Inherent uncertainty disguises attribution of reduced atmospheric CO2 growth to CO2 emission reductions for up to a decade

Abstract

The growth rate of atmospheric CO _2 on inter-annual time scales is largely controlled by the response of the land and ocean carbon sinks to climate variability. Therefore, the effect of CO _2 emission reductions to achieve the Paris Agreement on atmospheric CO _2 concentrations may be disguised by internal variability, and the attribution of a reduction in atmospheric CO _2 growth rate to CO _2 emission reductions induced by a policy change is unclear for the near term. We use 100 single-model simulations and interpret CO _2 emission reductions starting in 2020 as a policy change from scenario Representative Concentration Pathway (RCP) 4.5 to 2.6 in a comprehensive causal theory framework. Five-year CO _2 concentration trends grow stronger in 2021–2025 after CO _2 emission reductions than over 2016–2020 in 30% of all realizations in RCP2.6 compared to 52% in RCP4.5 without CO _2 emission reductions. This implies that CO _2 emission reductions are sufficient by 42%, necessary by 31% and both necessary and sufficient by 22% to cause reduced atmospheric CO _2 trends. In the near term, these probabilities are far from certain. Certainty implying sufficient or necessary causation is only reached after, respectively, ten and sixteen years. Assessments of the efficacy of CO _2 emission reductions in the near term are incomplete without quantitatively considering internal variability.

Keywords

• internal variability
• atmospheric CO2
• fossil-fuel emission reductions
• climate policy
• uncertainty
• Paris Agreement