Ocean acidification

There may be a movement in the debate, but in my view, ocean acidification may genuinely be the biggest danger facing humankind.

Wikipedia has a useful primer on the topic. (It is also surprisingly short, probably due to the very intermittent publicity the topic receives compared to global warming.)

The Royal Society study of 2005 gave the topic some momentum, and its lengthy report is not too difficult to read. Shorter summaries have been published by the Australian Department of Environment’s Antarctic Division, the Australian Academy of Science, and various other international bodies.

Here’s my quick review of the chemistry, as culled from those links:

– the average pH of surface ocean water has dropped over the last two centuries by about 0.1. Being a logarithmic scale, this equates to increased acidity of about 30 per cent. (Pure water is neutral with a pH of 7; oceans on average used to have a pH of 8.2.);

– estimates based on “business as usual” CO2 emissions are that by 2100 there could be a further drop in average ocean pH of 0.3 to 0.5, which at the top of that range equates to a 300 per cent increase in acidity. (Although even then the ocean is actually still alkaline.);

– with any substantial drop in pH, it will take thousands of years for natural ocean chemistry to get it back to pre-industrial levels;

– ocean pH is believed not to have been as low as even current levels for a very long time (hundreds of thousands, if not millions, of years.);

– crucially, changes of as little as 0.2-0.3 units can hamper the ability of key marine organisms such as corals and some plankton to calcify their skeletons, which are built from pH-sensitive carbonate minerals. (Calcium carbonate shells can be either in the form of calcite or aragonite, with the latter being more sensitive to lower pH.)

This chemistry seems well understood. It’s the effect on individual species of algae, other plankton, coral, molluscs and crustaceans, as well as the overall ecological consequences, that is trickier to quantify.

No one is saying the oceans will become utterly sterile: sea life of some kind has clearly survived much higher levels of atmospheric CO2 since the earth began. But ocean chemistry has been stable for a very long time, while recent ice ages have come and gone; now it is undergoing major change.

A major concern is the effect on coral reefs. Laboratory tests indicate that coral polyps might not actually die, but simply stop producing their hard shell and change in appearance to something resembling sea anemones.

The problem is, it’s the hard shell that creates and maintains a reef. Algae that are important for coral may not do well either. Just this week, there’s a report that corals indicate the ocean may be acidifying faster than expected.

Australians who value the Great Barrier Reef have good reason to worry: cold water takes in CO2 faster, and the Southern Ocean already has what scientists describe as “low saturation” with respect to aragonite.

(Basically, the lower the saturation, the bigger the problem for calcifying creatures). Increasingly undersaturated waters are expected to spread far north during this century.

Even if you are prepared to live with the possible loss of coral reefs, another major concern is the effect on the food chain. It has been said that only ten or so of the thousands of calcifying organisms have been tested for sensitivity to increased acidity, and some clearly do worse than others.

Pteropods (small swimming molluscs eaten by many types of fish) appear to be particularly sensitive, and scientists worry that they will be in real trouble in only 50 year’s time.

Source: Steven Watkinson, sciencealert.com