Budgets of air-sea and air-ice CO2 fluxes over the Arctic Ocean from the GreenFeedBack project

This data set and its companion description are presented in fulfilment of Deliverable 5.1 of the GreenFeedBack Research and Innovation Action. The GreenFeedBack project is designed to advance scientific understanding of greenhouse gas (GHG) dynamics across terrestrial, freshwater, and marine ecosystems. Within this framework, a significant effort was devoted to integrating air-sea and air-ice CO2 fluxes over the Arctic Ocean (AO) by leveraging observational data and model-based approaches.We are providing two independent assessments that integrate both air-sea and air-ice CO2 fluxes over the Arctic Ocean, and we are comparing them with a third assessment of air-sea CO2 fluxes derived from an Earth System Model. Attention has been paid to comparing different formulations of air-sea gas exchanges. We also provide for the first time an assessment of air-ice CO2 fluxes that covers all seasons. We estimated air-sea and air-ice CO2 fluxes over the RECCAP2 regions for the periods 2000-2017 and 2018-2022. Our first approach is based on the measurement of p(CO2) (partial pressure of CO2) of surface waters from the SOCAT (Surface Ocean CO2 Atlas) database. The p(CO2) measurements have been extrapolated using the Self Organising Map - Feed Forward Network (SOM-FFN) approach tailored for the Arctic Ocean in order to assess air-sea CO2 fluxes over the entire Arctic Ocean. We provide direct air-ice CO2 flux estimates for the compilation of available air-ice CO2 flux measurements, which were carried out using the enclosure method. The compilation has been initiated by Dr Daiki Nomura within the framework of the BEPSII working group and has been consolidated within the framework of the Greenfeedback project. This assessment has been merged with direct air-ice CO2 fluxes derived from a new compilation of measurements of air-ice fluxes carried out with the enclosure method (sea-ice). This allows us to budget both air-sea and air-ice CO2 fluxes, denoted as sea ice. Such a combined assessment has been compared to the assessment of air-sea and air-ice CO2 fluxes from atmospheric inversion (GCL), based on the GEOSChem atmospheric chemistry transport model, atmospheric CO2 observations from the global surface network measurements of NOAA ESRL, and the Local Ensemble Transform Kalman Filter approach. Finally, we compared these assessments of the Arctic Ocean CO2 sink with the GreenFeedBack-adapted version of OptimESM (GFB-ECE3), an improved version of the EC-Earth3 Earth system model (ECE3) with an activated carbon cycle representation.The data file provides monthly and yearly budgets for all RECCAP2 regions for the 5 products (SOM-FFN, GCL, GFB-ECE3, Sea ice, and SOM-FFN + sea ice). GCL product is available only for the period 2000-2017. The file also provides mean CO2 fluxes per square meter Over the period 2000-2017, we assess that the Arctic Ocean was a net sink of CO2 of 99.7 TgC yr-1 in the p(CO2) product (SOM-FFN) that can be adjusted to 94.8 TgC yr-1 if we consider the contribution of sea ice, 61.2 TgC yr-1 in the atmospheric inversion, and 80.3 TgC yr-1 in the GFB-ECE3 model. The sea ice was a net source of CO2 for the atmosphere of 4.9 Tg yr-1. The combined p(CO2) and sea ice product (SOM-FFN + sea ice) suggests an enhanced uptake of CO2 in the period 2018-2022, with an uptake of 115.85 TgC yr-1, approximately 16 TgC yr-1 larger than 2000-2017. The strengthening of the CO2 sink in the Arctic Ocean is supported by the GFB-ECE3 model, which also suggests an enhancement of 4.3 TgC yr-1, resulting in a total CO2 uptake of 84.6 TgC yr-1.