Image of a thriving coral reef and image of a dying one

Left: A healthy section of the Great Barrier Reef; Right: Bleached corals in the Great Barrier Reef

Gary Bell/Oceanwide/Minden Pictures (healthy reef); Brett Monroe Garner/Getty Images (bleached corals); Shutterstock.com (all other images)

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In Hot Water

Record-setting ocean temperatures are putting the Great Barrier Reef at risk.

February 3, 2025
By Talia Cowen

As You Read, Think About: How can the health of coral reefs affect other species?

Coral Reef Construction
Dive into the ocean to see how coral reefs form.

Jim McMahon/Mapman®

Ben Henley was 7 years old when he saw a coral reef for the first time. His father had taken him snorkeling in the Coral Sea, off the east coast of Australia. They explored one of Earth’s most spectacular natural wonders: the Great Barrier Reef. 

“It’s such a huge and beautiful place,” says Henley. He is now a scientist at the University of Melbourne in Australia.

The Great Barrier Reef stretches more than 1,400 miles along the shallow ocean floor. It’s the largest system of coral reefs in the world—and it’s in great danger. 

A study released last August found that temperatures in the Coral Sea are higher than they’ve been in 400 years. This is putting the Great Barrier Reef at risk. And it’s not the only reef being affected by rising ocean temperatures. 

“All coral reefs around the world are in trouble from climate change,” says Henley, who’s one of the authors of the study.

Gary Bell/Oceanwide/Minden Pictures

A healthy section of the Great Barrier Reef

Reefs at Risk

Coral reefs can be found in every ocean in the world. They’re among the planet’s most important ecosystems. Reefs cover only about 1 percent of the ocean floor, but they’re home to about one-quarter of all underwater species. More than 9,000 species live in the Great Barrier Reef alone, from turtles and seahorses to eels and sharks.

Although corals look like plants, they’re actually tiny animals that live in huge colonies. Their hard outer skeletons form reefs. Healthy corals come in every color of the rainbow. They get their colors from small plantlike organisms called algae, which also provide nutrients for corals. 

When oceans get too warm, corals release algae and turn white. This process, called coral bleaching, weakens corals and can cause them to starve. More than 70 percent of the Great Barrier Reef has been affected by bleaching, according to a report released last April. Some corals can recover from bleaching as the water cools in winter, but winter ocean temperatures are rising too.

“If corals die, the reef structures they build can crumble away,” Henley says. “In the future, there will probably be fewer places for fish and other animals to live.” 

Brett Monroe Garner/Getty Images

Bleached corals in the Great Barrier Reef

Saving Corals

Bleaching isn’t affecting only corals in the Great Barrier Reef. About 77 percent of the world’s reefs experienced some bleaching in 2023 and 2024.

“Large-scale coral bleaching events are now far more common and more severe than even a decade ago,” Henley says.

He and many other climate scientists say humans are largely responsible for warming oceans. For example, we burn fossil fuels to create electricity and power our cars. That releases greenhouse gases, which trap heat in Earth’s atmosphere. Experts say our production of too much of these gases is contributing to Earth’s rising temperatures.

Scientists are working to protect reefs. For instance, they’re developing corals that might be able to live at higher temperatures. But, Henley says, the climate is changing fast, so it’s a difficult race to save these precious reefs for future generations.

“Everything we can do to slow climate change will help the coral reefs of the world,” he says. “And that is a very good thing.”

Mega Coral Discovery

Manu San Félix/National Geographic

Last November, a team of scientists from National Geographic was exploring the Pacific Ocean near the Solomon Islands. A diver spotted what looked like a shipwreck beneath the surface. It turned out to be the single largest coral ever found! The mega coral measures 18 feet tall. It’s longer and wider than two basketball courts placed side by side.

The team was excited to find the coral in good health. 

“Just when we think there is nothing left to discover on planet Earth, we find a massive coral [that’s] pulsing with life,” said explorer Enric Sala.

Pictured: A diver swims above the mega coral.

1. Do you think “In Hot Water” is a good title for the article? Why or why not?

2. Based on the article, what is a main cause of the warming of oceans?

3. How does warmer ocean water affect corals?

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