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 develop a
data-driven metric for the GSNF using atmospheric carbon dioxide
concentration observations collected during the High-Performance
Instrumented Airborne Platform for Environmental Research (HIAPER)
Pole-to-Pole Observations (HIPPO) and Atmospheric Tomography Mission
(ATom) 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 ± 0.2 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 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 ± 15
Pg C, heterotrophic respiration to be 25 ± 11 Pg C, and net primary
productivity to be 28 ± 10 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.