Some of the light energy from the sun that strikes Earth's atmosphere is reflected back into space. The surface of the Earth absorbs the rest of the light energy and radiates it back again as heat. Some of this heat can be absorbed by the heat-trapping gases. Carbon dioxide is a major greenhouse gas. Natural cycles ordinarily keep the levels of carbon dioxide in the atmosphere from changing rapidly.
Some carbon is removed from the atmosphere through plant photosynthesis.
An equivalent amount of carbon is released into the atmosphere by animals and plants, including organisms that cause decay. These organisms give off carbon dioxide during a process called respiration. The respiration of decay organisms also releases methane, another heat-trapping gas, into the atmosphere. Throughout most of the planet's recent history, the concentration of carbon dioxide in the atmosphere has not changed from year to year as long as the processes of carbon release and absorption were in balance. Before the Industrial Revolution began in the mid-1700's, this release of carbon dioxide was gradual enough that the oceans could absorb any excess and there was little noticeable accumulation in the atmosphere.