Recent satellite records have revealed a global greening process over drylands, but to date, the reasons for this greening have been somewhat of a mystery. A new study just published on nature.com on February 12, 2016 by authors Xuefei Wang, Lixin Wang and Matthew F. McCabe concludes that
…higher concentrations of atmospheric CO2 induce plant water saving and that consequent available soil water increases are a likely driver of the observed greening phenomena.
Figure 1: Global climate map and a comparison of mean effect size and soil water response under elevated CO2.
(A) Site locations of the CO2 enrichment experiments together with globally distributed climate zones based on a standard aridity index formulation (precipitation/potential evapotranspiration); (B) Mean effect size of soil water content under elevated CO2 for the entire data set, under dryland and non-dryland regimes. The effect size was calculated as the natural log of the magnitude of an experimental treatment mean (the soil water under elevated CO2) relative to the control treatment mean (the soil water under ambient CO2); The dashed line indicates the threshold of statistically significant CO2 effect on soil moisture. The effect is positive when above the line and vice versa. (C) Enhancement of soil water content under elevated CO2 for dryland versus non-dryland regimes. The number of cases is shown in brackets. Error bars are bootstrapped confidence intervals (CI). All the statistics are significant at P < 0.05. The map was generated using ArcGIS for Desktop 10.3.1 (http://www.arcgis.com).
The 27% increase of CO2 in the atmosphere (from 315 ppm to approximately 400 ppm) over the period 1960-2015 is credited by the authors as being the only significant cause of the greening. While they investigated several other possible drivers of the observed changes, such as temperature increases and rainfall increases, none was shown to have a statistically significant impact on greening other than CO2.