Black and Bloom

Summary: Dark ice dynamics of the south-west Greenland Ice Sheet

We recently published a paper in The Cryosphere which examines the dynamics of dark ice in south-west Greenland.

Some of the largest declines in the albedo of the ice sheet have occurred along the south-western margin of the ice sheet, where we now see ‘dark’ ice appear in some years. Previous studies have revealed that a variety of light-absorbing impurities can be present on the surface, ranging from dust and black carbon, to biological materials such as cryoconite and ice algae. The darker (i.e. the lower the albedo) the ice sheet surface is, the faster it will melt for a given temperature above 0oC.

Satellite observations show that the extent of dark ice has varied significantly between recent successive melt seasons. However, we have not been able to understand what has been causing such large changes in the extent of dark ice between years. At present we therefore have no idea whether we will see more dark ice in the future, which means we cannot accurately estimate how much melting the ice sheet will undergo during this century. In turn, this makes upcoming sea level contributions from the ice sheet uncertain.

Here we use satellite imagery (from the MODIS sensor) to examine dark ice in the south-west of the ice sheet each year from 2000 to 2016. Not only does dark ice extent vary significantly between years, but so too does its duration, how dark it is and the timing of its first appearance. Comparison of dark ice with changes in the weather and meteorology reveals a few links. These include the sensible heat flux during June, July and August, the number of days in the melt season which have continuous melting 24 hours a day, and the timing each summer of when the snow is melted back to reveal the bare ice underneath.

Using these findings, we suggest that whilst the location of dark ice is best explained by impurities which are melting out of the underlying ice, these particulates alone do not drive dark ice dynamics. Instead, they may enable the growth of ice algae which cause visible surface darkening, but only when there is liquid meltwater around and plenty of sunlight for photosynthesis. Further field studies are required to understand the growth of ice algae and the likely link between algae growth and impurities.