Global Carbon Cycle

Abstract

Despite the low concentrations in the atmosphere relative to nitrogen or oxygen, carbon dioxide (CO2) plays a significant role in the Earth's life cycle and in controlling the global climate. The positive and negative feedback mechanisms associated with the exchange of atmospheric carbon with the ocean and organic material found on the land surface (called the terrestrial biosphere) appear to have been relatively well balanced for most of humankind's existence. Since the beginning of the industrial era, however, humans have been extracting fossil carbon from the Earth's interior and combusting it for energy production. Much of this fossil carbon is converted to CO2 gas and released into the environment where it is altering the carbon balance between the atmosphere, ocean and terrestrial biosphere. Recent changes observed in the global climate are consistent with the predicted response to increasing CO2 and other greenhouse gases in the atmosphere.

Key Concepts:

  • Carbon is an essential component of life on Earth.

  • Atmospheric CO2 concentrations varied considerably in the distant past depending on the balance of several geochemical processes.

  • Geochemical evidence suggests that atmospheric CO2 levels dropped below 300 ppm approximately 20 million years ago and have remained below that level until the beginning of the twentieth century.

  • The atmosphere, terrestrial biosphere and the ocean are the only systems that have natural exchanges of carbon on time‐scales relevant to human society.

  • Humans are in the process of altering the global carbon cycle at a rate that is unprecedented in Earth's history through land‐use practices and the burning of fossil fuels.

  • As we change the balance of the carbon cycle it is clear that the global climate is changing, but it is not clear how global ecosystems might respond.

Keywords: carbon dioxide; greenhouse gas; terrestrial biosphere; oceans; atmosphere; feedback mechanisms

Figure 1.

Atmospheric CO2 determined from air enclosed in ice cores and firn air (blue symbols; adapted from MacFarling‐Meure et al., ) and from direct atmospheric measurements at Mauna Loa, Hawaii (redlines on main plot and insert; data provided by P. Tans, NOAA/ESRL).

Figure 2.

Compilation of the EPICA Dome C CO2 ice core record and temperature anomaly more than the past 800 000 years. Reproduced with permission from Lüthi et al. (). © Nature Publishing Group.

Figure 3.

Schematic of carbon cycle on land. Black arrows represent preindustrial fluxes (in petagrams of carbon per year) and red arrows represent average anthropogenic fluxes for 1980s and 1990s. Values in brackets represent estimated reservoir sizes (in petagrams of carbon).? indicates that this is a very controversial and uncertain number. Adapted from Sabine et al. (). © Island Press.

Figure 4.

Photos of two FACE sites. The top photo is from a Sweetgum site in Tennessee (http://www.esd.ornl.gov/facilities/ORNL‐FACE/index.html). The bottom photo is from the Aspen FACE site in Wisconsin (http://aspenface.mtu.edu/). The white circles in both photos are towers that expose the encompassed region to elevated atmospheric CO2 levels.

Figure 5.

Schematic of ocean carbon cycle. Black arrows represent preindustrial fluxes (in petagrams of carbon per year) and red arrows represent average anthropogenic fluxes for 1980s and 1990s. Values in brackets represent estimated reservoir sizes (in petagrams of carbon). Adapted from Sabine et al. (). © Island Press.

Figure 6.

Schematic of global carbon cycle. Black arrows represent preindustrial fluxes (in petagrams of carbon per year) and red arrows represent average anthropogenic fluxes for 2000–2009. Values in brackets represent estimated reservoir sizes (in petagrams of carbon). Adapted from Sabine et al. (). © Island Press.

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Further Reading

Archer D and Brovkin V (2008) The millennial atmospheric lifetime of anthropogenic CO2. Climate Change 90: 283–297.

Feely RA, Doney SC and Cooley SR (2009) Ocean acidification: present conditions and future changes in a high‐CO2 world. Oceanography 22: 36–47.

Field CB and Raupach MR (eds) (2004) The Global Carbon Cycle: Integrating Humans, Climate, and the Natural World. Washington, DC: Island Press.

Gruber N, Gloor M, Mikaloff Fletcher SE et al. (2009) Oceanic sources, sinks, and transport of atmospheric CO2. Global Biogeochemical Cycles 23: GB1005. doi: 10.1029/2008GB003349.

van der Werf GR, Morton DC, DeFries RS et al. (2009) CO2 emissions from forest loss. Nature Geoscience 2: 737–738. doi: 10.1038/ngeo671.

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How to Cite close
Sabine, Christopher L(Aug 2014) Global Carbon Cycle. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0003489.pub2]