"Ocean deoxygenation"

Our friend Caspar Henderson has been searching for a good short phrase to describe the increase the ocean's "oxygen minimum zones" expected to happen as a result of climate change. His query led to a pretty fascinating exchange between a number of experts and the suggestion of ocean deoxygenation.

Just what is this "deoxygenation"?

A lot of intermediate and deep parts of the open ocean are depleted in oxygen. It is natural. First, algae growth on the surface leads to a rain of dead matter to the bottom. Decomposition of that material consumes oxygen. The deep oxygen can be refreshed by oxygen diffusing out of the air into surface waters if there is a lot of vertical mixing. But if the water is highly "stratified" -- say, a light, warm layer lying about a cold, dense layer -- there is little vertical mixing of waters.

This mechanism alo explains coastal hypoxic zones like the famous Gulf of Mexico "Dead Zone". The waters on the continental shelf of the northern Gulf are very stratified, thanks to the influx of fresh, light water from the Mississippi River. Hypoxia develops in the bottom water during the summer in part because of the lack of vertical mixing. That's why when a hurricane blows through, and the water gets all mixed, the dead zone tends to dissipate, or at least decrease in severity and extent.

The "ocean deoxygenation" concern comes from the fact that if climate change heats up the surface ocean, we get more stratification, less vertical mixing, and expansion of the existing ocean minimum zones in the ocean. Recent evidence suggests that is just what is happening.

Large areas of the intermediate and deep ocean have "naturally" low levels of oxygen.