PMEL CO2: Global Carbon Cycle

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This figure presents a basic picture of the global carbon cycle as we understand it today. It includes the preindustrial (black) and anthropogenic (red) ocean-atmosphere and land-atmosphere exchange fluxes. The anthropogenic fluxes are average values for the 1980s and 1990s. This figure also shows components of the long-term geological cycle and the composite estimates of CO₂ emissions from geological reservoirs (i.e. fossil fuels and the production of lime for cement).

Land Carbon Cycle

The exchange of carbon between the terrestrial biosphere and the atmosphere is a key driver of the current carbon cycle. Global net primary production (NPP) by land plants is about 57 Pg C y^-1. Total NPP is approximately 40% of gross primary production (GPP), with the remainder returned to the atmosphere through plant respiration. Land plants contain slightly less carbon than the atmosphere; soils contain substantially more. The world energy system delivered approximately 380 EJ (10¹⁸ J) of primary energy in 2002. Of this, 81% was derived from fossil fuels, with the remainder from nuclear, hydroelectric, biomass, wind, solar, and geothermal energy sources. The fossil fuel component released 5.2 Pg C in 1980 and 6.3 Pg C in 2002. Cement production is the other major industrial release, which increased to 0.22 Pg C in 1999. The combined release of 5.9 Pg C shown in the figure above represents an average emission for the 1980s and 90s. Carbon emissions from land use and land management have increased dramatically over the last two centuries resulting from the expansion of crop land and pasture, infrastructure extension and other effects driven by market growth, pro-deforestation policies, and demographic pressures. Prior to about 1950, carbon emissions from land use change were mainly from temperate regions. In recent decades, however, carbon releases from land use change have been concentrated in the topics.

Ocean Carbon Cycle

The oceans contain about 50 times more CO₂ than the atmosphere and 10 times more than the latest estimates of the plant and soil carbon stores. CO₂ moves between the atmosphere and the ocean by molecular diffusion, when there is a difference between the CO₂ gas pressure (pCO₂) in the oceans and the atmosphere. For example, when the atmospheric pCO₂ is higher than the surface ocean pCO₂, CO₂ diffuses across the air-sea boundary into the seawater. The gross exchanges of CO₂ across the air-sea interface are much larger than the net flux. The global budget presented in the above figure shows the preindustrial oceans as a net source of ~0.6 Pg C y^-1 to the atmosphere partially offsetting the addition of carbon to the oceans from rivers. Today, the oceans are a net sink for approximately 1.3 Pg C y^-1 representing an anthropogenic uptake of about 1.9 Pg C y^-1. Over the long term (millennial time-scales), the ocean has the potential to take up approximately 85% of the anthropogenic CO₂ that is released to the atmosphere. The reason for the long time constant is the relatively slow ventilation of the deep ocean. Most of the deep and intermediate waters have yet to be exposed to anthropogenic CO₂. As long as atmospheric CO₂ concentrations continue to rise, the oceans will continue to take up CO₂. However, this reaction is reversible. If atmospheric CO₂ were to decrease in the future, the recently ventilated waters would start releasing part of the accumulated anthropogenic CO₂ back to the atmosphere, until a new equilibrium is reached.

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