What was done
In the words of the authors, "vegetation net primary production (NPP) derived from a carbon model (Carnegie-Ames-Stanford approach, CASA) and its interannual change in the Qinghai-Xizang (Tibetan) Plateau [26.5-39.5°N, 78.3-103.1°E] were investigated in this study using 1982-1999 time series data sets of normalized difference vegetation index (NDVI) and paired ground-based information on vegetation, climate, soil, and solar radiation."

What was learned
Over the entire study period, NPP rose at a mean annual rate of 0.7%. However, Piao et al. report that "the NPP trends in the plateau over the two decades were divided into two distinguished periods: without any clear trend from 1982 to 1990 (R = -0.16, P = 0.67) and significant increase from 1991 to 1999 (R = 0.89, P = 0.001)."

What it means
The three researchers say their findings suggest that "vegetation growth on the plateau in the 1990s has been much enhanced compared to that in [the] 1980s, consistent with the trend in the northern latitudes indicated by Schimel et al. (2001)." In addition, they say that "previous observational and NPP modeling studies have documented substantial evidence that terrestrial photosynthetic activity has increased over the past two to three decades in the middle and high latitudes in the Northern Hemisphere," and that "satellite-based NDVI data sets for the period of 1982-1999 also indicate consistent trends of NDVI increase," citing multiple references for each of these statements. Consequently, Piao et al.'s work is simply one more piece of evidence for the inexorable "greening of the earth" that is occurring in response to (1) the ongoing recovery of the planet from the growth-inhibiting chill of the Little Ice Age, which was likely the coldest period of the entire Holocene or current interglacial, (2) the aerial fertilization effect of the historical and still-ongoing rise in the atmosphere's CO2 concentration, and (3) the growth-promoting effect of ever-increasing anthropogenic nitrogen deposition.

Reference
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.