What is the Carbon Cycle?

The basic carbon cycle of life is: (1) the conversion of atmospheric carbon dioxide to carbohydrates by photosynthesis in plants; (2) the consumption and oxidation of these carbohydrates by animals and microorganisms to produce carbon dioxide and other products; and (3) the return of carbon dioxide to the atmosphere. On a global level, the total carbon cycle is more complex, and involves carbon stored in fossil fuels, soils, oceans, and rocks.

We can organize all the carbon on earth into five main pools, listed in order of the size of the pool:

1. Lithosphere (Earth's crust). This consists of fossil fuels and sedimentary rock deposits, such as limestone, dolomite, and chalk. This is far and away the largest carbon pool on earth. The amount of carbon in the lithosphere: 66 to 100 million gigatons (a gigaton is one million metric tons). Of this amount, only 4,000 gigatons consists of fossil fuels.
2. Oceans. Ocean waters contain dissolved carbon dioxide, and calcium carbonate shells in marine organisms. Amount of carbon: 38,000 to 40,000 gigatons.
3. Soil organic matter. Amount of carbon: 1,500 to 1,600 gigatons.
4. Atmosphere. This consists primarily of carbon dioxide, carbon monoxide, and methane. The amount of carbon in the atmosphere has increased from 578 gigatons in 1700 to about 766 gigatons in 1999, and continues to increase at the rate of about 6.1 gigatons per year.
5. Biosphere. This consists of all living and dead organisms not yet converted into soil organic matter. Amount of carbon: 540 to 610 gigatons.

Carbon moves back and forth among these various pools. Nearly all of the carbon on earth is locked up in the lithosphere as sedimentary rock deposits and fossil fuels. And about 99.999% of this carbon is fixed in place and essentially off the table as far as the carbon cycle is concerned. Only the amount stored as fossil fuels enters the carbon cycle, and only then through human activities.

A sizable percentage of the "free" carbon on Earth exists in the atmosphere. As the carbon cycle undergoes shifts and fluxes through the eons, the amount of carbon in the atmosphere tends to increase or decrease to buffer the changes. Currently, the atmospheric carbon pool is expanding by about 6.1 gigatons per year, and the fossil fuel carbon pool is shrinking by about 4 to 5 gigatons per year. This is one aspect of the carbon cycle that can be readily manipulated by human activity.

The ocean absorbs 2.5 gigatons of carbon more from the atmosphere than it gives off to the atmosphere. But that extra amount of carbon is utilized by marine biota and eventually gets incorporated into deep sea deposits and sediments. So the net level of carbon in the ocean remains roughly the same every year.

The soil organic matter pool is currently losing about 1 to 2 gigatons of carbon per year to the atmospheric pool. About 60 gigatons of carbon per year enters the soil organic carbon sink as decaying biomass remains in the soil. About 61 to 62 gigatons of carbon are lost from this pool as soil organic matter is oxidized by the atmosphere. This is the other main cycle that can be manipulated by human activity. Changes in land use patterns and agricultural practices can affect the amount of carbon released into the atmosphere from soil organic matter.

The biosphere represents a significant carbon pool on Earth. About 110 gigatons per year of carbon is absorbed by the atmosphere into plant life through the process of photosynthesis. Of that amount, about 60 gigatons of carbon is released into the atmosphere through respiration, decay, and gaseous waste elimination from living animal biomass, both on land and in the ocean. The other 50 tons is incorporated into soil organic carbon, part of which can be readily oxidized and part of which is relatively stable for many years.

Before the industrial revolution, the main source of fluctuation in atmospheric carbon was from changes in biomass and soil organic carbon. Now, fossil fuel burning is the greatest factor in atmospheric carbon fluctuations.

The bottom line of all this is that the amount of carbon in the atmosphere is increasing by about 6.1 gigatons per year, mostly due to fossil fuel burning and land use changes that destroy soil organic carbon. This increase needs to stop, or at least slow down, since carbon dioxide in the atmosphere traps heat and becomes a greenhouse gas that can lead to global warming.

The atmospheric carbon balance sheet looks like this:

So how can humans manipulate the carbon cycle so that the atmospheric carbon pool stops expanding? Let's look at the two ends of the equation: where atmospheric carbon goes, and where it comes from. This will give us an idea of what changes can be made to reduce carbon buildup in the atmosphere.

Where atmospheric carbon goes.

1. Diffuses into the ocean. This part of the carbon cycle is difficult to manipulate.
2. Into plant life. This can be increased by increasing plant growth through reforestation, changes in agricultural cropping systems, and reclamation of marginal land.
3. Into soil organic carbon. As plant life decays, part of its carbon is converted by microorganisms into soil organic matter. In the initial phases of this process, the organic matter is in a "short-term" pool and can be easily oxidized. Once this happens, the carbon is released back into the atmosphere. By changing agricultural practices, it is possible to increase the amount of soil organic carbon in the intermediate and long-term pools.

Where atmospheric carbon comes from.

1. Fossil fuel emissions. This is the largest source of carbon buildup in the atmosphere.
2. Soil organic carbon destruction. Through excessive tillage and soil erosion, soil organic carbon can be oxidized and lost to the atmosphere. The total amount of carbon stored as soil organic carbon is roughly equal to the sum of the amount in the atmosphere plus the amount existing in plant and animal life combined. So any changes in soil organic matter destruction or creation can have a significant impact on atmospheric carbon levels.
3. Deforestation. As forests are burned for land clearing or other reasons, a significant amount of carbon is released into the atmosphere.