Carbon dioxide

　　Carbon dioxide (CO2) is emitted in a number of ways. It is emitted naturally through the carbon cycle and through human activities like the burning of fossil fuels.
Natural sources of CO2 occur within the carbon cycle where billions of tons of atmospheric CO2 are removed from the atmosphere by oceans and growing plants, also known as ‘sinks,’ and are emitted back into the atmosphere annually through natural processes also known as ‘sources.’ When in balance, the total carbon dioxide emissions and removals from the entire carbon cycle are roughly equal.
Since the Industrial Revolution in the 1700’s, human activities, such as the burning of oil, coal and gas, and deforestation, have increased CO2 concentrations in the atmosphere. In 2005, global atmospheric concentrations of CO2 were 35% higher than they were before the Industrial Revolution. For more information on CO2 trends in the atmosphere, visit the page on Atmosphere Changes.
Greenhouse gases occur naturally in the Earth’s atmosphere in addition to being emitted through human activities. This natural “carbon cycle” includes carbon dioxide used in plants during photosynthesis and the exchange of carbon dioxide between the atmosphere and the oceans.
The primary natural processes that release CO2 into the atmosphere (sources) and that remove CO2 from the atmosphere (sinks) are:
Animal and plant respiration, by which oxygen and nutrients are converted into CO2 and energy, and plant photosynthesis by which CO2 is removed from the atmosphere and stored as carbon in plant biomass;
Ocean-atmosphere exchange, in which the oceans absorb and release CO2 at the sea surface; and
Volcanic eruptions, which release carbon from rocks deep in the Earth’s crust (this source is very small).