Weak northern and strong tropical land carbon uptake from vertical profiles of atmospheric CO₂

Measurements of midday vertical atmospheric CO₂ distributions reveal annual-mean vertical CO₂ gradients that are inconsistent with atmospheric models that estimate a large transfer of terrestrial carbon from tropical to northern latitudes. The three models that most closely reproduce the observed annual-mean vertical CO₂ gradients estimate weaker northern uptake of -1.5 petagrams of carbon per year (Pg C year^-1) and weaker tropical emission of +0.1 Pg C year^-1 compared with previous consensus estimates of -2.4 and +1.8 Pg C year^-1, respectively. This suggests that northern terrestrial uptake of industrial CO₂ emissions plays a smaller role than previously thought and that, after subtracting land-use emissions, tropical ecosystems may currently be strong sinks for CO₂.