What is the Earth's Carbon cycle?
Carbon is the building block of life. Without it, there would be no organic chemistry, and therefore no life as we know it.
How does it work?
Even though Carbon is fundamentally important to life on Earth, it isn't exactly one of the most abundant elements on the planet. Life, therefore, has developed an intricate system which pumps the limited resource from one reservoir to another. Carbon is exchanged between the atmosphere and the biosphere largely through the the plant processes of transpiration and respiration - animals get their supply of carbon from eating autotrophic plants. When plants and animals die the Carbon they have assimilated ends up getting buried in the lithosphere either directly as is the case with terrestrial organisms, or via the water cycle as with aquatic life. Eventually the buried Carbon will reenter the atmosphere via outgassing from a volcano, or more recently through the burning of fossil fuels (which can be thought of as the carbon from million year old dead organisms). Some carbon also leaves the land directly due to land cover type changes, such as that from a forest to an agricultural field.
The human factor
The human induced changes in the Carbon cycle - land use change, and fossil fuel burning - are at the forefront of today's climate change debate. Carbon Dioxide is a product of both deforestation and fossil fuel burning, and humans have been spewing it into the atmosphere at an alarming rate since the dawn of the industrial revolution. All of this extra Carbon Dioxide in the atmosphere acts to thicken the insulating blanket surrounding the Earth, and increase its temperature. This increase in temperature is only the beginning of the problem, as changing temperature regimes will lead to changes in precipitation, and eventually total shifts in the location of the worlds ecosystems. World leaders are beginning to understand the consequences of climate change and have begun developing methods of curbing it, the Kyoto protocol is one example of such legislation.
Artwork by Maija Swanson and Nick Olejniczak