No abstract

Mitigation scenarios that achieve the ambitious targets included in the Paris Agreement typically rely on greenhouse gas emission reductions combined with net carbon dioxide removal (CDR) from the atmosphere, mostly accomplished through large-scale application of bioenergy with carbon capture and storage, and afforestation. However, CDR strategies face several difficulties such as reliance on underground CO 2 storage and competition for land with food production and biodiversity protection. The question arises whether alternative deep mitigation pathways exist. Here, using an integrated assessment model, we explore the impact of alternative pathways that include lifestyle change, additional reduction of non-CO 2 greenhouse gases and more rapid electrification of energy demand based on renewable energy. Although these alternatives also face specific difficulties, they are found to significantly reduce the need for CDR, but not fully eliminate it. The alternatives offer a means to diversify transition pathways to meet the Paris Agreement targets, while simultaneously benefiting other sustainability goals.

The Paris Agreement-which is aimed at holding global warming well below 2 °C while pursuing efforts to limit it below 1.5 °C-has initiated a bottom-up process of iteratively updating nationally determined contributions to reach these longterm goals. Achieving these goals implies a tight limit on cumulative net CO 2 emissions, of which residual CO 2 emissions from fossil fuels are the greatest impediment. Here, using an ensemble of seven integrated assessment models (IAMs), we explore the determinants of these residual emissions, focusing on sector-level contributions. Even when strengthened pre-2030 mitigation action is combined with very stringent long-term policies, cumulative residual CO 2 emissions from fossil fuels remain at 850-1,150 GtCO 2 during 2016-2100, despite carbon prices of US$130-420 per tCO 2 by 2030. Thus, 640-950 GtCO 2 removal is required for a likely chance of limiting end-of-century warming to 1.5 °C. In the absence of strengthened pre-2030 pledges, long-term CO 2 commitments are increased by 160-330 GtCO 2 , further jeopardizing achievement of the 1.5 °C goal and increasing dependence on CO 2 removal.