Can dry get wetter even if rainfall declines?

Drylands are 57% of the terrestrial area of the world, and are disproportionally affected by climate change. This is particularly pertinent in so-called “climate-change hotspots” such as the Mediterranean, where temperature increases at a rate of up to 0.45 ^oC/decade and precipitation is expected to decline. Given the sparsity of studies in drylands and the consequent lack of understanding of the unique processes in drylands, the degree to which these projections are accurate for drylands is questionable. The fact that drylands, by definition, are classified according to the aridity index, exposes the inherent assumption that desert hydrology is primarily governed by precipitation and potential evapotranspiration (ET₀). There is increasing evidence, however, that non-rainfall water inputs (NRWIs; fog, dew, and water vapor adsorption) are a substantial source of water in multiple desert environments. In arid and hyper-arid drylands, water vapor adsorption is not only the least studied of the three NRWIs, but also likely the most common. In the Negev desert, Israel, the projected decrease in rainfall and increase in temperature, and therefore increase in ET₀, is expected to result in drier soils. This potentially will increase the amount of water vapor adsorption. Here we present the actual rate of warming and the corresponding changes in ET₀ in the Negev desert. We then elucidate, for the first time, the contribution of water vapor adsorption to desert hydrology and how it might be affected by climate change based on changes observed in the last 20 years.