Simulated CO2 concentrations at Barrow station for 1979-2020 by ORCHIDEE-LMDz6A model

Supplement to Zhu et al., 2023, Temperature dependence of spring carbon uptake in northern high latitudes during the past four decades, in final submission to Global Change Biology.

Abstract: In the northern high-latitudes, warmer spring temperatures generally lead to earlier leaf onsets, higher vegetation production and enhanced spring carbon uptake. Yet, whether this positive linkage has diminished under climate change remains debated. Here we used atmospheric CO₂ measurements at Barrow (Alaska) during 1979-2020 to investigate the strength of temperature dependence of spring carbon uptake reflected by two indicators, spring zero-crossing date (SZC) and CO₂ drawdown (SCC). We found a fall-and-rise in the interannual correlation of temperature with SZC and SCC (R_SZC-T and R_SCC-T), showing a recent reversal of the previously reported weakening trend of R_SZC-T and R_SCC-T. We used a terrestrial biosphere model coupled with an atmospheric transport model to reproduce this fall-and-rise phenomenon, and conducted factorial simulations to explore its potential causes. We found that a strong-weak-strong spatial synchrony of spring temperature anomalies per se has contributed to the fall-and-rise trend in R_SZC-T and R_SCC-T, despite an overall unbroken temperature control on net ecosystem CO₂ fluxes at local scale. Our results provide an alternative explanation for the apparent drop of R_SZC-T and R_SCC-T during the late 1990s and 2000s, and suggest a continued positive linkage between spring carbon uptake and temperature during the past four decades. We thus caution the interpretation of apparent climate sensitivities of carbon cycle retrieved from spatially-aggregated signals.

File descriptions:

These data are the simulated 3-hourly CO2 concentrations at the location of Barrow station by the ORCHIDEE terrestrial biosphere model coupled with the LMDz6A atmospheric transport model. Six factorial experiments (named CTRL, fixFlux, fixWind, T1, T2, T3) were conducted to reproduce the observations and to test the mechanisms underlying the changing temperature dependence of spring CO2 drawdown. Detailed descriptions of the simulations can be found in the linked manuscript.