In hot water: Global warming amplifies risk of marine heat waves

Tens of millions across the northern hemisphere have experienced record heat waves this summer, further driving home how climate change is already tilting the odds in favor of more severe extreme heat events in the years ahead.

A similar dynamic is emerging in our oceans, and this could have far-reaching implications. A new study, published today in the journal Nature, finds that marine heat wave days have doubled between 1982 and 2016, which cannot be explained by natural variability alone. What's new and significant, though, is what the study projects for the future.

Why it matters: Marine species that already live close to the upper end of their heat tolerance could perish in prolonged and more severe marine heatwaves in the near future — much as portions of the Great Barrier Reef did during the 2014 to 2017 global coral bleaching event.

• Recent studies have shown that the frequency of marine heat waves has increased in the past several decades, but a lot has remained unknown about these events, until now.

How they did it: The new study uses satellite observations of ocean temperatures during the period from 1982 to 2016, combined with a suite of Earth system computer models to provide more insight into the likelihood and severity of such phenomena. The researchers identified an event as a marine heat wave when the sea surface temperature exceeded its local 99th percentile.

What they found: The study's findings are grim, as they indicate that global warming is already sharply raising the risk of marine heat waves, and this is likely to get significantly worse.

According to Thomas Frölicher, lead author of the study and a researcher at the University of Bern in Switzerland, if greenhouse gas emissions continue on their present course and temperatures were to rise by 6.3°F (3.5°C) by 2100, "the number of marine heatwave days will be 41 times higher than in preindustrial times," he told Axios.

• "On average, the spatial extent of the heatwaves would be 21 times larger, their duration would increase to 112 days and their maximum intensity would rise to 2.5°C," or 4.5°F.
• "The probability of marine heat waves is projected to increase almost everywhere," Frölicher said.
• However, these numbers would decrease significantly if emissions are cut, and global warming were to be limited to just 1.5°C, or 2.4°F, above preindustrial levels, the study found.

Interestingly, the study found that the number of marine heat waves is increasing faster than the number of heat waves on land.

"Since sea surface temperatures warm slower than temperatures over land one would expect marine heatwaves to increase slowly. However, since temperatures are less variable in the ocean than over land, this occurs more rapidly."

How it works: Marine heat waves can trigger large-scale changes in marine ecosystems, killing some species or forcing them to move.

"Recent observed marine heatwaves, such as the marine heatwave in Western Australia in 2011 or the blob in the North Pacific from 2013 to 2015, demonstrated the high vulnerability of marine ecosystems to such events," Frölicher said.

"Reported biological impacts range from geographical species shifts and widespread changes in species composition to harmful algal blooms, mass stranding of mammals and mass mortalities of particular species. In several cases, marine heatwaves have led to the closing of commercially important fisheries and aquaculture industries."

What we're watching: The new study goes a long way toward attributing trends in marine heat waves and projecting what may lie ahead. However, according to Frölicher, scientists currently lack a full understanding of the processes behind the formation of a marine heat wave, as well as the decay of such events. Researchers and scientists alike will need to do more work to understand how marine ecosystems will respond to more of these events.

Eric Oliver, a physical oceanographer at Dalhousie University in Nova Scotia who led a prior study on marine heat waves, said the new research is noteworthy for "quantifying the future projected changes.”

“It’s a very important issue in part because it’s so easy to ignore. When we see heat waves on land it’s obvious that people are dying," Oliver said. "It’s just much more obvious. Whereas unless you spend your life underwater, which very few people do… it’s really hard to see the impacts.”

The bottom line: The finding that oceans are facing a similarly scorching future as land areas is yet another warning that without sharp cuts in greenhouse gas emissions, we're in for a far more volatile and damaging future than we've been accustomed to.

"Together with the projected increase in marine heatwaves with future warming, this provides a worrying picture for the global oceans," Fischer says.