Assessing the role of terrestrial ecosystems in Finland's total CO2 balance through a comparison of top-down and bottom-up estimates

Large uncertainties persist in quantifying regional CO2 emissions and removals from the land use, land use change, and forestry sector, critical for Finland’s climate targets. In support of national greenhouse gas inventories, the scientific community has developed independent top-down and bottom-up approaches to quantify and verify CO2 emissions and removals from biospheric processes. This study merges existing top-down and bottom-up research datasets, including process-based ecosystem models and high-resolution atmospheric inversions, to synthesize available estimates of biospheric CO2 balances from Finnish terrestrial ecosystems. According to the national greenhouse gas inventory, biosphere in Finland removed on average [annual minimum ... maximum] –4.6 [–7.6 … –1.6] Mt C (of CO2) annually between 2012 and 2020. During the same period, regional high-resolution top-down ensemble estimated a mean sink of –12 [–32 … +2.8] Mt C yr–1 (deviation pointing to the means of ensemble minimum and maximum) while global top-down ensemble reported a sink closer to inventory with larger deviation between the ensemble members –7.3 [–49 … +32] Mt C yr–1. Corresponding values for regional bottom-up approaches represented by an ensemble of terrestrial ecosystem models closely aligned with the inventory, reporting –4.6 [–21 … +13] Mt C yr–1. The global ecosystem model TRENDY ensemble, with a larger number of ensemble members, estimated an average sink of –9.9 [–30 … +15] Mt C yr–1. Accordingly, we conclude that independent top-down and bottom-up estimates have some consistency in relation to the national greenhouse gas inventory, but at present, large uncertainties found in country-level balances prohibit reliable verification of the inventory.