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The unsung heroes fighting against climate change? Fish in the 'twilight zone'

AYESHA RASCOE, HOST:

Heads of state from around the world are at an international summit in Dubai this month trying to figure out solutions to the climate crisis. But it turns out fish have been combating climate change all along - specifically, fish from the so-called twilight zone of the ocean, the layer of the ocean that's between 200 and 1,000 meters deep. Scientists estimate their movements capture billions of tons of carbon every year and stash them in the ocean depths so they don't heat up our atmosphere any more than we are already doing. Ken Buesseler has been studying this process as a senior scientist at the Woods Hole Oceanographic Institution in Massachusetts. Ken Buesseler, welcome to the program.

KEN BUESSELER: Thank you.

RASCOE: So first of all, I know that the scientific term for this ocean layer is the mesopelagic zone. I just learned that today. But it's sometimes referred to as the twilight zone, right? Tell me about what this zone is.

BUESSELER: Well, I like the twilight zone partly 'cause of the mysteries of that TV show - right? - that I...

RASCOE: Yes.

BUESSELER: ...Certainly grew up on. But also, it's the layer below where the light penetrates. So the plants can't live down in this dim lit zone - and not the deepest part of the abyss. So it's kind of that in-between layer. That's what the mesopelagic refers to.

RASCOE: What is so special about fish from this part of the ocean?

BUESSELER: Well, it's not just the fish. But in particular, small animals that try and hide away in that dark zone during the daytime so they aren't eaten come up to the surface to eat the plants, the phytoplankton that actually store the carbon, and that's their food supply. So if you go up at nighttime, you won't get eaten. You go back down in the daytime. You come back up. You go back down.

RASCOE: 'Cause the movement of these animals and fish up and down is called a carbon pump. And what does that mean?

BUESSELER: That's where the plants take up carbon dioxide, just like on land, as they grow. Photosynthesis, if you remember that story, is the plants taking up CO2 and growing, making organic carbon. So now we're taking more of the CO2 atmosphere into the plants and biologically moving that deeper, whether it's these active migrators or just the sinking of those dead animals or the fecal matter from after they're eaten.

RASCOE: And is that because it's the poop or the byproducts? Like, the fish or the animals that eat the plants - then they, you know...

BUESSELER: Yeah.

RASCOE: ...It comes out as waste, and then the waste goes down deep into the ocean. I'm sure somebody might be eating right now, but you know what I'm saying.

BUESSELER: No, that's my favorite word. I use poop a lot because, essentially, you're tracking the flow of carbon through different animals as it sinks down. Actually, a better word sometimes is marine snow. That's the carbon coming from all those plants and animals - their debris, their carcasses, their poop, fecal matter - and it's that marine snowfall that's really so important to getting carbon out into the deep ocean.

RASCOE: Is that what's kept the Earth from overheating even more than what it already has?

BUESSELER: Yeah, absolutely. I mean, if we turned off this biological carbon pump, we would more than double these horrible temperature increases we've seen now. And so it plays a big role in taking up, locking up some of this carbon dioxide into the deep ocean.

RASCOE: That's Ken Buesseler, senior scientist at Woods Hole Oceanographic Institution. Ken, thank you so much for joining us.

BUESSELER: It's been my pleasure. Thank you.

(SOUNDBITE OF LOS MARIACHIS' "OCTOPUS'S GARDEN") Transcript provided by NPR, Copyright NPR.

NPR transcripts are created on a rush deadline by an NPR contractor. This text may not be in its final form and may be updated or revised in the future. Accuracy and availability may vary. The authoritative record of NPR’s programming is the audio record.

Ayesha Rascoe is a White House correspondent for NPR. She is currently covering her third presidential administration. Rascoe's White House coverage has included a number of high profile foreign trips, including President Trump's 2019 summit with North Korean leader Kim Jong Un in Hanoi, Vietnam, and President Obama's final NATO summit in Warsaw, Poland in 2016. As a part of the White House team, she's also a regular on the NPR Politics Podcast.