Quantifying CO₂ Fertilization Effect on the Increase of CO₂ Seasonal Cycle Amplitude at Northern High Latitudes

L. Hu¹, S.A. Montzka¹, P.P. Tans², J. Miller¹, A. Kaushik^3,1, J. Ma⁴, M. Krol⁵, M. Remaud⁶, K. Schuldt³, S.E. Michel⁷, C. Sweeney¹, Y. Liu⁸, I. Vimont^3,1, A. Crotwell^3,1, B. Hall¹, M. Chen⁹ and A. Andrews¹

¹NOAA Global Monitoring Laboratory (GML), Boulder, CO 80305; 720-319-7805, E-mail: lei.hu@noaa.gov
²NOAA Global Monitoring Laboratory (GML) Retired, Boulder, CO 80305
³Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, CO 80309
⁴Institute for Marine and Atmospheric research Utrecht, Utrecht University, Utrecht, The Netherlands
⁵Wageningen University, Department of Meteorology and Air Quality, Wageningen, The Netherlands
⁶Laboratoire des Sciences du Climat et de l'Environnement (LSCE), Institut Pierre-Simon Laplace, Orme des Merisiers, France
⁷Institute of Arctic and Alpine Research (INSTAAR), University of Colorado, Boulder, CO 80309
⁸School of Earth Sciences, The Ohio State University, Columbus, OH 43210
⁹Department of Forest and Wildlife Ecology, University of Wisconsin-Madison, Madison, WI 53706

Atmospheric observations show an enhanced increase of atmospheric CO₂ seasonal cycle amplitude (SCA) at northern high latitudes relative to lower latitudes (Fig. 1). This enhancement coincides with intensified temperature increase in this region, which was suggested to be the primary driver in causing the CO₂ SCA amplification at northern high latitudes. Here, we present new analyses using long-term observations of CO₂, COS, and δ13CO2 made by NOAA and its partner institutions to quantify the impact of CO₂ fertilization effect on the CO₂ SCA increase at northern high latitudes. We used these observations to directly estimate the trends in ecosystem processes such as stomatal conductance of CO₂, the ratio of intercellular to ambient CO₂ mole fractions, photosynthetic CO₂ uptake, and ecosystem respiration. Our results suggest that CO₂ fertilization has played a much more important role than previously thought in enhancing photosynthetic CO₂ uptake over this region. Furthermore, the increase in photosynthetic CO₂ uptake has exceeded the trend in ecosystem respiration to be a dominant driver to the high-latitude CO₂ SCA increase. 

Figure 1. Average seasonal cycle of detrended atmospheric CO₂ measured in NOAA collected whole-air flask samples during 1978 – 1982 (black) and 2017 – 2021 (red) at Barrow, Alaska (BRW) and Mauna Loa, Hawaii (MLO).