An Updated Synthesis of the Impacts of Ocean Acidification on Marine Biodiversity
Ocean acidification, often referred to as the “other CO2 problem”, is a direct result of rising atmospheric carbon dioxide (CO2) concentrations due to the burning of fossil fuels, deforestation, cement production and other human activities. As atmospheric CO2 increases, more enters the ocean across the sea surface. This process has significant societal benefits: by absorbing around a quarter of the total human production of CO2, the ocean has substantively slowed climate change. But it also has less desirable consequences, since the dissolved CO2 affects seawater chemistry, with a succession of potentially adverse impacts on marine biodiversity, ecosystem services and human society.
The starting point for such changes is an increase in seawater acidity, resulting from the release of hydrogen ions (H+). Acidity is measured on the logarithmic pH scale, with H+ concentrations* at pH 7.0 being ten times greater than at pH 8.0. Since preindustrial times, the mean pH in the surface ocean has dropped by 0.1 units, a linear-scale increase in acidity of ~26%. Unless CO2 emissions are rapidly curtailed, mean surface pH is projected – with a high degree of certainty – to fall by a further ~0.3 units by 2100, representing an acidity increase of around 170% compared to pre-industrial levels. The actual change will depend on future CO2 emissions, with both regional and local variations in the oceanic response (Chapter 3).
Very many scientific studies in the past decade have unequivocally shown that a wide range of marine organisms are sensitive to pH changes of such magnitude, affecting their physiology, fitness and survival, mostly (but not always) in a negative way. The consequences of ocean acidification for marine food webs, ecosystems, biogeochemistry and the human use of marine resources are, however, much less certain. In particular, ocean acidification is not the only environmental change that organisms will experience in future, since it will occur in combination with other stressors (e.g., increasing temperature and deoxygenation). The biological effects of multiple stressors occurring together cannot be assumed to be additive; instead, due to interactions, their combined impacts may be amplified (through synergism) or diminished (antagonism). Furthermore, there is now evidence that some – but not necessarily all – organisms may show genetically mediated, adaptive responses to ocean acidification.
This review provides an updated synthesis of the impacts of ocean acidification on marine biodiversity based upon current literature, including emerging research on the geological history of natural ocean acidification events, and the projected societal costs of future acidification. The report takes into consideration comments and feedback submitted by Parties to the Convention on Biological Diversity, other Governments and organizations as well as experts who kindly peer-reviewed the report.