Low latency carbon budget analysis reveals a large decline of the land carbon sink in 2023

In 2023, the CO2 growth rate was 3.37 +/- 0.11 ppm at Mauna Loa, which was 86% above that of the previous year and hit a record high since observations began in 1958, while global fossil fuel CO2 emissions only increased by 0.6% +/- 0.5%. This implies an unprecedented weakening of land and ocean sinks, and raises the question of where and why this reduction happened. Here, we show a global net land CO2 sink of 0.44 +/- 0.21 GtC yr-1, which is the weakest since 2003. We used dynamic global vegetation models, satellite fire emissions, an atmospheric inversion based on OCO-2 measurements and emulators of ocean biogeochemical and data-driven models to deliver a fast-track carbon budget in 2023. Those models ensured consistency with previous carbon budgets. Regional flux anomalies from 2015 to 2022 are consistent between top-down and bottom-up approaches, with the largest abnormal carbon loss in the Amazon during the drought in the second half of 2023 (0.31 +/- 0.19 GtC yr-1), extreme fire emissions of 0.58 +/- 0.10 GtC yr-1 in Canada and a loss in Southeast Asia (0.13 +/- 0.12 GtC yr-1). Since 2015, land CO2 uptake north of 20 degrees N had declined by half to 1.13 +/- 0.24 GtC yr-1 in 2023. Meanwhile, the tropics recovered from the 2015-2016 El Ni & ntilde;o carbon loss, gained carbon during the La Ni & ntilde;a years (2020-2023), then switched to a carbon loss during the 2023 El Ni & ntilde;o (0.56 +/- 0.23 GtC yr-1). The ocean sink was stronger than normal in the equatorial eastern Pacific due to reduced upwelling from La Ni & ntilde;a's retreat in early 2023 and the development of El Ni & ntilde;o later. Land regions exposed to extreme heat in 2023 contributed a gross carbon loss of 1.73 GtC yr-1, indicating that record warming in 2023 had a strong negative impact on the capacity of terrestrial ecosystems to mitigate climate change. This work reveals an unprecedented weakening of the land carbon sink in 2023 using low-latency atmospheric inversion, dynamic vegetation models, and AI-powered ocean CO2 flux emulators, highlighting the impact of extreme climate events like Amazon droughts and Canadian wildfires.