Strong relation between atmospheric CO2 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 = -0.68, p < 0.01) between these quantities over 2001-2023
indicates that drier years correspond to a higher CGR, suggesting
reduced land uptake. We then show regional TWS-CGR correlations and use
a new metric to assess their contributions to the global correlation.
The tropics account for the entire global TWS-CGR correlation, with
small cancelling contributions from the Northern and Southern Hemisphere
extratropics. Tropical America explains 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.
An ensemble mean of four atmospheric CO2 flux inversion products also
indicate a 74% tropical contribution to CGR variability, with tropical
America and Africa each contributing 30% and 27%. Regarding land cover
type, semi-arid/tropical forests contribute almost equally (37%/35%)
to CGR variability, although tropical forests cover a smaller surface
area (25%/10%). Timeseries of global and regional TWS and CO2 flux
inversions through 2001-2023 also show changing regional contributions
between global CGR events, which are discussed in relation to regional
drought and ENSO events.