Microbes in the sediment of the meltwater stream can help increase the island's contribution to sea level rise
PICTURE: SUPRAGLACIAL STREAM AND SEDIMENT FLOODPLAIN IN THE SOUTHWEST GREENLAND. View More CREDIT: SASHA LEIDMAN
According to Rutgers scientists, bacteria are likely to cause stronger melting on the Greenland ice sheet and possibly increase the island's contribution to sea level rise.
This is because the microbes cause sunlight-absorbing sediment to clump together and build up in the meltwater streams. This comes from a study conducted by Rutgers – the first of its kind – in the journal Geophysical Research Letters. The results can be fed into climate models, which leads to more accurate predictions of melting, say scientists.
"These streams can be seen all over Greenland and are a bright blue color that causes further melting as they absorb more sunlight than the surrounding ice," said lead author Sasha Leidman, a PhD student in the lab of co-author Asa K. Rennermalm, Associate Professor at the Institute of Geography, School of Arts and Sciences at Rutgers University-New Brunswick. "This is compounded as dark sediments build up in these streams, which absorb even more sunlight and cause more melting, which can increase sea level rise."
The Greenland ice sheet covers approximately 656,000 square miles – most of the island and three times the size of Texas, according to the National Snow & Ice Data Center. Global sea levels would rise by an estimated 20 feet if the thick ice sheet melted.
With climate change, sea level rise and coastal storms threaten low-lying islands, cities and countries around the world.
Most scientists ignore sediments in glacial streams that form on the Greenland ice sheet when meltwater flows to the ocean, but the Rutgers-led team wanted to find out why they accumulated so much sediment. In 2017, scientists flew drones over an approximately 400-meter-long stream in southwest Greenland, taking measurements and collecting sediment samples. They found that sediments cover up to a quarter of the creek floor, far more than the estimated 1.2 percent that would exist if organics and cyanobacteria didn't cause granules of sediment to clump together. They also showed that streams have more sediment than predicted by hydrological models.
"We found that sediment can only collect in these streams if bacteria grow in the sediment and clump it into spheres 91 times the size of what it was originally," said Leidman. “If bacteria didn't grow in the sediment, all of the sediment would be washed away and these currents would absorb significantly less sunlight. This sediment aggregation process takes longer than human history. "
The solar energy absorbed by currents is likely to depend on the health and longevity of the bacteria, and further warming in Greenland may lead to larger sediment deposits in glacial currents, the study said.
"A decrease in cloud cover and a rise in temperature in Greenland are likely to lead to increased growth of these bacteria, resulting in more sediment-related melting," Leidman said. “As climate change is causing more of the ice sheet to be covered by streams, this feedback may increase Greenland's contribution to sea level rise. By including this process in climate models, we can more accurately predict how much melting will occur, although it is not certain how much more melting will occur compared to the melting predicted by climate models. It probably won't be negligible. "
Rutgers co-authors include PhD student Rohi Muthyala and engineering professor Qizhong (George) Guo. A scientist at the University of Colorado Boulder contributed to the study.