Unexplainable - Can these corals take the heat?

Episode Date: August 17, 2026

Warming waters are an existential threat to coral reefs, and for people who want to restore them. But in the Republic of Palau, researchers are testing out an approach to help restored reefs survive hotter temperatures — before it's too late. To watch this episode on video, find us on Netflix! Guests: Benji Jones, senior environmental correspondent at Vox For show transcripts, go to ⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠vox.com/unxtranscripts⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠For more, go to ⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠vox.com/unexplainable⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠And please email us! ⁠⁠⁠[e-mail]⁠⁠⁠We read every email.Support Unexplainable (and get ad-free episodes) by becoming a Vox Member today: ⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠⁠vox.com/members⁠⁠⁠⁠⁠⁠⁠⁠Thank you! Learn more about your ad choices. Visit podcastchoices.com/adchoices

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But I don't think it's any secret that coral reefs are struggling. By one estimate, the world has lost half of its live core. since the 1950s. And my colleague, Benji Jones, has been reporting on this. Long-time listeners of the show will know him from episodes about discovering new species or the economic value of a bat. But recently, he went on a reporting trip to Palau to cover a project that's trying to turn things around for coral and is trying to do it fast.

Benji, thank you for being here. I'm so happy to be here. It is funny to me that we both went to Palau. Like, anyone who listens to the show knows that I also went to Palau, like, a month before you went to Palau. It's a special place. I found it interesting because whenever we've talked in the past, you are, like, the most cheerful person delivering the most grim news. Like, you always have this exact smile on your face. And they're like, everything's dying.

But this, you said, was like maybe a slightly more hopeful story? Yeah. I recently went to this island nation in the Pacific called Palau. And I was there to tell a story about coral reefs. And, yeah, I feel like it was more hopeful than I felt in a long time about these particular ecosystems. So I felt good. I felt good. And it was, like, so much fun.

Yeah, let's get into it. Like, when I was there, I sort of got a chance to actually snorkel. And it was just, I mean, it was stunning. It is literally, like, the most incredible reef ever. I mean, I'm so glad that you snorkel there so we, like, can bond over Palau's reefs because they really are so special. And, like, out of, like, a calendar almost, like the watercolor, like, it literally is just so, feels like fake almost. It's so beautiful.

Looking down through, basically it was like I put this mask on, put my face in the water, and it felt like, you know, those videos of like people seeing color for the first time

where they'll like put on the glasses and I was like, oh my gosh, I'm seeing like color for the first time. I like wanted to cry. Yeah, I mean, coral reefs are these magical underwater rainforests. They're so full of life. And I've been diving since I was like 15, I think. I love getting to like sink into this other world. And it's like, I want everyone to have that experience because it is so special. And it makes me really actually sad that reefs are in such bad shape. What's happening to them? Is it just climate change? Or? There are several different problems that reefs are facing.

There's like pollution, so like sewage and nitrogen from farmland running off into the water. There are diseases that affect corals. There's overfishing, which is bad for the reef. To what you're saying, climate change today, I would say is like the leading threat that reefs are facing. Like there was a report that came out in like 2018 or something that said with 1.5 degrees Celsius of warming relative to like pre-industrial times, we could lose 70 to 90% of coral reefs. 70 to 90%? Yeah, and like, so that is truly existential for these ecosystems.

And so much depends on them. They're not just cool to look at. They're not just cool for divers and snorkelers and tourists. They're incredibly important for protecting shorelines. They're incredibly important for maintaining fisheries for folks. So, yeah, there's like a lot at stake here. Why is heat specifically such an issue for them? Okay, so...

Benji's getting into like... I hope you're ready for me to tell you about coral polyps because I can talk about this for a long time. So, first of all, let's get the basics out of the way. Sure. Coral is an animal. Yes. So I like that sometimes they're like affectionately,

or maybe not affectionately called rocks, but they are very much an animal. And not only that, they are a colony of animals. So each individual coral that you're saying, scene. So, like, one of those branches, for example, is a colony of little animals called polyps. Polyps, like, they're, in my mind, they look like little sacks, like little goo sacks. Yeah, they're basically goose sacks. Nice.

