Why is the Greenland Ice Sheet melting faster?

Why is the Greenland Ice Sheet melting faster? 1
Greenland Ice Sheet

The cryosphere plays a vital role in affecting climate changes. The largest part of the cryosphere consist of ice and snow on land and includes the ice sheets in Greenland and Antarctica, along with the ice cap, glaciers, areas of snow, and permafrost. 

The Greenland Ice Sheet covers 1.7 million km2 – second in size only to the Antarctic Ice Sheet – and it is melting at an increasing rate.

If the Ice Sheet was to melt fully, it would add c. 7.2 m to global sea levels, a devastating event. A useful video illustrates the world-wide effects of just part this Ice Sheet melting.

The rate of melting of the Greenland Ice Sheet has increased since 1992. Therefore, there has been an urgent need for more quantitative data to enable predictions of future Ice Sheet melting to be made. The NERC funded Black and Bloom project has been a major effort to provide this data.

The project was set up to quantify how two factors – not included in earlier studies – affect the rate at which the Greenland Ice Sheet is melting:

  • Soot (black carbon) derived from wild fires or human sources, can darken the surface of the Ice Sheet reducing its albedo (reflectivity) resulting in increased melting. 
  • Microorganisms, many of which are photosynthetic, growing in the melt water can also reduce the albedo of the ice sheets increasing melting.

The Science

The project has carried out field expeditions along the western margin of the Greenland Ice Sheet where the albedo reduction has been the greatest. 

The project was divided into 4 working packages:

Package 1:  The Microbiology

This package has examined the growth and evolution of microbial communities on the surface of the Ice Sheet, and studied how the populations change over the seasons. At the same time, temperatures, sunlight and nutrient concentrations were measured to determine what controls the growth and survival of the microbial populations. 

The pigments contained by the microorganisms were also measured to link the data to other work packages.

Package 2:  The Particles

This package has characterised and quantified particles on the surface of the Ice Sheet across the seasons. Black carbon and aerosol size distribution were measured near the project camp, and air samples were filtered for later analysis in the UK.

The interactions between the microorganisms present and the particles were also quantified. It is thought that this interaction can retain the particles on the surface increasing the effect on the albedo. 

Package 3:  The Albedo

Variations in the albedo have been related to the results of Packages 1 and 2 using surface measurements, ice coring and UAV (unmanned aerial vehicle) flights. Work included reflectance measurements across the electromagnetic spectrum looking at how these were affected by albedo-reducing processes.  A model was prepared to enable predictions of albedo changes to be calculated under a range of different climate scenarios. 

Package 4:  The Scaling-Up and Modelling

This package has brought together the results of the others 3 packages, linking field and laboratory measurements with large-scale satellite imagery of changes in the albedo. This enabled projections to be made for surface melt under future climate scenarios.

The team

The project has been carried out by an impressive interdisciplinary team of scientists:

Package 1 – Microbiologists

Package 2 – Particulate Experts

  • Professor Liane Benning – GFZ Potsdam – Package Leader
  • Dr Jim McQuaid – Leeds University – Package Co-Leader
  • Dr Jenine McCutcheon – Leeds University – Post-Doctoral Research Associate
  • Dr Stefanie Lutz – GFZ Potsdam

Package 3 – Albedo Experts

Package 4 – Remote Sensors and Modellers

Advisers and Collaborators

Results

The team published a paper in 2017 that summarised some of the results covering the south-west sector of the Ice Sheet.  Field observations revealed that a variety of light-absorbing impurities (LAIs) were present on the surface, ranging from inorganic particulates to cryoconite (windblown powdery dust containing small rock particles, soot and microbes), and ice microalgae. Satellite observations showed that the areal extent of dark ice varied significantly between successive melt seasons. The authors concluded that they were unable to project how the albedo of bare ice sectors of the Ice Sheet will evolve in the future, causing uncertainty in the projected sea level contribution from the Ice Sheet over the coming decades. 

MODIS satellite imagery was used to examine dark ice dynamics on the south-west Ice Sheet each year from 2000 to 2016. The dark ice was quantified in terms of its annual extent, duration, intensity and timing of first appearance. Not only does dark ice extent vary significantly between years but so too does its duration, intensity and the timing of its first appearance.

The spatial distribution of dark ice was best explained by outcropping of particulates from eroding ice. These particles may enable the growth of pigmented ice algae which cause visible surface darkening, when liquid meltwater is present along with sufficient light.

Another paper published in 2018 described the microorganisms that were identified as part of the project. More than 99% of the microalgae identified fell with 8 taxa. The most common organism was Anclyonema nordenskioldii, followed by Raphidonema sempervirens, Chloromonas alpina, and Chloromonas polyptera. The microbial community composition was found to vary with the stage of the melt season.

Further results of the project are available online.

Although much work remains to be done, the Black and Bloom project has shown that microorganisms play an important role in affecting the climate even in the extreme temperatures of the Greenland Ice Sheet.

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