ocean sequestration and acidification

A topic of grave concern, in light of global climate change, is ocean acidification. As the planet warms in response to enhanced concentrations of greenhouse gases in the atmosphere, the oceans will naturally absorb more CO2. This is a factor of both the increased atmospheric concentration of this gas as well as the tendency for warmer waters to be able to hold greater amounts of dissolved gases (picture a warm can of coke vs. a cold can of coke).

In addition to the natural increase in oceanic CO2 concentrations that will likely occur due to global warming, humans are seeking to enhance this effect. Various proposed mechanisms for sequestering excess atmospheric CO2 involve putting it into the ocean. For instance, as described in the April 9th post on this blog, the sequestration method of ocean fertilization involves greater uptake and subsequent burial/storage of CO2 in marine reservoirs through the photosynthetic activities of phytoplankton. Other mechanisms are not so discrete, for example, the Carbon Sequestration Leadership Forum (an international initiative to promote education and dissemination of information regarding CO2 sequestration) cites direct injection of CO2 into the ocean as a viable sequestration mechanism for the future. A direct quote from clsforum.org touts the potential virtues of the ocean as a sink for CO2:

"CO2 is soluble in ocean water, and oceans both absorb and emit huge amounts of CO2 into the atmosphere through natural processes. It is widely believed that the oceans will eventually absorb most of the CO2 in the atmosphere. However, the kinetics of ocean uptake are unacceptably slow. The program will explore options for speeding up the natural processes by which the oceans absorb CO2 and for injecting CO2 directly into the deep ocean."

When one views the earth's systems in a simple way, sequestration of atmospheric CO2 in the oceans seems like an easy fix to the "global warming" problem. If we could only ratchet up the "unacceptably slow" uptake rates of the ocean, we could get that pesky greenhouse gas out of the air, and it would stop warming the planet... great! However, when one steps back to consider earth's systems in a realistic way, it becomes evident that it is not so easy to fix our problems. By transferring carbon from beneath the ground to the air and then to the oceans, we are drastically altering the planet's delicate, natural carbon cycle.

The U.S. Energy Information Administration (as cited on the National Energy Technology Laboratory website) predicts U.S. Co2 emissions to reach 8,800 million tons by 2030. To put this in perspective, let's do some back-of-the-envelope calculations. Before the industrial revolution, the earth's ocean carbon reservoirs were as follows: the surface ocean contained about 700 gigatons of carbon, and the deep ocean held about 38,000 gigatons. If we put 8.8 gigtons of CO2 into the atmosphere, it amounts to roughly .02% of the deep ocean reservoir, and 1% of the surface ocean reservoir... and that's just the United States (and I'm guessing it's a conservative estimate, considering the source)! Maybe that sounds like small beans, but the chemical dynamics of the ocean are extremely sensitive, and adding extra CO2 could have major implications for marine life.

When the ocean becomes supersaturated in the dissolved equivalents of carbon dioxide, some marine creatures will no longer be able to create the calcium carbonate shells necessary for their survival. Corals will begin to dissolve. Some plankton and pteropods will not be able to live. A 2005 article in Nature, by James C. Orr and others, suggests that these changes will take place over the next few decades... a startlingly short timescale.

The moral of the story is that, although ocean sequestration of CO2 may seem like an enticing solution to our global warming quandary, it is essential to consider all of the consequences of human actions before implementing them. There is no easy fix to this problem. We must instead consider the more difficult task of drastically cutting our emissions of greenhouse gases, and we need to to do as soon as possible.