Model-based scenarios represent a backbone for future climate change analyses, aiming to inform policy responses and designs. Emission scenarios play a crucial role, e.g., in assessing mitigation strategies evaluated by the Intergovernmental Panel on Climate Change’s (IPCC) Working Group III (WG3). Their output (emissions, aerosols, etc.) is used as input for climate scenarios, which, in turn, informs impact scenarios, vital for assessing adaptation strategies. Despite increasing scientific publications and improved scenario quality, scenario analysis has not been translated into sufficient policy actions. Three critical concepts—credibility, legitimacy, and salience—underpin discussions on knowledge value in the science-policy interface. This thesis focuses on scenario developments over time (supporting legitimacy assessments), the accuracy of projections (credibility), and policy relevance (salience) of the emission scenarios informing the IPCC assessment reports 1990–2022, as these are also the most prominent scenarios in the scientific literature. It aims to cover three research gaps: First, despite a growing literature on the emission scenario critique, assessments, and responses. No attempt has been made to assess the critique systematically. Second, emission scenario ranges need to be periodically updated and assessed against historical trends, last done in 2006 and partly in 2013. Third, the usefulness and policy relevance have been discussed scientifically, while the perceptions and needs of policymakers regarding scenarios are not explicitly explored in the literature. The results show that peer-reviewed scenario critiques and responses (280) focus on four areas: key scenario assumptions (40%), the emissions range covered by the scenarios and missing scenarios (25%), methodological issues (24%), and policy relevance and handling of uncertainty (11%). The critiques have shaped scenario narratives and development methods over time. Some areas of critique have decreased or become less prominent (probability, development process, convergence assumptions, and economic metrics). Other areas have become more dominant over time (e.g., policy relevance & implications of scenarios, transparency, Negative Emissions Technologies (NETs), and missing scenarios) (Chapter 3). Second, comparing projected and historical CO2 emissions shows that global historical emissions followed a medium-high emissions pathway well within scenario ranges of scenarios (Chapter 4). It may not automatically be the case in the future. Thus, several aspects, such as regional scenarios and consumption emissions, need further assessment (Chapter 5). Regarding salience, the UNFCCC delegate survey results demonstrate that policymakers focus on understanding and using the scenario tools for negotiations and national policy (Chapter 6), contrasting the scientific focus on the quality of assumptions, processes, approaches, and methods (Chapter 3). Thus, there is space for increasing scenario legitimacy concerning scenario narratives, e.g., reconsidering the divergency of values and cultural beliefs between policymaker-researcher and global North-South perspectives. In the larger perspective, the results show that for the upcoming COP28 and successive COPs, the scientific community developing scenarios may increase the usability of scenarios in policymaking in two ways: By providing a more detailed understanding of policymaker needs and closing the knowledge gaps between the high- and low- income regions. This also comprises increasing research in the low-middle-income regions and setting up scenario training and modeling institutions (capacity building).
