Coral reefs undergo what may become the most extensive bleaching event on record

LEILA FADEL, HOST:

The world's coral reefs are undergoing a mass bleaching event, which could soon be the worst on record.

MICHEL MARTIN, HOST:

Ocean temperatures have been unusually hot, which turns the corals a ghostly white. It is threatening an ecosystem that supports thousands of species and, by extension, millions of people. So scientists are trying to help corals survive.

FADEL: Lauren Sommer from NPR's climate desk is here to explain. Hi, Lauren.

LAUREN SOMMER, BYLINE: Hi there.

FADEL: So what exactly happens when corals bleach? And how bad is it for reefs?

SOMMER: Yeah. So corals are very sensitive to heat, and bleaching happens because too much heat upsets a very important relationship. It's between a coral and these tiny microscopic algae that live inside the coral. I mean, those algae are kind of like really good roommates. They make food for the coral by photosynthesizing - you know, making energy from the sun. But when the ocean gets hot, the corals get stressed, and that kind of roommate relationship just falls apart. The algae get kicked out, and the corals begin to starve.

FADEL: So do corals die from bleaching?

SOMMER: You know, not necessarily. If the heat doesn't last too long, corals can recover because those roommates - you know, the algae - they move back in. But when the heat drags on, corals do die. And that's what's happened in Florida last summer. There was a mass mortality event there. Australia's Great Barrier Reef is undergoing bleaching right now. And, you know, they're still waiting to see how many corals will die there. This is the second global bleaching event in the last decade, and it's expected to be the worst on record.

FADEL: So what does that mean for the future of coral reefs if these events get more common with climate change?

SOMMER: Yeah, I mean, many of the coral scientists I've talked to just have watched this bleaching event with dread because the outlook going forward for corals is just more of the same. Here's how Terry Hughes describes it. He's a coral scientist at James Cook University in Australia.

TERRY HUGHES: By mid-century, we'll have bleaching every consecutive summer. And if we do nothing about rising temperatures, if we continue with business-as-usual greenhouse gas emissions, then the world's coral reefs will simply be degraded and no longer recognizable for the next generation.

SOMMER: I mean, that would impact thousands of other marine species because reefs are some of the most biodiverse places on the planet. It also hits human communities because millions of people rely on reefs for food security and their livelihoods. So, you know, scientists are developing ways to help corals survive better. They're actually creating super corals.

FADEL: Super corals. OK. What are their superpowers?

SOMMER: So the idea is to develop corals that can handle heat better, basically speeding up evolution. You know, scientists in Australia and elsewhere in the world are finding the best corals at surviving heat, and then they're breeding those. They're also breeding the corals' algae. It's known as assisted evolution. And the idea is to develop corals that can be used to restore reefs.

FADEL: So could they survive in a hotter climate?

SOMMER: Yeah. I mean, that's the question, right? There won't be a bionic coral that can survive no matter what. This is not a get-out-of-jail-free card. I mean, coral scientists are very clear. It's about buying time - you know, years, maybe decades at the most. It's really so corals can hang in there until humans start to slow climate change. The most crucial thing for reefs, scientists say, is curbing emissions from burning fossil fuels and stopping temperatures from rising.

FADEL: Lauren Sommer from NPR's climate desk. Thank you, Lauren.

SOMMER: Thanks.

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