Carbon Cycle

It is believed that most of the carbon now on Earth was originally released from the interior of the Earth as CO₂, a gas which now makes up about 0.03 to 0.04 percent by volume of air, and is responsible for maintaining the Earth as a greenhouse with temperature conditions suitable for life. CO₂ is the most available form of carbon for living organisms. Molecules containing carbon may keep the carbon fixed over millions of years or may cycle the carbon through quickly. The atmospheric cycling and effects of CO₂ on climate are discussed in the Atmospheric System.

Thus, carbon exists in the biosphere as the central element of life, in the lithosphere as coal (carbon) or limestone (Calcium Carbonate, CaCO₃ ), in the atmosphere as CO₂, in the hydrosphere as dissolved CO₂ , as well as in other complex forms. The versatility of carbon compounds and the vital role of carbon as the basis of life is described in Biological/Ecological Systems. The atmosphere contains about 750 billion tons of carbon in the form of CO₂. Photosynthesis by plants removes about 120 billion tons of carbon from the air per year, but plant decomposition returns about the same amount. Living plants and animals contain 560 billion tons of carbon (mostly forest trees). Plant remains and organic matter buried in the soil contain about 1400 billion tons. About 11,000 billion tons are trapped in compounds which are complexes of methane (CH₄) and water, found on ocean floor. The oceans contain another 38,000 billion tons of carbon, most of it in the form of dissolved CO₂.

With the onset of the Industrial Revolution about 200 years ago, we began burning massive amounts of fossil fuels and releasing large amounts of the earthbound carbon into the atmosphere, primarily as CO₂. The burning of fossil fuels adds about 22 billion tons of CO₂ per year (?), containing about 6 billion tons of carbon. Deforestation adds a further 1.6 to 2.7 billion tons, by not removing this amount. The rapid growth of synthetic organic chemicals contributes to the amount of CO₂ released.

The main reservoirs for carbon are sedimentary rocks, fossilized organic carbon including the fossil fuels, the oceans, and the biosphere. Carbon goes primarily through three cycles with different time constraints:

1. A long-term cycle involving sediments and the depths of the lithosphere.
2. A cycle between the atmosphere and the land.
3. A cycle between the atmosphere and the oceans.

The last two cycles are faster and subject to human intervention.

Carbon Cycle One: Long-term Cycle

This cycling between atmosphere, oceans, and sediments involve a slow dissolution of atmospheric carbon and carbon from rocks via weathering into the oceans. In turn, the oceans deposit sediments, and then some of the sediments are thrown back into the atmosphere through volcanic action.

Figure C1: Carbon Cycle One.

This cycle occurs over hundreds of millions of years. A larger portion of sediments is calcium carbonate (CaCO₃) because the ocean contains large amounts of calcium.

Carbon Cycle Two: Air and Land Cycle

The second cycle between the atmosphere and biosphere occurs over different time scales ranging from days to decades. Carbon dioxide is the basic "food" of the biosphere and thus the biosphere is the agent for this cycling. Photosynthesis (synthesizing starches and sugars using light) is a main mechanism for cycling carbon by the biosphere. The chemical reaction of photosynthesis may be represented as:

CH₂O represents a unit of organic matter; six of the CH₂O unit would be C₆H₁₂0₆ which makes the simple sugar (glucose or fructose) and 11 of these units make C₁₁H₂₂O_11, a more complex sugar, sucrose, formed by the combination of one glucose and one fructose. Thousands of glucose molecules combine to form a molecule of starch, or of cellulose. (Molecule examples) Thus photosynthesis takes the atmospheric carbon in CO2 and "fixes" it into the biosphere. The subsequent cycling of the carbon in the biomass is created.

Figure C2: Carbon Cycle Two.

Thus 750 Gt-C in the atmosphere cycling at the rate of 80 Gt C/yr means that the lifetime of the carbon in the atmosphere reservoir is about 9 years.

When the organic matter is oxidized through respiration, the reverse of photosynthesis takes place.

Respiration releases CO2 into the atmosphere. Respiration and photosynthesis occur at nearly equal rates over one year. Buried biomass--eventually becoming fossils, including coal--have historically had an effect of keeping the carbon in the land. The accelerated burning of fossil fuels is, however, releasing these large stores into the atmosphere as combustion products.

Burning of biomass-based fuels such as methanol and ethanol has been suggested an alternate to fossil fuel combustion. Biomass fuels have no net release of carbon dioxide. The effects of fossil fuel burning is discussed in the Atmospheric System.

Carbon Cycle Three: Air and Sea Cycle

The oceans contain much more carbon than the atmosphere. Carbonates washed down from the rocks, over thousands of years, dissolved CO₂, and carbon in the oceanic biomass constitutes this reservoir. The carbon from the top layers of the ocean cycles faster whereas the carbon in deep waters may take thousands of years.

The summary of the three cycles is shown in Figure C3.

Figure C3: Three Carbon Cycles combined.

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