An integrated approach to quantifying uncertainties in the remaining carbon budget

Abstract The remaining carbon budget quantifies the future CO 2 emissions to limit global warming below a desired level. Carbon budgets are subject to uncertainty in the Transient Climate Response to Cumulative CO 2 Emissions (TCRE), as well as to non-CO 2 climate influences. Here we estimate the TCRE using observational constraints, and integrate the geophysical and socioeconomic uncertainties affecting the distribution of the remaining carbon budget. We estimate a median TCRE of 0.44 °C and 5–95% range of 0.32–0.62 °C per 1000 GtCO 2 emitted. Considering only geophysical uncertainties, our median estimate of the 1.5 °C remaining carbon budget is 440 GtCO 2 from 2020 onwards, with a range of 230–670 GtCO 2 , (for a 67–33% chance of not exceeding the target). Additional socioeconomic uncertainty related to human decisions regarding future non-CO 2 emissions scenarios can further shift the median 1.5 °C remaining carbon budget by ±170 GtCO 2 .