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

MPS-Authors
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Spring,  Aaron
IMPRS on Earth System Modelling, MPI for Meteorology, Max Planck Society;
Ocean Biogeochemistry, The Ocean in the Earth System, MPI for Meteorology, Max Planck Society;

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Ilyina,  Tatiana
Ocean Biogeochemistry, The Ocean in the Earth System, MPI for Meteorology, Max Planck Society;

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Marotzke,  Jochem
Director’s Research Group OES, The Ocean in the Earth System, MPI for Meteorology, Max Planck Society;

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Citation

Spring, A., Ilyina, T., & Marotzke, J. (2020). Inherent uncertainty disguises attribution of reduced atmospheric CO2 growth to CO2 emission reductions for up to a decade. Environmental Research Letters, 15: 114058. doi:10.1088/1748-9326/abc443.


Cite as: http://hdl.handle.net/21.11116/0000-0005-467C-2
Abstract
The growth rate of atmospheric CO2 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 CO2 emission reductions to achieve the Paris Agreement on atmospheric CO2 concentrations may be disguised by internal variability, and the attribution of a reduction in atmospheric CO2 growth rate to CO2 emission reductions induced by a policy change is unclear for the near term. We use 100 single-model simulations and interpret CO2 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 CO2 concentration trends grow stronger in 2021-2025 after CO2 emission reductions than over 2016-2020 in 30% of all realizations in RCP2.6 compared to 52% in RCP4.5 without CO2 emission reductions. This implies that CO2 emission reductions are sufficient by 42%, necessary by 31% and both necessary and sufficient by 22% to cause reduced atmospheric CO2 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 CO2 emission reductions in the near term are incomplete without quantitatively considering internal variability