One of the larger unknowns when it comes to the impact of human carbon dioxide emissions is the degree to which living things will be harmed by more acidic oceans. This is occurring because water with more CO2 dissolved in it is more acidic. There are concerns that overly acidic sea water might compromise the ability of organisms with shells made of calcium carbonate to build and maintain their bodies. Other affects on marine ecosystems are anticipated, though it is challenging to assess what their magnitude will be and when they will occur.
Scientists recently completed a study of a place where such effects are occurring naturally due to carbon dioxide venting from the sea floor:
Around the vents, [pH] fell as low as 7.4 in some places. But even at 7.8 to 7.9, the number of species present was 30% down compared with neighbouring areas.
Coral was absent, and species of algae that use calcium carbonate were displaced in favour of species that do not use it.
Snails were seen with their shells dissolving. There were no snails at all in zones with a pH of 7.4.
Meanwhile, seagrasses thrived, perhaps because they benefit from the extra carbon in the water.
The latest IPCC estimate is that global pH will fall from 8.1 today to about 7.8 by 2100. Greater than expected CO2 emissions would cause a larger change. Coral reefs are especially likely to suffer.
Oceanic acidification is the inevitable result of adding CO2 to the atmosphere, but it not otherwise causally connected to climate change. It does add a complication to any plan that seeks to reduce global temperature change through a means other than reducing CO2 emissions; even if more energy could somehow be reflected or dissipated into space, the marine consequences of acidic oceans would endure.