Increasing CO₂ Seasonal Cycle Amplitude in the Arctic Proportional to Rising Atmospheric CO₂ Levels

L. Hu^1,2, K. Schuldt¹, S.A. Montzka², E. Dlugokencky², S.E. Michel³, B.H. Vaughn³, K. Thoning², P.P. Tans², J.B. Miller², C. Sweeney² and A.E. Andrews²

¹Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, CO 80309; 303-497-5238, E-mail: lei.hu@noaa.gov
²NOAA Global Monitoring Laboratory (GML), Boulder, CO 80305
³Institute of Arctic and Alpine Research (INSTAAR), University of Colorado, Boulder, CO 80309

Atmospheric observations show an enhanced increase of atmospheric CO₂ seasonal cycle amplitude (SCA) in the Arctic relative to lower latitudes.  This enhancement coincides with intensified temperature increase in the Arctic and rising atmospheric CO₂ concentrations globally, both of which were suggested to be important in causing the Arctic CO₂ SCA amplification.  Here, we present analyses of five-decades of atmospheric CO₂ measurements at Barrow, Alaska (BRW) and Mauna Loa Observatory, Hawaii (MLO).  At BRW, atmospheric CO₂ SCA increased at a rate of 0.602 ± 0.057 % yr^-1 between 1971 and 2021, whereas it increased at a rate of 0.226 ± 0.068 % yr^-1 at MLO (Fig. 1).  However, when normalizing their SCA by annual atmospheric CO₂ mole fractions, the trend of CO₂ SCA at both sites mostly disappears (Fig. 1), suggesting the SCA trend was likely primarily driven by increasing atmospheric CO₂.  We also analyzed atmospheric CO₂, δ13CO₂, and carbonyl sulfide (COS) measurements from 11 long-term Arctic sites and the 4-decades of marine boundary layer reference, which is a smoothed representation of atmospheric CO₂ at different latitudes and constructed from atmospheric measurements at remote locations around the globe; results are overall consistent.  To better understand the cause for the proportionality between the enhanced CO₂ SCA in the Arctic and increasing atmospheric CO₂ and to disentangle the regional contributions of fluxes and atmospheric transport to Arctic CO₂ SCA amplification, we analyzed NOAA CarbonTracker (CT2019B) posterior fluxes between 2000 and 2018. Global simulations were conducted, where we traced the impact of regional fluxes by source categories (terrestrial CO₂ uptake fluxes, fossil fuel emissions, ocean fluxes, and fire emissions) on measured atmospheric CO₂ SCA at different sites.

Figure 1. Seasonal cycle amplitude (SCA) of atmospheric CO₂ measured at BRW (black) and MLO (red). Left panel: SCA in parts per million (ppm). Right panel: SCA normalized by annual average CO₂.