08 Apr 2022
08 Apr 2022
Status: this preprint is currently under review for the journal BG.

Controls of intermodel uncertainty in land carbon sink projections

Ryan S. Padrón1, Lukas Gudmundsson1, Laibao Liu1, Vincent Humphrey1,2, and Sonia I. Seneviratne1 Ryan S. Padrón et al.
  • 1Institute for Atmospheric and Climate Science, Department of Environmental Systems Science, ETH Zurich, Zurich, 8092, Switzerland
  • 2Department of Geography, University of Zurich, Zurich, Switzerland

Abstract. Over the last decades, land ecosystems removed from the atmosphere approximately one third of anthropogenic carbon emissions, highlighting the importance of the evolution of the land carbon sink for projected climate change. Nevertheless, the latest cumulative land carbon sink projections from eleven Earth system models participating in the Coupled Model Intercomparison Project Phase 6 (CMIP6) show large differences, even for a policy-relevant scenario with mean global warming by the end of the century below 2 °C relative to preindustrial conditions. We hypothesize that this intermodel uncertainty originates from model differences in the sensitivities of net biome production (NBP) to (i) atmospheric CO2 concentration, (ii) air temperature and (iii) soil moisture, as well as model differences in average conditions of (iv) air temperature and (v) soil moisture. Using multiple linear regression and a resampling technique, we quantify the individual contributions of these five terms for explaining the cumulative NBP anomaly of each model relative to the multi-model mean. Results indicate a primary role of the response of NBP to interannual temperature and soil moisture variability, followed by the sensitivity to CO2, and lastly by the average climate conditions, which also show sizeable contributions. We find that the sensitivities of NBP to temperature and soil moisture, particularly in the tropics, dominantly explain the deviations from the ensemble mean of the two models with the lowest carbon sink (ACCESS-ESM1-5 and UKESM1-0-LL) and of the two models with the highest sink (CESM2 and NorESM2-LM). Overall, this study advances our understanding of why land carbon sink projections from Earth system models differ globally and across regions, which can guide efforts to reduce the underlying uncertainties.

Ryan S. Padrón et al.

Status: open (until 29 May 2022)

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  • RC1: 'Comment on bg-2022-92', Anonymous Referee #1, 26 May 2022 reply

Ryan S. Padrón et al.

Ryan S. Padrón et al.


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Short summary
The answer to how much carbon land ecosystems are projected to remove from the atmosphere until 2100 is different for each Earth system model. We find that differences across models are primarily explained by the annual land carbon sink dependence on temperature and soil moisture, followed by the dependence on CO2 air concentration, and by average climate conditions. Our insights on why each model projects a relatively high or low land carbon sink can help to reduce the underlying uncertainty.