Asymmetry in the climate-carbon cycle response to positive and negative CO2 emissions

The majority of emissions scenarios that limit warming to 2°C, and nearly all emission scenarios that do not exceed 1.5°C warming by the year 2100 require negative CO₂ emissions. Negative emission technologies (NETs) in these scenarios are required to offset emissions from sectors that are difficult or costly to decarbonize and to generate global ‘net negative’ emissions, allowing to compensate for earlier emissions and to recover a carbon budget after overshoot. It is commonly assumed that the carbon cycle and climate response to a negative CO₂emission is equal in magnitude and opposite in sign to the response to an equivalent positive CO₂ emission, i.e. that the climate-carbon cycle response is symmetric. This assumption, however, has not been tested for a range of emissions. Here we explore the symmetry in the climate-carbon cycle response by forcing an Earth system model with positive and negative CO₂emission pulses of varying magnitude and applied from different climate states. Our results suggest that an emission of CO₂into the atmosphere is more effective at raising atmospheric CO₂than a CO₂removal is at lowering atmospheric CO₂, indicating that the carbon cycleresponse is asymmetric, particularly for emissions/removals > 100 GtC. The surface air temperature response, on the other hand, is largely symmetric. Our findings suggest that the emission and subsequent removal of a given amount of CO₂ would not result in the same atmospheric CO₂concentration as if the emission were avoided. Furthermore, our results imply using simple models used to estimate negative emission requirements may result in underestimating the amount of negative emissions needed to attain a given CO₂concentration target.