Corals are bleaching more and more often around the world because of warming ocean waters, a new study shows. Since bleaching can cause corals to die, this means that coral reefs — which provide food and profits for thousands of people — risk disappearing in the future if we don’t stop climate change.

Researchers analyzed data about bleaching events at 100 reef locations around the world from 1980 to 2016. They found that the rate of bleaching has increased more than fourfold in the past four decades — from once every 25 to 30 years back in the 1980s, to once every six years by 2016. That’s because ocean waters are warming up, according to the study published today in Science.

Bleaching occurs when the colorful algae that live inside the corals are expelled. That can happen because the water is too warm or too cold, or because of extreme low tides. But bleaching is disastrous for coral reefs, because the algae provide about 90 percent of the coral’s energy. Without it, the coral goes white as it starves. Previous studies have shown that global warming is causing corals to bleach and sometimes die. This latest study gives hard numbers on how often these bleaching events are now happening compared to the past.

“The amount of acceleration that we saw was really surprising,” says study co-author Mark Eakin, a coral reef expert and the coordinator of NOAA’s Coral Reef Watch. It “is mind-boggling and very frightening when you consider what that means for the future of coral reefs.”

Global temperatures have already increased by 1.8 degrees Fahrenheit since the 1800s. That’s because we’re pumping heat-trapping greenhouse gases like carbon dioxide into the atmosphere by burning fossil fuels. The warming ocean waters are causing corals to bleach more often — and not just during El Niños, a recurring climate pattern that brings warm waters to the tropical Pacific Ocean, affecting weather all over the globe. The latest global bleaching event, for instance, began in June 2014, when El Niño hadn’t fully formed yet, says Eakin. That bleaching continued for three years, and finally ended in May 2017.

Before the 1980s, coral bleaching was unheard of, says study co-author Terry Hughes, the director of the ARC Centre of Excellence for Coral Reef Studies at James Cook University in Australia. In the 1980s and 1990s, it happened mostly during particularly strong El Niños, but now ocean waters are so warm that “it doesn’t require an El Niño to have that level of coral bleaching even on a global scale,” Eakin tells The Verge. In fact, the new study shows that today, sea temperatures during the cooling climate pattern called La Niña are warmer than they were during El Niños 40 years ago, Hughes writes in an email to The Verge.

Losing coral reefs is bad for the environment — and people. Over 25 percent of marine species somewhat depend on coral reefs to survive, Eakin says. Reefs also protect shorelines from storms, and provide food for people in countries like Indonesia and the Philippines. In Australia, the Great Barrier Reef provides nearly $6 billion in revenue from tourism and fishing, including employment for almost 70,000 people.

The most important way to protect coral reefs is by tackling climate change. Limiting greenhouse gas emissions to meet the Paris agreement goals of keeping global warming below 2.7 to 3.6 degrees Fahrenheit is key, Hughes says. Reducing water pollution, overfishing, and habitat destruction can also help. Scientists are also trying to breed super corals that can survive warming waters, and researchers in Australia are looking at ways of cooling ocean waters around reefs when bleaching is severe to contain the damage, Eakin says.

For coral researchers like him, seeing the reefs bleach and die takes an emotional toll. It’s “absolutely shocking,” he says. A bleached reef loses all of its colors and fish, becoming “eerily silent.” The bleaching corals also leave a film of dead flesh and tissue on the surface of the water. “You come up to that and you get out of the water and everything stinks. You stink,” Eakin says. “You can’t get rid of that smell — that smell of death of the reef.”

“It really has worn me out,” he adds. “This has been a very stressful time.”