from long-term measurements describing key processes. Until science comes closer to this, our ability to predict the sensitivity of the Greenland ice sheet to global warming will remain limited.
Further temperature increases can accelerate the ice loss because of positive feed-back mechanisms, like thinning of the ice sheet that exposes larger areas to melting. It is hard to say how strong these mechanisms are, how rapidly the ice sheet will react to them and whether ice loss will be irreversible.
Throughout the earth's history, the ice sheets have shrunk in response to warming and grown in response to cooling. Deep ice core drillings reveal past climates and can give some indications of how they have changed. The Eemian era was an interglacial period 120 000 years ago when temperatures over Greenland were about 5 oC warmer than today. But the Greenland ice sheet did not melt completely. Sea level rose to about 5 m above today's level, with melting Greenland ice contributing 1-2 m (Dahl Jensen, pers. com.). Because global warming is amplified near the poles, a future temperature rise of 5 oC in Greenland may be reached when global average temperature rises by around half of this, which is within the range of IPCC projections for this century.
The ice sheets of Greenland and Antarctica have previously been associated with slow climate responses over thousands of years. But the acceleration of ice movement has caused a rethinking of how rapidly they respond to warming. Paleodata show periods of rapid melting of the large continental ice sheets after the last ice age, resulting in an average rise of sea level of 1 cm/year and peak rates up to 4 cm/year (UNEP, 2007). Shrinkage seems to be a faster process than growth, probably because accelerated ice flow plays an important role in retreat. A better understanding of these processes is of vital importance for assessing how much we can expect the flow of meltwater from the Greenland ice sheet to increase.
Greenland ice-sheet volume changes
provided by NASA Goddard Space Flight Center
Snowmelt extent and duration on the Greenland ice sheet
provided by University of Colorado (CIRES)
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