Ocean acidification is the result of CO2 released into the atmosphere finding its way into ocean waters. Estimates are that around one third of all human emissions of CO2 are currently absorbed this way. While that is good news for the problem of an enhanced greenhouse effect causing global warming, it is not good news for marine ecosystems.
In fact it is extremely bad news, and make ocean acidifications one of the most neglected aspects of the whole climate change policy debate. According to the article:
Since the beginning of the industrial era, the pH of surface waters has decreased slightly but significantly from 8.2 to 8.1, and it continues to decrease. Scientists project the pH of surface water will decrease by the year 2100 to a level not seen on Earth over the past 20 million years, if not longer.
20 million years! At least. And like the change in atmospheric chemistry, the calamitous part of that change is its rapidity. Some tens of millions of years ago it was higher than we are headed for, but it did not get there in one century. Marine life will not have the chance to adapt quickly enough. And let's not forget that marine life is already dealing with our pollution and overfishing.
55 milion years ago there was a huge spike in ocean acidity leading to the extinction of 30-40% of marine species and a major turnover in mamalian species. This is known as the Paleocene-eocene thermal maximun event. Google it, it is likely an excellent preview of what we are doing to the biosphere today.
Also from the article:
Since most corals live in shallow waters, coral reefs, some of the most biologically diverse places on Earth, are particularly vulnerable. "They are already under assault from warming water, over-fishing and habitat degradation," says Doney. "Environmental stress is leading to more incidents of 'coral bleaching,' which occurs when the symbiotic algae that lives inside the coral leaves or dies, and from which reefs often do not recover. Ocean acidification may push corals over the edge."
So it is a danger for corals as well, which already suffer from warming water.
All in all it is really a bleak picture and is why adaptation should not be on the table as a plan to deal with the emissions problem, though at this point it is a necesity to deal with the changes already well under way. It is also why geoengineering schemes, such as pumping aerosols into the stratosphere or sending sun shades into orbit to reduce insolation, are not going to solve the underlying problem of biodiversity loss. Any proposal that leaves the carbon in the atmosphere is not going to help us out of this mess.
Anyway, that's my two cents, but the source article is what you should read and pass on.
[update: 90% extinction of marine life in the original version was incorrect, that rate only applied to foraminifera. The text has been ammended.]
55 milion years ago there was a huge spike in ocean acidity and marine ecosystems saw a species extinction rate of perhaps 90%, the worst in the fossil record.
Hold on. Wasn't the worst extinction the Permo-Triassic extinction, the end of the Paleozoic? I've heard an extinction rate over 90% quote for the Permo-Triassic boundary - and I'm not a paleontologist - but I thought that the extinction rates within the Cenozoic were lower than that.
Which isn't to say that the PETM wasn't a horrible extinction event, especially in the oceans. And our current rate of extinctions is horrifying. But I think the worst geologic case goes back further. (I've seen recent papers arguing that CO2 was partly the culprit for the Permo-Triassic extinction, too, however.)
I believe I am correct as I did specify for marine ecosystems. I could perhaps have been more clear about that. Overall, my understanding is as yours that The Great Dying was worst overall, land and sea combined.
I will try to double check that 90% figure.
I guess my sense (as a geologist who has taught in Paleozoic rocks) is that the Permo-Triassic extinction was very much a marine event, too. (When I think of fossils, I think of marine invertebrates - they're most of what I had to study in paleontology classes. I've only got a very vague sense of what lived on land during the Paleozoic. Nonmarine rocks just don't preserve very well, geologically.)
I have trouble explaining the Great Dying to intro students because it involved a lot of now-extinct marine invertebrates. (One of my students made some comment about the organisms not being very important. I wish I had a better background in biology, so I could explain why charismatic megafauna are far from the entire story.)
I'm trying to remember a paper I read recently about the species changes during the Permo-Triassic extinction. (I think it compared the marine ecosystems occupied by brachiopods, molluscs, and some species of corals, and used the information to test how important oceanic anoxia and acidification may have been during that extinction.) I suspect it was in Science, because I don't read the paleotological literature otherwise, but I can't find the reference. It was an interesting paper, and it made me even more worried about what we're doing to the oceans.
Kim, you are correct, that figure was too high. I mixed up a rate I had read that applied only to foraminifera with an overall marine extinction rate. I ammended the article, thanks for calling it to attention!
Discussing the various extinctions is a distraction. The PETM is important and relevant because it was probably had a similar cause to the current Great Extinction. A sudden huge increase in atmospheric CO2 and other GHGs, from a flood basalt eruption in the fledgling North Atlantic ocean. Except that it extended on thousands of years, not a couple of centuries and it wasn't compounded by pollution, deforestation, soil degradation, over fishing, etc.
We're like tenants trashing a house with a huge boozy party. The landlord won't be happy.