Heat waves have spiked in recent years. The United States is now scorched by about six per year, compared to just two annually in the 1960s. At sea, marine heat waves such as the Blob, which warmed waters off the U.S. West Coast from 2013 to 2016, are becoming hotter over time.
Now, scientists have discovered that more intense, longer-lasting heat waves on continental shelves can strike the ocean bottom independently from the surface. Excess heat disrupts oceanic ecosystems and thwarts the ocean’s twin promises of cooling and carbon sequestration.
“These things can happen without any clear surface signature,” said Dillon Amaya, a climate scientist at NOAA who led the recent study published in Nature Communications. Unfortunately, ocean temperatures have historically been measured mainly at the surface, leaving scientists in the dark about what’s happening in the deep.
“That’s the scary part,” Amaya said. “Often, we won’t know that these things are happening until after the fact, when the ecosystem impacts are more apparent.”
Toastier Times in the Deep
Focusing on continental shelves—the offshore parts of a continent—Amaya and his colleagues combined observations and climate models to evaluate bottom marine heatwaves in nine large marine ecosystems off North American coasts from 1993 to 2019. The group combined sparse existing observations with climate modeling to approximate missing data through time.
The researchers tracked average temperatures in the ocean from the surface to the seafloor of each continental shelf (up to 400 meters deep). They defined a bottom marine heat wave as temperatures in the top 10% within the study’s 26-year range.
The data revealed bottom marine heat waves that lasted up to 6 months and were 0.5°C–3°C warmer than average. These spikes are enough to stress or kill species that live on continental shelves: lobsters, Dungeness crab, Pacific cod, oysters, clams, and other bottom dwellers, Amaya said. The hot spells sometimes occurred concurrently at the surface, but not always.
The data also showed that heat wave intensity varies with ocean bottom depth, an observation that could aid future heat wave predictions.
“To get long-term observations below the surface is tricky,” said Amandine Schaeffer, a physical oceanographer at the University of New South Wales in Sydney. She pointed out that this new study is not the first to measure heat waves below the surface; previous studies demonstrated similar phenomena on a smaller scale near Australia and in the tropical western Pacific.
The new study, Amaya said, is the first to do this at a large scale and shows that heat wave observations from past studies can be applied on a broader scale.
Continental shelves are nearshore seafloor environments, so the group’s findings don’t apply to deep or abyssal ocean ecosystems, which can be up to 4.5 kilometers (2.8 miles) deep.
Amaya said he hopes investigations will detail the physical mechanisms that drive bottom marine heat waves, allowing scientists to build tools to forecast temperature variability. Testing whether those mechanisms hold up across physically diverse regions will be key, he said.
The new study “shows the need for subsurface understanding and subsurface observations to really see what’s happening in terms of extremes below the surface,” Schaeffer said. She added that it usually takes 3 decades of data just to establish a baseline of what constitutes an extreme temperature, and it is more difficult to track temperatures below the surface—obstacles to broadening this research.
Whether marine heat waves will be more frequent or intense in the future is still an open question, Amaya said, partially because scientists don’t yet agree on the best way to analyze them. “But the absolute water temperature of these [heat waves] is definitely getting hotter,” he said.
However they are defined, heat waves at the surface or hundreds of meters below will continue to shape marine ecosystems as climate change intensifies in the coming years.
—Robin Donovan (@RobinKD), Science Writer