Land–atmosphere feedbacks a mechanism of dryland expansion

Just in the last four decades, ~5 million sq.km of humid land has transformed into a dryland. A rapidly warming climate is expected to further accelerate this dryland expansion. Consequently, not only will societies face permanent water insecurity, but plant and animal biodiversity will be under threat. Despite these consequences, very little is known about the physical mechanisms which cause irreversible drying of humid regions. As a result, our ability to predict the future expansion of drylands and its impact remains limited. Learning how drylands have expanded historically from humid regions could hold the key to predicting how they might expand in the future. So far, dryland expansion has been attributed to shifts in atmospheric circulation, topography, and orbital cycles. However, these changes occur at timescales reaching up to millions of years and thus do not fully explain the current rate of expansion. In this regard, the role of vegetation response to atmospheric drying and the consequent changes in land–atmosphere interactions have been largely ignored as possible mechanistic pathways of dryland expansion. Here, by tracking the air flowing over drylands, I show that the warming and drying of that air by changes in dryland vegetation-driven land–atmosphere feedback contributes to dryland expansion in the downwind direction. As they dry, drylands contribute less moisture and more heat to downwind humid regions, reducing precipitation and increasing atmospheric water demand, which ultimately causes their aridification. In ~40% of the land area that recently transitioned from a humid region into a dryland, self-expansion accounted for >50% of the observed aridification. Our results highlight the urgent need for climate change mitigation measures in drylands and provide a scientific basis for land-based interventions to prevent irreversible drying of humid regions