Background
The authors write that "many studies based on analyses of satellite images have detected a greening trend at global (Myneni et al., 1997; Nemani et al., 2003; Potter et al., 2007; Zhou et al., 2001) and regional scales (Donohue et al., 2009; Fang et al., 2004; Herrmann et al., 2005)." However, they say that "the response of vegetation to climatic changes widely differed by biome (Fang et al., 2005; Piao et al., 2006) and bioregion (Verbyla, 2008)."

What was done
Focusing on the grassland-oasis-desert complex of northwest China, Zhao et al., as they describe it, "investigated spatio-temporal changes in vegetation growth and their responses to a changing climate by biome and bioregion, using satellite-sensed Normalized Difference Vegetation Index (NDVI) data from 1982 to 2003, along with corresponding climate data."

What was learned
As a result of their efforts, the four Chinese researchers found that over the 22 years of their study, when annual mean temperature increased by 0.06°C/year, "about 30% of the total vegetated area showed an annual increase of 0.7% in growing season NDVI," which trend "occurred in all biomes and all bioregions except Sawuer, a sub-region of the study area with no significant climate change." And breaking this result into three sub-periods, they report that the NDVI increase was remarkable during 1982-1988, then tended to be slight, and finally actually declined a bit from 1998 to 2003, which pattern largely resembles the concomitant pattern of global air temperature change, which could have been responsible for the shifts in regional precipitation that appeared to be driving the observed shifts in NDVI. And in further support of this connection, Zhao et al. note that "previous analyses of satellite-measured vegetation growth suggested a greening trend of vegetation in the central United States (Wang et al., 2001, 2003) and the Sahel (Anyamba and Tucker, 2005; Herrmann et al., 2005) due to the effects of increasing precipitation at seasonal or annual scales."

What it means
As with most of the rest of the world: As global temperatures go, so goes the greening of the earth ... driven directly by the ongoing rise in the air's CO2 content, and possibly indirectly by warming-induced alterations in global precipitation patterns.

References
Anyamba, A. and Tucker, C.J. 2005. Analysis of Sahelian vegetation dynamics using NOAA-AVHRR NDVI data from 1981-2003. Journal of Arid Environments 63: 596-614.

Donohue, R.J., McVicar, T.R. and Roderick, M.L. 2009. Climate-related trends n Australian vegetation cover as inferred from satellite observations, 1981-2006. Global Change Biology 15: 1025-1039.

Fang, J.Y., Piao, S.L., He, J.S. and Ma, W.H. 2004. Increasing terrestrial vegetation activity in China, 1982-1999. Science in China Series C: Life Sciences 47: 229-240.

Fang, J.Y., Piao, S.L., Zhou, L.M., He, J.S., Wei, F.Y., Myneni, R.B., Tucker, C.J. and Tan, K. 2005. Precipitation patterns alter growth of temperate vegetation. Geophysical Research Letters 32: 10.1029/2005GL024231.

Herrmann, S.M., Anyamba, A. and Tucker, C.J. 2005. Recent trends in vegetation dynamics in the African Sahel and their relationship to climate. Global Environmental Change 15: 394-404.

Myneni, R.B., 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.

Piao, S.L., Mohammat, A., Fang, J.Y., Cai, Q. and Feng, J.M. 2006. NDVI-based increase in growth of temperate grasslands and its responses to climate changes in China. Global Environmental Change 16: 340-348.

Potter, C., Kumar, V., Klooster, S. and Nemani, R. 2007. Recent history of trends in vegetation greenness and large-scale ecosystem disturbances in Eurasia. Tellus B - Chemical and Physical Meteorology 59: 260-272.

Verbyla, D. 2008. The greening and browning of Alaska based on 1982-2003 satellite data. Global Ecology and Biogeoraphy 17: 547-555.

Wang, J., Price, K.P. and Rich, P.M. 2001. Spatial patterns of NDVI in response to precipitation and temperature in the central Great Plains. International Journal of Remote Sensing 22: 3827-3844.

Wang, J., Rich, P.M. and Price, K.P. 2003. Temporal responses of NDVI to precipitation and temperature in the central Great Plains, USA. International Journal of Remote Sensing 24: 2345-2364.

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.