Ocean upwelling zones are acidifying at a frightening pace, threatening fisheries that supply 20% of the world's catch and putting millions of livelihoods at risk, Scottish academics have warned.

Because atmospheric CO2 and ocean pH (acidity) are tightly coupled, the more CO2 that is released into the atmosphere, the more is absorbed by seawater, making the ocean progressively more acidic. 

Scientists from the University of St Andrews analysed ocean acidification mechanisms in the California Current and found upwelling systems - natural processes where deep waters rise to the surface - are accelerating acidification beyond effects caused solely by rising atmospheric CO2 concentrations.

Deep ocean waters turn acidic as microorganisms break down organic matter, releasing CO2. When upwelling brings these acidic waters to the surface, they interact with atmospheric CO2, triggering a chain reaction that slashes pH levels even further.

Researchers reconstructed historical acidity changes by studying 20th century coral skeletons, analysing boron isotope composition in their structures. The data confirmed upwelling zones have shown abnormally high acidification rates for several decades.

Computer modelling of 21st century acidity changes showed the situation will worsen as natural processes combine with human impact. The key driver is deep-water acid interacting with excess atmospheric CO2, creating a chain reaction of acidification.

Co Author Dr Hana Jurikova, Senior Research Fellow in from the School of Earth and Environmental Science, said: “Predicting how upwelling systems will respond to climate change is highly complex, as anthropogenic influences interact with natural sources of ocean acidification. Our research shows that such interactions can amplify environmental change in the California Current System, highlighting the need for similar studies in other regions to better anticipate future change.”  

Co Author Dr James Rae, Reader in the School of Earth and Environmental Science, said: “the ocean becoming more acidic poses major risks to marine ecosystems and the communities and economies they support. The solutions we now have for climate change, like heat pumps and electric vehicles, also fix ocean acidification, so it’s critical that we support them”.   

The threat extends beyond the California Current to the Humboldt Current off Peru, the Benguela Current off southwest Africa, the Canary Current off northwest Africa, and the Somali Current off eastern Africa.