Study co-author Nathan Chrismas collecting surface ice for analysis. Credit: Karen Cameron/Sara Penrhyn Jones

New research shows that algae growing on the Greenland ice sheet, the Earth’s second-largest ice sheet, significantly reduce the surface reflectivity of the ice sheet’s bare ice area and contribute more to its melting than dust or black carbon. The new findings, presented by the American Geophysical Union, could influence scientists’ understanding of ice sheet melting and projections of future sea level rise, according to the study’s authors.

Glaciologists have long known materials such as mineral dust and black carbon can darken the surface of large ice sheets. Scientists study these impurities because they reduce the sheet’s albedo, or the extent to which it reflects light, which increases melting of the ice and affects projections of sea level rise. But few studies had examined the darkening effect of algal cells, which naturally grow on the ice sheet.

The new study quantitatively assessed how surface ice algae contribute to darkening of the ice sheet, and found the algae reduce the ice sheet’s albedo significantly more than non-algal materials, like mineral particles and black carbon. Algal darkening is responsible for 5 percent to 10 percent of the total ice sheet melt each summer, according to the new research published in Geophysical Research Letters, a journal of the American Geophysical Union.

The findings sharpen the way glaciologists think about melting of ice sheets and how ice reflects light, according to Marek Stibal, a cryosphere ecologist at Charles University in Prague, Czech Republic, and one of the lead authors of the new study. A warming climate could also increase algal growth in the future, potentially boosting algae’s influence on ice sheet melting, he said.

“The novel aspect of our study is that we discover biological processes play an important role in ice sheet behavior,” Dr. Stibal said. “Glaciologists usually only look at inorganic materials when studying light reflectance and ice melt because biological processes are often too complicated to capture. But we find organisms can have a large-scale effect on a system that was previously studied in an abiotic context.”

Dr. Stibal and his colleagues used portable spectrometers and albedometers to measure the reflectivity spectrum of the bare ice surface each day. They also collected samples of surface ice and used a field microscope to characterize the algae and count the number of algal cells in each sample. They analyzed the relationship between the growth of the algae and the amount of light being reflected by the ice sheet surface.

Nozomu Takeuchi, professor of earth sciences at Chiba University in Japan, said the new research highlights the importance of accounting for biological processes in the cryosphere, and believes the study will have an impact on glaciological research of the Greenland ice sheet. “The major implication of their findings is that the ice sheet is not a simple abiotic system of snow and ice, but rather an ecosystem,” he said. “Understanding this biological process more quantitatively could induce a new perspective on other climate cycles of the Earth, such as glacial-interglacial cycles.”