Carbon Accumulation in Tropical Dry Forests and Savannas Despite Widespread Land-Cover Change, Revealed by Radar Remote Sensing

Accurate mapping of woody biomass and its temporal change is critical for understanding the global carbon balance, informing restoration and supporting evidence-based policy on climate mitigation and livelihoods. The vegetation of the dry tropics, comprised largely of dry forests and savannas, holds substantial woody carbon stocks, but both stocks and change over time remain uncertain. Where Earth Observation (EO) data have been used to estimate change over time, different products give conflicting results on the direction, magnitude and location of change. We present pan-dry-tropical maps of aboveground carbon density (AGCD) and AGCD change at 100 m resolution for the years 2015–2024, developed using ALOS ScanSAR satellite radar observations calibrated with in situ measurements (n = 237). These maps suggest an aboveground woody carbon pool (AGC) in the dry tropics of 25.3 ± 2.0 PgC, ~13.0% of global AGC. Net AGC has increased by 0.43 ± 0.07 PgC yr−1, ~20.0% of the global net land sink. The overall net increase (0.43 PgC yr−1) is comprised of gross losses from deforestation and degradation (0.27 ± 0.05 PgC yr−1) and gains due to densification and extensification of 0.59 ± 0.1 PgC yr−1, with the balance accounted for by small changes (change < 20% of AGCD; 0.11 PgC yr−1). Losses are equally split between degradation and deforestation, while gains are divided roughly 2:1 between the densification of woody ecosystems and expansion into previously grass-dominated areas. These new maps, created with a dry-tropics-specific EO method, highlight the importance of tropical dry forests and savannas in the global carbon cycle. The prominent role of degradation-related losses suggests that climate change mitigation will need to focus much more on reducing losses from overharvesting than at present. The widespread increases in carbon density and the increase in tree biomass into open systems present major challenges for biodiversity conservation, restoration and livelihoods.