Background
The authors write that "field observations and time series of vegetation greenness data from satellites provide evidence of changes in terrestrial vegetation activity over the past decades for several regions in the world," and they note that these changes in vegetative vigor "may consist of an alternating sequence of greening and/or browning periods," which can obscure what one could call the "big picture."

What was done
In a study designed to determine the net long-term trend in vegetative vigor of the entire planet, De Jong et al. employed "detection of trend changes in normalized difference vegetation index (NDVI) satellite data between 1982 and 2008," based on "time series of 648 fortnightly images [that] were analyzed using a trend breaks analysis procedure," which feat was accomplished for fourteen different classes of land cover (biomes).

What was learned
The four researchers report that short-term greening and browning trends were observed over portions of the studied period for almost 15% of earth's land surface; but they found that for the entire time period, "net greening was detected in all biomes," and that "the net global figure - considered over the full length of the time series - showed greening since the 1980s."

What it means
In discussing the current state of knowledge in this area, De Jong et al. write that "over the last few decades of the 20th century, terrestrial ecosystems acted as net carbon sinks, as evidenced by ecosystem process models and satellite vegetation data (Myneni et al., 1997; Schimel et al., 2001; Zhou et al., 2001)." And they say that "the easing of climatic constraints on plant growth as a result of increased CO2 concentrations and higher temperatures is a likely explanation for this effect (Nemani et al., 2003)." Thus, it can readily be appreciated that the twin evils of the world's climate alarmists - rising atmospheric CO2 concentrations and global warming - have actually been what has fueled the last quarter-century's greening of the earth.

References
Myneni, R.C., Keeling, C.D., Tucker, C.J., Asrar, G. and Nemani, R.R. 1997. Increased plant growth in the northern high latitudes from 1981 to 1991. Nature 386: 698-702.

Nemani, R.R., Keeling, C.D., Hashimoto, H., Jolly, W.M., Piper, S.C., Tucker, C.J., Myneni, R.B. and Running. S.W. 2003. Climate-driven increases in global terrestrial net primary production from 1982 to 1999. Science 300: 1560-1563.

Schimel, D.S., House, J.I., Hibbard, J.I., Bousquet, P., Ciais, P., Peylin, P., Braswell, B.H., Apps, M.J., Baker, D., Bondeau, A., Canadell, J., Churkina, G., Cramer, W., Denning, A.S., Field, C.B., Friedlingstein, P., Goodale, C., Heimann, M., Houghton, R.A., Melillo, J.M., Moore III, B., Murdiyarso, D., Noble, I., Pacala, S.W., Prentice, I.C., Raupach, M.R., Rayner, P.J., Scholes, R.J., Steffen, W.L. and Wirth, C. 2001. Recent patterns and mechanisms of carbon exchange by terrestrial ecosystems. Nature 414: 169-172.

Zhou, L., Tucker, C.J., Kaufmann, R.K., Slayback, D., Shabanov, N.V. and Myneni, R.B. 2001. Variations in northern vegetation activity inferred from satellite data of vegetation index during 1981-1999. Journal of Geophysical Research 106: 20,069-20,083.