Coral reefs are humming hubs of biodiversity. Although reefs cover less than 1 percent of the ocean floor, about 25 percent of known marine life is connected with them in some way, says Amy Apprill, an associate scientist at the Woods Hole Oceanographic Institution. They are also one of the clearest examples of climate change’s impact on the natural world.
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“If you ask a kid to draw a picture of what climate change looks like, they will often draw a bleached coral,” she says.
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Reef ecosystems face a variety of growing threats. Local issues such as overfishing, irresponsible tourism, and poor water quality threaten the reefs. But the biggest danger to corals is climate change, and the ocean acidification and warming water temperatures it brings. In summer and fall 2023, for example, high temperatures caused mass die-offs of corals in the Florida Keys and across the Caribbean Sea (1). The Intergovernmental Panel on Climate Change in 2018 predicted that most reefs would disappear if global warming reached 1.5 degrees Celsius, and effectively all of them would die if it reached 2 degrees (2).
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Such a loss wouldn’t just be a calamity for biodiversity. It’s terrible news for humans, too.
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Apprill says corals provide the framework for a sprawling undersea ecosystem, like trees in a forest. “You have this really large structure and it provides a habitat for lots of other organisms,” she says. “That habitat includes nooks and crannies for small invertebrates such as shrimps and crabs and snails. A coral reef is also a vibrant nursery hub for a lot of fish, even fish that go to other places for the rest of their lifetime. And then larger predators too, like sharks. Even whales will sometimes come by a reef.”
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This wealth of species is an economic engine for communities near reefs. Fishing for species that live in or near reefs is a multi-billion-dollar industry that feeds millions of people.3 And reefs are an eco-tourist attraction: more than 2 million people per year visit the iconic Great Barrier Reef off the Australian coast (4). The biodiversity of coral reefs also makes them a rich site for research, including into new chemicals to use in pharmaceuticals, Apprill says. “We've had cancer drugs come from reefs. There's a covid therapy that's in study right now that was developed from a deep sea coral,” she says.
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Coral also protects coastal lands from the tumult of the sea, which is crucial as climate change makes powerful storms more common.
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“They're really good at protecting coastlines,” says Apprill. “In fact, a coral reef will dissipate about 97 percent of wave energy. If you've ever flown over a coral reef, you can often see that—waves breaking, and then inside of the reefs next to the coastline, there's just still water. As we have more storms and more intense hurricanes, we need that infrastructure. It's like a breakwater to help protect our homes.”
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And human activity is making climate change even more perilous for corals, Apprill says. When corals die, they are quickly covered by algae. In a thriving reef ecosystem, fish could eat much of that algae and help the reef to recover—but many reef residents have been overfished. Without their help, the ecosystem can pass a tipping point beyond which the reef cannot recover, and all the associated species lose their habitat.
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Although reefs won’t be safe as long as we keep warming the planet, Apprill says it is still possible to help them locally. Preventing boats from dumping waste too close to a reef relieves stress on these ecosystems, as do measures to prevent chemicals in soil runoff from reaching the corals. These interventions don’t solve the problems of warming or more acidic oceans, but improving water quality makes an important difference.
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“I think by fixing these smaller things, we might help make the corals more resilient to climate change, too, because they're getting hit with multiple stressors,” Apprill says. “It's sort of like if you have a work week when five things go wrong. It's hard to recover. But if you had just one thing go wrong, you can be resilient.”

Published November 16, 2023

FOOTNOTES

1 NASA Earth Observatory: "Stressful summer for coral reefs," October 16, 2023.

2 "Global Warming of 1.5° C." Intergovernmental Panel on Climate Change, 2018.

3 Conservation International: "Economic Values of Coral Reefs, Mangroves, and Seagrasses: A Global Compilation." Center for Applied Biodiversity Science, Conservation International, Arlington, VA, USA, 2008.

4 Great Barrier Reef Marine Park Authority: Tourism visitation data. Accessed November 16, 2023.