Carbon makes up no more than 0.27% of the mass of all elements in the universe and only 0.0018% by weight of the elements in the earth's crust. Yet, its importance to living organisms is far out of proportion to these figures. In contrast to its relative scarcity in the environment, it makes up 19.4% by weight of the human body. Along with hydrogen, carbon is the only element to appear in every organic molecule in every living organism on earth.

The series of chemical, physical, geological, and biological changes by which carbon moves through the earth's air, land, water, and living organisms is called the carbon cycle.

In the atmosphere, carbon exists almost entirely as gaseous carbon dioxide. The best estimates are that the earth's atmosphere contains 740 billion tons of this gas. Its global concentration is about 350 parts per million (ppm), or 0.035% by volume. That makes carbon dioxide...