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Book Review

Watermelon snow – Colorful algae speed up the melting of glaciers

 

Lutz, S., A. M. Anesio, R. Raiswell, A. Edwards, R. J. Newton, F. Gill, and L. G. Benning (2016), The biogeography of red snow microbiomes and their role in melting arctic glaciers, Nature Communications, 7, 11968, doi:10.1038/ncomms11968.

 

White reflects a lot of sunlight so you stay cool and snow stays frozen. Black absorbs a lot of light so you heat up and snow melts.

White reflects a lot of sunlight so you stay cool and snow stays frozen. Black absorbs a lot of light so you heat up and snow melts.

Think about choosing a t-shirt to wear on a hot summer day. You would probably go for a white shirt over a black one. That’s because you have an intuitive understanding of an important difference between the shirts: their “albedo.” Albedo is a measure of reflectance: your white t-shirt has a high albedo and reflects a lot of sunlight. Your black shirt has a low albedo and absorbs a lot of sunlight, making you hot. The same thing happens with snow and ice. Much of the sunlight shining on a mound of fresh snow will reflect off of it, but if that mound is covered in dirt, the dark dirt particles will absorb the sunlight and melt the snow around it.

Map of Greenland colored by the difference in the number of days ice was melting in 2016 compared to the 1981-2010 average. Reds signify above average numbers of melt days in 2016. It’s normal for the Greenland ice sheet to melt a little every summer when temperatures rise above freezing, but during the last few years there have been many more days that the ice is actively melting. Source: http://nsidc.org/greenland-today/

Map of Greenland colored by the difference in the number of days ice was melting in 2016 compared to the 1981-2010 average. Reds signify above average numbers of melt days in 2016. It’s normal for the Greenland ice sheet to melt a little every summer when temperatures rise above freezing, but during the last few years there have been many more days that the ice is actively melting. Source: http://nsidc.org/greenland-today/

 

 

 

Over the last few years, the Greenland ice sheet has seen record melting rates as temperatures have climbed. In an effort to predict how much ice mass we’re losing and how quickly it is disappearing, scientists have been running models that include every possible factor that will affect melt rates, including the amount of dust and soot particles on the ice. The particles, called “black carbon”, blow in from deserts or forest fires and can significantly increase melting by increasing the albedo of the ice sheet.

Recently, researchers have found that there are plants and other microorganisms living in the snow that also increase albedo. Some of the most abundant types of snow-dwelling organisms are algae. Arctic snow algae stay dormant for most of the year when temperatures are below freezing and the sun is low, but when summer comes around, they form massive blooms on the snow and color it in greens and reds. The red comes from a pigment that protects the algae against the sun’s harsh UV rays. The term “watermelon snow” has been used to describe the oddly appetizing sight of a red algae bloom.

Algae come in various colors. Green tends to indicate a lot of chlorophyll, a pigment that harvests light, while red coloring comes from a pigment that protects against UV radiation. Some algae even start of green at the beginning of the summer and turn red as the sun gets stronger. Source: https://blackandbloom.org/

Algae come in various colors. Green tends to indicate a lot of chlorophyll, a pigment that harvests light, while red coloring comes from a pigment that protects against UV radiation. Some algae even start of green at the beginning of the summer and turn red as the sun gets stronger. Source: https://blackandbloom.org/

 

 

A team of scientists led by Stefanie Lutz at the University of Leeds set out to determine how much of an effect these red algae have on melting rates. They looked at 40 different sites across Greenland, Sweden, Svalbard, and Iceland. They chose sites in various environments: above and below the Arctic circle, at high and low elevations, near the coast and near the center of the continent. They wanted to sample as many environments as possible to determine the conditions under which red algae flourished.

 

The locations of the sampling sites across the Arctic. Each of the red circles contained several sites.

The locations of the sampling sites across the Arctic. Each of the red circles contained several sites.

 

 

 

 

As it turned out, red algae thrived throughout the range of environments. They did well everywhere. The team took measurements at different times throughout the spring and summer and found a strong relationship between the total amount of red algae found in snow and the albedo – algae decreased the snow albedo wherever they were found, but where there were more algae, the albedo was lower.

 

 Even without algae present, albedo decreases throughout the summer. At the beginning of summer, glaciers are covered with pure white snow, which has a very high reflectance, but as the snow gets wet and begins to melt, it begins to absorb more light, and this accelerates the melting process. The presence of algae speeds up that process even more. They start to wake up from their nine-month hibernation just as the snow is beginning to melt. The big colorful blooms they form decrease the snow albedo and increase the melting rate. Eventually the clean top layer of snow is gone, exposing the dirtier ice beneath, which absorbs even more light and melts even more ice.

Surface albedo over time for snow without algae (black) and snow with algae (red). Algae significantly decreased the surface albedo over the summer season leading to a total integrated decrease of 13%.

Surface albedo over time for snow without algae (black) and snow with algae (red). Algae significantly decreased the surface albedo over the summer season leading to a total integrated decrease of 13%.

The researchers found that over a 100 day melt period the presence of red algae on the snow would decrease the overall albedo by 13%. This is a significant reduction, which will have important effects on the total melt rate. It’s important that we really understand how quickly the Greenland ice sheet is melting – if the entire thing melted, global sea levels would rise by seven meters. This would put several of New York’s neighborhoods and many San Francisco Bay towns underwater. (To see how rising sea levels will affect your area, check out this tool from Surging Seas.) As temperatures continue to rise and the melt season continues to lengthen, the algae will have more time to stay active and darken the snow. And so, the work to quantify the importance of these and other algae continues: the team will spend the next two summers on the Greenland ice sheet working on the “Black and Bloom” project.. They set up a field camp in early July and will stay through late August to study the “black” (particles) and “bloom” (microbes) processes affecting glacial melt.

 

 

 

 

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