The seventh planetary boundary has been breached. There are nine in total, representing the extent to which our lifestyle pressures the planet's ecosystems. Exceeding these thresholds breaches the planetary boundaries. Six planetary boundaries have already been transgressed by human activity in recent years: climate change (CO2 levels and radiative forcing), biosphere integrity (ecosystems and biodiversity), land-system change, freshwater change, biogeochemical flows (nitrogen, phosphorus), and novel entities. The globally recognised 'Planetary Boundaries Science Lab' at the Potsdam-Institut für Klimafolgenforschung (PIK) has now determined that a seventh planetary boundary has currently been breached: ocean acidification.
The ocean is a core planetary life support system - not just for humans. It is less about food obtained through fishing; rather, the entire marine ecosystem is essential due to its stabilising effect on the water cycle, weather, atmospheric composition, and global warming. If marine ecosystems lose their equilibrium, the ocean will undergo a regime shift ("tip") and will no longer be able to perform these critical functions. This has a more rapid impact on terrestrial life than most people realise. For instance, nearly two-thirds of the oxygen required by terrestrial animals for respiration is produced in the oceans by phytoplankton (algae).
The oceans buffer immense volumes of anthropocenically generated CO2 from combustion. This gas is sequestered, meaning it fortunately no longer contributes to atmospheric warming. However, in seawater, it behaves like carbonic acid in a carbonated drink: it increases acidity, as CO2 in water forms carbonic acid. Consequently, the oceans are becoming increasingly acidic.
Using vinegar or lemon juice to remove limescale is a common household practice. Both function because their acidity dissolves calcium carbonate. The implications of carbonic acid (CO2) on marine calcifying systems are thus readily apparent. Fish bones, corals, mollusc shells, and marine gastropod shells are composed of calcium carbonate, and the water is now beginning to dissolve these vital structures at a rate higher than evolutionary adaptation can accommodate.
There is therefore an additional critical reason to limit CO2 emissions by reducing fossil fuel combustion: both for the climate and for the oceans, which are steadily warming and acidifying.
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