Background
Piao et al. set the stage for their study by noting that "enhanced terrestrial vegetation growth in the middle and high latitudes of the Northern Hemisphere over the past two decades has been well documented (Zhou et al., 2001; Nemani et al., 2003)," but they report that "the mechanisms for this phenomenon are still under debate."

What was done
Using a leaf area index data set for the period 1981-2000, which was created from satellite-derived observations of the normalized difference vegetation index parameter for land areas above 25°N latitude, the authors investigated "spatial patterns of mechanisms controlling current enhanced vegetation growth in the Northern Hemisphere," focusing on "how recent changes in precipitation, temperature [and] atmospheric CO2 concentration have influenced vegetation growth."

What was learned
Over the final two decades of the 20th century, the researchers found the mean rate of increase in growing-season leaf area index to have been 0.0041/year, and that 13% of that increase was provided by increases in precipitation, 31% was provided by increases in temperature, and that fully 49% was provided by the increase in the atmosphere's CO2 concentration.

What it means
In response to what climate alarmists describe as unprecedented increases in the air's CO2 content and temperature, which they characterize as phenomena worse than nuclear warfare and global terrorism, the bulk of the terrestrial vegetation of the Northern Hemisphere north of 25°N has not only not suffered because of them, it has actually grown more robust.

References
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.

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.