Carbon dioxide (CO2) is the primary greenhouse gas contributing to the enhanced greenhouse effect. While CO2 has a large variety of natural sources and sinks, human activities are responsible for the considerable increase of CO2 in the atmosphere occurred since the industrial revolution.

CO2 is naturally present in the atmosphere as part of the Earth's carbon cycle, i.e. the natural circulation of carbon among the atmosphere, hydrosphere and biosphere. Human activities are altering the carbon cycle either by adding CO2 to the atmosphere or by influencing the ability of natural sinks to remove CO2 from the atmosphere. The primary source of anthropogenic CO2-emissions is the combustion of fossil fuels (coal, oil, natural gas) and wood. Natural sinks are affected by deforestation and other land use changes.

According to IPCC (5th Assessment Report, 2014), anthropogenic greenhouse gas emissions have increased since the pre-industrial era and are now higher than ever. Between 1750 and 2011, cumulative anthropogenic CO2 emissions to the atmosphere were 2040 ± 310 GtCO2, and about half of these emissions have occurred in the last 40 years.

About 40% of the anthropogenic emissions have remained in the atmosphere. The rest has been removed from the atmosphere and stored on land in plants and soils, and in the oceans. The oceans have absorbed about 30% of carbon, with negative side effects such as ocean acidification. However, it is not clear how effectively these CO2 sinks will operate in the future under a changing climate and increasing human impacts.

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