(Scripps Institution of Oceanography, University of California at San Diego)

Heat waves over the world’s oceans are becoming longer and more frequent, damaging coral reefs and creating chaos for aquatic species. A study published Tuesday in the journal Nature Communications found a 54 percent increase in the number of days in which heat waves have cooked the oceans since 1925.

The rise in these marine heat waves has occurred while ever more heat is stored in the ocean because of accumulating greenhouse gases in the atmosphere. The ocean heat content in 2017 was the highest in recorded history, noted Kevin Trenberth, a climate scientist at the National Center for Atmospheric Research. Trenberth said in an email that ocean heat waves will necessarily increase given the building stockpile of heat, which has been measured from the surface down to more than a mile deep (2,000 meters).

Tuesday’s Nature study concurred that “we can expect further increases in marine heatwave days under continued global warming.”

The detection of a rise in heat waves over the ocean is unsurprising because they’ve already been documented over land thanks to global warming. Many studies have examined the potential consequences of increasing land heat waves including an uptick in heat-related illnesses and deaths, in the absence of adaptation. Extreme heat can also worsen air quality, stress infrastructure and decrease the productivity of outdoor workers, among other effects.

But far less attention has been paid to heat waves over the ocean, which can have consequences of their own for both the marine environment and people.

Effects on coral reefs


An aerial view of a bleached coral reef between Cairns, Australia, and Papua New Guinea from March 2016. (Arc Center of Excellence for Coral Reef Studies/European Pressphoto Agency)

“This increase in marine heat waves is having devastating impacts on coral reefs around the world,” said Mark Eakin, a coral reefs specialist at the National Oceanic and Atmospheric Administration.

Eakin said damage to reefs from these heat waves was first seen in 1983 in coral bleaching events, which have increased in frequency and intensity.

In fact, Eakin and colleagues published a study in January that found coral reefs are bleaching four to five times as frequently as they did three decades ago.

Effects on aquatic ecosystems


A beach in the San Diego neighborhood of La Jolla on June 11, 2015. (Scripps Institution of Oceanography, University of California at San Diego)

In 2014, sea surface temperatures in the northeast Pacific Ocean were 5 to 6 degrees above normal, and marine biologists were stunned by the onslaught of outsider species up and down the West Coast of the United States.

Among those spotted were a live ocean sunfish and warm-water blue shark in the Gulf of Alaska, mahi mahi off the coast of Oregon, a Pacific sea turtle common in the Galapagos near San Francisco, and marlin in the waters off Southern California. The next year, abnormally warm waters washed a swarm of red crabs ashore around San Diego.

The sprawling area of toasty waters was known as “the blob,” just one of several “notable” ocean heat wave events referenced in Tuesday’s study.


Sea surface temperature difference from normal June 15, 2015. (NOAA)

“The warm blob in the northeast Pacific caused increased mortality of sea lions, whales and seabirds, very low ocean primary productivity, an increase in warm-water copepod species in the Northern California region, and novel species compositions,” noted a separate commentary on the “emerging risks” from marine heat waves, published in Nature Communications in February.

Because “the blob” abutted the West Coast and many people were able to see some of its effects firsthand, it offered a teachable moment about the implications of ocean heat waves.

But most of the effects of these heat waves are “invisible” to humans, said Kim Cobb, a professor of atmospheric science at Georgia Tech, and can have important implications for ocean food supply and its sustainability.

Some of the effects of marine heat waves, which may be less obvious to people, include sustained loss of kelp forests, reduced surface chlorophyll levels, which affect phytoplankton populations, mass mortality of marine invertebrates and reshaping of the community structure of ocean species, Tuesday’s study said.

“These heat waves will only get worse, with untold impacts on ocean resources that we all depend on for food, recreation and other ecosystem services,” Cobb said.

Connections to ocean-atmosphere cycles

The presence of these ocean heat waves is closely linked to ocean-atmosphere cycles such as the El Niño Southern Oscillation and the Pacific Decadal Oscillation (PDO). When these patterns are in their warm phase (El Niño and the positive PDO phase), ocean heat waves tend to be more persistent, Tuesday’s Nature study found. But the study authors said that when they removed the effects of these patterns,  the long-term trend toward more frequent and longer-duration ocean heat waves remained.