Net ecosystem exchange (NEE) estimates 2006-2019 over Europe from a pre-operational ensemble-inversion system

Three-hourly net ecosystem exchange (NEE) is estimated at spatial scales of 0.25 degrees over the European continent, based on the pre-operational inverse modelling framework CarboScope Regional (CSR) for the years 2006 to 2019. To assess the uncertainty originating from the choice of a priori flux models and observational data, ensembles of inversions were produced using three terrestrial ecosystem flux models, two ocean flux models, and three sets of atmospheric stations. We find that the station set ensemble accounts for 61 % of the total spread of the annually aggregated fluxes over the full domain when varying all these elements, while the biosphere and ocean ensembles resulted in much smaller contributions to the spread of 28 % and 11 %, respectively. These percentages differ over the specific regions of Europe, based on the availability of atmospheric data. For example, the spread of the biosphere ensemble is prone to be larger in regions that are less constrained by CO2 measurements. We investigate the impact of unprecedented increase in temperature and simultaneous reduction in soil water content (SWC) observed in 2018 and 2019 on the carbon cycle. We find that NEE estimates during these 2 years suggest an impact of drought occurrences represented by the reduction in net primary productivity (NPP), which in turn leads to less CO2 uptake across Europe in 2018 and 2019, resulting in anomalies of up to 0.13 and 0.07 PgC yr(-1) above the climatological mean, respectively. Annual temperature anomalies also exceeded the climatological mean by 0.46 degrees C in 2018 and by 0.56 degrees C in 2019, while Standardised Precipitation-Evaporation Index (SPEI) anomalies declined to -0.20 and -0.05 SPEI units below the climatological mean in both 2018 and 2019, respectively. Therefore, the biogenic fluxes showed a weaker sink of CO2 in both 2018 and 2019 (-0.22 +/- 0.05 and -0.28 +/- 0.06 PgC yr(-1), respectively) in comparison with the mean -0.36 +/- 0.07 PgC yr(-1) calculated over the full analysed period (i.e. 14 years). These translate into a continental-wide reduction in the annual sink by 39 % and 22 %, respectively, larger than the typical year-to-year standard deviation of 19 % observed over the full period.

Automated summary

Three-hourly net ecosystem exchange (NEE) estimation and uncertainty assessment on the European continent at a spatial scale of 0.25 degrees revealed significant impact from the selection of atmospheric stations, with smaller contributions from biosphere and ocean ensembles. Unprecedented temperature increase and drought events in 2018 and 2019 led to reduced CO2 uptake and weaker sink of CO2 in Europe.