Chronic ocean warming is driving a shockingly large toll on marine life, a new study shows, with fish populations shrinking as warming accelerates by just 0.1°C per decade. The impact isn’t a blip: biomass declines of around 7.2% per decade, and in some cases the yearly drop can be as steep as 19.8% when warming persists over time.
Researchers analyzed year-to-year changes across 33,000 populations in the Northern Hemisphere from 1993 to 2021, separating the long-term warming trend of the ocean floor from short-lived events like marine heatwaves. Their findings indicate that chronic heating drains marine life steadily, even when short-term heat spikes temporarily boost certain species in some places.
Lead author Shahar Chaikin, a marine ecologist with the National Museum of Natural Sciences in Spain, explained: “In plain terms, faster warming of the seafloor accelerates the rate at which we lose fish.” He added that while a 7.2% decline per 0.1°C per decade might seem modest, the effect compounds over time across vast ocean basins, producing a troubling overall loss of marine life.
The Nature Ecology & Evolution study also notes that heatwaves can cause short-term population surges in some areas, effectively masking the longer-term damage caused by ongoing climate change.
For example, sprat populations at the warm edge of their range in the Mediterranean could plummet during a heatwave, whereas cooler regions like the North Sea might see a temporary increase at the cold edge of their range.
Species in colder waters tend to gain more from these shifts than those in warmer waters, but these transient gains do not erase the broader trend of decline due to sustained warming.
Carlos García-Soto of the Spanish National Research Council and a co-author of the UN’s World Ocean Assessment described the findings as a worrying pattern for ocean policy. He noted that while warming lowers overall fish biomass, heatwaves can briefly inflate numbers, potentially leading to misinterpretations when making management decisions.
Guillermo Ortuño Crespo, a marine biologist and co-director of a high seas group for the International Union for Conservation of Nature, praised the study’s solid methods and usefulness, but urged caution in attributing biomass changes primarily to climate breakdown. He emphasized that overfishing has long been the dominant driver of declines in many fisheries and that warming and deoxygenation are intensifying this pressure.
The research comes amid rising concerns about how fossil-fuel-driven temperature increases affect ocean ecosystems. Scientists have consistently warned that every fraction of a degree matters as global temperatures push toward the 1.5°C limit endorsed by world leaders.
Chaikin concluded, “Our results lay bare the biological cost beneath the waves. If ocean warming speeds up by even another tenth of a degree per decade, we should expect substantial, perhaps unfixable, losses in global fish stocks.”
