The Carbon Cycle

Changes in the carbon cycle and other biogeochemical cycles play a key role in modulating atmospheric and oceanic concentrations of CO2 and other GHGs. Scientists have learned a great deal over the past 50 years about the exchange of carbon between the atmosphere, ocean, and biosphere and the effects of these changes on temperature and other climate change (CCSP, 2007a). However, key uncertainties remain. For example, we have an incomplete understanding of how interacting changes in temperature, precipitation, CO2, and nutrient availability will change the processing of carbon by land ecosystems and, thus, the amount of CO2 emitted or taken up by ecosystems in the decades ahead (see Chapter 9). As noted in Chapters 2 and 6, some of these feedbacks have the potential to dramatically accelerate global warming (e.g., the possibility that the current warming of permafrost in high-latitude regions will lead to melting of frozen soils and release huge amounts of CO2 and CH4 into the atmosphere). Changes in biogeochemical processes and biodiversity (including changes in reflectance characteristics due to land use changes) also have the potential to feed back on the climate system on a variety of time scales. Models and experiments that integrate knowledge about ecosystem processes, plant physiology, vegetation dynamics, and disturbances such as fire are needed, and such models should be linked with climate models.

As the ocean warms and ocean circulation patterns change, future changes in the ocean carbon cycle are also uncertain. For example, it is unclear whether the natural "biological pump," which transports enormous amounts of carbon from the surface to the deep ocean, will be enhanced (Riebesell et al., 2007) or diminished (Mari, 2008) by ocean acidification and by changes in ocean circulation. Recent observational and modeling results suggest that the rate of ocean uptake of CO2 may in fact be declining (Khatiwala et al., 2009). Because the oceans currently absorb over 25 percent of human-caused CO2 emissions (see Chapter 6), changes in ocean CO2 uptake could have profound climate implications. Results from the first generation of coupled carbon-climate models suggest that the capacity of the oceans and land surface to store carbon will decrease with global warming, which would represent a positive feedback on warming (Friedlingstein et al., 2006). Improved understanding and representation of the carbon cycle in Earth system models is thus a critical research need.

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