Feedback mechanisms play a crucial role in the climate system, amplifying or dampening the climate response to enhanced concentrations of greenhouse gases from anthropogenic perturbations. Many of these feedbacks are known, but most of them only potentially. This article evaluates the role of a number of these feedback processes within the climate system. 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). Unlike previous studies, this study describes the scenario- and time-dependent role of biogeochemical feedbacks.
A number of simulation experiments are performed with IMAGE to project climate changes. Besides estimates of their absolute importance, the relative importance of individual biogeochemical feedbacks is considered by calculating the gain for each feedback process. This study focuses on feedback processes in the carbon cycle and the methane (semi-) cycle. Modeled feedbacks are then used to balance the past and present carbon budget. This results in substantially lower projections for atmospheric carbon dioxide than the Intergovernmental Panel on Climate Change (IPCC) estimates. The difference is approximately 18% from the 1990 level for the IPCC “Business-as-Usual” scenario.
Furthermore, the IPCC's “best guess” value of the CO2 concentration in the year 2100 falls outside the uncertainty range estimated with our balanced modeling approach. For the IPCC “Business-as-Usual” scenario, the calculated total gain of the feedbacks within the carbon cycle appears to be negative, a result of the dominant role of the fertilization feedback. This study also shows that if temperature feedbacks on methane emissions from wetlands, rice paddies, and hydrates do materialize, methane concentrations might be increased by 30% by 2100. The total effect of the methane feedbacks and the carbon dioxide feedbacks modeled can be expressed in the carbon dioxide-equivalent concentrations. Our simulated CO2-equivalent concentrations are lower than the IPCC estimates.