Midwest U.S. croplands determine model divergence in North American carbon fluxes

Large uncertainties in North American terrestrial carbon fluxes hinder regional climate 26 projections. Terrestrial biosphere models (TBMs), the essential tools for understanding 27 continental-scale carbon cycle, diverge on whether temperate forests or croplands 28 dominate carbon uptake in North America. Evidence from novel photosynthetic proxies, 29 such as those based on chlorophyll fluorescence, has cast doubt on the “weak cropland, 30 strong forest” carbon uptake patterns simulated by most TBMs. However, no systematic 31 evaluation of TBMs has yet been attempted to pin down space-time patterns that are 32 most consistent with regional CO2 observational constraints. Here, we leverage 33 atmospheric CO2 observations and satellite-observed photosynthetic proxies to understand 34 emergent space-time patterns in North American carbon fluxes from a large suite of 35 TBMs and data-driven models. To do so, we evaluate how well the atmospheric signals 36 resulting from carbon flux estimates reproduce the space-time variability in atmospheric 37 CO2, as is observed by a network of continuous-monitoring towers over North America. 38 Models with gross or net carbon fluxes that are consistent with the observed CO2 39 variability share a salient feature of growing-season carbon uptake in Midwest U.S. 40 croplands. Conversely, the remaining models place most growing-season uptake in boreal 41 or temperate forests. Differences in model explanatory power depend mainly on the 42 simulated annual cycles of cropland uptake—especially the timing of peak uptake—rather 43 than the distribution of annual mean fluxes across biomes. Our results suggest that 44 improved model representation of cropland phenology is crucial to robust, policy-relevant 45 estimation of North American carbon exchange.