Contributions to Sea Level Rise: Greenland Ice Sheet

Having focused in the last post on the contributions of glaciers and ice caps to SLC, this week focuses on the one of the two largest stores of freshwater the Greenland Ice Sheet. Along with the Antarctic Ice Sheet it holds 98% of the freshwater stored in ice and so its stability or otherwise could be crucial to SLC in the future. The behaviour of ice sheets creates the most significant uncertainty for future SLC, as their response to climate change is poorly understood and they are notoriously difficult to model.

The volume of ice in Greenland is equivalent to 7 m of SLR

From this statement it is clear that melting of ice from Greenland (below) has potential to catastrophically affect the world. However what is the likelihood of significant ice loss from this source?

The Greenland Ice Sheet with some major outlet glaciers indicated. Source

Current situation

The Greenland Ice Sheet, despite being significantly smaller than the Antarctic Ice sheet has in recent decades contributed more to global SLR due to increased surface melting (Rignot et al., 2011).  Straneo & Heimbach (2013) suggest mass loss has quadrupled in the last 20 years and contributes up to 25% of global SLR. This is attributed to increased surface runoff due to summer surface melting (see below) as a result of the warming temperatures in the region. Temperatures have increased rapidly by up to 5 degrees in recent years which increases the propensity to melt (Applegate et al., 2014). Until recent years this has been offset by increases in precipitation (Hanna et al., 2007).

Increased area of surface melting in the last 10 years. There is some interannual variability but a trend for increased melting. Source

However the increased run off has been linked to a subsequent increase in sliding at the base of the glacier which increases velocity and therefore calving in the ocean terminating glaciers (Zwally et al., 2002). The graph below summarises the recent situation of the Greenland Ice Sheet, with the decrease in Surface Mass Balance (red) particularly worrying and suggesting SLR.

Graph showing mass flux of Greenland Ice Sheet. Blue line (Precipitation), Green line (Melting), Red line (Surface Mass Balance), Orange Line (Run off). Dashed lines are trends since 1990. Source

Greenland has only a few outlet ice streams that reach the sea, and act as the major deliverer of melt to the sea. Indeed some of Greenland’s outlet glaciers such as Jakobshavn have been dubbed ‘the fastest glaciers in the world’. As the image below shows, Jakobshavn has retreated rapidly in the past 150 years. This is worrying as Jakobshavn drains 6.5% of Greenland’s Ice sheet area. This picture is repeated across a number of outlet glaciers with rapid retreat and increasing contribution to SLR.

The retreat of the Jakobshavn Glacier since 1950. Source

Increasing ocean temperatures around Greenland result in rapid calving at the front of the glaciers as this video shows, attributed to rapid submarine melting due to the increased ocean temperatures (Rignot et al., 2010). This speedup continues with it estimated to move up to 46 m per day! As the clip from the film 'Chasing Ice' shows, calving can be rapid and dynamic causing SLR.

The Future

The recent acceleration in ice loss from the Greenland Ice Sheet means it is contributing more and more to GMSL rise. This contribution is projected to increase as more surface melt due to occurs over more sections of the Greenland Ice Sheet, which increases ice loss. The IPCC suggests it will contribute 0.01- 0.17 m to GMSL rise (or 0.2 - 2 mm/yr) by 2100 but these estimates may be an underestimation due to limits in the understanding of glacier flow, leading to uncertainty as shown in the graph below. 

Projections of Greenland's contribution to SLR by 2100. C is Solid Ice Discharge (ie. calving), D is Surface Mass Balance (melting). Source

Irreversible Loss?!

Some GCMs have projected a non-linear response to warming for the Greenland Ice Sheet; as the surface melts it lowers and this warms the near surface which would further melt the ice sheet. This could lead to vastly increased SLR as the Ice Sheet dynamically thins. This is as yet uncertain and projected as unlikely in the 21st Century but further forward could lead to massive reduction in size of the Greenland Ice Sheet and significant contribution to SLR.