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Estimated change in annual mean sea surface pH between the 1700s and the 1990s. Graphic: <a href="http://en.wikipedia.org/wiki/File:WOA05_GLODAP_del_pH_AYool.png">Plumbago</a>, Creative Commons, some rights reserved
Estimated change in annual mean sea surface pH between the 1700s and the 1990s. Graphic: Plumbago, Creative Commons, some rights reserved

Natural Selections: How rising CO2 levels are changing our oceans

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Most of the concern about carbon dioxide is focused on the quantity in the atmosphere and its effect on climate. But rising CO2 levels in the oceans can have equally significant effects on the ecosystems of the seas. Martha Foley and Curt Stager discuss the changing aquasphere.

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Martha Foley: I read another grim story about carbon dioxide pretty recently in the New Yorker Magazine, written by Elizabeth Colbert, and it hit me pretty hard. She is talking really about the other CO2 problem, and that is that this excess of the carbon dioxide is making the ocean more acid, and I want to talk about that. Why is that so important?

Curt Stager: Yeah, it is kind of hard to believe. You hear about acid and you think of acid rain in the lakes and stuff—and how could the oceans possibly...

MF: The oceans are huge.

CS: They’re huge. And also, if you study oceanography at all, you’d say there is also a buffering system that should resist changes in pH and acidity, too. It’s pretty complicated and hard to explain on the radio, but it has to do with the carbonate (CO3) system in there, and if you add acids, it tends to neutralize it and stabilize it. So how could this be happening? Plus, the ocean is far from acidic now.

MF: It’s salty.

CS: It’s very salty, but in terms of the acidity scales it's strongly basic—the opposite of acid—so how in the world is this going to happen?

MF: Is it just that there is so much CO2?

CS: Yeah, it turns out that this new thing is—that when CO2 goes into water—it makes a weak acid called carbonic acid. It combines with water molecule. So we often hear about the greenhouse effect and the buildup of CO2, and over the long term eventually (maybe) the oceans are going to “save us” by soaking up this extra CO2.

MF: They talk about carbon sequestration in the ocean, actually pumping excess CO2 down into the depths, like "out of sight, out of mind."

CS: It turns out that when it goes into the ocean, it lowers the pH, makes it more acidic—or more accurately—it will never get truly acidic, but it will get less basic. And neutralize it. Who cares about this? One expectation—it depends on what our scenarios are—but it seems pretty likely the mean pH is going to drop like half of a pH unit, or have a threefold acidity increase by the end of the 21st century.

MF: Ok, so why?

CS: Well, a lot of marine life makes their shells out of calcium carbonate minerals to form zeragonite or calcite.

MF: Like coral, and they make a shell?

CS: Corals, any mollusk you can think of, any barnacles, and then things you may have heard of—these little plankton animals. Elizabeth Colbert’s article talked about pteropods, which are these tiny little swimming mollusks.

MF: And they just have this tiny little shell?

CS: Tiny little shell, forams [foraminiferas] and other things like that, too. Some of them use this aragonite version and some use the calcite version of the carbonate. These things will fizz if you put acid on them and they will decompose.

MF: So the shell dissolves?

CS: The shells dissolve, like if you take hydrochloric acid or vinegar and put it on limestone, on a clam shell, chalk—you see it neutralize. Kind of a slower version of that will happen as the pH changes in the ocean.

MF: So they won’t be able to make their shells?

CS: Well, two things will happen. Once we get about mid-century, especially around the polar regions, and in the deep sea where it is colder and the CO2 can build up better, the effect will be this—things that mostly use aragonite the shells will start to dissolve, actually dissolve. The ones that use the calcite, the little-bit-more resistant, will also be affected. When they are trying to build their shells it will slow them down and they won’t grow as well, and in that case, it’s the corals which are going to have a tougher time building their skeletons.

So, it’s like, "Okay, that can’t be true." But if you start thinking about it, the affects will cascade down, too. The deep sea life forms don’t have sunlight; they depend on the plankton dying and falling down on them from above. And these pteropods and forams and others like that are the base of their food chain, so these things happening up at the surface are going to have gigantic effects on the ecosystem down at depth, too.

MF: This is another food chain problem, as far as that goes. And so for the little creatures, they will cease to exist?

CS: We look back at the past records in geologic time and this kind of thing has happened from volcanic eruptions and mass disasters, and it took millions of years for life forms to recover from those. So the news doesn’t look too good. There’s still a lot of work to be done on exactly what is really likely to happen with different species and different ecosystems, but it is something that we are going to be hearing a lot more about in the near future.

MF: So there is no silver lining here? No happy spin?

CS: It’s kind of hard to hear about it. If we stabilized and stopped increasing our CO2 we may have effects on the global warming we are worried about. But if we stabilized at what we think CO2 levels are going to be in the next decade or two, it will still have effects in the deep sea and around the polar regions.

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