COVID-19 induced reduction of fossil-fuel emissions in 2020 altered the seasonal cycle of atmospheric CO2 at high latitudes

• The impacts of fossil-fuel emissions on CO 2 seasonal cycle were investigated based on atmospheric-transport simulations. • The CO 2 seasonal cycle amplitude significantly increased in 2020 compared to 2018−2019 at high-latitude sites. • The fossil-fuel emission declines accounted for 0.43 ± 0.12 ppm of the increases in the CO 2 seasonal cycle amplitude at high-latitudes in 2020 The seasonal cycle of atmospheric CO 2 is commonly assumed to be dominated by terrestrial carbon uptake and minimally affected by fossil-fuel CO 2 emissions (FF). However, the magnitude and extent of the influence of FF on the CO 2 seasonal cycle remain elusive. In the first half of year 2020, the outbreak of the coronavirus disease 2019 led to large reductions in FF, providing a testbed for understanding the impact of FF on the CO 2 seasonal cycle. Here, we utilized CO 2 observations from the global surface network and conducted atmospheric transport simulations with daily FF data to assess the effects of FF reductions in 2020 on the CO 2 seasonal cycle. We first found widespread increases of 1.10 ± 0.67 ppm in the CO 2 seasonal cycle amplitude (SCA) at high-latitude sites in 2020 compared to 2018−2019, which were equal to 170 ± 11% of the inter-annual variability of detrended SCA over the previous 20 years. Further, we attribute FF reductions to 0.43 ± 0.12 ppm of the observed SCA increases at high-latitude sites in 2020. At the Barrow site, FF reductions account for 31.9% of the SCA increase in 2020. The critical impact of FF on SCA increase does not suggest a larger decline in FF at high latitudes than mid-latitudes. Rather, the FF contributes more to SCA anomalies in magnitude at mid-latitudes than high latitudes. The localized impacts of anomalies in land-atmosphere carbon exchange fluxes generally exceeded the FF impacts on SCA anomalies at mid-latitudes, inducing varying changes in SCA across different mid-latitude sites. Our study thus presents a framework that may be extended further, where the knowledge of FF for the year 2020 versus other years, enables refinement of how atmospheric CO 2 concentration vary seasonally.