An Assessment of CO2 Uptake in the Arctic Ocean From 1985 to 2018

As a contribution to the Regional Carbon Cycle Assessment and Processes phase 2 (RECCAP2) project, we present synthesized estimates of Arctic Ocean sea-air CO2 fluxes and their uncertainties from surface ocean pCO(2)-observation products, ocean biogeochemical hindcast and data assimilation models, and atmospheric inversions. For the period of 1985-2018, the Arctic Ocean was a net sink of CO2 of 116 +/- 4 TgC yr(-1) in the pCO(2) products, 92 +/- 30 TgC yr(-1) in the models, and 91 +/- 21 TgC yr(-1) in the atmospheric inversions. The CO2 uptake peaks in late summer and early autumn, and is low in winter when sea ice inhibits sea-air fluxes. The long-term mean CO2 uptake in the Arctic Ocean is primarily caused by steady-state fluxes of natural carbon (70% +/- 15%), and enhanced by the atmospheric CO2 increase (19% +/- 5%) and climate change (11% +/- 18%). The annual mean CO2 uptake increased from 1985 to 2018 at a rate of 31 +/- 13 TgC yr(-1) dec(-1) in the pCO(2 )products, 10 +/- 4 TgC yr(-1) dec(-1) in the models, and 32 +/- 16 TgC yr(-1 )dec(-1) in the atmospheric inversions. Moreover, 77% +/- 38% of the trend in the net CO(2 )uptake over time is caused by climate change, primarily due to rapid sea ice loss in recent years. Furthermore, true uncertainties may be larger than the given ensemble standard deviations due to common structural biases across all individual estimates.