The seventh planetary boundary has been breached. There are nine in total, indicating the extent to which our lifestyle strains the planet (nature). If we exceed the healthy limit, we cross planetary boundaries. Six of our planet's boundaries have already been breached by human activity in recent years: climate change (CO2 concentration 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 central life-support system for all life on Earth - not just for humans. This is less about food from fishing. What is essential is the entire ecosystem with its stabilising effect on the water cycle, weather, atmospheric air, and also on global warming. If the marine ecosystem loses its balance, the ocean "tips" and can no longer perform these vital functions. This impacts life on land much faster than most people realise. For example: almost two thirds of the oxygen that we land animals need to breathe is produced in the seas by phytoplankton (algae).
The oceans buffer immense amounts of the CO2 generated by human combustion. It is sequestered and, fortunately, does not contribute to further global warming. However, in the sea, it behaves like carbonic acid in a soft drink: it introduces acidity; because CO2 is nothing other than carbonic acid. As a result, the ocean is becoming increasingly acidic.
We use vinegar or lemon as a household remedy against limescale? Both work because the acid dissolves calcium carbonate. It is therefore easy to anticipate what carbonic acid (CO2) does to calcifying systems in the ocean. Fish bones and corals are made of calcium carbonate, as are bivalve shells or sea snail shells - and the water is now suddenly beginning to attack these vital structures faster than evolutionary adaptation can assist the organisms.
Consequently, there is indeed another key reason to further limit CO2 emissions by burning fewer fossil fuels. For the climate - and for the oceans, which are becoming warmer and increasingly acidic.
Sources:
