The central Arctic Ocean (CAO) plays an important role in the global carbon cycle, but the current and future exchange of the climate-forcing trace gases methane (CH₄) and carbon dioxide (CO₂) between the CAO and the atmosphere is highly uncertain. In particular, there are very few observations of near-surface gas concentrations or direct air–sea CO₂ flux estimates and no previously reported direct air–sea CH₄ flux estimates from the CAO. Furthermore, the effect of sea ice on the exchange is not well understood. We present direct measurements of the air–sea flux of CH₄ and CO₂, as well as air–snow fluxes of CO₂ in the summertime CAO north of 82.5^∘ N from the Synoptic Arctic Survey (SAS) expedition carried out on the Swedish icebreaker Oden in 2021.

Measurements of air–sea CH₄ and CO₂ flux were made using floating chambers deployed in leads accessed from sea ice and from the side of Oden, and air–snow fluxes were determined from chambers deployed on sea ice. Gas transfer velocities determined from fluxes and surface-water-dissolved gas concentrations exhibited a weaker wind speed dependence than existing parameterisations, with a median sea-ice lead gas transfer rate of 2.5 cm h⁻¹ applicable over the observed 10 m wind speed range (1–11 m s⁻¹). The average observed air–sea CO₂ flux was −7.6 , and the average air–snow CO₂ flux was −1.1 . Extrapolating these fluxes and the corresponding sea-ice concentrations gives an August and September flux for the CAO of −1.75 , within the range of previous indirect estimates.

The average observed air–sea CH₄ flux of 3.5 , accounting for sea-ice concentration, equates to an August and September CAO flux of 0.35 , lower than previous estimates and implying that the CAO is a very small (≪ 1 %) contributor to the Arctic flux of CH₄ to the atmosphere.