The positive trend in vegetation greenness has been observed through satellite images, but the reasons for it had been unclear.
After analyzing 45 studies from eight countries, Lixin Wang, assistant professor of earth sciences in the School of Science at Indiana University-Purdue University Indianapolis, and a Ph.D. student in Wang's group, Xuefei Lu, concluded the greening likely stems from the impact of rising levels of atmospheric carbon dioxide on plant water savings and consequent increases in available soil water.
While recent findings based on satellite records indicate a positive trend in vegetation greenness over global drylands, the reasons remain elusive. We hypothesize that enhanced levels of atmospheric CO2 play an important role in the observed greening through the CO2 effect on plant water savings and consequent available soil water increases. Meta-analytic techniques were used to compare soil water content under ambient and elevated CO2 treatments across a range of climate regimes, vegetation types, soil textures and land management practices. Based on 1705 field measurements from 21 distinct sites, a consistent and statistically significant increase in the availability of soil water (11%) was observed under elevated CO2 treatments in both drylands and non-drylands, with a statistically stronger response over drylands (17% vs. 9%). Given the inherent water limitation in drylands, it is suggested that the additional soil water availability is a likely driver of observed increases in vegetation greenness.