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We explore the feasibility of limiting global warming to 1.5°C without overshoot and without the deployment of carbon dioxide removal (CDR) technologies. For this purpose, we perform a sensitivity analysis of four generic emissions reduction measures to identify a lower bound on future CO2 emissions from fossil fuel combustion and industrial processes. Final energy demand reductions and electrification of energy end uses as well as decarbonization of electricity and non-electric energy supply are all considered. We find the lower bound of cumulative fossil fuel and industry CO2 emissions to be 570 GtCO2 for the period 2016–2100, around 250 GtCO2 lower than the lower end of available 1.5°C mitigation pathways generated with integrated assessment models. Estimates of 1.5°C-consistent CO2 budgets are highly uncertain and range between 100 and 900 GtCO2 from 2016 onwards. Based on our sensitivity analysis, limiting warming to 1.5°C will require CDR or terrestrial net carbon uptake if 1.5°C-consistent budgets are smaller than 650 GtCO2. The earlier CDR is deployed, the more it neutralizes post-2020 emissions rather than producing net negative emissions. Nevertheless, if the 1.5°C budget is smaller than 550 GtCO2, temporary overshoot of the 1.5°C limit becomes unavoidable if CDR cannot be ramped up faster than to 4 GtCO2 in 2040 and 10 GtCO2 in 2050.
This article is part of the theme issue ‘The Paris Agreement: understanding the physical and social challenges for a warming world of 1.5°C above pre-industrial levels’.

Conclusion. The answer to the question whether it is still possible to limit warming to 1.5°C without overshoot and CDR depends strongly on the remaining 1.5°C budget from 2016 onwards. For a 1.5°C CO2 budget up to 550 GtCO2, overshoot will be inevitable; CDR will be required to return to the 1.5°C limit if the limiting cases formulated in our analysis hold. For budgets between 550 and 650 GtCO2, we find CDR trajectories that allow to stay below 1.5°C without overshoot in the steepest FFI CO2 emissions reduction cases. For budgets of 650 GtCO2 and higher, the steepest emissions reduction cases are sufficient to limit warming to 1.5°C without CDR deployment. However, these steepest cases are based on limiting cases for carbon intensity improvements of electricity and non-electric energy supply, electrification of energy end use, final energy demand reductions, and CDR deployment. They are designed to describe outer bounds beyond which developments are very unlikely. But they themselves may also be unlikely to obtain, with the exception of power sector decarbonization. Thus the passage to limiting warming to 1.5°C can further narrow if socio-economic, political and sustainable development considerations are taken into account. The benchmark values provided by the limiting cases will be a useful device to connect to bottom-up assessments of deep decarbonization pathways in individual sectors. Limiting warming to 1.5°C is an enormous challenge. To tackle this challenge, every tonne of CO2 that is not emitted into the atmosphere counts. In the scenarios analysed here 200 GtCO2, a third of the CO2 budget in the limiting case, are already used up until 2020. This calls for a parallel approach to strengthen action as quickly as possible and at the same time invest in the development of critical mitigation options like carbon-neutral liquids and radical energy efficiency measures that will be needed to reach carbon neutrality.

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