Evaluating Northern Hemisphere Growing Season Net Carbon Flux in Climate Models Using Aircraft Observations

Abstract

Understanding terrestrial ecosystems and their response to anthropogenic climate change requires quantification of land‐atmosphere carbon exchange. However, top‐down and bottom‐up estimates of large‐scale land‐atmosphere fluxes, including the northern extratropical growing season net flux (GSNF), show significant discrepancies. We developed a data‐driven metric for the GSNF using atmospheric carbon dioxide concentration observations collected during the High‐Performance Instrumented Airborne Platform for Environmental Research Pole‐to‐Pole Observations and Atmospheric Tomography Mission flight campaigns. This aircraft‐derived metric is bias‐corrected using three independent atmospheric inversion systems. We estimate the northern extratropical GSNF to be 5.7 ± 0.3 Pg C and use it to evaluate net biosphere productivity from the Coupled Model Intercomparison Project phase 5 and 6 (CMIP5 and CMIP6) models. While the model‐to‐model spread in the GSNF has decreased in the CMIP6 models relative to that of the CMIP5 models, there is still disagreement on the magnitude and timing of seasonal carbon uptake with most models underestimating the GSNF and overestimating the length of the growing season relative to the observations. We also use an emergent constraint approach to estimate annual northern extratropical gross primary productivity to be 56 ± 17 Pg C, heterotrophic respiration to be 25 ± 13 Pg C, and net primary productivity to be 28 ± 12 Pg C. The flux inferred from these aircraft observations provides an additional constraint on large‐scale gross fluxes in prognostic Earth system models that may ultimately improve our ability to accurately predict carbon‐climate feedbacks.

Neutral Tropical African CO2 Exchange Estimated From Aircraft and Satellite Observations

“…It has also been used by the global carbon cycle, oceanography, and atmospheric chemistry communities to constrain, often the magnitude and spatial distribution rather than a projection, of greenhouse gas fluxes, ocean heat transports, oxidation rates, ecosystem carbon stocks, etc (e.g., Bian et al, 2023;W. Li et al, 2017;Loechli et al, 2023;Long et al, 2021;Patra et al, 2014;Resplandy et al, 2016;Stephens et al, 2007). Our approach is consistent with this latter group.…”

“…The emergent constraint approach has been widely used in the prognostic climate modeling community to constrain future climate projections based on observations of the current system and emergent relationships from Earth system model ensembles (e.g., Cox, 2019; Hall et al., 2019; Simpson et al., 2021; M. S. Williamson et al., 2021). It has also been used by the global carbon cycle, oceanography, and atmospheric chemistry communities to constrain, often the magnitude and spatial distribution rather than a projection, of greenhouse gas fluxes, ocean heat transports, oxidation rates, ecosystem carbon stocks, etc (e.g., Bian et al., 2023; W. Li et al., 2017; Loechli et al., 2023; Long et al., 2021; Patra et al., 2014; Resplandy et al., 2016; Stephens et al., 2007). Our approach is consistent with this latter group.…”

Atmospheric CO2 inversion models overestimate northern extratropical land and ocean carbon uptake as assessed at background in-situ sites

“…However, subsequent studies have shown dominant impacts of transport uncertainty on our ability to estimate the northern hemisphere land CO 2 sink [13][14][15] . Use of independent observations for validation of inversion estimated CO 2 uxes was promoted in the past decade; however, the regional or semi-hemispheric ux ranges could be narrowed down with limited success over some regions only [16][17][18][19][20][21] .…”