Like, like all of us. And, like us, they have skeletons. Okay. And so, they're these little, like, gooey things with tentacles. And they have, they excrete a skeleton. And as they grow. Like, if we oozed a skeleton. Yeah. And it looked like, a giant rock. Yeah. Okay. Cool. Yeah.

And like, it is that secretion of the skeleton that creates all this beautiful, this three-dimensional terrain that's so important. So you have the polyp, the coral polyp, this little animal, it's in a colony. They're all genetically identical. So all the little polyps in a piece of coral are genetically identical. They clone themselves over and over and over again when they grow. A coral polyp has this incredible partnership with a kind of algae, a beneficial algae. that lives inside of its tissue.

And that algae is basically like a solar power plant that generates most of the energy that coral polyps need to survive. So like the algae is basically photosynthesizing. Exactly, like a plant. Just like a plant does. And then that like photosynthesis creates a bunch of nutrients and it like hands off some of the nutrients to...

Yeah, like sugar basically. It's giving to the coral. And the algae in return, gets like a place to live near the sun. It can photosynthesize. It's paying rent. It's giving a lot of its sugar to the coral polyp. So why this matters is because when it gets too hot in the ocean,

that partnership between the polyp and their power plant algae breaks down, scientists think that the algae might be secreting some kind of toxic substance. And so the coral polyp like ejects the algae. And so that is bleaching when all those. algae cells, leave the coral, and then it turns white because it doesn't have algae in it, which gives it a lot of its color. Okay. Basically, that's why we get these, like, almost the images that I've seen look like.

Haunting. Skeletons, like a graveyard of coral, essentially. But it's coral, from what you're telling me, like, slowly starving. Yeah, exactly. So it's bleached coral is starving, and not dead, but it will be dead if the temperatures don't come back down again. And so basically bleaching is like a very sick coral. What have people been doing about this bleaching? So people are basically planting coral on reefs that are dying to try to regrow them.

Okay. In the same way that you would plant like saplings or seeds to regrow forests, which people do too. They're doing that for reefs. And there are actually a lot of people who are doing this replanting of reefs or coral restoration as like scientists call it. I've seen like one source tracking this globally and there are like 900 plus projects around the world to restore coral reefs. So basically like there are these like agents

all around the world that are trying to rebuild reefs, which is like very exciting, very hopeful because people really care. It shows you that people really care in the same way that I care because like I love these ecosystems. So people are really, trying to help. Is it super effective? Or? I mean, so this is like, this is the problem.

This is what I really like want to talk to you about, which is that because the oceans are getting hotter and hotter, if you're putting corals out into that same ocean, they are going to be more likely to bleach in the future just because the oceans are heating up. So it's like, why are we planting trees if the forest is just going to burn anyway? Because I remember you doing some reporting a couple years ago about an effort like this, right? Yeah. So in 2022, I went down to Florida with a photographer, Jenny Adler, and we were there to observe a lot of this restoration work in practice. There's this organization called the Coral Restoration Foundation, and like they do pretty cool work. They have these, like, beautiful trees, they call them, that they sink

underwater, they're like made a PVC pipe, and then they hang corals from them. It was just cool to be diving on these reefs that had been replanted. And so it was like, okay, scientists are like literally regrowing this coral reef. And I'm seeing the results of that. So that was in 2022. Uh-oh. But this was like devastating. So in 2023, there was a record-breaking heat wave that started that summer. And I was like literally cruised. over fields of planted coral that was all bleached white. Oh, no. Yeah, it was really sad.

It was sad because, like, I want Florida to have its reef because especially in a place like that, that's really vulnerable to storms, it's so important. 90% of the live coral over the last, whatever, 50 years is gone. So, like, being able to replant it is so important. But I was also sad because all these researchers and this organization that had, put blood, sweat and tears into planting this reef, were watching their work over years. I mean, they had planted, like, 40-plus thousand pieces of coral. All that work sort of, like, disappeared, or most of it did, a lot of it did.

And so, yeah, it was just sad, like, surfacing with some of the people from that organization. Like, they were just, like, devastated. It wasn't just their work that was disappearing. It was also this, it represented a challenge to the whole idea of restoring coral reefs, because if we know that heat waves are getting worse, why put coral back out there? Why put all this time and money into planting coral if it's just going to die anyway? So is there anything that we can do then? Yeah, so that's what I was doing in Palau, was meeting with a researcher, was watching a researcher work who, is trying to fix this problem.

