Strong relation between atmospheric CO ₂ growth rate and terrestrial water storage in tropical forests on interannual timescales

Abstract. The atmospheric CO2 growth rate (CGR) is characterised by large interannual variability, mainly due to variations in the land carbon uptake, the most uncertain component in the global carbon budget. We explore the relationships between CGR and global terrestrial water storage (TWS) from the GRACE satellites. A strong negative correlation (r = −0.70, p < 0.01, based on monthly data) between these quantities over 2002–2023 indicates that drier years correspond to a higher CGR, suggesting reduced land uptake. We then show regional TWS-CGR correlations and use a metric to assess their contributions to the global correlation. The tropics can account for the entire global TWS-CGR correlation, with small cancelling contributions from the Northern and Southern Hemisphere extratropics. Tropical America makes the dominant contribution (69 %) to the global TWS-CGR correlation, despite occupying < 12 % of the land surface. Aggregating TWS by MODIS land cover type, tropical forests exhibit the strongest CGR correlations and contribute most to the global TWS-CGR correlation (39 %), despite semi-arid and cropland/grassland regions both having more interannual TWS variability. Tropical forests exhibit the strongest CGR correlations due to their high productivity and sensitivity to water stress, which strongly influences interannual variations in carbon uptake. An ensemble mean of eight atmospheric CO2 flux inversion products also indicate a 66 % tropical contribution to CGR variability, with tropical America/Africa contributing 27 %/28 % respectively. Regarding land cover type, semi-arid/tropical forests contribute most (37 %/28 %) to CGR variability, although tropical forests cover a very much smaller surface area (25 %/10 %). Time series of global and regional TWS and CO2 flux inversions through 2002–2023 also show changing regional contributions between global CGR events, which are discussed in relation to regional drought and ENSO events. Our study advances previous work by providing a more detailed analysis of regional contributions and doing a temporal breakdown of contributions.