Chronic ocean heating is fuelling a “staggering and deeply concerning” loss of marine life, a study has found, with fish levels falling by 7.2% from as little as 0.1C of warming per decade.
Researchers examined the year-to-year change of 33,000 populations in the northern hemisphere between 1993 and 2021, and isolated the effect of the decadal rate of seabed warming from short shifts such as marine heatwaves. They found the drop in biomass from chronic heating to be as high as 19.8% in a single year.
“To put it simply, the faster the ocean floor warms, the faster we lose fish,” said Shahar Chaikin, a marine ecologist at the National Museum of Natural Sciences in Spain and the study’s lead author.
“A 7.2% decline for every tenth of a degree per decade might sound small,” he added. “But compounded over time, across entire ocean basins, it represents a staggering and deeply concerning loss of marine life.”
The study, published in Nature Ecology & Evolution on Wednesday, also found marine heatwaves were leading to short-term booms in some populations that masked long-term harm from climate breakdown.
For instance, a heatwave that could cause sprat populations to fall in the Mediterranean Sea, which is at the warm edge of their natural range, would lead to a boom in the North Sea, at the cold edge of their range.
Fish in cold areas are better able to capitalise on these shifts than those in warm areas, the researchers found, but these temporary cold-water gains mask “widespread loss” due to ocean warming.
Carlos García-Soto, a scientist at the Spanish National Research Council and co-author of the UN’s world ocean assessment, said the study revealed a “concerning” dynamic for ocean governance.
“Overall warming reduces fish biomass, while heatwaves can generate temporary increases that mask the underlying trend,” said García-Soto, who was not involved in the study. “This combination introduces a clear risk of poor interpretation when taking decisions.”
Guillermo Ortuño Crespo, a marine biologist who co-directs a high seas specialist group with the International Union for Conservation of Nature, said the study was “methodologically sound and highly valuable” but cautioned against making climate breakdown the main explanation for biomass changes in marine species.
“Historically, overfishing has been the main driver of biomass declines in many of the world’s fisheries [and] according to the FAO [UN Food and Agriculture Organization] the proportion of overfished stocks globally continues to rise,” said Ortuño Crespo, who was not involved in the study. “The current challenge is that this overfishing crisis is being further exacerbated by ocean warming and deoxygenation.”
Marine life is extremely vulnerable to the shifts in temperature brought on by the fossil fuel pollution that clogs the atmosphere. Scientists have repeatedly warned that “every fraction of a degree matters” as temperatures race dangerously close to the 1.5C threshold to which world leaders have promised to limit global heating by the end of the century.
“Our research proves exactly what that biological cost looks like underwater,” said Chaikin. “If we allow the pace of ocean warming to speed up by even a 10th of a degree per decade, we are expecting great losses to global fish populations that no management plan can easily fix.”
