01.25.2021 07:00 AM

Sun-Loving Bacteria May Be Accelerating Glacial Melting

Scientists find that cyanobacteria cause sediments on glaciers to clump, thus absorbing more sunlight. It's not great news for fans of lower sea levels.

The butterfly effect describes how a tiny input, like the flapping of an insect’s delicate wings in South America, can kick off a series of snowballing events, like the formation of a tornado in North America. At least, that’s the idea in the mathematical world of chaos theory. While atmospheric scientists will tell you that it’s not particularly likely that a butterfly has such powers—no doubt a relief to conscientious butterflies everywhere—the effect in general is real: Seemingly inconsequential events can trigger a cascade of knock-on effects that grow in size and significance. On the Greenland ice sheet, scientists say they’ve found an agent of such change that’s far smaller than a butterfly, but whose proliferation could have far more consequence than a tornado.

Cyanobacteria—photosynthetic microbes that live in meltwater—are likely growing more abundant here, thanks to warmer temperatures and decreased cloud cover. When these bacteria come into contact with sediments (largely made of quartz) on a glacier, they make the particles clump together to form balls 91 times their original size. So instead of the little particles washing away in meltwater, they start accumulating in streams atop glaciers, which are more formally known as supraglacial streams.

Photograph: Sasha Leidman

“This sediment is very dark, and therefore absorbs a lot of sunlight,” says Rutgers University hydrologist Sasha Leidman, lead author on a recent paper describing the findings in the journal Geophysical Research Letters. “What the paper found is that this sediment would not be there without the fact that there's bacteria growing in the sediment and clumping it together so it can't be washed away.” More dark grit, then, could be absorbing more of the sun’s energy, and accelerating the melting of the ice sheet.

And in case you haven’t been reading the news: Melting glaciers are bad.

These microscopic bacteria could have big implications for the planet. Greenland’s ice sheet covers over 650,000 square miles, and if it melted entirely, global sea levels would rise 24 feet, according to NASA. That’s not plausible anytime soon, but NASA further estimates that Greenland lost 3.8 trillion tons of ice between 1992 and 2018, contributing 0.4 inches to global sea level rise in that time.

To be clear, the presence of bacteria on Greenland’s ice sheet is nothing new. Microbes are intertwined with sediments that either make their way up the ice from exposed land around the base of the glaciers, or blow in from farther away. As this dust accumulates on the glacier, it forms what scientists call cryoconite holes: The darker sediment absorbs the sun’s energy, heating the ice to melt away a divot, which you can see below.

Photograph: Sasha Leidman

Mixed in with this grit and melted ice are the cyanobacteria, which run on sunlight. As a cryoconite hole gets deeper, its bottom moves out of direct sunlight, meaning there’s less energy available for the cyanobacteria living within it. But, Leidman says, “when it rains, or there's a heavy melt event, the sediment in those cryoconites gets washed out and washed into these supraglacial streams, where they accumulate in floodplains.”

Now the bacteria are exposed to all the sunlight they could ever dream of, especially given the decreased cloud cover over Greenland. As they proliferate, the cyanobacteria have two ways of darkening that sediment. For one, they themselves produce a dark substance, a combination of humic acids and what scientists call extracellular polymeric substances. The former comes from the degradation of dead bacteria, and may offer surviving bacteria UV protection. The latter is a glue-like ooze that helps the cyanobacteria stabilize their local environments.

The second way, says Leidman, is that “they change the structure of the sediment, clump it together so that it can more easily hold water and more easily stick to surfaces. So just the fact that it’s clumped together means that it can absorb more sunlight.” The accumulated buildup in the supraglacial streams is significantly darker than the ice itself.

By flying drones around Greenland’s ice sheet, Leidman and his colleagues found that the sediment can cover up to 25 percent of a stream’s bottom. (Check out their beautiful footage below.) In addition, they estimated that without the bacteria acting to gather the grit, just 1.2 percent of the bottom would be covered, because the smaller loose particles would wash away instead of settling.

The researchers are still grappling with many unknowns, though. Given that the cyanobacteria run on sunlight, they’ll likely proliferate as Greenland warms. But how warm is too warm? “We don't really know whether these bacteria will survive with higher temperatures or greater flow rates, or how the rivers will be changing their shape,” says Leidman. But, he adds, “as the temperature increases, there's likely going to be more bacterial growth. So while it's definitely not the leading cause of increases in melt rates, it most likely is a non-negligible factor.”

This research shines light not only on a seemingly significant phenomenon unfolding atop glaciers, but could also give scientists a better understanding of how glacial melting fits into the larger picture of climate change, says Kyra A. St. Pierre, a biogeochemist at the University of British Columbia who studies Arctic hydrology but wasn’t involved in this new work. “I think the more that we spend time within these systems, the more we're going to find these processes, and the better we're going to be able to actually predict what will happen over time,” she says.

The trick will be scaling this up, modeling how cyanobacteria might be influencing melt across larger glacialized areas, then incorporating that into global models of climate change to improve their accuracy in predicting sea level rise.

The work may also help change the very way we characterize glaciers: They’re a whole lot more than stagnant giant chunks of ice, as it turns out. “We did definitely think of glaciers as these cold, kind of dead environments, in a lot of cases,” says St. Pierre. “And this is testament to the fact that there's a lot more going on in these systems than I think we originally thought.”