Aridity change and its correlation with greening over drylands

A drying trend and an expansion of drylands through history, as well as an expectation to continue under the future climate, have been inferred by recent studies. However, this seems to conflict with observed greenness over drylands. In this study, the changes in aridity over drylands from 1982 to 2011 were examined using the aridity index (ratio of precipitation and potential evapotranspiration), precipitation minus actual evapotranspiration, and soil moisture derived from a combination of observation and modelling. In addition, corresponding changes in vegetative greenness and their relationships with aridity changes were explored. The results show that above three indicators all point to a little change in aridity of drylands over the past three decades, and their trends in spatial patterns agree well each other. Simultaneously, significant greening (p < 0.05) occurred in more than 36% of vegetated drylands, in contrast to the 7% with significant browning (p < 0.05). Drylands as a whole, vegetative greenness demonstrated a significant positive relationship (p < 0.05) with precipitation change. At the biome scale, significant relationships (p < 0.05) were observed for vegetative greenness and precipitation for all other vegetation types except forests, suggesting that the increasing precipitation was one of the main drivers of greening over drylands. In addition, the largest increase of greening was observed for croplands, implying a strong impact from agricultural activities particularly irrigation practices. Results from this study provide clues for understanding the greening of drylands and are also crucial for the understanding of environmental changes in the context of climate change.