Reduced northern mid-latitude carbon sink estimates from atmospheric CO2 data

Abstract Terrestrial ecosystem in the Northern Hemisphere is characterized by a substantial carbon sink in recent decades1–3. However, the carbon sink inferred from atmospheric CO2 data is usually larger than process- and inventory-based estimates, resulting in carbon release or near-neutral carbon exchange in the tropics. The atmospheric approach is known to be uncertain due to systematic biases of coarse atmospheric transport model simulation. Compared to a coarse-resolution inversion estimate in the integrated region of N. America, E. Asia, and Europe from 2015 to 2018, the carbon sink estimate from a native high-resolution atmospheric inversion is significantly reduced from -3.0±0.08 gigatons of carbon per year (GtC yr-1) to -2.15±0.08 GtC yr-1 due to prominent more carbon release during the non-growing seasons. The major reductions concentrate in the mid-latitudes (20 °N~45 °N) where the mean land carbon sinks in China and USA are reduced from 0.64±0.03 and 0.35±0.02 GtC yr-1 to 0.14±0.03 and 0.15±0.02 GtC yr-1, respectively. The high-resolution transport reduces the annual flux bias mainly by better reproducing the vertical CO2 gradient of the strong and persistent fossil fuel emissions. These results suggest that high-resolution inversion with accurate vertical and meridional transport is urgently needed in targeting national carbon neutrality.