Most people have experienced a heat wave on land. But heat waves can strike in the ocean, too. And as the planet continues to warm, marine heat waves are growing longer and deadlier, hurting the seafood supply that billions of people worldwide rely on for their food and livelihoods.
Researchers at Michigan State University say a controversial strategy to shield Earth from some of the sun’s rays would offer unequal protection for the oceans and communities that depend on them, leaving 25% to 75% of the world’s seas still at risk from overheating, models suggest.
The findings were published in the journal Environmental Research: Climate.
First author Lala Kounta started studying marine heat waves in 2020, when unusually warm waters off the coast of Senegal where she grew up triggered a toxic algal bloom that made hundreds of fishermen sick.
Senegal is far from alone. Worldwide, heat waves in the planet’s oceans have gotten longer and more frequent than they were a century ago, previous research shows.
“The consequences are already visible,” said Kounta, a physical oceanographer and postdoctoral scholar working with professor Phoebe Zarnetske in MSU’s Ecology, Evolution and Behavior Program.
Scientists say a series of marine heat waves is partly to blame for the die-off, in the last 30 years, of more than half the living coral in Australia’s Great Barrier Reef.
Sea lions and seabirds starved as their food species shifted to cooler waters during a massive marine heat wave that took hold off the West Coast of the United States from 2014 to 2016.
High ocean temperatures off the coast of southern Chile in 2016 fueled a toxic algal bloom that wiped out 100,000 tons of salmon and trout, making it the largest fish farm mortality ever recorded.
One study of 34 marine heat waves worldwide found that the economic toll from a single event can run into billions of dollars.
In the search for solutions, slow progress on cutting emissions has prompted some to ask: can we engineer our way out of the climate crisis?
One strategy that’s been proposed is to artificially cool the planet by deliberately dimming the light from the sun.
The idea, a form of climate intervention called stratospheric aerosol injection, would involve using planes to release small particles or gases such as sulfur dioxide high above Earth into the stratosphere, where they could deflect some of the sun’s rays — like drawing a thin gauzy curtain over the face of the Earth.
Shooting sun-blocking particles into the stratosphere may sound fantastical. The idea is to mimic the natural cooling effects of volcanic ash and gas. When volcanoes erupt, they launch millions of tons of sulfur dioxide and other gases miles into the sky. Once there, they combine with water in the atmosphere to create small droplets that drift around the globe and remain aloft for several years.
These aerosols reflect incoming sunlight and have a temporary cooling effect on the planet. When Mt. Pinatubo erupted in the Philippines in 1991, for example, global temperatures dipped by roughly half a degree (0.6 degrees Celsius) for two years.
In the past 30 years, the U.S. National Academies has issued three reports calling for more research on the potential consequences of solar geoengineering, to better understand the full range of options for dealing with climate change. But while most research has focused on what the technique would mean for warming in the atmosphere, the impacts on our oceans remain unclear.
For the study, Zarnetske, Kounta and collaborators in the NSF-funded Climate Intervention Biology Working Group used computer simulations to forecast marine heat waves under future scenarios with and without solar geoengineering.
In her office, Kounta pointed to a graph showing what ocean temperatures could look like in the next 10, 20, 40 years. The future they depict looks bleak.
If current trends continue, the team’s modeling found, average ocean temperatures are on track to rise by 1 degree Celsius by 2069, with marine heat waves becoming hotter and longer across 97% of the world’s oceans.
In contrast to this “business as usual” scenario, the simulations suggest that solar geoengineering could offer some cooling, but the benefits wouldn’t be felt everywhere.
In one set of simulations, the team modeled a world in which enough sun-blocking particles are pumped into the stratosphere to keep Earth’s temperature at 1.5 degrees Celsius above preindustrial levels, a threshold scientists say the planet is already beginning to exceed.
In another, more aggressive scenario, the team examined what would happen to ocean temperatures if enough sun-blocking particles were released to reduce atmospheric warming to less than 1 degree Celsius.
Kounta pulled up a map of the world’s oceans, showing the pattern of temperature changes the model predicted for the period from 2035 to 2069.
“The degree of protection depends on how aggressively it is deployed,” Kounta said.
Under the moderate scenario, only 20% to 25% of the world’s oceans were shielded from worsening heat waves. But under the more aggressive scenario, that figure increased to 75%.
The regions that benefitted the most were the Tropical Atlantic, the Indian Ocean, the Arctic Ocean and the South Atlantic Ocean, “including the West African coast I know so well,” Kounta said.
“But other regions tell a very different story,” she added.
Even under the more aggressive approach, the simulations suggest that nearly 25% of the world’s oceans would still face longer, hotter heat waves in the decades to come, straining fishing communities in the North Atlantic, the Northern and Tropical Pacific and parts of the Southern Oceans.
Despite the intervention, some regions could even stop suffering occasional spikes and enter a constant state of overheating by the end of the century.
“The geography of protection is deeply unequal,” Kounta said.
These patterns illustrate the central policy challenge when considering solar geoengineering: “Who decides whether to deploy?” Kounta asked.
Countries would need to reach an agreement regarding how to manage the technique and how to stop or make adjustments if the results aren’t as expected.
“You need geopolitical collaboration and importantly, you also need exit ramps if this was ever to be deployed,” Zarnetske said.
Critics of the strategy say it masks the symptoms of climate change rather than curing its root cause. That’s because it does nothing to alleviate the buildup of CO2 and other heat-trapping gases.
As CO2 levels continue to rise, they diffuse into the ocean, making the water more acidic and corrosive to shelled animals such as mussels, clams, sea urchins and sea snails that form the base of the ocean food chain.
And if a large-scale solar geoengineering project were deployed and then abruptly halted, some models predict a sudden, rapid spike in global temperatures if the hidden greenhouse warming were to hit the planet all at once.
“It's not a substitute for reducing emissions; reducing emissions is still the priority and is the most effective action to mitigate climate change,” Zarnetske said.
Important questions remain regarding other potential consequences of solar geoengineering, for everything from crop yields to rainfall.
“There's very little known about the ecological impacts,” said Zarnetske, who heads MSU’s Spatial and Community Ecology Lab, or SpaCE Lab.
That’s a lot of “what ifs.”
“We’re studying this topic to learn about its potential impacts,” Zarnetske said. “As scientists, it's our responsibility to provide the best available science to inform decisions.”
The study was conducted with researchers from Louisiana State University, Colorado College, Cornell University, the U.S. National Center for Atmospheric Research, Duke University and University of Dakar.
This research was supported by the U.S. National Science Foundation, MSU’s African Futures Research Leadership Program and by a seed grant from MSU’s Ecology, Evolution and Behavior program.
This story was originally featured on the Ecology, Evolution and Behavior website.
