Emergent constraints on climate—carbon cycle feedbacks from tropical atmospheric aridity

The vulnerability of the terrestrial tropical carbon cycle to changes in climate, especially temperature and moisture, remains one of the largest sources of uncertainty in future climate projections. Harnessing new satellite-driven global carbon reanalysis, we show here that tropical atmospheric aridity, which is directly related to the atmospheric vapor pressure deficit (VPD), is a causal driver of the interannual variability of the tropical net carbon balance and consequently the CO2 growth rate with observed present-day sensitivities of -3.2 ± 0.62 GtC mb-1 yr-1. Our results provide evidence that a large part of tropical net biome exchange variability is indirectly driven by land-atmospheric coupling via VPD variations that cannot be explained by tropical temperatures alone. Furthermore, we find that there is an emergent relationship between the sensitivity of the tropical carbon balance to VPD and the long-term response of tropical-land carbon storage to increase in VPD across an ensemble of Earth System Models used in the Climate Model Intercomparison Project 6 (CMIP6). Employing a hierarchical emergent constraint, the global carbon—climate feedback from aridity is -22±11 GtC mb-1 which represents a substantial reduction in uncertainty relative to the CMIP6 ensemble. Our findings show that atmospheric aridity is an important proxy for the combined effects of both water and temperature on the terrestrial carbon balance and a key predictor of carbon—climate feedbacks.