Heat waves aren’t just for land lubbers. Climate change has turned the oceans into cauldrons of scalding water, upending marine ecosystems around the world.
A new paper in Nature shows how marine heat waves have become more common and intense in recent decades, largely due to climate change. What’s worse, it shows that even if the world manages to limit warming to two degrees Celsius, the trend will continue and humans will ultimately be the main driving force for virtually every marine heat wave. If we let the world warm past that mark, the results could be catastrophic for the high seas.
The role of human-caused climate change in intensifying land-based heat waves is now well-established. But less research has focused on the ocean, where extreme heat events in recent years have wiped out portions of the Great Barrier Reef, caused bull sharks to migrate further north, and left scientists scrambling to find ways to save coral. Individual events like the 2016 heat wave in the Great Barrier Reef have been tied to climate change (it made the heat wave 175 times more likely), but there hasn’t been a big picture look at the topic featuring projections into the future.
That’s what led a team of Swiss researchers used a mix of models and satellite measurements to get a handle on how marine heat waves have already changed and what the future holds for the globe. The satellite record, which runs from 1982-2016, helped ground truth the models they used to create a pre-industrial baseline—what the oceans looked like without all the extra heat trapped by greenhouse gas emissions. They then used the models to understand how climate change is affecting the extent, duration, and intensity of marine heat waves as well as project future changes.
The findings show the number of marine heat wave days doubled between 1982 and 2016, while heat waves also increased in extent and intensity. Moreover, they show that 87 percent of marine heat waves can be attributed to climate change, meaning they would not have occurred without it.
“We were expecting that marine heatwave will likely change under global warming,” Thomas Frolicher, a climate researcher at the University of Bern who led the research, told Earther. “However, we show for the first time, that the increase in the number of marine heatwave days over the satellite period is much larger than what we can expect from natural variability alone.”
That’s not even the bad news. Climate change is projected to keep cranking the heat up. With the majority of the heat trapped by greenhouse gases ending up in the ocean, the impacts will become particularly profound there.
The research shows that if we allow the world to warm two degrees Celsius, what policymakers have outlined as a “safe” limit of global warming, the odds of marine heat waves will increase 23 fold. If the world meets the wholly inadequate pledges to the Paris Agreement, the planet would likely warm 3.5 degrees Celsius. That uptick in temperatures would make marine heat waves would become 41 more times likely, and the portion that would be attributed solely to humans would be upwards of 97 percent.
Frolicher said that reefs would face high risk of bleaching on a regular basis, but that we don’t really know enough about other marine species to predict the exact impacts. He also warned that it could take years for the heat to trickle down to the deep ocean, where we know even less about the ecosystems and their responses to heat.
“Retrospective studies, innovative laboratory experiments and interdisciplinary approaches are needed to ultimately understand the impact of marine heatwaves on marine ecosystems and their socio-economic services,” he said.
“Societal values regarding the continued existence of certain species that are particularly sensitive to marine heatwaves, like coral, and the sustainability of industries that rely on marine resources will affect a range of policy decisions at national, state, and even local levels,” Kathy Mills, a researcher at the Gulf of Maine Research Institute, told Earther.
Indeed as Mills points out, the seas provide a bounty of services from protecting coastal communities from storm surge to giving them an economic boost through fisheries and tourism. Given the impacts we’ve already seen from heat waves, it behooves us to consider how we’ll respond to what comes next.