The frequency of ocean heat waves has surged more than 50 percent since the early 20th century in a threat to fish, corals and other marine life stoked by global warming, an international study showed on Monday.
Abrupt local spikes in temperatures, far less researched than heat waves on land, add to pressures on marine life such as over-fishing and plastic pollution, they wrote in the journal Nature Climate Change.
Around the world's oceans, the number of days of marine heat waves per year rose 54 percent in the period 1987-2016 from 1925-54, according to the scientists in Britain, Australia, Canada, New Zealand, Japan, Spain and the United States.
"Extreme temperature events may be one of the most important stresses on the oceans in coming decades," lead author Dan Smale of the Marine Biological Association of the United Kingdom told Reuters.
"Whether it's seaweeds or corals, fish, seabirds or mammals, you can detect the adverse effects of marine heat waves," he said.
Marine heat waves, defined as at least five days with temperatures far above average, are caused by heat from blazing sunshine and by shifting warm currents.
Among impacts, a 2011 marine heat wave off western Australia killed abalone stocks and a 2012 heat wave off the eastern United States drove lobster stocks north towards Canada.
Many tropical corals have suffered from harmful "bleachings" in recent years.
The scientists said marine heat waves were "emerging as forceful agents of disturbance" that could "restructure entire marine ecosystems," disrupting livelihoods and food supplies for millions of people.
Most previous studies about climate change in the oceans have focused on a gradual rise in average temperatures, which hit a new record annual high in 2018, forcing fish to swim towards the poles or into the cooler depths.
Heat waves often have natural causes but the report said "there is growing confidence that the observed intensification is due to human activities," led by the burning of fossil fuels.
"Multiple regions in the Pacific, Atlantic and Indian Oceans are particularly vulnerable to marine heat wave intensification," they wrote.
Joaquim Garrabou, of the Institute of Marine Sciences in Barcelona, Spain, who was not involved in the study, said the Mediterranean was also at risk because creatures cannot shift north in an almost landlocked sea.
In recent years "massive impacts" in the Mediterranean, especially on corals, sponges and mollusks in a devastating 2003 heat wave, he told Reuters.