Ocean heat waves—defined as periods of extreme temperatures lasting five days or more—have become increasingly common in recent decades. In fact, as a new study published in Nature Climate Change finds, Earth’s number of annual ocean heat wave days spiked by around 54 percent between 1987 and 2016, with bouts of abnormally high temperatures not only occurring more frequently, but also lasting for longer periods of time.
As Damian Carrington explains for the Guardian, underwater heat waves pose a significant threat to marine ecosystems, which are already at risk due to issues including overfishing and rampant plastic pollution. Sweeping through oceans much like wildfires blaze through forests on land, extreme temperatures exact damage on foundational organisms such as kelp forests, seagrass meadows and coral reefs. Given that these framework species provide shelter and food to many other ocean creatures, the study’s authors warn that such destruction will likely have cascading consequences for marine biodiversity.
To assess the effects of ocean heat waves, researchers led by ecologist Daniel Smale of Great Britain’s Marine Biological Association turned to 116 previously published academic studies. Combined, National Geographic’s Sarah Gibbens notes, the papers yielded data from more than 1,000 ecological records, enabling the team to hone in on multiple recorded instances of unusually high temperatures.
Reflecting on eight specific heat waves, the scientists identified regions and species deemed most vulnerable to temperature surges. As Mary Papenfuss writes for the Huffington Post, areas of the Pacific, Atlantic and Indian Oceans topped the list, with the Caribbean’s coral reefs, Australia’s seagrass and California’s kelp forests emerging as particular concerns.
In terms of species, Pacific Standard’s Kate Wheeling adds, the team notes that stationary plants and animals were the hardest hit, while tropical fish and mobile invertebrates were able to cope with the heat by moving to different habitats. Interestingly, John Timmer reports for Ars Technica, the researchers actually observed heightened levels of fish diversity during periods of above-average temperatures, likely due to the animals’ mass migration toward friendlier waters. The same trend did not prove true for sea-dwelling birds, however, as shifting habitats limited the avian creatures’ access to prey.
According to Reuters’ Alister Doyle, marine heat waves are triggered by heat from the sun and shifting warm currents. Wheeling further explains that because the phenomenon is measured relative to average ocean temperature, it can occur in any region at any point during the year. El Niño—a regularly occurring climate pattern that makes the waters of the central and eastern Pacific warmer than normal—appears to exacerbate incidents of extreme heat, but as The New York Times’ Kendra Pierre-Louis and Nadja Popovich point out, heat waves can (and do) occur without the presence of El Niño.
Although the researchers’ findings are most consequential for marine ecosystems, Pierre-Louis and Popovich explain that damage to ocean habitats will also affect humans who rely on fishing and fish farming.
“Certainly there’s going to be changes with climate change to marine communities, but still the sun is going to shine, and plankton is going to grow, and things are going to eat that plankton, so it's not like the oceans are going to become the dead sea," Nick Bond, a climatologist at the University of Washington who was not involved in the study, tells Pacific Standard.
“It's just that, as a consequence of what we're doing to the oceans, there's going to be different marine communities in different places than what we're used to,” Bond concludes. “Obviously that is a problem because we're sort of set up for what the climate is now rather than what it is going to be in the future."