The Greenland Ice Sheet

Similarly to Arctic sea ice, the extent of Greenland Ice Sheet (GIS) melting was above average during 2016, ranking as the 10^th highest in the 38-year satellite record. Summer air pressure was higher than average, a trend observed during the last several years, inducing drier and warmer conditions and surface melting, particularly along the western coast. Areas of darker ice with lower albedo, frequent along the west coast, were exposed acting to further enhance warming and ice sheet melting (NSIDC, 2016). 

(1) Average daily melt area anomaly for Greenland comparing melt area in each year to the 1981-2010 average
(2) Cumulative melt day area for 2015 and 2016 against the highest ice melting extent years (2007, 2010 and 2012) (NSIDC, 2016)

Why is the GIS so important?

The GIS covers most of Greenland (~81%) and at around 1.7 million km² is the second largest ice body, following the Antarctic Ice Sheet. The biggest concern over Greenland is that the complete melting of the ice sheet could cause a 7m+ sea level rise, enough to submerge 1.14 million km² of land with populations of 375 million people. 

However, sea level rise is not the only major consequence of the ice sheet melting. A fast rate of melting would produce large quantities of freshwater in the North Atlantic on top of the heavier salt water. Such an alteration in sea salinity could depress the Gulf Stream and fundamentally alter ocean circulation (Driesschaert et al., 2007). Atmospheric circulation would also be liable to significant change, with declines in heat transfer from equatorial to polar regions because of reduced global temperature difference.

Response to warming

Mass loss of the GIS is occurring through two main mechanisms: integrated surface mass balance decreases (difference between surface accumulation and ablation) and glacial acceleration inducing increased discharge (Fyke et al., 2014).

Furthermore, with the enhanced melting of the GIS, an increase in the formation of supraglacial lakes (meltwater in the surface depressions of an ice sheet) during the melt season has been apparent over the past two decades. The presence of supraglacial lakes leads to a reduction in albedo and, acting as a positive feedback, enhances surface melt (Ignéczi et al., 2016). Water reaching the ice sheet base through the drainage of these lakes transports warmth to the base of the ice sheet, enhancing basal sliding and ice velocity (Johansson et al., 2013).

The video below nicely summarises the mechanisms contributing to the decline of the GIS and the observations that have been recorded over the last few decades:

The carbon cycle

In addition to releasing vast quantities of freshwater, the melting of the GIS is exporting many nutrients, including dissolved silica. Dissolved silica is essential in sustaining diatom communities, which act as a carbon pump in the oceans. Meire et al. (2016) recently investigated the extent of dissolved silica export through the physical and chemical weathering by glaciers. Previously considered inactive in the global silica cycle, meltwater from the GIS was shown to strongly enrich surface waters with silicate. An increased supply of between 20-160% to coastal areas was predicted by the end of this century.

The decay of the GIS may also have carbon cycle feedbacks counteracting the reduction in CO2 caused by potentially higher ocean primary productivity. Ryu and Jacobson (2012) investigated the significance of glacial meltwater in transporting inorganic and organic carbon to the oceans and atmosphere. They found that, although rather modest compared to other sources, rivers draining the GIS were contributing to increased CO2 in the atmosphere. It is likely that soils, vegetation, and microbial metabolism have created this carbon reservoir beneath the GIS. Scenarios inferred by simple models suggest that CO2 released from the GIS could act as a rapid positive feedback mechanism. However, many of the parameters used are presently highly uncertain, for example the size of the CO2 reservoir beneath the ice sheet, rate of chemical weathering, and timing and trend of melting. Uncertainties are also apparent in the scaling of carbon fluxes measured from the Akuliarusiarsuup Kuua River (drains the Isunnguata and Russell Glaciers) to the entire GIS.