The Physical Carbon Pump

Introduction

Sketch illustrating the concept of vertical deep mixing, where carbon dioxide is transported from the ocean surface to the ocean depths by sinking cold water in the high latitudes. If brought to the surface (for instance, by upwelling) the cold water will warm up and release some of its carbon dioxide to the atmosphere.

Now that we have introduced biogeochemistry and the way atmospheric carbon dioxide levels have changed with time, we will spend the next four sections investigating the details of the marine and terrestrial carbon cycles. The exchange of carbon between the atmosphere and the ocean takes place in a number of ways. The most important of these mechanisms is through physical mixing of the ocean, a process called “vertical deep mixing” by scientists. It occurs when warm water in oceanic surface currents are carried from low latitudes to high latitudes on Earth and cooled, making them heavy enough to sink below the surface layer and, in some places, all the way to the bottom. When seawater is cooled it takes up more carbon dioxide. Of course, when cold water returns to the surface and warms up again, it loses carbon dioxide to the atmosphere. In this fashion, vertical circulation makes sure that carbon dioxide is constantly being exchanged between the ocean and the atmosphere (We will learn more about the mechanisms that drive ocean circulation in another lesson). Vertical circulation is ultimately responsible for the fact that cold water fills the depths of the ocean. Also as mentioned previously, cold water holds more carbon dioxide than warm water. Thus, vertical circulation acts as an enormous carbon pump, giving the ocean a lot more carbon than if equilibrium with the surface ocean were the only mechanism controlling the sharing of carbon between atmosphere and ocean.

Warming the Oceans: A Thought Experiment

Let us design a thought experiment to guess at how the transport of carbon dioxide would change upon warming of the ocean. Warming of ocean waters takes place at the top, where we have the sunlight, so at first a little more carbon dioxide is released into the air from the water. The water is not as cool the water it used to be, and when it reaches high latitudes, where it gets ready for sinking, it takes up less carbon dioxide than it would otherwise and, in addition, it does not sink as deeply.

In this simple experiment, the ocean will yield some of its carbon dioxide and slow its uptake of carbon dioxide from the atmosphere. We also note, however, that the deep cold water no longer participates very actively in the vertical circulation and tends to stagnate. Oxygen (O₂) is used up while carbon dioxide is being produced from organic matter on the sea floor and from organic matter still falling down from above. In places where oxygen is entirely used up, nitrate (NO₃) is used by the bacteria as an oxygen source instead. In this process, nitrous oxide (N₂0) and molecular nitrogen (N₂) are made while nitrate is being destroyed.

So, by warming the oceans and cutting down on the physical pump we have created a deep ocean reservoir rich in carbon dioxide and poor in nutrients. Whenever this water is returned to the surface (by deep mixing activity) it will now bring carbon dioxide back to the atmosphere, without the means to recapture it by photosynthesis (for which nutrients are needed). Such a process could have contributed to the pulsed nature of the carbon dioxide rise during deglaciation, as seen in the ice cores.

Cooling the Oceans: Another Thought Experiment

Now let us make another thought experiment to make a guess which way things would go upon cooling the ocean. Cooling takes place at the top, where we can get rid of heat by evaporation and by radiation of infrared to the sky. For cooling to occur, a heat deficit is necessary. As it cools, the water will take up more carbon dioxide. Also, it will readily mix vertically, other factors being equal. Since cold water is heavier than warm water, it will sink to the depth level appropriate for the density of the sinking water. The entire water column will be exposed to the air. As deep water comes to the surface it will warm and release carbon dioxide. As it moves to high latitudes, it will cool again and take up carbon dioxide, more so if it is now colder.

On the whole, in this simple experiment, the carbon dioxide of the atmosphere is drawn down, so the cooling process favors further cooling from the loss of greenhouse gas. This is a classic case of positive feedback. A corollary to this experiment is that the water column, after cooling, is quite well mixed, which was not necessarily the case before. If the mixing was slower before, during the warm stage, carbon dioxide could have accumulated in intermediate waters within the subsurface layer of water (called the thermocline). Initially, with intensified mixing, the thermocline would give off additional carbon dioxide to the atmosphere, counteracting the effect from cooling. This might help explain why, during the initial phase of reglaciation, the carbon dioxide values in the atmosphere as seen in the ice cores tend to stay high upon cooling.

The main use of the thought experiments is to illustrate how complicated things get when considering the exchange of carbon dioxide between ocean and atmosphere upon changing the climate. Whether the scenarios outlined in the thought experiments have much resemblance to reality is another matter. Perhaps they do. Maybe they don't. But it is this kind of thinking that needs to go into the mathematical models to make them responsive to climatic change.