Heat waves in the planet's oceans are happening more frequently and lasting longer than they did a century ago, a new study shows.

The study, published Tuesday in the journal Nature Communications, brought together scientists from Canada, the U.S., the U.K. and Australia to examine ocean temperature data dating back to 1900.

Marine heat waves occur when temperatures are higher than expected and remain high over a period of at least five days.

Eric Oliver, an assistant professor of oceanography at Dalhousie University and the lead author of the study, said in the early 20th century, there was an average of two marine heat waves per year globally, but now there are three or four. While they used to last 10 days on average, they now last for an average of 13 or 14 days.

This map shows the ocean temperature anomalies for the summer of 2015. (Eric Oliver)

The number of marine heat wave days globally has increased by 54 per cent from 1925 to 2016, the study found.

The findings are worrisome because marine heat waves can have lasting effects, Oliver said.

"I find them unsurprising, but consistent with what we know about climate change, and therefore alarming," he said of the study's results.

"The whole package of global warming that we're experiencing is generally alarming because we're seeing impacts on our natural environment that in some cases we're not going to be able to undo, or will take much longer to undo than it took to cause them."

Eric Oliver is an assistant professor of oceanography at Dalhousie University and the lead author of the study. (Cecilia Carrea)

The scientists pulled daily temperature data from satellite records dating back to the early 1980s and from six locations where ocean temperatures have been monitored and recorded daily for a century, including sites in Norway, the U.K., Canada and the U.S.

"There's only a handful around the world where they've been recording daily temperature — literally going out to the shore with a bucket or at the end of the pier with a bucket for the last 100 years every day. Those are really valuable records."

Oliver and the study's co-authors, who are part of an international working group, also retrieved data dating back about 100 years from more infrequent measurements taken by ships.

'The Blob'

In 2016, a marine heat wave led to severe bleaching of the Great Barrier Reef in Australia.

One of the largest marine heat waves on record, nicknamed "The Blob," developed in 2014 and lasted until 2016, stretching from Mexico to the Bering Sea. The patch brought sea creatures that usually live in warmer waters to northerly locations and some scientists believe it may have been responsible for a toxic algae bloom on the West Coast.

A thermal map of the North Pacific shows what scientists have nicknamed 'The Blob.' (CBC)

In 2011, a marine heat wave off Australia lingered for about a month, prompting fish kills and the closure of the lucrative abalone fishery. The hot spot also caused the area's kelp forests to collapse, which were permanently replaced by seagrass meadows.

"So we have this event that comes and goes and then the environment … returns to normal, but the ecosystem has actually switched from one state to another," Oliver said.

"It definitely is alarming, and in a place like here in Nova Scotia, where we rely on the ocean not just for recreation, but for income in terms of fisheries and aquaculture and things like that, it supports a lot of people's lives. It's definitely, definitely a concern."

A bleaching coral, seen on the left, is surrounded by mostly dead reef. (Danielle Claar/University of Victoria)

Marine heat waves are part of the natural climate system and can be caused by ocean currents or high atmospheric temperatures.

But Oliver said the scientists found the trend of more frequent and longer heat waves has intensified in recent decades.

"The 116-year time series shows that it's not a linear increase, that it's an accelerating increase. It was something like three-quarters of the change occurred over the last 30 or so years."

The trend is unlikely to slow any time soon.

"The probability of it continuing is, I would say, almost certain, because the trend is basically explained by rising ocean temperatures and we know that that is not going to stop rising," Oliver said. "Even if we stop emitting greenhouse gases now, there's so much inertia in the system that it's not going to stop warming for a while."