Description: As a means of countering climate change, some scientists have proposed that climate engineering, which is a deliberate action designed to alter the Earth’s climate, could be done. In this chapter an overview is given of the proposed climate engineering methods that involve the direct manipulation of marine systems. This includes methods that enhance the ocean’s natural physical, chemical, and biological CO2 sequestration pathways, as well as purely technical ones that either use the ocean as a carbon storage reservoir or alter it’s properties to affect the Earth’s radiation budget. Few methods have been thoroughly evaluated and there are still many unknowns, at both the level of basic understanding and as to whether or not it would even be technologically feasible to implement any of them. Research so far has shown that some CE methods do have the potential to alter certain aspects of the climate system. Some have more potential than others and most of them appear to have significant side effects.

Description: Temperature targets of the Paris Agreement limit global net cumulative emissions to very tight carbon budgets. The possibility to overshoot the budget and offset near-term excess emissions by net-negative emissions is considered economically attractive as it eases near-term mitigation pressure. While potential side effects of carbon removal deployment are discussed extensively, the additional climate risks and the impacts and damages have attracted less attention. We link six models for an integrative analysis of the climatic, environmental and socio-economic consequences of temporarily overshooting a carbon budget consistent with the 1.5 °C temperature target along the cause-effect chain from emissions and carbon removals to climate risks and impact. Global climatic indicators such as CO2-concentration and mean temperature closely follow the carbon budget overshoot with mid-century peaks of 50 ppmv and 0.35 °C, respectively. Our findings highlight that investigating overshoot scenarios requires temporally and spatially differentiated analysis of climate, environmental and socioeconomic systems. We find persistent and spatially heterogeneous differences in the distribution of carbon across various pools, ocean heat content, sea-level rise as well as economic damages. Moreover, we find that key impacts, including degradation of marine ecosystem, heat wave exposure and economic damages, are more severe in equatorial areas than in higher latitudes, although absolute temperature changes being stronger in higher latitudes. The detrimental effects of a 1.5 °C warming and the additional effects due to overshoots are strongest in non-OECD countries (Organization for Economic Cooperation and Development). Constraining the overshoot inflates CO2 prices, thus shifting carbon removal towards early afforestation while reducing the total cumulative deployment only slightly, while mitigation costs increase sharply in developing countries. Thus, scenarios with carbon budget overshoots can reverse global mean temperature increase but imply more persistent and geographically heterogeneous impacts. Overall, the decision about overshooting implies more severe trade-offs between mitigation and impacts in developing countries.