20. Carbon Dioxide

The ultimate source of CO₂ is thought to be volcanic emissions. However, most CO₂ is continuously recycled as part of the carbon cycle. CO₂ is taken up by plants, and through photosynthesis is fixed as organic matter. The organic matter is then used by plant or animals to obtain energy, releasing the CO₂ back into the atmosphere. CO₂ is a greenhouse gas, intercepting and re-emitting the radiation from Earth back into space. It is thought that CO₂ has been the Earth’s major thermostat mechanism through time. Higher levels result in a greenhouse condition, lower levels an icehouse state. CO₂ is ultimately removed from the system by the weathering of silicate minerals, an exceedingly slow process. Because CO₂ reacts with water to form carbonate and bicarbonate ions it is thousands of times more soluble than oxygen. Although the changes in the icehouse state from glacial to interglacial are forced by variations in Earth’s orbit, atmospheric CO₂ levels follow, acting as a positive feedback. Since the Industrial Era, there has been an almost exponential increase in the amount of CO₂ in the atmosphere, almost all from the burning of fossil fuels. Today’s atmospheric concentrations are as much higher than pre-industrial levels as those were above the levels of the Last Glacial Maximum. CO₂ is the major greenhouse gas in the polar regions, concentrating global warming in the high latitudes.