Their research shows that annual marine heatwave days have increased by 54% from 1925 to 2016, with an accelerating trend since 1982.

“A marine eco-system that used to experience 30 days of extreme heat per year in the early 20th century is now experiencing 45,” says lead author of the study Dr Eric Oliver of Dalhousie University in Canada.

“That extra exposure time to extreme heat can have detrimental effects on eco-system health.”

Marine heatwaves, prolonged periods of unusually warm water at a particular location, have important implications for marine biodiversity, said Dr Oliver.

Dr Dan Smale of the Marine Biological Association (MBA) in Plymouth, who co-convened the working group and co-wrote the paper, said that such a heatwave off Western Australia in 2011 “caused an entire eco-system to shift from being dominated by kelp forests to being dominated by turfy weeds”.

This had in turn affected the fish and invertebrates that used the kelp forests as habitat.

“We also observed mass mortality events of corals, abalone and fish in response to extreme temperatures,” he said.

More recently, a persistent area of warm water in the North Pacific nicknamed “The Blob” had caused fishery closures, mass strandings of marine mammals and harmful algal bloom outbreaks along the coast.

Daily ocean-temperature measurements have been available globally via satellite only since the 1980s, so the researchers also collated data from the few sites with daily records stretching back to the early 20th century, and monthly measurements of sea-surface temperatures over a similar period.

By piecing the three sources together they obtained what they say is the first-ever single global record of marine heatwaves – and now want to gain a better understanding of how eco-systems will respond to the changes in future.

Climatologists, oceanographers and marine biologists from Canada, the UK, Australia and the USA took part in the study.

Divernet - The Biggest Online Resource for Scuba Divers