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Why is the Atmospheric Carbon Reservoir so Small?

The Earth’s Carbon Reservoirs
The amount of carbon in the atmosphere is surprisingly small. What keeps it at a low level? Why is carbon dioxide a trace gas (about 367 ppmv) rather than making up most of the atmosphere, as is the case for the sibling planets of Earth, Venus and Mars? To tackle these questions, we first need a little background.

Sizes of reservoirs are given in mass units. For example, the atmospheric reservoir of carbon (mostly in the form of carbon dioxide) is about 750 GtC (Gigatonnes of carbon – see the glossary of scientific units for further clarification). The ocean is near 40,000 GtC; the biosphere is near 610 GtC; and, depending on how it is defined, soil is almost 1600 GtC. We can immediately see that the ocean is extremely important in the study of atmospheric carbon dioxide since it is so large a reservoir and is in intimate contact with the air.

Also, when considering there is about 5000 GtC in the form of fossil fuels ready to be burned, we immediately realize that the atmosphere could be easily overwhelmed by all the carbon available for industrial use. Also we realize that planting trees, while a good thing, could not solve the problem of carbon emissions for long. While the biosphere (mostly trees) has roughly the same mass as the atmosphere, doubling the mass of trees would help out with about 10 percent of the potential problem. Doubling the mass of trees, of course, would be a major undertaking in itself, especially when considering that deforestation is occurring at a rapid pace in the tropics.

An important point in this scheme of reservoirs and fluxes is that they differ greatly in size and in their ability to respond to changes, a property called “reactivity.” Large reservoirs with small fluxes in and out (called “input” and “output”) are not very reactive. Small reservoirs with relatively large fluxes in and out are very reactive - as far as carbon is concerned, the atmosphere is such a Reservoir. Fortunately, the atmosphere is closely coupled to the ocean, a large Reservoir that can offset this problem and stabilize the atmosphere. Unfortunately, the atmosphere's dependency on the ocean Reservoir has a drawback: if the ocean reacts to climate change by giving off a small proportion of its carbon dioxide, the atmosphere, with its low concentrations of carbon dioxide, greatly amplifies the effect. In other words, what seems a small adjustment for the ocean results in a big change in the atmosphere.

Why So Little Carbon in the Atmosphere?

Reservoirs of carbon (in GtC) in the ocean (blue labels), in biomass in the sea and on land (tan and green labels), in the atmosphere (light blue label) and in anthropogenic emissions. Fluxes of Carbon between Reservoirs are depicted by the arrows, the numbers represent GtC. (From: IPCC)
In the atmospheres of our sister planets, Mars and Venus, carbon dioxide is dominant. On both planets, there is more CO2 in the air than on Earth. On Mars it's about 30 times more, while on Venus it is about 300,000 times more! While the Earth does have enormous amounts of carbon nearly all of it is tied up in carbonate sediments, coal, and other organic matter, rather than being stored in the atmosphere.

Plants, algae and shell-making organisms are ultimately responsible for the large-scale solidification of carbon dioxide within carbonate minerals (stored in limestone rock) and organic materials. Making coal and other organic matter has also led to splitting the carbon from the oxygen, with much of the oxygen staying in the air. This has produced an atmosphere fundamentally different from those of Venus and Mars — one that is chemically out of balance and therefore "unsustainable" were it not for Earth’s ongoing life processes.

When looking at the system in this way, we see that the low carbon dioxide values in the Earth’s atmosphere are a result of the biologically-mediated movement of CO2 from reactive reservoirs (the atmosphere and ocean) to much less reactive reservoirs (limestones and organic matter). Although these long-term reservoirs can be heated (through subduction by plate tectonics) re-releasing the carbon dioxide into the atmosphere, weathering and life processes then cycle them back into the long-term storage, continuously keeping the atmospheric values low.

An Early Approach to Carbon Reservoirs
An early approach to understanding the fundamental process of moving carbon dioxide out of the atmosphere and into long-term reservoirs was first formulated by Harold Urey (1893-1981) in his book, “The Planets,” published in 1952. He argued that the amount of carbon dioxide in our atmosphere was governed by an equation:

CO2 + CaSiO4 → CaCO3 + SiO2.

This equation describes the weathering process occurring when slightly acidic rain water brings dissolved carbon dioxide to the surface of fresh igneous rocks, which contain calcium-bearing silicate minerals (whose chemical formula is CaSiO4). In Urey's equation, the calcium in the rocks and carbon dioxide in the water combine to make CaCO3 (calcium carbonate in the limestone rocks) while the silicate is released to make SiO2 (silica in opal and chert minerals). Urey then argued that the concentration of carbon dioxide in our atmosphere corresponds to the equilibrium expected for this reaction. Thus, according to Urey, the amount of carbon dioxide in the atmosphere is set by the presence of the water on Earth.

But is this equilibrium approach valid? Actually, a number of factors have to be considered when determining the atmosphere’s carbon dioxide value. Carbon dioxide also comes out of volcanoes, as a result of reactions within the Earth at high temperatures and pressures. The rate at which this happens is presumably independent from the surface reactions described in Urey’s equation, which occur at low pressures and temperatures. After entering the atmosphere, some of the carbon dioxide is concentrated in the soil by the action of plants and other organisms (bacteria, fungi). The reactions of carbon dioxide with silicate minerals within the soil, therefore, do not proceed according to the concentration of carbon dioxide in the air. In addition, the rate of dissolution of rocks is contingent not only on the presence of water, but also the presence of microscopic organisms on the surface of the rocks, as well as the presence of roots exuding acid. Moreover, the precipitation of the carbonate and silica is made possible not only by inorganic processes but also by organisms (algae, corals, molluscs, and foraminiferans produce carbonate and diatoms, radiolarians and sponges make silica). This analysis of Urey’s chemical equilibrium approach should make clear how important other factors, like life, influence carbon dioxide levels. The reactions that govern the long-term storage of carbon are rate-dependent and these rates are determined mainly by plate tectonics and by life processes, factors not included in Urey’s model.

What we are looking at when we are studying the carbon cycle are carbon atoms in transit between reservoirs: from volcanic sources to limestones and organic matter, with the atmosphere and oceans forming a conduit between the two (as in the figure above). The time that an atom spends in a reservoir is called its “residence time.” The residence time is simply equal to the content of the reservoir divided by the rate of input (or the output, which is the same). The more we can speed the carbon atoms along in the process of turning them into sediment, the fewer there will be in the atmosphere-and-ocean system and the shorter their residence times will be. The less readily we get rid of them, the more they will pile up in the ocean and in the air (increasing their residence times) until they will be so dense that the Earth heats up and speed the reactions. This can be envisioned like a freeway between two cities: when the traffic is moving, there are fewer cars in the freeway “reservoir” that connects the two. However, when traffic is clogged, the freeway “reservoir” fills up quickly. Likewise when carbon is not moving from volcanoes to sediments quickly enough, it will fill up the atmosphere and the ocean reservoirs that transport it, a process we are witnessing today.

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