Piao, S., Mohammat, A., Fang, J., Cai, Q. and Feng, J. 2006. NDVI-based increase in growth of temperate grasslands and its responses to climate changes in China. Global Environmental Change 16: 340-348.
What was done
Noting that the climate of China's temperate grasslands "experienced dramatic change in the past several decades," the authors thought it important to investigate the impact of that climate change on the productivity of the country's grasslands. This they did by analyzing Normalized Difference Vegetation Index (NDVI) data obtained from the U.S. National Oceanic and Atmospheric Administration's very high resolution radiometer at a spatial resolution of 8 x 8 km and at 15-day intervals from January 1982 to December 1999, comparing those results with temperature, precipitation and Thornthwaite (1948) moisture index data generated from 680 well-distributed climate stations across China.
What was learned
Piao et al.'s results indicated little to no increase in precipitation and moisture index, respectively, over the period of their study. In the case of temperature, however, there was a least-squares linear warming of 0.89°C between 1982 and 1999 with R = 0.59 and P = 0.009, which they described as "dramatic." Even more dramatic, however, was their finding that "mean growing season NDVI increased significantly (R = 0.73, P = 0.001) from 0.25 in 1982 to 0.28 in 1999," or by approximately 12%.
What it means
At first glance it would appear that the dramatic increase in temperature is what drove the dramatic increase in grassland productivity. However, more detailed analyses revealed, in the researchers' words, that "the positive effect of temperature on the growth of grassland decreased [our italics] as temperature rose [our italics]."
So what might have compensated for the temporally-decreasing strength of the growth enhancement provided by the warming of the region? As possible contenders for this honor, Piao et al. mention the "atmospheric CO2 fertilization effect, increased nutrient deposition, and human activity such as grazing management [and] land abandonment due to migration into urban areas."
Be that as it may, and whatever the answer is, one thing is clear: over the period of time that the world's climate alarmists claim the planet experienced "unprecedented increases" in both the CO2 content and temperature of the atmosphere (which are the twin evils that are continually demonized by the radical environmentalist movement), the productivity of China's temperate grasslands marched ever onward and upward to new and greater heights.
Thornthwaite, C.W. 1948. An approach toward a rational classification of climate. Geographical Review 38: 55-94.