Land-atmosphere feedbacks drive dryland drought and expansion under climate warming

Drylands cover ∼45% of Earth’s land surface, support ∼40% of the global population, and harbor ∼30% of endangered species. However, anthropogenic climate change increasingly dries drylands through multiple processes and feedback mechanisms. From a thermodynamic perspective, global warming elevates atmospheric vapor pressure deficit (VPD) in drylands, accelerating moisture loss from vegetation and bare soils, thereby exacerbating aridity in drylands. This mechanism is further amplified by local land-atmosphere feedbacks: soil desiccation and vegetation dry out typically result in a reduced evaporation fraction. This, in turn, decreases the relative humidity yet further boosts the water demand of air. Furthermore, human-induced shifts in large-scale atmospheric circulations, reduce local precipitation and further aggravate aridity in subtropical drylands.

Despite various physical processes of dryland changes being documented in recent decades, the role of land-atmosphere feedbacks in shaping drylands’ water supply has remained elusive. In Science, Koppa and colleagues1 introduced an observationally constrained physical Lagrangian transport model to elucidate how land-atmosphere feedbacks drive the humid-dry transition on our warming Earth. They highlight the crucial role of upwind land-atmosphere feedbacks in regulating downwind hydroclimatic conditions under anthropogenic change over multi-decadal timescales. This influence is also significant for individual events in drylands that are amplified by upwind land-atmosphere conditions. Schumacher et al.2 systematically quantified the contribution of land-atmosphere feedbacks in influencing 40 major droughts worldwide, revealing their key role in propagating soil droughts across drylands. Together, we underscore the growing importance of land-atmosphere feedbacks not only in local drylands but also in their broader impacts on downwind regions across different timescales in the context of anthropogenic warming.

Long-term land-atmosphere feedback enables dryland self-expansion

Numerous studies have shown that climate warming since the 1950s has caused significant drying trends (aridification) and widespread expansion of global drylands, particularly in southern Africa and the Sahel. While dryland expansion and aridification are often attributed to rising VPD in a warming climate—reflecting both the increased evaporation potential and the inability of evaporation to offset atmospheric drying—the detailed mechanisms and quantitative contributions of these drivers remain uncertain.

A universal atmospheric transport model provides an opportunity to unravel the contributions of upwind land-atmosphere feedbacks to dryland self-expansion. This Lagrangian modeling framework traces the trajectories of global air parcels to examine heat and moisture source-receptor relationships. Koppa et al.1 applied this approach to identify the influence of land-atmosphere feedbacks on global drylands from 1980 to 2018. In doing so, they demonstrated that land-atmosphere feedbacks—the processes whereby upwind drylands cause aridification in downwind humid regions, termed dryland self-expansion—account for more than half of aridity increases in ∼40% of global humid-dry transition areas.

In dryland self-expansion, it is the drying of upwind drylands that determines the downwind humid-dry transition, despite the upwind drylands not being the primary moisture supply of downwind areas. Even in the regions where moisture and heat primarily originate from humid areas, such as Australia and eastern Eurasia, upwind dryland drying still drives downwind aridification. The interconnection of moisture and heat transport between drylands and adjacent humid regions fuels these land-atmosphere feedbacks. More explicitly, the drying of drylands limits local evaporation and elevates sensible heating over the long term. This process inflicts downwind aridification through air advection over the course of years and decades, further reducing downwind evaporation and increasing downwind potential evaporation (conceptually linked to enhanced sensible heating), resulting in persistent dryland self-expansion. It should be noted that this upwind dryland aridification includes two pathways—dampened precipitation and rising potential evaporation—and differs across regions. Reduced precipitation in upwind regions is the primary driver of dryland self-expansion in the subtropics and the Southern Hemisphere, while enhanced potential evaporation in upwind areas predominantly drives self-expansion in the Northern Hemisphere mid-latitudes.