Toward an observational constraints on the Transient Climate Response to Cumulative Emission

The relationship between CO2-induced warming and global mean temperature, known as the Transient Climate Response (TCR) to cumulative CO2 emissions (TCRE), is anemergent property of the Earth system. It allows to derive allowable CO2 emissions, or carbon budget, for a given anthropogenic warming threshold, such as the Paris Agreement warming target. The assessment of the TCRE in IPCC AR6 makes use of the theoretical framework as proposed by Jones and Friedlingstein (2020), which separate TCRE in two major drivers: the Transient Climate Response (TCR) and the airborne fraction (AF) of anthropogenic CO2 emissions. While published works have allowed to account for a constrained TCR range in the assessment of the TCRE, estimated of AF results only from unconstrained multi-model outputs.

The present work applies a novel methodology based on Bayesian statistics to integrate multiple lines of historical evidences to constrain future AF. Bayesinas statistics allows to exploits the time-varying relationship between the total anthropogenic emissions of CO2 and AF over the historical period and propagate this relationship in the future to constrain the AF range at CO2 doubling. The narrower very likely range for AF at CO2 doubling 41-59% (50% as Best estimates) results in a constrained very likely range for the TCRE, 1-2.1 K EgC-1 (1.5 as Best estimates). This constrained range is about 20% smaller than the latest assessed range for the TCRE and shines light on how novel observations and monitoring of anthropogenic emissions and airborne fraction of CO2 could results in even stronger constrain on TCRE estimates in the near future.