The European Carbon Budget - declining forest sinks and regional carbon sources

An alarming decline has been observed in the European carbon sink in recent years, thought to be a combination of increased harvest, severe droughts, bark beetle infestations, tree mortality and slow-down in tree growth causing reduced CO₂ uptake. These developments are challenging European forests in their capacity to remain carbon sinks. Meanwhile the European Union’s climate policy is counting on this large natural carbon sink to meet their agreed climate targets.

Although Europe is an extensively studied region, several knowledge gaps remain in the European greenhouse gas (GHG) budgets, importantly a lack of assessments of carbon losses from recent forest disturbances. Due to the highly fragmented European landscapes, data at very high resolution of tree cover and biomass are needed to capture disturbance and heterogeneity at small scales. Large uncertainties also remain in the contribution of land-cover and land-use change and fires.

Here we investigate the recent changes in the European carbon sink by combining new high-resolution Earth Observation-based estimates of forest biomass, forest disturbance and recovery, atmospheric CO₂ inversions, and national GHG inventory (NGHGI) approaches to ask (i) which European regions have already turned into carbon sources, (ii) in how far this is due to decreased carbon uptake or increased carbon release, and (iii) how these changes relate to natural and anthropogenic disturbances. We highlight regional differences across the continent and discuss advantages and challenges of the complementary data streams.