Quantifying non-CO₂ contributions to remaining carbon budgets

The IPCC Special Report on 1.5 °C concluded that anthropogenic global warming is determined by cumulative anthropogenic CO₂ emissions and the non-CO₂ radiative forcing level in the decades prior to peak warming. We quantify this using CO₂-forcing-equivalent (CO₂-fe) emissions. We produce an observationally constrained estimate of the Transient Climate Response to cumulative carbon Emissions (TCRE), giving a 90% confidence interval of 0.26–0.78 °C/TtCO₂, implying a remaining total CO₂-fe budget from 2020 to 1.5 °C of 350–1040 GtCO₂-fe, where non-CO₂ forcing changes take up 50 to 300 GtCO₂-fe. Using a central non-CO₂ forcing estimate, the remaining CO₂ budgets are 640, 545, 455 GtCO₂ for a 33, 50 or 66% chance of limiting warming to 1.5 °C. We discuss the impact of GMST revisions and the contribution of non-CO₂ mitigation to remaining budgets, determining that reporting budgets in CO₂-fe for alternative definitions of GMST, displaying CO₂ and non-CO₂ contributions using a two-dimensional presentation, offers the most transparent approach.