No projected global drylands expansion under greenhouse warming

Model projections of future drylands distribution using a proxy based on atmospheric aridity show expansion under climate change, but may not be an accurate representation. An alternative index based on ecohydrological variables such as water limitation shows no global expansion of drylands. Drylands, comprising land regions characterized by water-limited, sparse vegetation, have commonly been projected to expand globally under climate warming. Such projections, however, rely on an atmospheric proxy for drylands, the aridity index, which has recently been shown to yield qualitatively incorrect projections of various components of the terrestrial water cycle. Here, we use an alternative index of drylands, based directly on relevant ecohydrological variables, and compare projections of both indices in Coupled Model Intercomparison Project Phase 5 climate models as well as Dynamic Global Vegetation Models. The aridity index overestimates simulated ecohydrological index changes. This divergence reflects different index sensitivities to hydroclimate change and opposite responses to the physiological effect on vegetation of increasing atmospheric CO2. Atmospheric aridity is thus not an accurate proxy of the future extent of drylands. Despite greater uncertainties than in atmospheric projections, climate model ecohydrological projections indicate no global drylands expansion under greenhouse warming, contrary to previous claims based on atmospheric aridity.

Automated summary

Model projections using an atmospheric aridity proxy show future expansion of drylands under climate change, but an alternative index based on ecohydrological variables suggests no global expansion. The aridity index overestimates changes compared to the ecohydrological index, indicating differing sensitivities to hydroclimate change and opposite responses to increasing atmospheric CO2. Despite greater uncertainties, climate model ecohydrological projections do not support global drylands expansion under greenhouse warming as previously claimed based on atmospheric aridity.