Limiting Ocean Acidification Under Global Change

Largely as a result of human activities such as the burning of fossil fuels for energy and land-use changes such deforestation, the concentration of carbon dioxide in the atmosphere is now higher that it has been at any time over the last 800,000 years. Most scientists believe this increase in atmospheric carbon dioxide to be an important cause of global warming.

According to Dr Toby Tyrrell of the University of Southampton's School of Ocean and Earth Science (SOES) based at the National Oceanography Centre, Southampton, UK, the oceans absorb around a third of carbon dioxide emissions, which helps limit global warming. Uptake of carbon dioxide by the oceans also increases their acidity, with potentially harmful effects on calcifying organisms such as corals and the ecosystems that they support and increased ocean acidification is also likely to affect the biogeochemistry of the oceans in ways that we do not as yet fully understand.

Tyrrell and his colleagues, in collaboration with researchers at the Met Office, used computer models to quantify the likely response of ocean acidification to a range of carbon dioxide emission scenarios, including aggressive mitigation. Collectively, these models take into account ocean-atmosphere interactions (such as air-sea gas exchange), climate, ocean chemistry, and the complex feedbacks between them.

Global mean ocean surface pH has already decreased from around 8.2 in 1750 to 8.1 today (remember than a decrease in pH corresponds to an increase in acidity). The simulations suggest that global mean ocean pH could fall to between 7.7 and 7.8 by 2100 if carbon dioxide emissions are not controlled.

However, if an aggressive emissions control scenario can be adopted, with emissions peaking in 2016 and reducing by 5% per year thereafter, the simulations suggest that mean surface ocean pH is unlikely to fall below 8.0 by 2100. But even that represents a large change in pH since the pre-industrial era.

A clear message from the study is that substantial emission reductions need to occur as soon as possible and that further reductions after atmospheric carbon dioxide concentration peaks will be needed if ocean pH is to be stabilised.

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