Can long-term tropical land carbon-climate feedback uncertainties be constrained from interannual variability?

Whether tropical land carbon sink will persist in the future to slow climate change remains elusive in Earth System Model (ESM) projections, largely due to carbon-climate feedback uncertainties. Unraveling drivers of interannual variability (IAV) of the land carbon cycle can inform tropical land carbon-climate feedbacks. Here we utilize two generations of factorial ESM experiments to show that the IAV of the tropical land carbon uptake under both present and future climate is consistently dominated by terrestrial water variations in ESMs. The magnitude of this interannual sensitivity of tropical land carbon uptake to water variations (γ_IAV,W) under future climate shows a large spread across the latest 16 ESMs (2.3 ± 1.5 PgC/yr/Tt H₂O). Based on the identified significant emergent relationship between γ_IAV,Wunder future climate and present climate, the mean and spread of future γ_IAV,Ware reduced by about 41% and 44%, respectively (1.3 ± 0.8 PgC/yr/Tt H₂O), using observations and the emergent constraint methodology. However, the long-term tropical land carbon-climate feedback uncertainties in the latest 16 ESMs can no longer be directly constrained by land carbon cycle IAV compared with previous generations of ESMs, given that additional important processes such as tree mortality are not well represented in IAV but could determine long-term tropical land carbon storage. This result highlights the importance of recommended out-of-sample testing for validating previously diagnosed emergent constraint. In summary, our results suggest the limited implication of IAV for long-term tropical land carbon-climate feedbacks and help isolate remaining uncertainties with respect to the effects of water limitation on tropical land sink in ESMs.