Dark Ice and Bio-albedo: Seminar Sum-Ups


The Greenland Ice Sheet has experienced an increase in melting since the early 1990s due to global climate change especially from atmospheric warming. This melting has huge implication for sea level rise. Therefore, if we can better understand surface melting of the Greenland Ice Sheet we can quantify the impact on sea level rise in the future. For climate change mitigation and the safety of our planet it is essential we act now.

This was the motivation for the research Andrew Tedstone from the University of Bristol explained in a recent seminar I attended. Andrew works as part of a team named BLACK and BLOOM and their focus is to understand how dark particles and microbes can darken the Greenland Ice Sheet (Figure 1) and potentially accelerate surface melting.
Figure 1: Location of Greenland and the two field sites, (Google Maps and Satellite).

MOTIVATION


Note: Albedo – Albedo is the measure of reflected radiation from the sun. In light areas more radiation is reflected so the area is cooler in contrast dark areas absorb more radiation and are generally warmer. 

 Surface melting can be enhanced by areas of dark ice as they reduce the albedo of the ice sheet. Numerous sources for this variable darkening of Greenland ice have been proposed however BLACK and BLOOM believe Ice Algae are the important contributors. Ice Algae are tiny bacteria that coalesce together and need water, light and nutrients to survive. They also have a population that doubles every 4-5 days. 

Andrew explained the AIM of their research was to understand how these microbes affect albedo. With MOTIVATION coming from the fact changes in albedo affect melting of the Greenland Ice Sheet and in turn global sea level rise. 

Figure 2: Area of dark ice with snow accumulation in the crevasses. A: Surface algae on the Greenland Ice Sheet, B: Surface blooms, C, D, E, F, G: Photomicrographs of the algae cells, (meltfactor.org, Stibal et al., 2017) 

 They have significant evidence to believe it is Ice Algae contributing to the albedo effect, known as ‘bioalbedo’. Satellite imagery from MODIS (Moderate Resolution Imaging Spectroradiometer) indicates that the West margin of Greenland has seen a strong decline in albedo however, overall there is a large variability in the extent of dark ice from year to year. Although once it goes dark it stays until the end of the melt season. The darkness also increases over the season. As algae double their population rapidly this can explain the gradual darkening over time. Once dead the algae turn clear explaining the variance of darkness between years and seasons. The algae do need nutrients and water which means that dust from previous climates that is now being melted out from the ice sheet could provide this. They also implied there could be a positive feedback during these dark years as more melting occurs – more microbial growth happens so there is more darkness and the albedo lowers therefore more melting ensues. 

The team at BLACK and BLOOM established these pieces of evidence as proof that Ice algae influence dark ice and albedo of the Greenland Ice Sheet. This was ascertained even before getting to the field. 


FIELDWORK

They undertook three seasons of fieldwork in the western margin of the ice sheet and had a chance to really understand the heterogeneity of the dark ice even within a 100cm2 area. One puzzling feature was the daily variance in albedo they observed which could not be attributed to algal population increase. However, from further investigation they concluded that this is due to complex changes in the physical characteristic of the ice. The team also did more fieldwork in the north-west of the ice sheet where ice-algae was also in abundance demonstrating their effect is widespread.


APPLICATION

Regional Climate Models often poorly constrain bare ice albedo and no biological darkening is ever included. So Andrew and his team have used their fieldwork to build ice albedo into a regional climate model named MAR. Their results show albedo values that accurately align with the MODIS satellite results much better than previous models that neglect bioalbedo. This is evidence that using the ice algae effect to darken bare ice in a regional climate model effect outcomes more realistic albedo results. Upscaling to ice sheet wide reveals that ice algae could encourage 20% more melt in the dark ice years. This could have a substantial effect on sea level rise in the future. 



Wider context


The team at BLACK and BLOOM have demonstrated dark ice years on the Greenland Ice Sheet can be correlated with Ice Algae provided they have liquid water and nutrients to survive. Ice Algae provide an important positive feedback on albedo and surface ice melting. As the planet warms due to climate change melting of the Greenland Ice Sheet could have catastrophic consequences for global sea level rise. This work exemplifies that Ice Algae significantly lower albedo and subsequently enhance surface melting of the ice sheet. Further research by Andrew and his team will help to quantify the impact of Ice Algae on albedo, the future of the Greenland Ice Sheet and most importantly how this will impact sea level in the future.  



Meg







Links you might be interested in:



BLACK and BLOOM website -

https://blackandbloom.org/



Published papers on the topic -

Cook, J.M., Hodson, A.J., Gardner, A.S., Flanner, M., Tedstone, A., Williamson, C., Irvine-Fynn, T., Nilsson, J., Bryant, R. and Tranter, M., 2017. Quantifying bioalbedo: a new physically based model and discussions of empirical methods for characterising biological influence on ice and snow albedo. Cryosphere, 11(6), pp.2611-2632.

Tedstone, A.J., Bamber, J.L., Cook, J.M., Williamson, C.J., Fettweis, X., Hodson, A.J. and Tranter, M., 2017. Dark ice dynamics of the south-west Greenland Ice Sheet. Cryosphere, 11(6), pp.2491-2506.

 Stibal, M., Box, J.E., Cameron, K.A., Langen, P.L., Yallop, M.L., Mottram, R.H., Khan, A.L., Molotch, N.P., Chrismas, N.A., Calì Quaglia, F. and Remias, D., 2017. Algae drive enhanced darkening of bare ice on the Greenland ice sheet. Geophysical Research Letters, 44(22).


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