Impact of air-ice CO₂ fluxes on polar ocean carbon budgets from a bipolar data compilation

In ocean carbon budget assessments, sea ice is still treated as an impermeable barrier, rather than a dynamic interface mediating CO₂ exchange between ocean and atmosphere. We compiled more than 6000 chamber-based air-ice CO₂ flux measurements from the Arctic and Southern Oceans between 2003 and 2021, spanning diverse ice, snow, and seasonal conditions. These data show that sea ice releases CO₂ in winter and absorbs it in summer, with summer uptake offsetting winter emissions. On an annual basis, sea ice represents a small net CO₂ source of +4 Tg C yr⁻¹ in the Arctic and +2 Tg C yr⁻¹ in the Southern Ocean, challenging earlier views of a major sink. Although these fluxes are negligible at basin scales relative to open-ocean uptake, sea ice exchanges gases even in cold winter conditions, with implications for small-scale processes and other trace gases, underscoring the need for sustained, process-resolving observations.