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

The seasonal cycle of atmospheric CO2 is commonly assumed to be dominated by terrestrial carbon uptake and minimally affected by fossil-fuel CO2 emissions (FF). However, the magnitude and extent of the influence of FF on the CO2 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 CO2 seasonal cycle. Here, we utilized CO2 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 CO2 seasonal cycle. We first found widespread increases of 1.10 ± 0.67 ppm in the CO2 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 CO2 concentration vary seasonally.