The 3rd ICOS Nordic Symposium (23-25 November 2021, online) aims to foster the discussion on sources and sinks of greenhouse gases in Nordic countries, to stimulate interactions between research communities and between researchers and stakeholders. The ICOS Nordic Symposium is a bi-annual joint venture of the Nordic ICOS RI (Integrated Carbon Observation System) national networks ICOS Denmark, ICOS Finland, ICOS Norway and ICOS Sweden. ICOS stations observe greenhouse gas concentrations in the atmosphere as well as carbon fluxes between the atmosphere, the land surface and the oceans. The Nordic national ICOS networks cover northern climates and ecosystems which are undergoing the fastest climate change in the world. Climate warming could destabilize carbon stored in boreal forest soils, accelerate degradation of the mires, and acidification of the seas, with concomitant impact on economy and commitment to enhance carbon sinks. We invite researchers from atmospheric, ecosystem and ocean sciences to share their work through oral and poster presentations in the symposium.