The carbon cycle
There is a strong and well-known correlation between the level or concentration of carbon dioxide in the atmosphere (expressed in parts per million or ppm) and the average global surface temperature. It has been shown by the work of Michael Mann and others that as the level of atmospheric carbon dioxide has increased in the past 200 or so years, the global average surface temperature has increased. This is the basis for the theory of global warming.
Why has the level of carbon dioxide increased? It is widely agreed that two human activities are primarily responsible: 1) the burning of fossil fuels; 2) deforestation. Fossil fuels are "ancient carbon." Deforestation decreases the biosphere's ability to take up and store carbon. Why do these factors change the atmosphere?
The level of carbon dioxide in the atmosphere is regulated by the carbon cycle. This is a natural process whereby the element carbon is transferred between "natural sources" and "natural sinks". Sources and sinks are like pipes that are connected to "carbon stocks." The largest natural carbon stocks are the lithosphere (crude oil, coal, magma), the biosphere (all carbon-based life), the oceans and the atmosphere. Carbon is transferred among the stocks via flows between sources and sinks. The uptake of atmospheric carbon dioxide by plants is an example of a flow to a sink. Decomposition of organic material is another example of a flow. A volcanic eruption is another example. The amount of carbon in the cycle changes over geologic time, i.e., eons.
When humans burn coal or products derived from crude oil, the carbon dioxide from this burning is composed of carbon that was stored in the lithosphere eons ago. It was taken out of the carbon cycle and stored, or sequestered, in the lithosphere. Now it is being re-injected into the carbon cycle at a rate that is unprecedented in the history of the planet. The "ancient carbon" that is being emitted is ending up in the two stocks that are able to accept it: the oceans and the atmosphere. The signs of increased carbon levels in the ocean are increasing pH level or increasing acidity. This is having a profound effect on ocean life. The sign of increased carbon level in the atmosphere is increasing carbon dioxide concentration. The carbon dioxide level is now the highest it has been in the last 400,000 years, and possibly the last 20 million years. The rate of increase of the carbon dioxide level is unprecedented in the history of the planet.
Here is a link to the IPCC TAR discussion of the carbon cycle.
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