Modelling feedback mechanisms in the carbon cycle: balancing the carbon budget

Abstract

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Within the carbon cycle feedback mechanisms that amplify or damp the exchange of carbon dioxide between the different reservoirs to enhanced concentrations of carbon dioxide and increased temperature from anthropogenic perturbations play a crucial role. Quite a lot of these feedbacks are known, but most of them only potentially. This report evaluates the role of a number of these feedback processes within the carbon cycle. In order to assess their impact the feedbacks which at present can be quantified reasonably are built into the Integrated Model to Assess the Greenhouse Effect: IMAGE. Contrary to previous studies, this study describes the scenario- and time dependent role of biogeochemical feedbacks. A number of simulation experiments are performed with IMAGE to make future projections of the concentration of carbon dioxide. Key principle in this is that we use the modelled feedbacks to balance the past and present carbon budget. This results in substantially lower projections for atmospheric carbon dioxide than the IPCC-estimates: for the IPCC 'Business-as-Usual' scenario the difference is about 9%. Besides estimates of their absolute importance, also the relative importance of individual biogeochemical feedbacks is considered, by calculating the gain for each feedback process. Finally the total gain is calculated, which appears to be negative, caused by the dominant role of the fertilization feedback.