There are inherent uncertainties in the key assumptions and relationship about future population, socio-economic development and technical changes that are the bases of the IPCC SRES Scenarios. However the uncertain nature of these emissions paths have been well documented (Morita and Robinson, 2001).
The imperfect understanding of some of the processes and physics in the carbon cycle and chemistry models generate uncertainties in the conversion of emissions to concentration. To reflect this uncertainty in the climate scenarios, the use of AOGCMs that explicitly simulate the carbon cycle and chemistry of all the substances are needed.
The Hadley Centre has developed a version of the climate model that allows the effect of climate change on the carbon cycle and its feedback into climate, to be included. In the one AOGCM, the feedback is strong and positive.
There is another feedback that arises from the effect that climate change has on the chemical reactions between species in the atmosphere, particularly as the amount of water vapour in the atmosphere increases. This affects the levels of methane and ozone considerably. In the Hadley Centre model this feedback is small and negative.
These two feedbacks have been introduced in the Hadley Centre model to reduce the uncertainty in the conversion of emissions to concentrations.