An Exceptional Melting Season for the Greenland Ice Sheet
The Greenland ice sheet (GrIS) has shrunk by a record amount during the 2019 melting season. A new study, published in The Cryosphere, details the unprecedented atmospheric conditions which lead to record (or close-to-record) values of low surface mass balance (the ice sheet’s net balance that considers mass accumulated and loss), high runoff (water that flows over land instead of being absorbed or evaporating), and low snowfall. The study relied on remote sensing observations, regional climate model outputs, reanalysis data, and artificial neural networks to illustrate the exceptional nature of this past melting season. They were able to determine that 95.8% of the GrIs surface underwent melting during the summer of 2019, with a maximum daily melt covering 73% of the ice sheet’s surface reached on 31 July 2019. If this doesn’t alarm you, I’m not sure what will.
A number of factors are to blame for the shrinkage of the GrIS. For starters, the summer of 2019 was characterized by a low albedo AND reduced snowfall. This combination resulted in the premature exposure of bare ice and “enhanced the melt-albedo feedback by promoting the absorption of solar radiation”. A little geography recap: albedo is the proportion of incident light or radiation that is reflected by a surface. Something with a low albedo absorbs a larger amount of incident light or radiation, whereas something with a high albedo will reflect incident radiation.
But what led to these snowfall and albedo anomalies? Blue skies seem to be the largest culprit. The reduced cloudiness in the area, caused by anticyclonic conditions, was responsible for a knock-on effect of “below-average summer snowfall and albedo”. The researchers uncovered that it was “the persistency of high-pressure systems […] that promoted an increase in the absorbed solar radiation”. These high-pressure systems persisted for 63 of the 92 summer days — that’s equivalent to 68% of the summer! The increase in atmospheric circulation pattern changes can be attributed to climate change: when regional temperature and humidity is altered (by planetary warming), winds respond by changing the intensity and structure of their circulation. Moreover, these anticyclonic conditions stimulated a flow of warm, moist air to move along the ice sheet all the way to the northern portion — which has seen a record-breaking increase in surface melt.
Gaining an understanding of atmospheric circulation’s role in the shrinkage of ice sheets is essential for accurate sea level rise forecasting. Coastal cities and coastal habitats are increasingly at risk of major flooding events. Should the entirety of the GrIS melt, sea level would rise by an estimated 6 meters. Moreover, the addition of freshwater to ocean ecosystems would cause significant damage to saltwater organisms. Many glacier models are likely gravely underestimating future melt since they are not accounting for these changing circulation patterns. In fact, “If such high-pressure zones become a regular annual feature, future melting could be twice as high as currently predicted”. As demonstrated by this study, the persistence of high-pressure systems and atmospheric circulation anomalies, are a major driver in ice melt and should be researched further.
