The implementation of climate policies is often hindered by a lack of acceptability as well as the lack of long-term vision of stakeholders. Mitigation scenarios exercises allow the debate; however the scenario design process lacks transparency and stakeholders' (and citizens') involvement, which leads to a lack of credibility, legitimacy and ownership of mitigation scenarios. This paper presents a process for the collaborative design of an "acceptable" mitigation scenario through a public dialogue involving fifty stakeholders. Stakeholders selected the mitigation policies and measures they deemed acceptable. They significantly contributed to the development of the scenario methodology as well, by defining its specifications including data transparency, sensitivity analysis and the need to clarify certain types of economic and financial impacts for the evaluation of Factor 4 scenarios. The acceptable policies were integrated into the hybrid CGE model Imaclim-R France. This modeling platform, designed for the dialogue between modelers, stakeholders and experts, evaluates the social, economic and financial impact of climate policies. This attempt at a methodology exemplifies a collaborative scenario design, thereby proving that such a process can be implemented. The scenario based on the measures that stakeholders deemed acceptable reduce CO2 emissions by 68% by 2050 compared to 1990, which is a result close to the target of the Factor 4 (75%). The measures to reduce emissions are beneficial for employment and economic growth, except in the short term. They can also reduce rapidly and permanently the households' budget dedicated to energy services. Finally, the financial assessment of measures is positive throughout the period. This result is a solid foundation on which to build the acceptability of mitigation trajectories. Beyond the scenario, the methodological innovation offers the means to increase the involvement of stakeholders in decision-making.
