Most people learn about the carbon cycle in elementary school. They learn that respiration and combustion release carbon, and that plants take up carbon and release oxygen through photosynthesis, then they're off to the next unit. In the policy debates over global warming, the politicians, lobbyists, and reporters who define the discussion generally seem to operate within an understanding that does not go beyond the elementary school curriculum. We are accustomed to simplistic politicians, but even when we turn to genuine scientists, there is little agreement on the subject of the impact of human activities on the carbon cycle, and in turn, their effects on life on Earth. Why is this so hard?

Living things bind carbon. On this planet, biomass is a huge storage locker for carbon. A dried human being is about 50% carbon by weight. Most dead things relinquish their carbon through decomposition, usually into the atmosphere. Sometimes, the processes of life take up more carbon than they release. For instance, crustaceans make their shells out of calcium carbonate, which binds carbon long after they leave this veil of tears. These sorts of things are carbon sinks - that is, their net uptake of carbon is greater than their net release. Back in the misty past, lots of organic material got locked up underground, and decomposed into what we know today as petroleum. That process was a carbon sink back then, but we are releasing all that bound carbon now. That's what all the politicians are on about.

If you go back to the elementary school model, you picture all plants as carbon sinks, because they are sucking up the carbon we exhale, but it's not really so. Agriculture churns carbon through the biosphere very quickly. At least once a year, at planting time, you will see a field devoid of plant life. That's its net carbon binding. When a forest is converted to agricultural use, it represents a release of carbon equivalent to the mass held within the trees that get chopped down.

The Bad News:
When fixed carbon, such as that in petroleum or old-growth forests, is released, it is persistent. That is, it is present in the atmosphere or the biosphere until it finds its way back into a true carbon sink. Since 1850, human activities have released about 100 trillion kilograms of carbon through changes in land use alone. By the 1980's, the net release from all sources was up to about 7.1 trillion kilograms per year. For the most part, these releases are cumulative.

It seems that mature forests are not only a carbon store, but a true carbon sink, because their root systems drive carbon deep into the soil, where it reacts with calcium silicate to form silicic acid and calcium carbonate, which is quite a stable way of binding carbon. Conversion of forests to agricultural use both releases carbon, and eliminates an important carbon sink, though its true capacity is beyond our current ability to quantify.

The Good News:
Controlled studies have shown that plants respond positively to increases in atmospheric carbon. When more is present, they grow faster, storing more of it in the same timeframe. The shrinking rain forests are probably sequestering carbon more efficiently now than in the recent past.

Changing land use does not necessarily mean negative impact in terms of carbon emission. Consider a mature forest. Although it is a tremendous carbon store, it is probably a weak carbon sink. If it is replaced with a tree farm whose products go into permanent uses like furniture and housing, the carbon in all that wood is taken out of circulation. Of course, that would not apply to tree farms whose output goes to pulp mills. Most paper ends up in landfills, where it decomposes, releasing its carbon into the atmosphere.

The biggest player in the Earth's carbon cycle is the oceans. The oceans are a net carbon sink, to the tune of about 2 trillion kilograms per year. So far, humans have not impacted this capacity significantly.

Something else is going on. This is a closely studied area, but there are about 1.8 trillion kilograms of carbon per year missing. Some unidentified thing on this planet is sucking up that much. That still leaves a net accumulation of 3.3 trillion kilograms per year in the atmosphere, but it could be worse.

The Ambiguous News:
Many of the statistics cited in this writeup are somewhat speculative. For instance, the true net carbon uptake of the world's oceans is not known with any degree of confidence, nor are the mechanisms behind it fully understood.

Much of what we'd wish were firm in this field of study is actually quite squishy. For instance, environmentalists would have you believe that old growth forests are a more efficient carbon sink than tree farms, and agricultural foresters would have you believe the opposite.

In short, this is an area rich with possibility for those of you with curiosity, ingenuity, and the desire to set things straight.

Act swiftly, I beseech you.
etc - Node what you don't know

Along with the terrestrial carbon cycle, there is another process labeled the "Carbon Cycle". This is a process of stellar nucleosynthesis where carbon nuclei serve as a catalyst for the formation of helium out of hydrogen.

All main sequence stars create energy by fusing hydrogen into helium. In stars the size of our sun and smaller, they do this directly: protons collide, and eventually form helium-4 from the p-p chain. In larger stars, the carbon cycle becomes more predominant. In this cycle, a stable carbon-12 nucleus absorbs protons, cycling through isotopes of carbon, nitrogen and oxygen, until the growing nucleus divides into the original carbon nucleus and a helium-4 nucleus. The cycle then repeats.

There are three salient points to remember about the stellar carbon cycle. First, it is strictly catalytic. While the proportion of carbon, nitrogen and oxygen will change, the total amount of these three elements will not. (Very occasionally, some slight amounts of fluorine and neon will be created, but only in negligible amounts). The second important point is that the reaction is very temperature and pressure dependent. In a star the size of our sun, the carbon cycle plays a minimal role in fusion (about 1%), but it rapidly increases with size, and in a star the size of Sirius, twice the size of our sun, it is by far the predominant producer of energy. The third thing about the carbon cycle is that it (surprise!) requires the presence of carbon within the star. Our sun is about 1/300th carbon, which is a relatively high proportion. In a star where this proportion was much smaller, the carbon cycle would not contribute to energy generation, even if the size of the star is quite large.

Although seemingly a technical detail, the existence of the carbon cycle has large implications for chemistry, physics, cosmology and astronomy.

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