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Palau, I guess. Yeah. So Plow is not like one island. It is 340 islands. Yes. Not very many of them are inhabited. But they look like they're floating because the bottoms of these little islands. Some of them are only the size of like a house or a bus or something, a tiny.

But the bottoms of them are eroding. And so it looks like there's a gap between the bottom of the base of the island and the ocean. And so there are like moments when you're boating through these islands. And it feels literally like, I hope you got this reference, like the floating islands of Pandora and Avatar. Call me Basic. I don't care. It's a great movie. But like I could just sort of suspend disbelief for a little bit, like being on the boat.

Like the air, like the wind blowing through my hair, my not very much hair. Luscious locks. Yeah, exactly. What's interesting about being there for a story about rest of the same. restoration about coral reefs is that Palau's reefs are actually, as you saw, they're doing really well. There are a couple reasons that make it a really good place to study coral reefs. One of them is that there are loads of them. So, like, the flip side of it being healthy is that, like, there are all these coral reefs. There are tons of populations of coral. So if you want to study coral and study how

different corals survive or don't survive under warming, it's a good place just because you have a lot to access. There's a marine lab there. That's, you also, I think, went to this money. Yeah, Pickrick. So the Palau International Coral Reef Center. You got it, yeah. I believe. And they have a wet lab in the back end.

You can always expect to find big tanks full of sea creatures. And in this case, lots of coral and clams and urchins. But you were there to talk to one researcher in particular, right? I've known this guy for a while. His name is Steve Palumby. Okay. He's at Stanford. And he is really focused on improving coral reef restoration.

How can we make this better? Particularly, he's done a ton of work on heat tolerance within corals. Heat tolerance. So trying to basically identify which coral colonies are going to be more likely to survive should a heat wave come. And can we then use those more heat tolerant corals to rebuild reefs? So instead of just putting any corals out to restore a reef, let's use the ones that are hardier in the face of heat so that they won't die. So there's this whole field of ink. inquiry into why some corals seem to be more heat-resistant than others. And it's not simple.

There is a genetic basis to it. It is rooted to some extent in a coral's genome in its DNA. That algae that it has this partnership with, that matters too. So certain strains of the algae are more heat-resistant than others. So you have to get the right algae to be able to tolerate heat better. And there's also like conditioning. Like what did that particular colony experience in the past? Will that make it more likely to be resistant to heat in the future? So there are all these different scientific questions about what is it about these corals that we see surviving heat that makes them survive? What makes them so special? I mean, that sounds like a lot of work then if you're trying to tease out like the coral and the algae.

I mean, yes, I think like there is still a lot of unknown when it comes to the basis of heat tolerance. And the problem with that is that, like, we don't have time. We don't have time to, like, pursue just the science to try to figure out exactly what corals are more heat-resistant and be perfect about it, like, really understand the genetic basis, for example, or, like, understand everything about the symbiont, the algae in it. And so from Steve's perspective, this guy, the Stanford biologist, who's been working in Palau for a while, he's basically, like, we kind of know enough. right now to make restoration better, to start figuring out which corals are heat-resistant and planting them in the ocean. And so Steve is like, okay, what can we do right now to improve the way that we're planting corals so they survive if future heat waves come? What does that look like, essentially? What does kind of being like, we know enough to start doing stuff?

What is the stuff that he's doing? Yeah. So Steve's thing is basically finding a way to quickly test corals for their heat tolerance before planting them on the reef. And so what he has spent a lot of his time doing is trying to figure out how to create a quick heat test to run corals through that isn't going to take a ton of time and resources. So basically, he will take a lot of time. bits of coral off the wild reef, bring them back to Pickrick, this lab in Palau, and then essentially put them into picnic coolers. Picnic Coolers like the, like, oh, it's a little white thing with a red top. Like what you put, like, fruit and cheese in for a picnic in the park. And they'll take it to Prospect Park,

yeah. So these coolers are outfitted with, like, aquarium heater, a small pump, and there's some seawater in them. So he puts these little pieces of coral. into the coolers, and then essentially he just turns the temperature up and sees which ones bleach first and which ones stay colorful, indicating that they might have some advantage under warming conditions. Interesting. And the innovation here is A, it's pretty simple because it's a picnic cooler. Like you can buy that online, and same of the other parts, and it's pretty cheap.

