NEW YORK (NYTIMES) - When deadly heat waves hit on land, we hear about them.
But the oceans can have heat waves, too. They are now happening far more frequently than they did in the last century and are harming marine life, according to a new study.
The study, published on Monday (March 4) in the journal Nature Climate Change, looked at the effect of marine heat waves on the diversity of life in the ocean.
From coral reefs to kelp forests to sea-grass beds, researchers found that these heat waves were destroying the framework of many ocean ecosystems.
Marine heat waves are said to occur when sea temperatures are much warmer than normal for at least five consecutive days.
Scientists estimate that the oceans have absorbed more than 90 per cent of the heat trapped by excess greenhouse gases since mid-century.
Humans have added these gases to the atmosphere largely by burning fossil fuels, like coal and natural gas, for energy.
This excess heat is increasing not only baseline ocean temperatures, but also the frequency and duration of marine heat waves.
An earlier study by some of the same researchers found that, from 1925 to 2016, marine heat waves became, on average, 34 per cent more frequent and 17 per cent longer. Overall, there were 54 per cent more days per year with marine heat waves globally.
The most severe years tended to be El Niño years. Warmer ocean temperatures are one of the characteristics of an El Niño pattern.
"There's also some indication that El Niños have been getting more extreme with climate change," said Dr Eric C.J. Oliver, an assistant professor of physical oceanography at Dalhousie University in Halifax, Nova Scotia, who was a co-author of the study.
But regional marine heat waves can happen even without an El Niño, he said.
As ocean heat waves proliferate, problems will mount for people who depend on fishing and fish farming, or aquaculture, said Dr Gretchen Hofmann, chairwoman of the department of ecology, evolution and marine biology at the University of California, Santa Barbara.
"If you look at geography, lots of the world's population lives on the coast," said Dr Hofmann, who was not involved in the study. "Commercial fisheries, but also aquaculture facilities on the coast, will face challenges as ocean water changes.
Heat waves are not great for aquaculture production." The study's authors found many parts of the Pacific, Atlantic and Indian oceans where aquatic life was especially vulnerable. These areas were home to a lot of biological diversity and had plants and animals that were already living in the warmer parts of their ranges. They were also affected by other human effects like pollution and overfishing.
Even seabirds did not emerge from marine heat waves unscathed, most likely because their food sources either died off or moved in response to the warming.
But perhaps the biggest surprise was the significant loss of what biologists call foundational species, like coral reefs, sea grasses and kelp forests. They support the diversity of aquatic life by providing shelter from predators, moderating temperatures and acting as food sources. When they disappear, the entire ecosystem disappears along with them.
Climate change "may be a gradual process, but set on that backdrop are these extreme warming events that might be becoming more common that can have more dramatic consequences," said Dr Robert Miller, an associate research biologist at the Marine Science Institute of the University of California, Santa Barbara, who was not involved in the study.
"In some cases, ecosystems might not be able to recover from those events," he said. "And so, the effects of climate change might happen a lot sooner than we expect."