The Swim

Ocean Heat Waves Are on the Rise, Bleaching Coral Reefs and Disrupting Fisheries

Marine heat waves occur 34 percent more often and last 17 percent longer than they did in the early 20th century — and climate change is the likely culprit.

Silhouette of a turtle swimming underwater, Lady Elliot Island, Great Barrier Reef, Queensland, Australia | Sean Scott via Getty Images
Silhouette of a turtle swimming underwater, Lady Elliot Island, Great Barrier Reef, Queensland, Australia | Sean Scott via Getty Images

As global temperatures rise, landlubbers aren’t the only ones feeling the heat.

The number of heat waves in patches of the world’s oceans have been increasing over the last century, a new study of marine temperature records finds. Not only that, but those marine heat waves are getting longer and more intense, Eric Oliver, a physical oceanographer at Dalhousie University in Nova Scotia, told Seeker.  

“Basically, the time that an ecosystem is exposed to what would be warmer-than-average temperatures that might put it under stress at the very least is about 50 percent longer now than it was 100 years ago,” Oliver said.

In recent years, oceanic heat waves have caused the bleaching of much the world’s coral, including a large part of Australia’s Great Barrier Reef. They’ve disrupted fisheries as sea life moves to cooler temperatures, and affected weather on land.

Oliver and his colleagues at universities in Canada, Australia, and Britain have been examining the recurring hot spells since a 2011 event off western Australia. Their latest findings, published in the research journal Nature Communications, found heat waves occur 34 percent more often and last 17 percent longer than they did in the early 20th century — and climate change, driven by the carbon emissions from fossil fuels, is likely to drive those numbers upward further.

After that 2011 event, kelp forests in what’s typically cooler water collapsed and were replaced by sea grass. Fish that made their homes in the area swam away, and coral even took root in places it hadn’t been before. The heat wave only lasted about a month — but after it lifted, some of those changes persisted, Oliver said.

“The warming event actually triggered a change in the ecosystem,” he said. “And when it was dialed back to normal, the ecosystem stayed changed — and it has stayed changed since then.”

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The researchers define a marine heat wave as a stretch of five or more days in which surface temperatures are in the top 10 percent of the 30-year average between 1983 and 2012. There have been several notable events in the last few years in places ranging from the northwestern Atlantic Ocean in 2012, the northeastern Pacific in 2013-2015 and in the Mediterranean Sea in 2003 — the same summer a heat wave in Europe killed tens of thousands of people.

The study looked at satellite-derived temperature data since the 1980s; daily readings from coastal stations in the United States, Canada, Norway, and Britain, some of which date back to 1904; and ocean temperatures recorded by a variety of research, military, and merchant ships dating back to the early 20th century.

The record is consistent with human-caused climate change even when natural events like El Niño – the periodic warming of the equatorial Pacific that helped produce successive global temperature records in 2014, 2015, and 2016 — are factored in. Many of the changes in the extremes are driven by the fact that average temperatures have risen as well, Oliver said.  

Researchers hope to use the data in a follow-up study by feeding their data into computer models of climate to see what that will mean for the future under different scenarios for carbon emissions.

“While we can’t explicitly prove the hypothesis that anthropogenic climate change is responsible, everything is consistent and points toward that,” he said.