Siberian 2020 heatwave increased spring CO2 uptake but not annual CO2 uptake

Abstract Siberia experienced an unprecedented strong and persistent heatwave in winter to spring of 2020. Using bottom–up and top–down approaches, we evaluated seasonal and annual CO 2 fluxes of 2020 in the northern hemisphere (north of 30 °N), focusing on Siberia where the pronounced heatwave occurred. We found that, over Siberia, CO 2 respiration loss in response to the pronounced positive winter temperature anomaly was greater than in previous years. However, continued warming in the spring enhanced photosynthetic CO 2 uptake, resulting in the largest seasonal transition in net ecosystem CO 2 exchange; that is, the largest magnitude of the switch from the net CO 2 loss in winter to net CO 2 uptake in spring until June. However, this exceptional transition was followed by the largest reduction in CO 2 uptake in late summer due to multiple environmental constraints, including a soil moisture deficit. Despite a substantial increase of CO 2 uptake by 22 ± 9 gC m −2 in the spring in response to the heatwave, the mean annual CO 2 uptake over Siberia was slightly lower (3 ± 13 gC m −2 yr −1 ) than the average of the previous five years. These results highlight the highly dynamic response of seasonal carbon fluxes to extreme temperature anomalies at high latitudes, indicating a seasonal compensation between abnormal uptake and release of CO 2 in response to extreme warmth that may limit carbon sink capacity in high northern latitudes.