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Representation of Unaccounted Physical Couplings in Peak-shaving Solar Radiation Modification Scenarios 

Solar Radiation Modification (SRM) is a proposed method to halt global warming and related impacts. This method is gaining interest in the climate change community as a potential supplement to conventional mitigation (emission reduction and carbon dioxide removal) to avoid surpassing a given temperature threshold in an overshoot pathway. In this so-called peak-shaving framework, SRM would lower global mean temperature to below the specified threshold during the otherwise occurring overshoot until mitigation has sufficiently brought down atmospheric CO2. At present, the peak-shaving framework assumes that SRM can be added independently to conventional mitigation. This additivity assumption disregards potential interlinkages between the critical components of overshoot pathways, emission reductions and net-negative emissions, and an SRM intervention. The aim of this thesis is to assess the importance of these currently unaccounted physical couplings between mitigation and SRM. It is demonstrated that the range of uncertainty in future emission reductions and net-negative emissions leads to a wide spectrum of different SRM deployment trajectories, highlighting the uncertainty currently implied by the peak-shaving framework and potential implications of non-additivity. The additivity assumption of SRM and mitigation is subsequently scrutinized by examining the impact of SRM on the practicality of decarbonization with renewable energy and the change in negative emission burden under SRM due to carbon feedback modification. I find that the potential to reduce emissions with wind and solar renewable energy sources may become more challenging with SRM. However, the model simulates temporarily enhanced land and ocean carbon sinks, which imply that the negative emission burden could be reduced during the upscaling of SRM deployment which may somewhat compensate for the reduced decarbonization potential. Nevertheless, this carbon uptake benefit is temporary and turns into an additional burden during later stages of SRM deployment. The results of this thesis therefore suggest that the additivity assumption does not hold in terms of physical impacts of SRM on mitigation, since the deployment of it can significantly change the underlying emissions trajectory. This provides a step away from the highly idealized concept of the current peak-shaving framework towards a more comprehensive outlook on the uncertainties implied by such a deployment. This is important because the certainty that peak-shaving SRM is predicated on could be misleading and needs to be taken into account when moving towards more integrated assessments of SRM. The current landscape of models for integrated climate policy scenarios is not well suited for this purpose due to a lack of direct feedback from the climate impact of SRM to the underlying mitigation trajectory. This thesis highlights how this missing feedback between models is a major limitation in SRM simulations and a barrier to comprehensive SRM scenario assessments. 

Jury

M. Christopher Smith	University of Leeds	Rapporteur
M. Olivier Boucher	CNRS/Institut Pierre-Simon Laplace	Rapporteur
Mme Helene Muri	NTNU: Norwegian University of Science and Technology	Examinatrice
Mme Ines Camilloni	University of Buenos Aires	Examinatrice
M. Laurent Terray	CERFACS	Directeur de Thèse
M. Roland Séférian	CNRM (Université de Toulouse, Météo-France, CNRS)	Co-directeur de Thèse
M. Benjamin M. Sanderson	CICERO	Co-encadrant de thèse