Atmospheric inversions with more CO2 and ancillary observations better captured drought impacts on the European carbon uptake

Several severe droughts have hit Europe in recent years, impacting the terrestrial carbon uptake of this region. Yet quantitative estimates of carbon uptake anomalies are uncertain. Atmospheric CO2 inversion models (AIMs) provide observation-based estimates of the largescale carbon fluxes and their spatiotemporal variations, but their capacity for capturing drought impacts on the terrestrial carbon uptake is poorly known. Here we assessed the capacity of state-of-the-art AIMs for monitoring drought impacts on the European terrestrial carbon uptake over 2001–2015 using observations of environmental variability, measurements of vegetation function, and model estimates of carbon uptake anomalies. We found that classic global inversions with only surface CO2 observations give divergent estimates of drought impacts on the European carbon uptake. A set of regional inversions assimilating denser CO2 observations over Europe, for the EUROCOM project, demonstrated some improved consistency, with all inversions capturing a reduction in carbon uptake during the 2012 European drought. However, they show divergent estimates in interannual variability of carbon uptake for most years. Finally, we found that a set of global inversion systems that assimilate satellite-based XCO2 or environmental variables in addition to surface CO2 observations better captured annual and seasonal anomalies of the European carbon uptake induced by regional droughts in both 2012 and 2015. 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, for which increasing CO2 observations and incorporating ancillary data, typically space-based, are promising avenues.