Editor’s comments

Greenland’s ice sheet is the single largest body of freshwater ice in the Northern Hemisphere, containing around 3 million km**3 of ice. If it were to melt completely it would cause global sea level to rise by roughly 7 m.

The Greenland Ice Sheet has expanded and contracted many times in response to changes in the Earth’s climate.

The Eemian, the previous interglacial period, is a particularly interesting period to consider because in many ways it represents an analogue for what could happen to the present-day Greenland Ice Sheet as temperatures continue to rise. Air temperatures over Greenland during the Eemian were also relatively stable for several thousands of years but at about 5 °C higher than today. Sea level was roughly 4 to 6 m higher than today and reconstructions using data from ice cores suggest that a partial melting of the Greenland Ice Sheet may have been responsible for between 1 and 3 m of this increased sea level.

Recent changes in the ice sheet suggest that it is already responding dramatically to the changing climate, but the extent to which it will shrink over the coming decades remains uncertain. This assessment is the first with a comprehensive focus on the Greenland Ice Sheet under the currently changing climate. It forms an integral part of the Arctic Council project Climate Change and the Cryosphere: Snow, Water, Ice and Permafrost in the Arctic (SWIPA).

Experimental observations: temperature

Some long-term datasets from different parts of the ice sheet do exist. The longest series of weather measurements have been collected by the Danish Meteorological Institute at eight coastal sites in southern Greenland. Some of these series cover 100 years or more. Data have also been collected at a network of automatic weather stations installed across the ice sheet since the 1990s.

Coastal records since 1840 show the highest temperatures to have occurred during the 1930s and 1940s. These coastal datasets also show a general warming of about 2 to 4 °C since the end of the 1980s, mainly during winter. Recent warming at the western edge of the ice sheet is also clear in the automatic weather station data.

However, there are too few datasets, and those that do exist are still too short, to establish temperature trends within the vast interior of the ice sheet.

Experimental observations: precipitation

Precipitation adds mass at the surface of the ice sheet. Snowfall on the Greenland Ice Sheet has significantly increased over the past 50 years, mainly due to warmer near-surface air temperatures causing increased moisture in the air which leads to increased precipitation. Since 2000, the high interior of the ice sheet (the part above 2000 m) has thickened, gaining around 5 cm in height each year through this increase in snowfall. There were particularly heavy snowfall years in 2002/2003 (southeast Greenland) and 2004/2005 (West Greenland). Some scientists have suggested that heavy snowfall years may become more frequent in a climate with warmer winters.

Experimental observations: surface melting

The Greenland Ice Sheet loses mass through surface melting and, at the margins of the ice sheet, through icebergs and melting from ice surfaces in contact with warmer ocean water. Air temperature and winds just above the surface of the ice sheet are the most important factors influencing surface melting. Another important factor is the reflectivity of the ice surface to incoming sunlight – a property known as surface albedo. Particles of dust and soot (black carbon) in the atmosphere can be deposited directly onto the surface of the ice sheet and can reduce albedo in the melt zone.

Melting in the ablation zone (the low altitude area of a glacier or ice sheet where there is a net loss in ice mass due to melting) has been measured at a few sites around the margins of the ice sheet. How representative these few sites are and how well the surface processes are understood is uncertain. Some meltwater will run off directly, some will drain through cracks in the ice and some will refreeze.

Data from satellites show that the areas experiencing summer melting have increased significantly since 1979, with a record-breaking area of surface melting measured in 2007.

A new record was set in July 12-16, 2012 when the ice melt was observed over 97% of Greenland. The massive four day ice melt was detected independently by three satellites Oceansat-2, MODIS and a U.S. Air Force meteorological satellite. Such extensive melting across the ice sheet has not occurred since 1889, according to ice cores analysis.

Experimental observations: Ice outflow

The speed with which ice flows away from the accumulation zone on the upper part of the ice sheet becomes extremely variable as it approaches the margins – with areas of slow flow separated from fast-flowing outlet glaciers and ice streams. Vast amounts of ice are discharged into the sea from the ends of glaciers.

