Detecting changes in the high-latitude carbon seasonal cycle with a multi-model approach

Abstract:

High-latitude amplification of the CO₂ seasonal cycle has been well documented in long-term atmospheric observations since the 1960s. However, understanding the causes of the increasing amplitude remains challenging. Candidate theories include warming-induced earlier onset and enhancement of vegetation growth, increased carbon releases in the fall, and mid-latitude transport. To quantify the contributions of increasing gross primary productivity (GPP) and ecosystem respiration (ER) separately to the enhanced CO₂ seasonal cycle in the Arctic, we use state-of-the-science process-based, empirical and inverse modeling capabilities to partition net ecosystem exchange (NEE) into GPP and ER for the last two decades. The process-based Simple Biosphere Model (v4.2; SiB4) helps track and understand mechanisms controlling phenological plant responses to climate variables such as temperature and moisture. SiB4 diagnostics include carbonyl sulfide and SIF, which can be used to trace GPP directly, and stress functions that capture plant responses to climate variability. We also test a simpler, empirical framework with the Polar Vegetation Photosynthesis and Respiration Model (Polar-VPRM), which uses reanalysis climatology from ERA5, satellite-based solar-induced fluorescence and eddy covariance (EC) flux tower measurements to estimate parameters corresponding to particular vegetation types. Polar-VPRM parameters were also tuned with atmospheric inversion model-derived NEE and GPP fluxes, which were constrained by thousands of high-accuracy and high-precision in situ atmospheric CO₂ and COS observations that have much larger footprints than EC measurements. Finally, PVPRM and SiB4 fluxes were compared with FLUXCOM and FLUXSAT products to assess trends and regional differences between GPP and ER over the North American and Eurasian Arctic.