Negative emissions technologies in energy system models and mitigation scenarios - a systematic review

Achieving global commitments on climate mitigation necessitates a rapid transition of the global energy system away from fossil fuels. Modelled scenarios within Integrated Assessment Models (IAMs) and Energy Systems Models (ESMs) indicate that limiting global temperature rise to safe levels will require some dependence on negative emissions technologies (NETs). However, the representation of NETs varies significantly across models, leading to differences in their roles across mitigation scenarios. Bioenergy with carbon capture and storage (BECCS) and direct air carbon capture and storage (DACCS) are two possible solutions that are directly related to the energy system. Here, we perform a systematic review of the representation of BECCS and DACCS within ESMs and IAMs, exploring their roles in decarbonisation scenarios and identifying how NETs contribute to energy system transition pathways. In terms of model structure, we examine the sensitive factors like discount rate, and limitations of modelling NETs in ESMs and provide insights for best practices. The results show that the availability of BECCS and DACCS can facilitate the transformation of energy systems towards low- or net-zero emissions and reduce the marginal abatement cost (MAC) of CO2 for achieving ambitious climate targets in modelled scenarios. When NETs are not available, a more rapid and deep emissions reduction is required, including a larger-scale deployment of renewables and earlier phaseout of fossil fuels. More significant changes in energy demand, such as energy efficiency improvements, electrification of end-use sectors and lowering energy services demand, can reduce the dependency on NETs. In addition, although NETs deployment helps to reduce the energy system mitigation costs, sticking to technically feasible pathways also presents challenges as MACs remain high, and relying heavily on these technologies may result in risks, such as temperature overshoot and fossil fuel lock-in.