  In 2023, the CO2 growth rate was 3.37 +/- 0.11 ppm at Mauna Loa, 86% above
the previous year, and hitting 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, the weakest since 2003. We used
dynamic global vegetation models, satellites 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 2015-2022 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 South-East Asia
(0.13 +/- 0.12 GtC yr-1). Since 2015, land CO2 uptake north of 20 degree N
declined by half to 1.13 +/- 0.24 GtC yr-1 in 2023. Meanwhile, the tropics
recovered from the 2015-16 El Nino carbon loss, gained carbon during the La
Nina years (2020-2023), then switched to a carbon loss during the 2023 El Nino
(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 Nina's retreat in
early 2023 and the development of El Nino 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.
