Assessing the risks of ocean acidification

The regular world climate reports compiled by the Intergovernmental Panel on Climate Change (IPCC) form the most reliable basis for political decisions regarding climate change.

For the fifth issue, the IPCC defined five current Reasons for Concern (RFC), eight key risks as well as four risk levels. The “Burning Ember” diagrams – the name refers to the colour gradient – visualise risks related to climate change. The corresponding narrative complements the graph.

BIOACID results are summarised here in line with the IPCC approach in order to facilitate the integration into the upcoming sixth assessment report. Included are also results of a meta-analysis of hundreds of individual studies – including many investigations conducted by the project itself – that has been produced as part of BIOACID. By providing a global perspective on the impacts of ocean acidification and warming, the Burning Ember diagram and the narrative presented here complement the case studies described in this brochure. However, a global picture cannot be compiled without many regional and local findings.



Risks of harmful ecosystem effects of ocean acidification are considered moderate around current carbon dioxide (CO2) levels of about 380 ppm. At these levels, decreasing calcification due to anthro­pogenic ocean acidification is already observed in some foraminifera and pteropods.
In addition, negative impacts on pteropods and oyster cultures along the west coast of North America have been attributed to upwelling of acidified water shifted closer to shore combined with anthropogenic acidification.
Under ocean acidification only (left column, warming excluded), the transition to high risk occurs at a CO2 level of about 500 ppm, beyond which studies reflect onset of significantly negative effects and high risk in 30 to 50 per cent of extant calcifying taxa (corals, echinoderms, molluscs, calcifying macroalgae; in particular tropical species).
Risks are estimated to be very high with limited capability to adapt beyond about 700 ppm, based on a rising percentage of the calcifying taxa being negatively affected. For the calcifying invertebrate taxa, these conclusions are confirmed by observations at natural analogues (volcanic CO2 seeps, upwelling systems) and by similar sensitivity distributions among taxa during paleo-periods.
Current knowledge indicates that the combined pressures of ocean warming extremes and acidification lead to a shift in sensitivity thresholds to lower CO2 concentrations, as seen in corals and crustaceans.
For corals this comes with the risk that ocean acidification will increasingly contribute to the reduction in areal extent of coral ecosystems, already underway as a result of interacting stressors (extreme events, predation, bleaching). Knowledge of the long-term persistence of acidification impacts presently relies on findings in the paleo-records. Therefore, evidence that changes in extant ecosystems will persist is limited, especially for fish (fish seem to be primarily affected by habitat loss due to increasing water temperature). Additionally, knowledge is scarce on compen­satory mechanisms and their capacity and associated limits to long-term evolutionary adaptation under ocean acidification and warming.

Updated from: O’Neill et al., 2017, Nature Climate Change, 7, 28–37, doi:10.1038/nclimate3179

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