The same forces responsible for cli­mate change are also impacting on the oceans. Half of the carbon diox­ide emitted by human activity is ab­sorbed by the sea. In some ways this is a good thing, as it mitigates the impact of carbon dioxide emissions in our at­mosphere, but when the gas combines with water, it produces calcium car­bonic acid (that which gives soft drinks their fizz), which in turn decreases the amount of calcium carbonate in the water. Calcium carbonate is a key building block of calcifying creatures­ phytoplankton, echinoderms, crusta­ceans and molluscs—which need it to construct shells and skeletons.

This could be bad news for New Zealand organisms. As carbon dioxide is more soluble in colder waters, it is quite likely that acidification will impact on flora and fauna that live there first. Think of the Bluff oysters of Foveaux Strait, of the calcifying algae that cover 80 per cent of the Otago coast, of the deep-water corals, and of ocean-wide plankton that underpin the entire food web.

But the sea is complicated, and the creatures living within it are a nuanced collection with myriad idiosyncratic re­sponses to any environmental change. The worst-case scenario may never happen; while many studies show that increasing carbonisation reduces an or­ganism’s ability to calcify, some research also suggests that some organisms can take a bit of acidity in their stride, and others may even benefit from it.

And so while the press frightens us with headlines about the end of the Bluff oyster, scientists have limited evidence of what impact acidification is having on living organisms in the environment, and lab tests have been blighted by inconsistent experimental methods. “We know how much CO₂ is being emitted and how much is being absorbed by the ocean,” says Univer­sity of Otago scientist Philip Boyd. “It’s the biology that is letting the side down, where things are much more difficult to predict…identifying which species will prosper and which will not.”

One thing is more certain: there is currently no method of reversing oceanic acidi­fication, apart from burning fewer fos­sil fuels.