Do State-Of-The-Art Atmospheric CO2 Inverse Models Capture Drought Impacts on the European Land Carbon Uptake?

The European land carbon uptake has been heavily impacted by several recent severe droughts, yet quantitative estimates of carbon uptake anomalies are uncertain. Atmospheric CO2 inverse models (AIMs) provide observation-based estimates of the large-scale carbon flux dynamics, but how well they capture drought impacts on the terrestrial carbon uptake is poorly known. Here we assessed the capacity of state-of-the-art AIMs in monitoring drought impacts on the European carbon uptake over 2001-2015 using observations of environmental variability and vegetation function and made comparisons with bottom-up estimates of carbon uptake anomalies. We found that global inversions with only limited surface CO2 observations give divergent estimates of drought impacts. Regional inversions assimilating denser CO2 observations over Europe demonstrated some improved consistency, with all inversions capturing a reduction in carbon uptake during the 2012 drought. However, they failed to capture the reduction caused by the 2015 drought. Finally, we found that a set of inversions that assimilated satellite XCO2 or assimilated environmental variables plus surface CO2 observations better captured carbon uptake anomalies induced by both the 2012 and 2015 droughts. In addition, the recent Orbiting Carbon Observatory-2 XCO2 inversions showed good potential in capturing drought impacts, with better performances for larger-scale droughts like the 2018 drought. These results suggest that surface CO2 observations may still be too sparse to fully capture the impact of drought on the carbon cycle at subcontinental scales over Europe, and satellite XCO2 and ancillary environmental data can be used to improve observational constraints in atmospheric inversion systems. Plain Language Summary Atmospheric CO2 inverse models (AIMs) are useful tools for quantifying the response of large-scale carbon uptake to climate extremes, but their capacity for monitoring drought impacts, particularly at regional scales, is not fully explored. In this study, we assessed the capacity of state-of-the-art AIMs for monitoring drought impacts on the European land carbon uptake over 2001-2015 using a large array of observational and model data sets. We found: (a) global inversions with only limited surface CO2 observations face a great challenge in monitoring drought impacts on the European carbon uptake; (b) Regional inversions assimilated denser CO2 observations over Europe, for the EUROCOM project, demonstrated some improved consistency but are still deficient, showing divergent estimates in interannual variability of carbon uptake for most years; and (c) A set of inversion systems that assimilated satellite XCO2 or assimilated environmental variables plus surface CO2 observations better captured annual and seasonal anomalies caused by droughts. Our study demonstrates that surface CO2 observations may still be too sparse to fully capture the impact of drought on the carbon cycle at subcontinental scales over Europe, whereby satellite XCO2 and ancillary environmental data can offer observational constraints for improving the estimates.

Automated summary

The capacity of atmospheric CO2 inverse models (AIMs) to monitor drought impacts on the European carbon uptake is uncertain. Global inversions with limited surface CO2 observations give divergent estimates of drought impacts. Regional inversions assimilating denser CO2 observations and satellite XCO2 or assimilated environmental variables plus surface CO2 observations better capture carbon uptake anomalies induced by droughts. Surface CO2 observations may still be too sparse, and satellite XCO2 and ancillary environmental data can improve observational constraints in atmospheric inversion systems.