Atmospheric carbon dioxide (CO2) concentration has increased by 42% since the onset of the industrial revolution due to emissions from fossil fuel burning, cement production and land-use change, as reported in the WMO Greenhouse Gas Bulletin No. 10. As of 2010, the oceans had absorbed an estimated 155 ± 30 petagrams (Pg, 1 petagram = 1015 grams-force) of anthropogenic CO2 [Khatiwala et al., 2013], the equivalent of 28% of the total CO2 emissions during the same time. This factor limited the increase of CO2 in the atmosphere. Although this ocean CO2 uptake reduces climate change, it also comes with severe consequences for ocean chemistry and biology.
Since the beginning of the industrial era, human activity has added 4 kg of carbon dioxide per day per person on average to the ocean. This anthropogenic CO2 reacts with water to form an acid. As atmospheric CO2 continues to increase, more and more CO2 enters the ocean, which reduces pH (pH is a measure of acidity, the lower the pH, the more acidic the liquid) in a process referred to as ocean acidification. Along with the increase in acidity (higher concentrations of hydrogen ions, H+), there is also a simultaneous decrease in concentrations of carbonate ion (CO32-). Reductions in CO32- reduce the chemical capacity of the ocean to take up further CO2 while also degrading the ability of many marine organisms to produce and maintain shell and skeletal material.
Declines in surface ocean pH due to ocean acidification are already detectable and accelerating. Measurements gathered at biogeochemical time-series sites around the world reveal similar decreasing trends in ocean pH (reductions between 0.0015 and 0.0024 pH units per year), but datasets are only available for the last few decades. To estimate earlier changes, scientists have used both models and data-based extrapolations. Both approaches converge to indicate that since 1860, the pH of the ocean surface has dropped from 8.2 to 8.1, corresponding to a 26% increase in H+. The present rate of change can be put into context by looking at the paleoclimatic record. The current change appears to be the fastest in at least 300 million years, with the fastest known natural acidification event – occurring 55 million years ago – being probably ten times slower.
Under most emission scenarios, Earth system models project an acceleration in acidification at least until mid-century. When forced by the latest scenarios from the Intergovernmental Panel on Climate Change (IPCC), Earth system models that participated in Phase 5 of the Coupled Model Intercomparison Project (CMIP5) consistently indicated that reductions in surface pH will depend almost solely on the atmospheric CO2 pathway that will be taken. Between 1850 and 2100, under the most conservative IPCC scenario for the trajectory of greenhouse gas concentrations, the decline in global-mean surface pH among models ranges from 0.12 to 0.14, a 36% increase in acidity; under the worst scenario, it ranges from 0.41 to 0.43, a 165% increase in acidity. But pH is not the only concern.
Enhanced ocean CO2 uptake alters the marine carbonate system, which controls seawater acidity. As CO2 dissolves in seawater it forms carbonic acid (H2CO3), a weak acid that dissociates into bicarbonate (HCO3-) and hydrogen ions (H+). Increased H+ means increased acidity (lower pH). The rate of the ocean’s acidification is slowed by the presence of CO32-, which binds up most of the newly formed H+, forming bicarbonate. But that buffering reaction consumes CO32-, reducing the chemical capacity of the near-surface ocean to take up more CO2. Currently, that capacity is only 70% of what it was at the beginning of the industrial era, it may well be reduced to only 20% by the end of the century. The same models project that CO32- concentrations will reach levels that are so low by mid-century that tropical coral growth may become unsustainable. In the cold polar oceans, where CO32- is naturally less abundant, CO32- concentrations have already begun to dip below a critical level in which waters become corrosive to the calcium carbonate (CaCO3) mineral known as aragonite – the main component of shells, pearls, the exoskeleton of crustaceans, etc. By the end of the century, these corrosive conditions are predicted to expand throughout the polar oceans, causing concern for the fate of organisms such as pteropods – snails – that secrete aragonite to build their shells, and for food security, as many economically important organisms depend on availability of CO32- to survive.
Despite remarkably consistent projections for changes in surface ocean pH, there is less agreement among models about the magnitude of subsurface changes. Furthermore, current Earth system models have coarse resolution and are not designed to study changes in the coastal ocean where other anthropogenic influences can also affect changes in pH.
Tanhua T., Orr J. R., Lorenzoni L. & Hansson L., 2015. Monitoring ocean carbon and ocean acidification. WMO Bulletin 64(1). Article.
