Intense heat waves are bad for human health. They can lead to sometimes deadly conditions like dehydration and stroke. And just like extreme temperatures on land, marine heat waves can drastically alter life under the sea.

A new study published today in Nature Climate Change found that the occurrences of marine heat waves have substantially grown in the past three decades, and it’s becoming clearer how deadly warmer temperatures are for biodiversity.

Marine heat waves are periods when the average water temperature of a given region is exceptionally high. In the past 30 years marine heat wave days have increased by just over 54 percent, a trend the study’s authors found consistent with declines in oceans life.

High-profile marine heat waves like “the blob,” a huge mass of warm water that was present off the U.S. West Coast from 2014-2016, were included in the study. The blob was responsible for massive die-offs of everything from invertebrates to marine mammals.

“It is clear that extreme warming events can drive abrupt changes in entire ecosystems with widespread consequences,” says study author ecologist Daniel Smale.

A global snapshot

To get a global snapshot of how marine heat waves are changing life in the world's oceans, Smale and his research team looked at 116 previously published papers. It gave them data from over 1,000 different ecological records. Heat waves were quantified as any period longer than five days when the ocean warmed to abnormally high temperatures.

They then used existing datasets to quantify the amount of biodiversity in a given region. Particularly worrying to the scientists were regions with dense biodiversity that experienced warming. Those regions were especially at risk of life being damaged or dying off and having cascading effects on neighboring ecosystems.

Three regions were particularly hard hit by warming waters, the study notes: coral reefs in the Caribbean, seagrass in Australia, and kelp forests off the coast of California.

Warming disrupts the typical functions of such massive ecological habitats. Corals, for instance, become stressed when subjected to warmer-than-average temperatures. They then expel their symbiotic algae and undergo a process called coral bleaching where the normally colorful coral becomes sickly and turns a stark white.

The big picture

In 2005, the U.S. lost half of its Caribbean corals. On the Great Barrier Reef, more than half of the region's corals are already dead. When corals die, they can no longer support the hundreds of fish and other marine species that live on the reefs.

“Coral reefs that evolved with just a few weeks of above-average temperatures every decade or so are now suffering up to three months of extreme temperature every few years,” says ecologist Enric Sala, a National Geographic explorer who was not involved with the study.

“For example, tropical storms will create even more destruction because coral reefs won't be able to continue growing and protecting the shores from waves,” he says.

Shrinking biodiversity may also one day have profound impacts on food security and sea-based economies. Last week, a study published in the journal Science found that climate change is causing fish to disappear. Global populations of fish taken for human consumption by fisheries have decreased by around 4 percent. For some regions that experience warming waters as well as overfishing, the decline is more than 30 percent.

Smale also worries that the loss of critical regions like coral reefs, seagrass beds, and kelp forests will only add more carbon dioxide to the atmosphere. Scientists estimate the ocean has absorbed 26 percent of the carbon released into the atmosphere in the past decade. All of the carbon that was absorbed by underwater flora is released when the ecosystem dies.

What can be done?

“Ocean systems are facing a number of threats, such as plastic pollution and ocean acidification,” says Smale. “But it is clear that extreme warming events can drive abrupt changes in entire ecosystems with widespread consequences.”

He predicts that warming events will continue to threaten the balance of ocean life in the coming decades.

“The ultimate cause is something we have to address,” says Katie Matthews, the deputy chief scientist at Oceana. “If we don’t do that, everything else we work on will have little to no impact.”