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There are several important factors that influence the size of the carbon budgets that are consistent with the ambition of the Paris Climate Agreement ‘to limit global temperature increase to well below 2 °C above pre-industrial levels, and to pursue efforts to limit this increase even further to 1.5 °C’. Some of these factors concern scientific uncertainties (e.g. limitations on the understanding of the climate system), while others are policy choices (e.g. the likelihood with which the target should be achieved). However, under all assumptions and policy choices, negative CO₂ emissions seem crucial to achieve the 1.5 °C target.

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Rapid emission reductions needed

Scenarios show that, based on an assessment of technological potential, there are pathways that would still achieve the climate goals. These scenarios assume that it is possible to implement climate policies in most of the regions concerned, leading to a peak in global emissions within the next decade, followed by rapid reductions.

Median value for each category in the IPCC AR5 WGIII scenario database for CO₂ emissions (panel a), the increase in global mean temperature (panel b) and the probability of staying below 1.5 °C (panel c) for all scenarios included in the AR5 WGIII Scenario database.

The need for negative CO₂ emissions

Although the reliance on negative CO₂ emissions from the use of bio-energy combined with carbon capture and storage (BECCS) could be reduced, it will be very difficult to achieve the 1.5 °C target without negative CO₂ emissions. In reality, only relatively small investments are being made in BECCS technologies, and there are concerns about the implications for food production and biodiversity protection, in relation to their large-scale use. As the support for these technologies, currently, is low and experience in large-scale application is lacking, it is important to explicitly discuss the feasibility of these pathways and the possibility of setting up research and development programmes to increase our experience.