Since 2014, more than half of the ocean surface across the globe has recorded temperatures considered extreme compared with a historical average
1 February 2022
The world’s oceans passed a threshold eight years ago as marine heatwaves became the “new normal” due to climate change, with research showing extreme temperatures recorded across more than half of Earth’s seas since then.
Marine heatwaves, such as the “blob” of warm water in the Pacific Ocean between 2014 and 2016, and a smaller 2019 event, can cause algal blooms, coral bleaching and mass die-offs of fish and birds that feed on them.
While scientists have projected that heatwaves will become more intense and frequent under future climate change, it is less appreciated that they have already gone from a rare phenomenon worldwide to a common occurrence.
Kyle Van Houtan at Duke University in Durham, North Carolina, and Kisei Tanaka at the US National Oceanic and Atmospheric Administration have now found that 2014 marked the first time that more than half of the global ocean surface recorded temperatures were considered extreme compared with a historical baseline. Just three years later, such temperatures covered a high of 60 per cent of the oceans. The figure was less than a fifth in the early 1900s.
“Our new index of extreme marine heat shows the global ocean crossed a critical barrier in 2014 and it’s now normal. It’s arrived, it’s here,” says Van Houtan.
While the oceans as a whole crossed the threshold of 50 per cent only recently, some oceans hit it much earlier. The South Atlantic passed the milestone in 1998. “That was a long time ago. I think that’s really jarring,” says Van Houtan.
Definitions of a marine heatwave can vary. Van Houtan and Tanaka looked at two sets of global sea surface temperature data from 1870 to 2019, using the first 50 years to establish a historical baseline of what extreme marine heat looked like then for every pixel of ocean, for every month of the year. The hottest 2 per cent of temperatures were deemed extreme. The pair then used that as a yardstick to map the prevalence of those extremes up to 2019.
“It’s a nice piece of work,” says Amelie Meyer at the University of Tasmania, Australia. What’s different about the research is it looks at marine heat extremes rather than individual heatwaves or projecting the future, she says. “They are reinforcing the idea that climate change is already well in progress,” says Nick Bond at the University of Washington in Seattle.
One thing to bear in mind is that the baseline period was relatively cold, says Bond. There will also be some uncertainty in the historical benchmark because readings of sea surface temperatures recorded by ships were very scarce in early 20th century, says Alex Sen Gupta at the University of New South Wales in Sydney, Australia.
But he says looking at extremes rather than averages is important. “We are becoming increasingly aware that it’s temperature extremes rather than mean climate that have the most extreme effects on marine organisms.”
Journal reference: PLOS Climate, DOI: 10.1371/journal.pclm.0000007
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