Capturing climate actions in long-term scenarios: Integrated assessment modelling of state policies and non-state actions implemented to limit global temperature increase to 2 or 1.5 degrees Celsius

Thirty years ago, countries officially agreed to the global objective to stabilise greenhouse gas concentrations at a level that would avoid dangerous anthropogenic interference with the climate system. Since then, several attempts have been made to translate this into more concrete goals and targets, including binding country reduction targets (e.g. Kyoto and Copenhagen), mostly resulting in failure. In 2015, however, the Paris Agreement formulated a global goal to keep global temperature increase well below 2oC and pursue efforts to keep it below 1.5oC above pre-industrial levels. Ultimately, climate policies need to be implemented at the national level. This thesis examines how national actions can be aligned with global goals and routes to enhance ambition and implementation. First, it shows an ambition gap between Nationally Determined Contributions (NDCs) to the Paris Agreement and pathways compatible with the cost-optimal implementation of the global Paris goals. Second, it also indicates an implementation gap where current implemented policies at the national level are not even achieving the NDCs by 2030. To stay on track for achieving the Paris goals, total greenhouse gas emissions need to decrease by 35-45% relative to current policy implementation by 2030. Two routes were explored that could enhance ambition and implementation. One possible route is to enhance action from non-state actors such as cities, regions and businesses. They collectively pledge ambitious reduction targets in international cooperative initiatives, but individual actions to make these ambition concrete seem less strong. The contribution of these actions is estimated to decrease emissions between 1-2 GtCO2eq by 2030 relative to current policies. However, assessing the combined impact of national and non-state action is difficult as it depends on assumptions concerning overlap and the additionality of climate actions. Another route is to increase ambition by enhancing policy learning and replicate successful sectoral policies in different parts of the world. If nine successful sector policies could be replicated globally, this could half the emissions gap between current policies and the Paris goals by 2030. These examples include for example fuel efficiency standards and building codes. The presented results were obtained with scenarios created using integrated assessment models. Since the Paris Agreement, the main focus of climate policy assessments with these models has been changing from ‘where do we go’ to ‘how do we get there’. Including explicit representation of government policies and non-state climate actions in the models is the first step in this process. To support these assessments, a climate policy framework was developed that maps different stages from the climate policy cycle to specific scenarios with increasing policy stringency. In the current phase of climate policy, it is important to regularly update the policy scenario results to support the climate negotiations and to further include insights on policy implementation from social sciences such as sociology, psychology and political science