Quantifying the transient climate response to carbon dioxide removal

As anthropogenic greenhouse emissions continue to rise, limiting warming to 1.5°C has become elusive. Emissions pathways seeking to return to 1.5°C after overshoot will therefore require net negative emissions. A crucial question in this context is how much CO₂ needs to be removed from the atmosphere to achieve a given amount of cooling (say 0.1°C). Studies seeking to answer this question often resort to the Transient Climate Response to Emissions (TCRE), a measure of the warming effect of cumulative CO₂ emissions, neglecting that the climate may respond asymmetrically to CO₂ emissions and removals. This contribution draws on CDRMIP pulse CO₂ removal simulations to quantify the temperature response to CO₂ emissions and removals in a range of Earth system models of full and intermediate complexity. We find that the temperature response to an equivalent amount of CO2 emissions and removals differs in magnitude, with the sign of this difference being model dependent. We investigate the cause for these inter-model differences by quantifying the contribution of carbon cycle and physical climate response differences to the overall temperature asymmetry. Establishing a robust metric of the transient climate response to CO2 removal is key to our understanding of how climate will respond to net negative emissions and to quantifying the amount of removal needed to restore a given temperature target.