From :http://www.atmos.washington.edu/~dennis/Our_Changing_Climate.html
Numerous studies have shown the benefits from increasing CO2 levels on plant growth, but until recently, few studies have been conducted to understand how plants respond to the lower CO2 concentrations that were the norm during the glacial periods. Currently, CO2 levels are slightly above 400 ppm. However, during the last glacial period, ending about 20,000 years ago, CO2 levels were as low as 170 to 180 ppm. Below 200 ppm marks the very lowest level for CO2 since plants evolved and at these levels most plants are essentially starved for CO2.
