The role of the tropical carbon balance in determining the large atmospheric CO2 growth rate in 2023

The global annual mean atmospheric CO2 growth rate in 2023 was one of the highest since records began in 1958, comparable to values recorded during previous major El Niño events. We do not fully understand this anomalous growth rate, although a recent study highlighted the role of boreal North American forest fires. We use a Bayesian inverse method to interpret global-scale atmospheric CO2 data from NASA's Orbiting Carbon Observatory (OCO-2). The resulting a posteriori CO2 flux estimates reveal that from 2022 to 2023, the biggest changes in CO2 fluxes of net biosphere exchange (NBE) - for which positive values denote a flux to the atmosphere - were over the land tropics. We find that the largest NBE increase is over eastern Brazil, with small increases over southern Africa and Southeast Asia. We also find significant increases over southeastern Australia, Alaska, and western Russia. A large NBE increase over boreal North America, due to fires, is driven by our a priori inventory, informed by independent data. The largest NBE reductions are over western Europe, the USA, and central Canada. Our NBE estimates are consistent with gross primary production estimates inferred from satellite observations of solar-induced fluorescence and from satellite observations of vegetation greenness. We find that warmer temperatures in 2023 explain most of the NBE change over eastern Brazil, with hydrological changes more important elsewhere across the tropics. Our results suggest that the ongoing environmental degradation of the Amazon is now playing a substantial role in increasing the global atmospheric CO2 growth rate.