Ocean color satellites reveal glacier algae, insights for climate models

UNIVERSITY PARK, Pa. – The brownish-grey algae that darken the Greenland ice sheet in summer cause the ice to melt faster, but only recently have scientists measured these blooms in the field, and only at few sites. To measure algal blooms across large regions and understand their effects on melting over time, scientists are now turning to space.

“Scientists go into the field and sample one or two spots where these algal blooms occur, but we don’t really know how they change over time or over a large region,” said Shujie Wang, assistant professor of geography at Penn State. “To solve this problem, my research team and I borrowed the methodology used to measure algae in water, which uses ocean color satellites and has a long history.”

The algae that bloom on water differ from those that bloom on ice, but both species contain chlorophyll-a, which has a distinct reflected near-infrared radiation signature that satellite sensors can detect.

Mapping glacial algae over space and time can give researchers insights into how algae affect albedo, said Wang, who conducted the research as a postdoctoral scholar at Columbia University’s Lamont-Doherty Earth Observatory under the supervision of Marco Tedesco, Lamont research professor. Albedo is a parameter that says how much incoming solar radiation is reflected by a surface.

“Albedo is crucial for understanding how ice melts and what will happen in the future to the contribution of Greenland to sea level rise,” explained Tedesco. “Little is known of the effects of algae on this, and the work by Shujie is pioneering in this regard. It was amazing to have her as a postdoctoral scientist.”

Snow and ice have a high albedo, meaning that most of the incoming radiation is reflected to the atmosphere. When algae accumulate on the ice surface, however, they absorb the radiation, warming the ice and accelerating surface melt.

The researchers used data from the Medium Resolution Imaging Spectrometer (MERIS) on the European Space Agency’s Envisat satellite to quantify glacier algal blooms in southwestern Greenland from 2004 to 2011. They compared the data to measurements taken in the field and by the Moderate Resolution Imaging Spectrometer (MODIS), which measures surface albedo. They report their findings in the journal The Cryosphere.

The scientists found that chlorophyll-a signatures captured by MERIS matched field data, confirming that researchers can use ocean color satellite data to measure algal growth and see how it changes over the summer. The same held true for the albedo changes measured by MODIS.

“We came up with a rough estimate that if algae growth doubles, then albedo decreases between 2% to 4%,” said Wang, who also holds an appointment in Penn State’s Earth and Environmental Systems Institute. “This biological component has never before been incorporated into climate modeling.”

The researchers also found that years with fewer cloudy days, and therefore more solar radiation, saw more algae development.

“Algae need solar radiation to create energy through photosynthesis, so shortwave radiation may be an important factor controlling how much algae will grow or how they’re spatially distributed,” Wang said.

The interplay between climate, algae and ice creates a feedback cycle that accelerates the melting process. As the world warms and atmospheric patterns change, recently there has been a trend towards more sunny days. More sunny days means the ice will absorb more solar radiation and melt faster. The solar radiation will also promote algae growth, further accelerating melt.

Incorporating this biological component into regional climate models can help improve albedo projections, said the researchers.

“Algae and bacteria are the only life on the ice sheet, and it is fascinating to imagine that the areas where they thrive are among the closest thing we can imagine to extraterrestrial life. The project was, indeed, co-funded by the NASA Astrobiology program,” said Tedesco, who also holds a position at the NASA Goddard Institute for Space Studies.

The next step is to quantify how much algae development increases meltwater. Wang plans to address that question in subsequent research.

Also participating in this research were Patrick Alexander, Lamont-Doherty Earth Observatory and NASA Goddard Institute for Space Studies; Min Xu, University of South Florida; and Xavier Fettweis, University of Liege, Belgium.

The National Science Foundation, NASA Exobiology program, NASA Modeling, Analysis and Prediction program and the Heising-Simons foundation supported this research.