Assessment of major uncertainties in calculating regional contributions to climate change

During the negotiations on the Kyoto Protocol, Brazil proposed a methodology to link the relative contribution of Annex-I Parties to emission reductions with the relative contributions of Parties to the global-mean temperature increase. The proposal was not adopted during the negotiations,but reffered to the Subsidiary Body for Scientific and Technological Advice for consideration of its methodological aspects. In this context we analyze the impact of model uncertainties and methodological choices on the regionally attributed global-mean temperature increase. A climate assessment model was developed, which calculates changes in greenhouse gas concentrations, global-mean temperture and sea-level rise attributable to individual regions. The analysis shows the impact of the different choises in methodological aspects to be as important as the impact of model uncertainties on a region's contribution to present and future global temperature increase. Choices may be the inclusion of the anthropogenic non-CO2 greenhouse gas emissions and/or the CO2 emissions associated with land-use changes. When responsibility to global temperature change is attributed to all emitting parties, the impacts of modeling uncertainties and methodological choices are considerable. However, if relative contributions are calculated only within the group of Annex-I contries, the results are remarkably insensitive to the uncertainty aspects considered here