Canto: There's been a decrease in overall surface ocean pH over the past couple of hundred years, attributed to the uptake of carbon dioxide in the atmosphere, the high levels of which are attributed to human activity. As you might expect, the process of acidification, and its consequences, are complex, but it has much to do with the carbon cycle, particularly the cycle of inorganic carbon. Seawater temperature, along with alkalinity, affects the ratio of carbon dioxide dissolution. This is chemistry stuff - a reaction occurs with the water to form a balance of ionic and non-ionic forms, including 'dissolved free carbon dioxide', carbonic acid, bicarbonate and carbonate. Overall this increases the hydrogen ion content and decreases the pH. It's called acidification, but another way of looking at it is a reduction in alkalinity. The sea is still alkaline.
Jacinta: Yes, the pH of water is 7, that's neutral. They reckon the pH of the surface ocean has come down from a little under 8.2 in the 1700s to a little over 8.1 currently, but that the acidification is accelerating and the future is looking increasingly grim depending on which expert you consult. And the effects on marine life?
Canto: Shellfish produce calcium carbonate. It's a process called calcification, and it's essential to many marine organisms. I won't go into the chemistry here but unless the surrounding seawater is saturated with carbonate ions, the structures built out of calcification will simply dissolve. It's more complicated than this [for example, it depends on which calcium carbonate structure the organism relies on for calcification - there are two, aragonite and calcite, with varying degrees of solubility], but the saturation state of seawater, which varies with temperature and carbon dioxide levels, is vital to the health of the organisms inhabiting it.
Jacinta: So what organisms are we talking about here?
Canto: A very wide range indeed. Think of corals of course, but also molluscs and crustaceans. And perhaps most importantly, the very abundant single-celled micro-organisms with little calcium carbonate plates, called coccolithophores. However there's a lot of disagreement, both about the current extent of acidification, and about its long-term, and even short-term impacts. It may not all be doom and gloom, and we should remember the human tendency, generally a positive one, to motivate ourselves into action through these negative projections.
Jacinta: More research is required! More more more!
Canto: That’s right – all scientists want is the opportunity, and the funding, to get to the bottom of all these bottomless issues, to keep on with their research – and who can blame them.
Jacinta: And there are always new crises coming up to concern ourselves with. Have you heard about the methane clathrates under the ocean? They have the potential, apparently, to release such volumes of methane as to send global temperatures skyrocketing, just as might have happened at the time of the Paleocene-Eocene Thermal maximum. Watch this space!