Global detection of aridification or increasing wetness in arid regions from 2001 to 2013

Arid regions are highly vulnerable to climate change and human activity, and global warming in particular has the potential to increase the arid land area. One traditional way to evaluate the extent of climate change in dryland regions is to use the aridity index (AI), defined as the ratio of annual rainfall to annual potential evapotranspiration. However, the AI is a climatic index; it does not represent actual conditions of aridity in arid regions. In contrast, the satellite-based aridity index (SbAI), which is based on day/night land surface temperature changes, is considered to represent actual conditions of moisture availability. Arid regions during 2001–2013 were classified at global scale by comparing the SbAI with the AI, that is, within Turc space, which is based on the water balance concept. In addition, factors contributing to aridification or wetness changes detected in different global regions were examined by comparing the SbAI and AI result with the yearly maximum normalized difference vegetation index and past land use. As a result, dryland regions were newly classified into five zones. In the stable zone, land areas were classified into hyper-arid, arid, semi-arid, or dry sub-humid regions by both the SbAI and the AI. Areas in the transition zone toward dryness are moderately dry. Areas in the transition zone toward wetness include large river basins, oases, and wadis with little rainfall. In the moist zone, rainfed or irrigated farming is being successfully conducted. Many parts of the dry zone, however, are extremely dry.