Who is in the driver’s seat of IPCC evidence for mitigation policy? Exploring the political influence of Integrated Assessment Model Intercomparison Projects

Many boundary organizations operate on the science-policy interface for climate change, none more influential than the Intergovernmental Panel on Climate Change (IPCC). Widely considered the pinnacle of evidence for mitigation policy, the assessment work done by Workgroup 3 (WG3) is largely shaped by Integrated Assessment Model Intercomparison Projects (IA MIPs). However, only a limited number of models (and modelers) participate in the IA MIPs, and the participating models are criticized for their 'black-box' character, making them inaccessible for scrutiny by outside parties. We study the influence of IA MIPs on IPCC evidence for policy by reviewing all six IPCC WG3 Assessment Report (AR) Summaries for Policymakers (SPMs). The results show that half of the content of the latest SPMs is based on evidence from IA MIPs and the share has increased historically. We compare the thematic topics of evidence based on IA MIPs with the rest of the evidence in the SPMs and corresponding ARs. All in all, our findings imply a disproportionate political power distribution for evidence generation for the SPMs stemming from the strong role of IA MIPs for IPCC WG3. In other words, we show that IA MIP modelers have a disproportionately large influence on climate change mitigation evidence, and thus a strong influence on climate change policy. This is problematic not least because the evidence is created by a smaller number of people than one would anticipate given the IPCC’s mission to provide a comprehensive summary of all available scientific knowledge. But it is also problematic because of the value-laden models' assumptions, which are difficult to communicate and challenge. Finally, by comparing the thematic topics of the evidence, we also show the topics of the remaining evidence and discuss possible reasons underlying its limited political influence.