Carbon dioxide
Carbon dioxide (CO2) is a naturally occurring gas made of one carbon atom and two oxygen atoms. It moves through the carbon cycle and acts as a Greenhouse gas by absorbing outgoing infrared radiation. Human activities, especially fossil-fuel combustion, industrial processes, and land-use change, have increased its atmospheric concentration and driven long-term warming.
In simple terms
Carbon dioxide is exchanged among the atmosphere, oceans, soils, plants, and animals. Natural sources and sinks were broadly balanced over long periods, but human activities now add more CO2 than natural systems remove. Burning coal, oil, and gas is the largest human source globally; cement production and changes in land use also contribute. Reforestation and other land management can increase removals, while geological storage can retain captured CO2. Because Carbon dioxide persists and accumulates, warming depends strongly on cumulative emissions rather than only a single year’s release.
Why it matters
CO2 is the main driver of human-caused long-term warming and the reference gas used in Carbon dioxide equivalent calculations. Measuring its sources and removals supports Carbon accounting, target setting, and evaluation of mitigation options. A tonne emitted and a tonne claimed as removed must still be understood through timing, durability, system boundaries, and measurement quality; equal numbers do not automatically mean equal climate outcomes.
Example
For example, a fictional cement plant emits CO2 when fuel is burned and when limestone is chemically converted during production. Its Greenhouse gas inventory separates those sources, applies appropriate Emission factors or measurements, and reports the result. Efficiency may reduce fuel emissions, while process emissions require different technical responses.
How it differs
Carbon dioxide equivalent
Carbon dioxide is one specific gas measured by mass; Carbon dioxide equivalent is a common unit that converts different greenhouse gases according to a stated Global warming potential and time horizon.