Marine heat waves and acidification occurring more frequently than expected

New research indicates that marine heat waves and extreme ocean acidification events are occurring together much more frequently than statistical models would predict. An analysis of 43 years of surface ocean data from 1982 to 2024 reveals that these compound events happen globally approximately 2.5 times more often than if the two phenomena were independent.

In low to mid-latitudes, the frequency of these simultaneous stressors is even higher, occurring roughly three to four times more often than expected. This is largely due to permanent water stratification in tropical and subtropical regions, where surface heat directly alters water chemistry by increasing hydrogen ion release, thereby driving up acidity levels.

These combined stressors pose a severe threat to marine ecosystems. Previous instances, such as ‘the Blob’ in the northeastern Pacific between 2013 and 2015, resulted in closed fisheries, mass strandings of marine mammals, and significant seabird mortality. While compound events are less common in upwelling regions like the eastern equatorial Pacific or near the poles, the increasing synergy between heat and acidity remains a critical concern for global ocean health.