New light shed on the darkening of the Greenland ice sheet

Every year, the snow covering the ice sheet margins melts away to uncover the darker ice beneath. Bare ice surfaces are less reflective than snow, causing more solar radiation to be absorbed, and therefore more melt. 

In the southwestern margin of the ice sheet, the transition is even more abrupt, as snow uncovers a very dark ice region known as the Dark Zone, where melt rates are substantially enhanced. What make the ice so dark here is both a scientific mystery and a crucial missing piece in climate models. 

Researchers from the ERC Deep Purple Project set themselves the task of developing a method to investigate the darkening of the ice surface from the sky, not knowing they were embarking on a 4 year quest…

Scaling up to a satellite pixel: a long-term team effort

The core task was to understand how to extract information about the ice surface from spaceborne observations. Satellites are not only able to detect how dark the surface is, but also how darkness varies across the spectrum of light’s different wavelengths - creating a spectral signature that encodes information on the state of the ice surface, including the presence of particles such as dust, algae and black cryoconite.

The main challenge became decoding this signature – so the team developed a new methodology exactly fit for this purpose, blending physical and machine learning techniques.

Creating such a model would have been impossible without the 20+ years of accumulated data, crucial to the design, model calibration and validation steps. The study itself took several years to be completed, but in reality, it represents a cumulative effort of a science community spanning over 30 years. 

Uncovering ice darkening from pixel to regional scale

The model identifies how different types of particles are distributed across the melt zone of the ice sheet, how the ice structure changes spatially, and how each factor contributes to darkening the ice surface. 

These results represent a major advance in our understanding of the Dark Zone and our ability to study the state of the ice sheet surface.

The implications go well beyond the question of surface darkness - the model can be used to understand the accumulation of meltwater at the ice sheet surface, and the development and spread of the dark algal blooms, characteristic of the western margin of the ice sheet.

Looking into the future: from remote sensing to climate modeling

Each big step forward always opens doors to new challenges. The next big question is how to predict the ice surface state in the future and incorporate this knowledge into climate models.

This study demonstrates that various physical and biological processes will need to be accounted for, so that addressing the challenge of future darkening will require an even stronger interdisciplinary and collective effort.

Link to the scientific article: https://doi.org/10.1126/sciadv.ady9482