The Greenland ice sheet and surrounding ice caps routinely experience melt on a seasonal basis – they lose mass by melting and iceberg calving in the summer, and gain mass through snowfall in the winter. The net sum of this gain and loss is the ice sheet’s “mass balance” and is usually measured over a year.

In years of anomalously high melt, such as 2012 or as 2019 is turning out to be, the gain in mass over winter is insufficient to replenish the ice and snow melted in summer, and the annual mass balance is negative.

Greenland has had a negative annual mass balance since about the early 1990s, due to enhanced melting and calving of icebergs. However, the rate of mass loss increased markedly in the mid-2000s. Since 2010 the ice sheet’s mean annual mass balance has been about -300 billion tons, equivalent to a mean global sea level rise of about 0.8mm a year.

This year there have already been significant spikes in melt rate and extent in mid-June and early July.

Apart from the direct influence of the melting ice on sea level rise, the Greenland ice sheet also has a significant impact on regional and global climate. Increased fresh meltwater runoff from the ice sheet may affect ocean circulation in the North Atlantic Ocean, potentially weakening the northwards transport of heat from tropical latitudes.

Over longer timescales, thinning and shrinking of the ice sheet will change local and regional atmospheric circulation patterns, and reduce the reflectivity of the Earth’s surface leading to an increase in the amount of solar radiation absorbed.