Supraglacial lakes - What's that?

Recently, our GROCE colleagues from suprojects 7 and 8 published a new study in the science journal The Cryosphere: 'The distribution and evolution of supraglacial lakes on 79°N Glacier (north-eastern Greenland) and interannual climatic controls'. They describe how they mapped  the supraglacial lakes from high resolution images from the Sentinel-2 satellite. In addition, using atmospheric observations from weather stations and climate models, they investigated under which atmospheric conditions a supraglacial lake develops.

But what are supraglacial lakes? 

A report by Jenny Turton (07. September 2021)

Figure 1: Supraglacial lake on the 79°N Glacier. [Credit: Ole Zeising]

Figure 2: The spread of supraglacial lakes in July (purple) and August (pink) in summer 2016 (a), 2017 (b), 2018 (c), and 2019 (d) in Northeast Greenland. [Credit: Turton et al. 2021]

How do supraglacial lakes form?

Supraglacial lakes are large pools of melted ice on the surface of glaciers and ice sheets. You can see them clearly as big blue areas on photographs of the Greenland Ice Sheet in summer (Figure 1). At lower elevations, supraglacial meltwater lakes develop regularly during summer. However, in recent years the lakes are opening at higher locations on the ice sheet, especially during particularly warm summers like 2016. 

And suddenly the lakes are empty!

What is so special about these meltwater lakes? They can very quickly and suddenly drain to the bottom of the ice sheet! - You can imagine it like someone pulling the plug out of a sink. The meltwater from the lakes rushes into deep depths and is suddenly below the ice sheet. There, the water film ensures that the ice can slide better over the rock and the flow speeds of ice streams and glaciers accelerate. Ultimately, this means that more and more ice is transported faster to the coast, where it melts and is carried into the ocean as fresh water. But even under the ice sheet, the meltwater paves its way towards the ocean. Compared to the salty ocean, this meltwater is fresh (i.e. it has no salt content) and thus changes the temperatures and salinity when it mixes with the ocean water.

What did we investigate?

We investigated the number of lakes, the area that they cover and time that they develop in detail and compared them with the prevailing atmospheric conditions, especially the air temperature and snowfall. Summers 2016 and 2019 were characterised by very warm and dry conditions, which led to high total lake areas and the opening of lakes at 1600 m above sea level (Figure 2a/d). Conversely, in 2018, there was a very large snowpack prior to summer and the air temperatures were particularly cool (Figure 2c). 

There is some evidence that the supraglacial lakes are now opening at higher elevations (which are normally too cold for the surface to melt) than previously identified. Global climate models predict that the supraglacial lakes will develop at higher elevations in the northeast of Greenland under climate change. Perhaps we are already seeing the impact of global and local temperature rise in this region.