The functioning of Arctic ecosystems is not only critically affected by climate change,but it also has the potential for major positive feedback on climate. There is, however, relatively little information on the role, patterns, and vulnerabilities of CO2 ﬂuxes during the nonsummer seasons in Arctic ecosystems. Presented here is a year-round study of CO2 ﬂuxes in an Alaskan Arctic tussock tundra ecosystem, and key environmentalcontrolsonthese ﬂuxes.Importantcontrolson ﬂuxesvarybyseason.Thispaperalsopresents a new empirical quantiﬁcation of seasons in the Arctic based on net radiation. The ﬂuxes were computed using standard FluxNet methodology and corrected using standard Webb-Pearman-Leuning density terms adjusted for inﬂuences of open-path instrument surface heating. The results showed that the nonsummer season comprises a signiﬁcant source of carbon to the atmosphere. The summer period was a net sink of 24.3g Cm2, while the nonsummer seasons released 37.9g Cm2. This release is 1.6 times the summer uptake, resulting in a net annual source of +13.6g Cm2 to the atmosphere. These ﬁndings support early observations of a change in this particular region of the Arctic from a long-term annual sink of CO2 to an annual source from the terrestrial ecosystem and soils to the atmosphere. The results presented here demonstratethatnearlycontinuousobservationsmayberequiredinordertoaccuratelycalculatetheannual netecosystemCO2 exchangeofArcticecosystemsandtobuildpredictiveunderstandingthatcanbeusedto estimate, with conﬁdence, Arctic ﬂuxes under future conditions.