Comparative assessment of potential impacts, side-effects and uncertainties of CE measures and emission-reduction efforts (ComparCE-2)

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The main objective of the ComparCE project is a comprehensive assessment of the various Climate Engineering (CE) measures and emission-reduction efforts, taking into account uncertainties in the earth system model simulations on which the assessment is based. We want to address various urgent questions, that we feel are missing in the context of CE assessment so far. As a first and central step it will be investigated which assessment metrics and indicators are of importance in the CE context, and thus to the entire priority program, and how they differ from climate change assessment metrics. There has been research in this field for climate change but for the CE debate this assessment is missing. From exchanges with international CE research groups, it became clear, that a future deployment of CE would likely consist of a combination of different CE measures. Therefore in this project we address the question, of how the Earth system will react to a combination of CE measures and whether the distinct signals can still be attributed to a certain technique. In this context it will be addressed how the effectiveness of a CE measure depends on the background climate state, and how e.g. the timing of a deployment might be of relevance. Furthermore, robust patterns of changes due to CE will be investigated in order to enable regional analysis of CE impacts, as well as detection and attribution of signals of regional CE. This is of special importance since the regional climate extremes are of relevance for the local communities. This analysis will also inform the metrics finding process. Additionally, termination of CE will be addressed in the context of the rates of changes in the earth system to inform biological impact assessment studies. Finally, since the only tools to assess climatic responses to CE techniques are models, we feel that an important contribution to the CE debate is a thorough assessment of key model uncertainties. These will be quantified determining changes in the probability density functions of metrics e.g. the probability of passing a certain threshold given different future scenarios. Results of this project will provide a comprehensive assessment of joint mitigation-CE scenarios under uncertainty of models, scenarios and metrics, which will be discussed iteratively with partner projects in the SPP.