01/22/2021

Greenland’s Microbial Melt-Down

6:57 minutes

an aerial view of a glacier's movement and its deposit melt
Meltwater lakes form on the surface of Greenland’s Petermann Glacier, June 2019. Credit: NASA/USGS

The Greenland ice sheet covers nearly 700,000 square miles—three times the size of Texas. The ice sheet is estimated to have lost nearly 4 trillion tons of ice in the past three decades. A team of researchers recently investigated how the bacteria in the sediments on the ice sheet could be contributing to the melting of the ice. Their results were published in the journal Geophysical Research Letters

Producer Alexa Lim talks to glaciologist Asa Rennermalm about how the mix of bacteria and sediments can darken the ice, impacting how the ice sheet melts.


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Segment Guests

Asa Rennermalm

Asa Rennermalm is an associate professor of Geography at Rutgers, The State University of New Jersey in New Brunswick, New Jersey.

Segment Transcript

IRA FLATOW: This is Science Friday. I’m Ira Flatow. The Greenland ice sheet covers nearly 700,000 square miles. That’s a lot of ice– about three times the size of Texas. And the ice sheet is shrinking rapidly. Some estimates put the amount of ice lost at nearly 4 trillion tons in the past three decades.

One factor playing a part in this melting? The bacteria living in the sediments on the ice sheet. Producer Alexa Lim has more.

ALEXA LIM: Most people probably haven’t had a chance to venture out to Greenland or an ice sheet. I imagine a pristine white frozen field and crystal clear streams. But in reality, some parts of the Greenland ice sheet kind of look dirty. They’re covered in dark sediments that are teeming with bacteria.

A team of scientists wanted to figure out how these sediments and bacteria might affect the water and ice on the Greenland ice sheet. Their results were published in the journal Geophysical Research Letters. My next guest is one of the authors on that study. Asa Rennermalm is an associate professor of geography at Rutgers University in New Brunswick, New Jersey. Welcome to Science Friday.

ASA RENNERMALM: Thank you.

ALEXA LIM: I know you’ve studied the rivers of the Greenland ice sheet. Can you kind of describe what the floodplains in those rivers look like in the areas that you work?

ASA RENNERMALM: Most of it is covered by snow. But there at the edge, where you have most of the melting occurring, you have these like streams and rivers flowing on the ice. It’s a really beautiful and surreal landscape. It’s basically everything is just ice and water and ice.

But there’s also dark sediment on top of the ice. It kind of reminds you of flood plains in a normal river, where you have sort of a more flat, low-lying area. In parts of these rivers, we measured about a quarter is covered by this very thin layer of sediment that is like overlaying the ice.

ALEXA LIM: And did you notice that there was more sediment clumping in these rivers?

ASA RENNERMALM: A PhD student that I am working with, Sasha Leidman, he’s always been interested in sediment. And he wanted to know how much sediment is transported in these rivers? And so we went up to Greenland, and we measured how much water was flowing. And we took samples of the sediment.

And then when we came back to Rutgers, Sasha took his samples, and he analyzed the size of these sediment particles. And he came up with something super fascinating. It turns out that the sediment grain size he measured was too small to be deposited in the channels. It shouldn’t be any sediment in those channels at all. So we basically found more sediment in those channel floodplains than we would expect.

ALEXA LIM: So then in your study, you came up with an idea for the causes of all these sediments, and it was bacteria.

ASA RENNERMALM: Exactly, I know. I mean, I don’t know about you, but certainly I didn’t think there was any significant biology on the surface of the ice sheet. But people have discovered algae on the ice sheet. There is terms like “biological darkening” of the ice sheet– the fact that biological activity on the ice sheet, like bacteria, algae, can cause the ice sheet surface to become darker, which means it will absorb more of the solar radiation coming in.

That means if you have more solar radiation being absorbed instead of reflecting back to space, you’re going to have more melting, more water flowing out into the oceans, and the risk of more sea level rise. So we remember all of that research. And we’re like, hey, wait a minute. Could it be that it’s actually bacteria, algae, and stuff like that that is making the sediment larger than the mineral grains? Because what we measured was just the size of the minerals.

But then we realized these bacteria can actually clump it together, so it becomes these larger granules. And those granules fall apart when you go to– take them to the lab in New Jersey. The transport from Greenland to New Jersey– they don’t survive that.

ALEXA LIM: You have the bacteria that’s clumping up these sediments and kind of maybe clogging up the rivers or just finding their ways in there more. So then how does all that add up to impact the melting of the ice sheet?

ASA RENNERMALM: Because the particles are larger, they’re less likely to move away, be flushed away by the water. It stays there, and it makes the channels appear darker, right? And that means it absorbs more solar radiation, melts more. Yeah.

ALEXA LIM: So what other ways can bacteria affect the ice?

ASA RENNERMALM: Another term for is called cryoconite.

ALEXA LIM: Cryoconite, OK.

ASA RENNERMALM: Sounds maybe a little bit like a science-fiction movie. It’s just a mix of dust and mineral particles, organic matter. And it’s clumped together. When I go on the part of the ice sheet that’s melting every summer, which is called the ablation zone, it’s widespread. Like it’s on the surface.

It’s so dark compared to the ice that it melts down, right? You get sort of a hole that melts down. So what you see all over– it’s not this like clean ice surface. No, it’s like a perforated surface with these cryoconite holes everywhere. So it’s cryoconite that is just melted down into the ice– roughly like a feet or so.

And that– there’s a dynamics to that, too. That’s certainly influenced by weather conditions. They tend to form if you have strong solar radiation. And then if you have windy, overcast day, they will sort of erode out. So there is a connection between the cryoconite on the surface and the streams, so that it comes from the surface to the streams.

ALEXA LIM: There’s another layer that’s added on top of all this, which is climate change. How does climate change play a role in all of this?

ASA RENNERMALM: Because it’s changing so fast in Greenland, and Greenland has really been warming a lot over the last decades– the Arctic as a whole is warming at least twice the rate as global places. And because organic, like, bacteria, organic matter, is going to be sensitive to the environmental conditions around them, you could imagine that, you know, there could be increased biological activity. Or depending on what kind of changes you have with respect to cloud cover and solar radiation and so on, it’s hard to say exactly how it would change. But it probably would change. Future is unknown, but it’s going to change. We know.

ALEXA LIM: That’s the one known.

ASA RENNERMALM: That’s the known, yeah.

ALEXA LIM: That’s all the time we have, so thanks so much for joining us.

ASA RENNERMALM: Thank you.

ALEXA LIM: Asa Rennermalm is an associate professor of Geography at Rutgers University in New Brunswick, New Jersey. For Science Friday, I’m Alexa Lim.

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About Alexa Lim

Alexa Lim is a senior producer for Science Friday. Her favorite stories involve space, sound, and strange animal discoveries.

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