AAAS: That other carbon problem, ocean acidification

Friday morning’s first sessions included a series of talks on what was termed “the other carbon problem”—ocean acidification. When dissolved in water, carbon dioxide can combine with a single water molecule to form carbonic acid, a process that ultimately lowers the pH of the water. On a global scale, adding more CO2 to the atmosphere will trigger this process to occur on a massive scale. So far, the oceanic pH has dropped by .1 units, but it’s expected to drop another .3 points in the next century unless emissions are curbed.

Speakers in the session included Peter Brewer and James Barry of the Monterey Bay Aquarium, Ulf Riebesell of the Leibniz Instititue of Marine Sciences in Kiel, Germany, and Andrew Knoll of Harvard. The issues they discussed are complex and interrelated, so I’m going to rearrange some of their points to give this description a more logical flow.

The panel presented two reasons we should care about oceanic pH. The first is a matter of basic physiology, covered by Barry. Most respiration processes in higher organisms produce CO2, so basic chemistry says that the more CO2 that is present in the environment, the less efficiently these reactions will run. Ultimately, that will lead to the energy cost of basic living going up, and that price will have to be paid in terms of reduced growth and/or reproduction. Barry suggested that this cost will be paid in terms of reduced productivity of the oceans and longer generation times, making it harder for species to undergo evolutionary adaptation to the changing conditions.

Peter Brewer also noted that the dynamics of depth, pressure, and temperature would actually produce permanent “dead zones” at some depths, where the CO2-oxygen ratios are too low to support efficient respiration (this is apparently already happening in some tropical ocean regions). This could eventually play havoc with what he termed the planet’s largest animal migration: the migration of most oceanic life from the surface to the intermediate depths during daylight hours. Riebesell also pointed out that carbon’s ability to fertilize algal life might exacerbate this problem by creating growth booms that further deplete the oxygen.

The second reason for concern is for animals that rely on calcium carbonate for skeletal elements, as this mineral will dissolve in acidic media. Although this will impact shellfish and echinoderms, the big loser here is coral. Riebesell showed figures that indicated that, by 2070, its likely that the optimal growth conditions for warm-water coral will simply no longer exist, and about 60 percent of the environment for cold-water coral will be gone.

Riebesell also got into why we should care. He pointed out that, of 18 measures of environmental services evaluated by one group, corals provided 15, ranging from being a tourist draw to providing physical shelter from the ocean. More generally, Barry showed figures indicating that oceanic environments with reduced biodiversity are more prone to both boom-and-bust cycles and total collapse, meaning that this stress may ultimately limit humanity’s ability to rely on the ocean as a food source.

The geologic record, as presented by Knoll suggested that Brewer’s statement—”nobody’s been able to find anything really good about this”—was reasonably accurate. Brewer had noted that the oceans hadn’t seen pHs this low for 40 million years, but Knoll extended the record back ten times further. During the Ordovician, atmospheric carbon levels were much higher, but had risen gradually, allowing the oceans to remain saturated with calcium carbonate, and life had flourished.

But, 250 million years ago, the formation of the Siberian Traps through a massive volcanic eruption caused a sudden and massive shift in oceanic pH, and nearly 90 percent of oceanic species went extinct. He noted that the extinctions followed lines that were predictable; species we’d expect to be sensitive to carbonate concentrations died, while those that have finer control over their physiology largely made it through the extinctions.

His takehome is not that we have to worry about simply having lower oceanic pH, but that we do need to worry about the rate of change. Riebesell echoed that by noting that species with rapid generation times will be better able to adapt. The basic conditions will also produce some winners—cyanobacteria, green algae, and dinoflagellates were all mentioned. In the historic analysis, however, corals come out as losers. Not only do they have a relatively long generation time, but they evolved after the first high-carbon period Knoll spoke of, and so have never faced conditions like those we’re heading for.
The big uncertainties were discussed in two forms. The first is feedback loops, such as the carbon fertilization mentioned above. Several talks also mentioned how bacteria, when oxygen starved, start using other molecules for oxidation reactions, which could start pumping other greenhouse gases (various nitrogen oxides were mentioned) into the oceans and atmosphere. The other uncertainty was how to set a target point that we want to avoid reaching; here, the panelists agreed that there was no scientific way to put a hard number on what degree of change and extinction could be tolerated.

The discussion that followed ultimately provided the best perspective on what this means. It happened too quickly to attribute quotes to individuals, but I will share some of them below:

“We’re entering a new geologic era.”
“We talk about it calmly in a way that’s not appropriate for the scale of what we are doing.”
“Society right now is not willing to pay the tab for a lot of things… we are inflicting these changes and all this chaos on those who will come later.”
“Morally wrong and, I think, unnecessary.”

Generally, it was described as a tragedy of the commons, where the commons exists in time, in that the tragedy will be faced by generations that are not born yet.

John Timmer, Ars Technica, 15 February 2008. Article.

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