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.
Highlights
Substantial year-to-year variations in the growth rate of global atmospheric CO2 concentrations show variations that cannot be explained by land-use changes, fossil fuel emissions or the increase of carbon sink capacities due to increasing atmospheric CO2 concentrations (Friedlingstein et al 2019, Peters et al 2017)
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
In the context of potential future CO2 emission reductions, we ask whether atmospheric CO2 growth trend reductions in the near term can be attributed to a policy change
Summary
Substantial year-to-year variations in the growth rate of global atmospheric CO2 concentrations show variations that cannot be explained by land-use changes, fossil fuel emissions or the increase of carbon sink capacities due to increasing atmospheric CO2 concentrations (Friedlingstein et al 2019, Peters et al 2017). The variations of the tropical land carbon sink is dominated by the El Niño-Southern Oscillation (Jones et al 2001, Zeng et al 2005), and the pronounced Southern Ocean carbon sink is susceptible to changes in atmospheric circulation patterns (Landschützer et al 2015, McKinley et al 2017) This internal variability of the global carbon cycle in atmospheric CO2 may disguise the detection of potential CO2 emission reductions in atmospheric CO2 observations. We ask what the probability is that a slowdown in atmospheric CO2 growth is attributable to a policy change implementing CO2 emission reductions as the difference between Representative Concentration Pathway (RCP) 4.5 and RCP2.6, in the face of internal climate variability. We ask after how many years this policy change will cause atmospheric CO2 growth rates to slow down for certain
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