Increasingly negative tropical water-interannual CO 2 growth rate coupling.

Terrestrial ecosystems have taken up about 32% of the total anthropogenic CO ₂ emissions in the past six decades ¹ . Large uncertainties in terrestrial carbon-climate feedbacks, however, make it difficult to predict how the land carbon sink will respond to future climate change ² . Interannual variations in the atmospheric CO ₂ growth rate (CGR) are dominated by land-atmosphere carbon fluxes in the tropics, providing an opportunity to explore land carbon-climate interactions ^3-6 . It is thought that variations in CGR are largely controlled by temperature ^7-10 but there is also evidence for a tight coupling between water availability and CGR ¹¹ . Here, we use a record of global atmospheric CO ₂ , terrestrial water storage and precipitation data to investigate changes in the interannual relationship between tropical land climate conditions and CGR under a changing climate. We find that the interannual relationship between tropical water availability and CGR became increasingly negative during 1989-2018 compared to 1960-1989. This could be related to spatiotemporal changes in tropical water availability anomalies driven by shifts in El Niño/Southern Oscillation teleconnections, including declining spatial compensatory water effects ⁹ . We also demonstrate that most state-of-the-art coupled Earth System and Land Surface models do not reproduce the intensifying water-carbon coupling. Our results indicate that tropical water availability is increasingly controlling the interannual variability of the terrestrial carbon cycle and modulating tropical terrestrial carbon-climate feedbacks.
(© 2023. The Author(s).)