But it's also fast. So a lot of like the more traditional or like a lot of heat tests run for two weeks or a month. And the idea there is that it's supposed to be like better mimicking natural conditions. But what Steve is trying to show is that even if you just have four days, you can heat up the water really quickly in an increment. So yeah, like 30 degrees Celsius is the baseline. And then you ramp up to 34 and then to 35 the next day and then 36 the next day. Just do that for four days. And then each morning you check the corals in your cooler and see which ones.

are still colorful, which ones are not. And the ones that are still colorful are the champions. They're the winners in this stress test, in this heat test. And so once he's figured out, like, this one's extreme, like, this one's a champion, this one's maybe, like, middle of the pack, whatever, what does he then do with those corals? Like, where do they go? Yeah, so I'll just briefly say he works with this other researcher at Stanford named Courtney Claypack, who does a lot of the, like, implementation of the restoration.

So she's trying to build this project out. But the idea is that because they're still trying to understand how well these tests work, these quick four-day tests work, they are planting all of those corals that they've tested on the reef, including the low heat-tolerant ones, the medium-heat-tolerant ones, and the high-heat-tolerant ones, and then seeing what happens in the wild. What does the nursery look like?

So it's, like, desperate to get out to this one, spot on the reef where they had been adding all these heat-resistant, according to these tests, these quick tests, heat-resistant corals. They're trying to regrow a reef in Palau that one of the few reefs that's in bad shape for the local community, the local community, the governor, this guy Sharp, was like, this used to be an amazing fishing ground. We have since lost the fish here. Can you help us rebuild the reef? Asked Pickrick, got in touch with Steve. Steve's like, okay, let's make this the spot of a nursery where a lot of our climate, A lot of our heat-tolerant corals can go so that over time they might have a higher chance of surviving if there are marine heat waves in the future.

So we got to snorkel and see this cool, like, restoration site. What was it like? What did the nursery look like? We were, like, a couple miles from shore, but it was pretty shallow. That's, like, one of the cool things about coral reefs in a place like this. Like, you can be really far offshore, and it's really shallow because the reef has grown up. So, yeah, we jumped in, and on the seafloor, there were a bunch of these metal structures. that look a little bit like sort of like spiders on the floor of the ocean. And they're really common in restoration.

You actually attach the corals with like zip ties to those, they're called Mars stars. And I'm snorkeling out over this, this like garden of these metal structures covered in coral. And it was just cool because I had known like what they had been through. Like the other, the cool thing about this work and why it's different than a lot of, like, the restoration that I've seen is that there were like 26 different species of coral. So it was like really diverse. And some of them look like these like green fuzzy mittens. Some of them look like these, that like kind of incredible neon blue like antlers. Some of them just like kind of lumpy and purple.

Those are my favorites. There were fish too. There were giant clams there. There were sea cucumbers. or a parrot fish. What's next? Do they, like, take hardy plowin coral to other places? Do other places do this test?

Like, what could this teach us for other environments? Yeah. It's a good question because we know that, like, yeah, this particular reef is not in desperate need of restoration, but so many other ones are. Basically, like, right now, there are already similar projects. going on in different parts of the Pacific. So there's one in Maui, there's one in the Federated States of Micronesia,

there's one in the Marshall Islands. So they're testing this approach in a bunch of different places. In the future, the idea is that because this is so affordable, and it's pretty straightforward, four days, picnic coolers, et cetera, things you can buy online. Anyone can implement this, like, heat testing protocol into their restoration. So basically, like, why don't we just add this one step of, like, let's test the corals that you're going to put out so that we don't just, I don't know, roll the dice to see if they have what it takes to survive. So, like, let's be more strategic about what we are planting on a reef.

