Earth's Greening Continues - Peer reviewed paper.

From a paper published in Biogeosciences 10: 

Giraffes in the African Savanna Image 123RF

Global changes in dryland vegetation dynamics (1988–2008) assessed by satellite remote sensing: comparing a new passive microwave vegetation density record with reflective greenness data
N. Andela, Y. Y. Liu, A. I. J. M. van Dijk, R. A. M. de Jeu, and T. R. McVicar

Abstract. Drylands, covering nearly 30% of the global land surface, are characterized by high climate variability and sensitivity to land management. Here, two satellite-observed vegetation products were used to study the long-term (1988–2008) vegetation changes of global drylands: the widely used reflective-based Normalized Difference Vegetation Index (NDVI) and the recently developed passive-microwave-based Vegetation Optical Depth (VOD). The NDVI is sensitive to the chlorophyll concentrations in the canopy and the canopy cover fraction, while the VOD is sensitive to vegetation water content of both leafy and woody components. Therefore it can be expected that using both products helps to better characterize vegetation dynamics, particularly over regions with mixed herbaceous and woody vegetation. Linear regression analysis was performed between antecedent precipitation and observed NDVI and VOD independently to distinguish the contribution of climatic and non-climatic drivers in vegetation variations. Where possible, the contributions of fire, grazing, agriculture and CO₂ level to vegetation trends were assessed. The results suggest that NDVI is more sensitive to fluctuations in herbaceous vegetation, which primarily uses shallow soil water, whereas VOD is more sensitive to woody vegetation, which additionally can exploit deeper water stores. Globally, evidence is found for woody encroachment over drylands. In the arid drylands, woody encroachment appears to be at the expense of herbaceous vegetation and a global driver is interpreted. Trends in semi-arid drylands vary widely between regions, suggesting that local rather than global drivers caused most of the vegetation response. In savannas, besides precipitation, fire regime plays an important role in shaping trends. Our results demonstrate that NDVI and VOD provide complementary information and allow new insights into dryland vegetation dynamics.

From CO2 Science:

What was learned
The five researchers report that "NDVI was more sensitive to herbaceous vegetation changes and short-term precipitation variations," while VOD "was more sensitive to changes in woody vegetation and longer-term precipitation variations." And, as a result, they remark that "co-trends between NDVI and VOD provide evidence of widespread woody vegetation encroachment at the expense of the herbaceous vegetation component in arid regions, and arid shrublands in particular." And as their ultimate conclusion about the matter, they concluded that the "spatial distribution of trends suggests that a global driver (e.g., CO2fertilization) is causing a change in relative performance of woody vegetation compared to herbaceous vegetation," while further noting that evidence for woody thickening and encroachment was also found for some semi-arid drylands.

What it means
In spite of the postulated growing negative impacts of man and climate alike, the greening of the earth continues - and in places where it's toughest of all to be green (arid lands) - with the proposed impetus for the phenomenon being the likely-enabling role of anthropogenic-induced atmospheric CO2 enrichment.