Multifaceted characteristics of dryland aridity changes in a warming world

Abstract

Drylands are an essential component of the Earth System and are among the most vulnerable to climate change. In this Review, we synthesize observational and modelling evidence to demonstrate emerging differences in dryland aridity dependent on the specific metric considered. Although warming heightens vapour pressure deficit and, thus, atmospheric demand for water in both the observations and the projections, these changes do not wholly propagate to exacerbate soil moisture and runoff deficits. Moreover, counter-intuitively, many arid ecosystems have exhibited significant greening and enhanced vegetation productivity since the 1980s. Such divergence between atmospheric and ecohydrological aridity changes can primarily be related to moisture limitations by dry soils and plant physiological regulations of evapotranspiration under elevated CO2. The latter process ameliorates water stress on plant growth and decelerates warming-enhanced water losses from soils, while simultaneously warming and drying the near-surface air. We place these climate-induced aridity changes in the context of exacerbated water scarcity driven by rapidly increasing anthropogenic needs for freshwater to support population growth and economic development. Under future warming, dryland ecosystems might respond non-linearly, caused by, for example, complex ecosystem–hydrology–human interactions and increased mortality risks from drought and heat stress, which is a foremost priority for future research. Estimates of global dryland changes are often conflicting. This Review discusses and quantifies observed and projected aridity changes, revealing divergent responses between atmospheric and ecohydrological metrics that can be explained by plant physiological responses to elevated CO2. Atmospheric, agricultural, hydrological and ecological indices of aridity reveal strongly divergent trends since 1950 and into the near future. Warming-driven increases in vapour pressure deficit hasten evaporative water loss and deplete surface moisture, in turn amplifying atmospheric drying through land–atmosphere feedbacks. Plant stomatal closure under elevated CO2 reduces transpiration and compensates for the adverse effect of higher vapour pressure deficit for plant growth, explaining the co-occurrence of ecosystem greening and atmospheric drying in drylands. The physiologically induced lowering of evapotranspiration under rising CO2, along with the strong limitation by soil moisture, disconnects atmospheric drying and hydrological responses in drylands. With rapid climate change and population growth, anthropogenic water demand in drylands is projected to increase by ~270% by the 2090s, exacerbating current water resource scarcity. As future water deficits are driven mainly by increasing water demand, sustainable water resource management and water conservation technologies are needed to balance the socio-economic demands for water resources while maintaining healthy dryland ecosystems. Atmospheric, agricultural, hydrological and ecological indices of aridity reveal strongly divergent trends since 1950 and into the near future. Warming-driven increases in vapour pressure deficit hasten evaporative water loss and deplete surface moisture, in turn amplifying atmospheric drying through land–atmosphere feedbacks. Plant stomatal closure under elevated CO2 reduces transpiration and compensates for the adverse effect of higher vapour pressure deficit for plant growth, explaining the co-occurrence of ecosystem greening and atmospheric drying in drylands. The physiologically induced lowering of evapotranspiration under rising CO2, along with the strong limitation by soil moisture, disconnects atmospheric drying and hydrological responses in drylands. With rapid climate change and population growth, anthropogenic water demand in drylands is projected to increase by ~270% by the 2090s, exacerbating current water resource scarcity. As future water deficits are driven mainly by increasing water demand, sustainable water resource management and water conservation technologies are needed to balance the socio-economic demands for water resources while maintaining healthy dryland ecosystems.