The element carbon can exist in a gaseous form like carbon dioxide, in a solid form when carbon is combined with other elements such as carbonates which are present in most types of rocks constituting the earth's crust. The combustion process transforms the element carbon from a solid or liquid form to carbon dioxide which is classified as a greenhouse gas as it can congregate in the upper atmosphere. The process by which carbon has been cycled between various forms is called the carbon cycle and this cycle has been implicated in many of the changes in the earth's climate over geological time scales.

The carbon cycle

Carbon dioxide is absorbed by plants and trees and used in the process of photosynthesis which results in the production of oxygen which humans and animals need to breathe. Carbon dioxide is also absorbed by the oceans where it is used by phytoplankton, the organism at the start of the oceanic food chain, to form their skeletons.

If more carbon dioxide is absorbed than given off, then the carbon dioxide level in the atmosphere will fall and this has been associated with the onset of previous ice ages in which global temperatures have fallen. The current ice caps of Greenland and Antarctica and Alpine glaciers are remnants of the last ice age. Conversely, if more carbon dioxide is given off than absorbed, then the carbon dioxide concentration in the atmosphere will rise and there is clear evidence over geological time that this has resulted in periods of global warming.

The figure below shows the rise in levels of carbon dioxide over the past 1000 years.

Levels of carbon dioxide in the atmosphere, measured in parts per million over 1000 years.

Greenhouse gas and global warming	Economics of sustainable energy use
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