In the debate over global warming, little has been said about its affect on oxygen balance. Atmospheric oxygen serves two crucial purposes for sustaining life on earth. In the upper atmosphere it takes the form of ozone and blocks ultraviolet radiation. Without this layer, no life on the surface of the earth could exist. The second purpose is as an oxidant for respiring life forms. Most of the oxygen on earth is sequestered in rocks like silicon dioxide. Atmospheric oxygen comes exclusively as a waste product of photosynthesis of plants and prokaryotic organisms. The bulk of atmospheric oxygen comes from photosynthesizing bacteria near the surface of the oceans.
The air is comprised of 78% nitrogen, 21% O2 and the rest as trace gases such as argon and carbon dioxide. The oceans also contain a large supply of dissolved carbon dioxide and oxygen. As water warms, it can hold a smaller amount of oxygen. Thus, as the oceans warm due to global warming, they are actually outgassing some oxygen (Keeling and Garcia, PNAS 99:7848-7853, 2002) . At the same time, oxygen in the atmosphere is also slightly decreasing due to combustion of fossil fuels.
Right now the decrease of oxygen in the ocean is 0.7 µmol per kg per decade. For comparison , the concentration of oxygen in the ocean is on the order of a few hundred µmol per kg. So the decrease is small but not insignificant. Given that one hypothesis for the extinction event 250 million years ago known as the Great Dying (see my previous entry) was due to a lack of oxygen in the oceans, I think that this is of some concern. We simply don't know enough to predict the consequences of global warming. However, I really think we need to take it seriously. Affecting the carbon balance could have implications beyond the melting of glaciers and the increase of hurricanes. It could affect the oxygen that we breathe.