An increasing Arctic-boreal CO₂sink offset by wildfires and source regions

The Arctic-Boreal Zone (ABZ) is rapidly warming, impacting its large soil carbon stocks. We use a new compilation of terrestrial ecosystem CO 2 fluxes, geospatial datasets and random forest models to show that although the ABZ was an increasing terrestrial CO 2 sink from 2001 to 2020 (mean ± standard deviation in net ecosystem exchange: −548 ± 140 Tg C yr -1 ; trend: −14 Tg C yr -1 , p<0.001), more than 30% of the region was a net CO 2 source. Tundra regions may have already started to function on average as CO 2 sources, demonstrating a critical shift in carbon dynamics. After factoring in fire emissions, the increasing ABZ sink was no longer statistically significant (budget: −319 ± 140 Tg C yr -1 ; trend: −9 Tg C yr -1 ), with the permafrost region becoming CO 2 neutral (budget: −24 ± 123 Tg C yr -1 ; trend: −3 Tg C yr -1 ), underscoring the importance of fire in this region.