Given that it's like, it's not easy. Like, I just said that, like, this testing approach is easy. It's easy relative to other tests. But, like, doing coral restoration is, like, really labor intensive. I mean, it's, like, you have to have boats. You have to have a staff. You have to have the equipment. Seawater, like, kind of, like, erodes.

destroys everything on land. Like, it is hard to, like, raise coral. And so finding ways to make that easier is, like, helpful. So the idea is basically that, like, other people around the world could do this and just take the winners from those tests to plant. So it really strikes me that this is not, it's not just sort of, like, restoration of the reef as it was. Such a good point. Restoration is not a great word for what we need to do because we can't, we're not trying to restore reefs to some former state. Like restore is like, let's bring back. Whereas it is no longer enough to look to the, to like try to bring what we had back. It's not enough. It won't work. That's what lots of people have tried and then the coral dies. We now need to like

recreate or re-engineer what these ecosystems look like because things have just gotten so bad. Yeah. And so when you say this about like, oh, we need to engineer them. We have to just sort of like build a new kind of reef. Part of me is like, well, I guess in the time frame that we have, like, that's what we have to do. But then there is a part of me that's also like, do we know enough? I mean, I love that question. I think we, the simple answer is that we definitely do not know enough to create, recreate any ecosystem. I mean, I think about, like, there could be tradeoffs, and there's been a fair amount of

research on it. So maybe a coral that's really heat-tolerant is, like, I can resist the worst warming doesn't grow very quickly. Yeah. Or it gets hit by disease and dies more easily. So what are those trade-offs? We have, like, scientists have a pretty decent sense that these trade-offs are not so extreme that, like, you're really hampering the growth of a reef by using just the, like, heat-tolerant corals.

But those are questions that, like, do matter. still when you're thinking about scaling up these interventions. I think the reason why it's still worth doing something like this is that we know what it looks like to do nothing. Yeah. So this is a hopeful story to you. It is. I mean like hopeful A because it gives restoration hope. Right. It is potentially solving one of the problems that is plaguing coral restoration, plaguing this grand global effort to regrow reefs. Also, it's a hopeful story to me because plow is amazing. There are still reefs that are in great shape. It's like thinking of this one particular spot that we went to, which was like, you just, it was like pretty shallow. We jumped off the boat.

We saw at least a couple sea turtles, sharks, and then at the end of this dive, we came across one colony of coral that was probably the size of. of a trailer, maybe bigger, maybe bigger, like a small building size, one individual colony of coral easily 500 years, maybe a thousand years. It's like seeing the like these ancient trees of like California or whatever. And so I don't know, that dive in particular just like left me feeling very hopeful. And I don't think the message is, okay, like this reef is fine. Like why are we worrying?

It's like there's something that's still worth saving. Okay. Benji, hopeful for once. Thank you so much for coming on the show. I'm hopeful that you'll come on again soon. And if anyone wants to read your article on Fox.com, it is fantastic, has so many photos, and I think it's a great read.

Oh, thank you. If you'd like to watch this episode and see some of Benji's fantastic footage, You can find our show, Unexplatable, on Netflix. You can also find Benji's excellent reporting on all this on our site, Vox.com. This episode was produced by Valerie Schenckman and by me, Bird Pinkerton. It was edited by the wonderful Joanna Solitarev. The video edit was done by Shelby Smith, Valerie Schenkman, and Alex Coles,

with animation from Kareem Karea. Ibrahim Ocich and Joe Nebris were our studio engineers. Christian Ayala did the mixing and the music. sound design for the podcast. Melissa Hirsch, check the facts. Noam Hassenfeld does our music. Meredith Hodnott is the fact that some corals make themselves a kind of sunscreen. And thanks always to Brian Resnick for co-creating the show with me at Noam. If you have ideas about coral you want us to think about or any other topics you'd like us to explore, we are at Unexplanable at vox.com.

If you'd like to support the show and the journalism that Vox does, we would love it. if you would become a member. It is very easy to do. Just go to vox.com slash members. You will get access to all of Vox's journalism, but you will also know that you are supporting all of Vox's journalism. And for those of you who have emailed to let us know that you signed up because of Unexplainable, thank you. Thank you also to those of you who have left us a nice review on your podcast platform or just told someone in your life about the show. You are all the best. Unexplanable is part of the Vox Media Podcast Network,

and we will be back very soon with another episode about everything that we do not yet know.