Fig. 4.

CMIP5 ESM behavior compared with the monthly mean optimized CO₂ flux from our analysis. (A) Growing season net CO₂ flux (in teragrams of carbon per year) against the CO₂ flux seasonal amplitude (in teragrams of carbon per year) indicate three model groupings. (B) Annual net CO₂ flux (in teragrams of carbon per year) against early winter (September through December) CO₂ flux. The arrow indicates the early winter flux of the GFDL-ESM2G model (annual net flux is −328 TgC⋅y⁻¹). (C) Time series of CO₂ flux for the model with the closest matching carbon fluxes in A and B (MIROC-ESM-CHEM) and the aircraft optimized CO₂ flux. The modeled peak carbon uptake in summer is too large and a month too early compared with the aircraft optimized CO₂ flux. Negative fluxes represent uptake by the biosphere.