Background
The authors write that "in recent decades considerable interest has been focused on trends in vegetation phenology," due largely to climate-alarmist predictions that human-induced increases in CO2 emitted to the atmosphere will lead to climate changes that have largely negative impacts on earth's natural and managed terrestrial plant life.

What was done
Working with Normalized Difference Vegetation Index (NDVI) data derived from satellite-assessed measurements of the optical reflectance of sunlight in the red and near-infrared wavelengths, as described by Rouse et al. (1974) and Tucker (1979) - which were obtained and developed by the Global Inventory Modeling and Mapping Studies (GIMMS) group at NASA's Goddard Space Flight Center from the Advanced Very High Resolution Radiometer (AVHRR) instruments that were part of the National Oceanic and Atmospheric Administration (NOAA) Polar Operational Environmental Satellite (POES) series - Eastman et al. developed a 30-year (1981-2011) series with a spatial resolution of approximately 8 km at the equator and a bi-weekly temporal resolution.

What was learned
The five researchers report that over half of all of the planet's land area experienced a significant trend in seasonality, and that of this group of significant trends, almost half of them were of three specific types: those experiencing a uniform increase in NDVI throughout the year, those with increasing NDVI in the green season and a decreasing NDVI in the brown season, and those with increasing NDVI in the green season only. In addition, they say that the first of these groups was composed of forests, and more especially broadleaf forests, while the second group was mostly composed of grasslands and shrublands, and the third group was associated with high-latitude regions of the Northern Hemisphere.

What it means
In concluding, the U.S. research team declares that "from this remarkable 30-year archive of satellite imagery, we thus see evidence of a greening trend," which clearly indicates that the net result of the climatic and physiological effects of atmospheric CO2 enrichment on earth's terrestrial plant life has in the mean been decidedly beneficial.

References
Rouse Jr., J.W., Haas, R.H., Deering, D.W., Schell, J.A. and Harlan, J.C. 1974. Monitoring the Vernal Advancement and Retrogradation (Green Wave Effect) of Natural Vegetation. NASA GSFC, Greenbelt, Maryland, USA.

Tucker, C.J. 1979. Red and photographic infrared linear combinations for monitoring vegetation. Remote Sensing of the Environment 8: 127-150.