Design principles for the IPCC emission scenario ensemble

Working Group III (WGIII) of the Intergovernmental Panel on Climate Change (IPCC) regularly assesses collections of emission scenarios submitted by the scientific community, linking long-term projections about socio-economic conditions and technology rollout with the changing climate. Current protocols for assembling and evaluating submitted scenarios have been shaped by the IPCC's institutional position at the science-policy interface, particularly its commitment to delivering a policy-neutral scientific assessment. While these protocols allow WGIII experts to address an important array of questions about climate change mitigation, they limit opportunities for drawing policy-relevant information from scenarios in other ways. In preparation to future IPCC reports, this paper considers alternative approaches to the construction and evaluation of WGIII's ensemble. A framework is proposed for clarifying and evaluating the available options, based on three dimensions: ownership, logic, and purpose.