English Abstract Within the climate system feedback mechanisms that
amplify or damp the climate response to enhanced concentrations of
greenhouse gases from anthropogenic pertubations, play a crucial role.
Quite a lot 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. 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 climate change. Besides estimates of their
absolute importance, also the relative importance of individual
biogeochemical feedbacks is considered, by calculating the gain for each
feedback process. We focus on the feedback processed in the carbon cycle
and the methane (semi-)cycle. With respect to the carbon cycle we use the
modelled feedbacks to balance the past and present carbon budget. For
atmospheric carbon dioxide this results in substantially lower projections
than the IPCC-estimates: for the IPCC 'Business-as-Usual' scenario the
difference is about 9%. For the IPCC 'Business-as-usual' scenario the
calculated total gain of the feedbacks within the carbon cycle appear to be
negative, caused by the dominant role of the fertilization feedback. As to
the methane feedbacks, this study shows that, if temperature feedbacks on
methane emissions from wetlands, rice paddies and hdyrates do materialize,
the methane concentration might be enhanced with 80% in 2100. The total
effect of the methane feedbacks and the carbon dioxide feedbacks modelled,
can be expressed in the carbon dioxide equivalent concentrations. Our
simulated CO2-equivalent concentrations are about the same as the
IPCC-estimates. This has to do with the resulting positive gain of the
methane feedbacks, counteracting the negative gain of the feedbacks within
the carbon cycle at the end of the next century. Over the whole period,
however, the total gain of all feedbacks modelled appears to be negative for
the IPCC 'Business-as-Usual' scenario.
