Multidimensional assessment of global dryland changes under future warming in climate projections

Drylands are the homes to over one-third of the world's population, and are vulnerable to anthropogenic climate change. Based on climate projections, recent studies reported a substantial expansion of global drylands in the coming decades and attributed that expansion to future warming. However, the expansion of drylands contradicts a widespread vegetation greening and a slight runoff increase in dryland ecosystems in the same climate models. Here we re-examine changes in global drylands for the coming century and at two future warming targets (1.5 degrees C and 2 degrees C warming relative to the preindustrial level) based on outputs of climate models who participated in the Fifth Coupled Model Intercomparison Project (CMIP5). In addition to aridity index (AI) that has been widely used to measure the atmospheric aridity, we also assess changes in drylands from the hydrologic and agro-ecological perspectives, using runoff (Q) and leaf area index (LAI) as indicators, respectively. Our results show that when the impact of elevated atmospheric CO2 concentration ([CO2]) on vegetation water consumption is considered in the estimation of potential evapotranspiration (E-P) and AI, the expansion of atmospheric drylands is at a much slower rate (similar to 0.16% per decade under RCP4.5 and similar to 0.30% per decade under RCP8.5) than previously reported. Moreover, the additional 0.5 degrees C warming does not lead to an evident further expansion of atmospheric drylands. In terms of hydrologic and agro-ecological drylands, both of them show significant shrinks over the coming decades, suggesting reduced hydrologic and agro-ecological aridity in the region. Finally, contrasting with previous perceptions, our results demonstrate that warming only plays a minor role in altering global drylands from all three perspectives. Increases in net radiation are primarily responsible for the expansion of atmospheric drylands, and increases in P and [CO2]-induced increases in vegetation water use efficiency are the key drivers of changes in hydrologic and agro-ecological drylands.