Determination by MODIS satellite-based methods of recent global trends in land surface aridity and degradation

The Aridity Index (AI; the ratio of precipitation to potential evaporation), is an indirect, essentially climatic index that has been used to identify global distributions of arid regions. Since the turn of the century, the increasingly widespread availability of satellite data has led many researchers to use it to assess desertification. In this study we used a satellite-based aridity index (SbAI) to investigate global changes of land surface aridity from 2000 to 2017. Degraded land areas were identified by using SbAI and the normalized difference vegetation index (NDVI). Here, degraded land include existing desert and the land having both permanent and temporal dust erodibility. Our results showed that actual land condition by SbAI became dryness than that derived from the climatic AI indicator in arid, semi-arid, and dry sub-humid regions and that the dryness was reflected in an increase in the extent of areas classified as hyper-arid. From 2000 to 2017, however, the annual extent of the wetter areas within arid regions (semi-arid and dry sub-humid regions) increased, and that of drier areas (hyper-arid and arid regions) decreased. The global area of degraded land decreased slightly between 2000 and 2017, and the annual average area for that period was 13.5×106 km2 (9.2% of total land area).