Signs of reduced biospheric activity with progressing global warming: evidence from long-term records of atmospheric CO₂ mixing ratios in Central-Eastern Europe

Abstract. The recent rise of temperatures across the globe, mainly attributed to the raising anthropogenic emissions of greenhouse gases, is predicted to have an increased impact on ecosystems over the next century and beyond. One of the manifestations of this anthropogenic global warming will be the increased occurrence of prolonged droughts in the temperate climate zones, specifically in Northern America and Europe. Drought events that took place in Europe in 2003 and 2010 are known to have led to significant reduction of carbon dioxide sink, due to simultaneous occurrence of water stress limiting the photosynthetic activity and increase of respiration under higher temperatures. In the current study we present the evidence of an increased impact of droughts on the annual cycle of carbon dioxide over Central-Eastern Europe, based on long-term observations (1995–2018) of mixing ratios conducted at two continental sites: Kasprowy Wierch mountain station (KAS, Southern Poland) and Hegyhatsal tall tower (HUN, Hungary). Analyses of the smoothed, detrended annual cycles from both sites reveal a gradual reduction of annual amplitudes towards lower magnitudes, with simultaneous reductions of annual maxima (KAS: −0.13 ± 0.05 ppm/yr, HUN: −0.08 ± 0.12 ppm/yr) and increases of minima (KAS: 0.09 ± 0.04 ppm/yr, HUN: 0.08 ± 0.08 ppm/yr). By comparing the area of influence of both stations (established by analyses of footprints calculated with Hysplit Lagrangian model) to the regions of drought extent (established by analysing the temperature and soil moisture anomalies), we attribute the observed rising trend of annual minima to the increased frequency of large-scale drought events that reduce mean summer assimilation rates over Central-Eastern Europe. This conclusion is further corroborated by comparison to the biogenic fluxes calculated by the regional inversion system CarbonTracker-EU, albeit the statistical uncertainty is non-negligible (CO₂ biogenic flux over Europe Transcomm region is equal to 0.03 ± 0.03 PgC/yr). On the other hand, reduction of anthropogenic CO₂ emissions (−0.07 ± 0.02 PgC/yr over Europe) seem to at least partially explain the trend of reduced winter maxima of CO₂ at the observation sites.