Okay, so we all know that carbon dioxide has this property of being selectively transparent, and that it is accumulating at greater concentrations in Earth's atmosphere because the rate that it is being produced by human activities greatly exceeds the rate it is removed by natural processes. That's the global warming issue in a nutshell. But there's another aspect to climate change that hasn't gotten as much press: ocean acidification.

Much of the carbon dioxide produced since the Industrial Revolution has been absorbed by the oceans. Global warming would have been as noticeable as it is today much earlier had the oceans not acted as a "carbon sink" in this fashion. But the oceans can't absorb CO2 forever without consequence. When CO2 dissolves in H2O, it produces H2CO3, also known as carbonic acid. (top image)

Caldeira and Wickett published a study in 2003 in which the explored the consequences of adding all this extra acid to the oceans. The oceans are large, so changes to their pH take place slowly, but it looks like the ocean's pH is dropping (becoming more acidic) as it absorbs the extra CO2 from the atmosphere. They made some predictions (second image) about how projected emissions of CO2 will influence the amount of CO2 in the atmosphere (shown here as pCO2, which translates as the "partial pressure of carbon dioxide in the atmosphere), and then how that would influence the ocean's pH over a range of depths over the next millennium. As you might expect, their model shows surface waters becoming acidic first, because they are in direct contact with the CO2-rich atmosphere. Oceanic mixing propagates the acidic waters to the depths over longer timescales. They predict a maximum reduction of 0.7 pH units in surface waters, starting around the year 2200.

How will this effect marine life? Remember that lots of marine creatures make their skeletal material (hard parts) out of the mineral calcite, and calcite dissolves in acid. (In my classes, we put a drop of hydrochloric acid on a rock sample to determine if it is calcite.) Consider the effects on two kinds of plankton: coccolithophores and pteropods. The third and fourth images here show scanning electron micrographs of how skeletal material reacts to acidified conditions. The third image is from a study by Ulf Reibesell of the University of Norway, who grew coccolithophores in a series of model "ocean" tanks that had equilibrated to an "atmosphere" containing 300 ppm and 800 ppm CO2. For reference, pre-Industrial CO2 values were about 280 ppm, and today's CO2 values are about 380 ppm. You can see that the calcareous plates of the coccolithophores are smaller, thinner, and more degraded in the more acidic water. The fourth image shows the results of a similar experiment on a pteropod, by Orr, et al. in 2005. (A pteropod is a kind of planktonic snail.) The pteropod was placed in a tank of water undersaturated with respect to aragonite (a polymorph of calcite) for 48 hours. Sub-images b, c, and d show degradation of the snail's shell in those acid waters, and sub-image e shows a the surface of a normal pteropod shell for comparison.

Here's some model predictions of ocean pH from Scott Doney in a 2006 paper in Scientific American. Note that the northern Pacific Ocean becomes marginally saturated with respect to aragonite by the end of the century, and the Southern Ocean will be undersaturated by then. The skeletons of organisms with calcareous shells in those waters will begin to dissolve! So far, the pH drop has been only about 0.1 pH unit, but it is expected to hit around 0.3 pH units by 2100. It's hard to imagine how fundamental a change this will be to oceanic ecosystems!

Now, a new study in Science by the Coral Reef Targeted Research Group concludes that it's not just these high-latitude ocean water. Global warming kills tropical coral reefs, too. They consider the effects of ocean acidification as well as the effects of "bleaching" (when warm corals eject their symbiotic algae).

Labels: climate change, CO2, ocean acidfication