Evidence synthesis methods to enhance climate policy evaluation for IPCC assessments

Climate policy research has greatly accelerated in recent years, resulting in the publication of tens of thousands of policy evaluations. With immense efforts, the Intergovernmental Panel on Climate Change (IPCC) assesses this research, aiming to provide policymakers with the best available evidence to assist the choices and designs of climate policies. Yet, in the wider policy evaluation community, evidence synthesis practices, which could support the existing process, are largely missing.

In this thesis, I adopt and refine three evidence synthesis methods from other scientific disciplines to test their applicability and showcase their usefulness for the systematic assessment of the state of knowledge on climate policy questions, specifically on the empirical evidence on carbon pricing. I map out the available evidence on carbon pricing policy evaluations, assessing the state of research and identifying research gaps; conduct a systematic review and meta-analysis on the effectiveness of carbon pricing policies in reducing greenhouse gas emissions; and assess under what conditions and why carbon pricing is or is not effective using a realist synthesis method.

The evidence syntheses conducted here reveal a number of policy relevant research findings that have been missed by previous IPCC reports. The mapping identifies more than 4,000 evaluations of carbon pricing policies, predominantly focused on only few policy schemes, with considerable research gaps for many other schemes. The meta-analysis shows that carbon pricing has effectively reduced greenhouse gas emissions in at least 17 out of 21 reviewed carbon pricing schemes, with reductions of between -5% and -21%. The realist synthesis captures nine causal mechanisms on how carbon pricing triggers emissions reductions, with varying relevance across sectors and country contexts.

A more widespread application of evidence synthesis methods to address climate policy questions could greatly support the IPCC in providing a meaningful and comprehensive assessment of the climate policy options.