Heat waves and acidification: how is climate change transforming the oceans?

Oceans are experiencing unprecedented heatwaves and acidification, according to research published in August 2026 in the scientific journal AGU Advances. Scientists from ETH Zurich reconstructed 43 years of ocean history, from 1982 to 2024, using observational data, satellite information, and statistical models. They found that combined episodes of extreme heat and acidification occur four times more frequently in low and mid-latitudes than would be expected by chance alone. These dual stressors threaten marine life such as fish, corals, mollusks, algae, and other organisms. The study highlights that while many of these events are relatively small, covering less than one million square kilometers and lasting around a month, some have been exceptionally large. One notable example was the "Blob," a massive marine heatwave in the northeastern Pacific in 2015, which covered 12.6 million square kilometers. Another was the North Atlantic heatwave of 2023-2024, which lasted nearly a year. The most intense combined episode occurred off Western Australia in 2011. The combination of heat and acidification poses particular risks because their interaction can amplify effects. For instance, an organism near its thermal tolerance limit might become more susceptible to chemical changes in water. Acidification specifically affects organisms that build shells or skeletons from calcium carbonate, including mollusks and corals. Research published in Nature Climate Change indicates that the global ocean is undergoing simultaneous changes in multiple variables, including temperature, salinity, dissolved oxygen, and pH, in large regions and at different depths. According to the authors, a significant portion of the ocean has already transitioned into a climate state distinct from that recorded decades ago. One striking finding is the extent of acidification: nearly all of the world's ocean surface shows a clear decrease in pH since the mid-1990s, beyond natural variability. Between 25% and 40% of the upper layers of the ocean exhibit significant simultaneous changes in at least two environmental properties, with approximately a quarter showing alterations in three variables simultaneously. These findings are supported by studies showing real-world impacts. A 2026 study published in PeerJ followed intertidal reef communities along Brazil’s southeastern coast from 2017 to 2022. Researchers found a correlation exceeding 80% between temporary changes in macroalgae coverage and marine heatwaves. Stronger and longer-lasting events led to a loss of up to 38% in macroalgae coverage. Even after signs of recovery following extreme events, the marine community did not return to its previous composition. Another study published in 2026 suggests that warming oceans could significantly affect the size of marine animals. Research indicates that two-thirds of marine animal groups could experience reductions in body size by 2100, potentially impacting food security for humans. Some species, however, might grow larger, such as octopuses. The implications of these findings underscore the complex and multifaceted nature of ocean change driven by climate factors. As scientists continue to monitor and analyze these shifts, the need for comprehensive strategies to address the challenges posed by a rapidly changing marine environment becomes increasingly urgent.