Marko Scholze

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.