One of the results of recent scientific studies is the finding that annual ice discharge for the entire Greenland ice sheet increased by 30% over the past decade: from 330 gigatonnes (Gt) in 1995 to 430 Gt in 2005. This increase was due to faster ice flow by the outlet glaciers and ice streams.

A gigatonne (1 000 000 000 tonnes) is roughly a block of ice 1.1 km**3 in size.

Many outlet glaciers, including Jakobshavn Isbræ on the west coast and Kangerdlugssuaq Glacier and Helheim Glacier on the east coast, have experienced recent and dramatic increases in the amount of ice discharged each year. Ice discharge from many southern outlet glaciers increased rapidly between 1995 and 2000 and by 2005 this pattern had extended to outlet glaciers in northern areas. There is some regional variation, however, with discharge from the Helheim and Kangerdlugssuaq glaciers falling back to previous levels in 2006, while the high loss of ice from Jakobshavn Isbræ has continued.

Faster flowing ice in outlet glaciers is causing widespread retreat of these glaciers, as discharge continues to outstrip supply. This out-of-balance loss of ice to the sea from many southern glaciers has caused extensive areas of thinning (‘drawdown’) near the edge of the ice sheet. Because major sections of the ocean-terminating glaciers are floating, relatively warm ocean currents are believed to be particularly important.

Experimental observations: mass balance

Up until 1990 – before the recent speeding-up of ice flow in the outlet glaciers and the trend towards increased mass loss from surface melting – the Greenland Ice Sheet seemed to be roughly in balance. The total amount of ice added and lost each year seemed to be around 500 Gt; of the 500 Gt added in snowfall ~50% was lost in run-off from surface melting and ~50% was discharged in icebergs.

Total Mass Balance = INPUT – (OUTPUT) = Accumulation – (Surface Runoff + Iceberg Production + Bottom Melting)

Recent measurements show that this balance has now shifted and that there have been quite large and rapid changes in surface melting and ice discharge.

Because past changes in the mass balance of the ice sheet influence the present flow of ice in the ice sheet, predicting future changes in the mass balance of the ice sheet requires the previous changes in mass balance to be taken into account. Whether the ice sheet is growing or shrinking depends on the balance between the processes that add or remove ice. The overall balance between the mass gain by snow accumulation and the mass loss through iceberg calving and runoff from melting each year is known as the ‘total mass balance’. This is effectively a measure of the ‘health’ of the ice sheet. If, over time, the ice sheet loses more mass than it gains then the total mass of the ice sheet will gradually decrease.

There are three techniques for estimating or observing changes in total mass balance: the mass budget approach, repeat altimetry, and changes in gravity. Only gravity measurements directly measure changes in ice mass, the first two approaches measure changes in other quantities (e.g., melt, surface height) and then convert these to changes in ice mass.

• The mass budget approach calculates the total mass balance from individual estimates of ice gains (snowfall) and ice losses (melt and icebergs).
• Satellite radar altimetry and laser altimetry from airplanes have been widely used to measure changes in the height of the ice sheet. Changes in the height of the ice sheet reflect changes in the total amount of ice it contains.
• Changes in the Earth’s gravity field are directly related to changes in mass. Since 2002 the GRACE (Gravity Recovery and Climate Experiment)mission has measured changes in the Earth’s gravity field.

A compilation of mass balance estimates all indicate that the Greenland Ice Sheet has experienced an overall loss of ice since the early 1990s. Between 1995 and 2000 the estimated annual loss of ice averaged about 50 Gt; but, by 2003–2006 the rate of loss had increased dramatically, to an average of about 160 Gt per year. This is equivalent to an annual rise in sea level of 0.44 mm. The mass loss observed since 1990 is a direct consequence of the warming climate over the Greenland Ice Sheet. The future strong warming that is predicted to occur in the high northern latitudes will lead to increased mass loss from the Greenland Ice Sheet with local, regional and global impacts.

The Greenland Ice Sheet in a Changing Climate: Snow, Water, Ice and Permafrost in the Arctic (SWIPA) 2009. Arctic Monitoring and Assessment Programme (AMAP), Oslo 2009