In classical carbon cycle model studies, emissions from fossil fuel burning are prescribed and the model computes the time evolution of atmospheric CO2 as the residual between emissions and uptake by land and ocean. Because the global carbon cycle is intimately embedded in the physical climate system, several feedback loops exist between the two systems (Friedlingstein et al., 2003). For example, increasing CO2 modifies the climate, which in turn impacts on ocean circulation and therefore on oceanic CO2 uptake. Similar effects are expected to occur on land with rising temperatures (e.g. higher soil carbon respiration rates). When a climate or carbon cycle feedback results in an increase in the atmospheric CO2 accumulation rate and thus in enhanced climate change, it is referred to as a positive feedback. A change that reduces atmospheric CO2 is a negative feedback.
The quantitative assessment of these feedbacks necessitates the use of coupled carbon cycle climate models. Three coupled models, Hadley Centre, Institute Pierre-Simon Laplace (IPSL) and Climate System Modelling Initiative (CSMI 4), have recently examined the feedbacks based on Intergovernmental Panel on Climate Change (IPCC) scenarios between 1850 and 2100 (Cox et al., 2000; Dufresne et al., 2002; Fung et al., 2005). By 2100 the results show dramatically different climate-carbon cycle sensitivities. These models simulate an enhanced increase of atmospheric CO2 as a result of the climate change impacts on the carbon cycle. However, the magnitudes of the feedbacks vary by a factor of four between the simulations. Without the feedbacks the models reach an atmospheric concentration of ~700 ppm by 2100. When the feedbacks are operating, the Hadley Centre model (Cox et al., 2000) reaches 980 ppm, leading to an average near-surface warming of +5°C, the IPSL model (Dufresne et al., 2002) attains only 780 ppm and a warming of+3°C and the CSMI 4 model reaches an atmospheric CO2 concentration of 792 ppm with a warming of +1.4°C. This different behaviour can be traced back to the land carbon cycle climate sensitivity of the Hadley Centre model being much larger than either the IPSL or CSMI 4 models as well as to the geochemical oceanic uptake being much larger in the IPSL model than in the Hadley Centre model.
These pioneering model simulations are subject to important limitations. In these models key biological processes on land and in the ocean are highly parameterized and poorly constrained (see Fung et al., 2005). Proper modelling of the coupled carbon and climate system, however, requires an improved understanding of the two primary classes of feedbacks: that of the carbon cycle and that of the carbon-climate system.
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