From:http://onlinelibrary.wiley.com/doi/10.1111/j.1469-8137.2010.03441.x/pdf
Higher concentrations of CO2 also have another beneficial effect upon plant growth. As CO2 levels increase up to certain limits, plants are able to use water more efficiently. The reason for this is that the stomata, the pores on the leaves of plants, must remain open longer at low CO2 concentrations to allow sufficient CO2 to enter the plant. CO2 is one of crucial ingredients in the photosynthetic process. While the stomata are open, water vapor escapes as transpiration. The longer the stomata remain open to allow sufficient CO2 to enter the leaf for photosynthesis to occur, the more water escapes. Thus, plants are more drought resistant at higher CO2 concentrations.
Many plants species evolved at much higher CO2 concentrations than we are experiencing currently. The predecessors to Gymnosperms, or evergreens, evolved about 360 million years ago when CO2 levels were about 4000 ppm, 10 times today’s levels. The Angiosperms, or flowering and deciduous trees, evolved about 160 million years when CO2 levels were about 2200 ppm, over five times current levels. So at the levels experienced during the last glacial period of 180 ppm, the plant kingdom was clearly under great stress. We are all quite aware that the survival of the more advanced members of the animal kingdom which includes humans, are clearly dependent upon the well-being of the plant kingdom. So it would appear that during recent glacial periods, much of the life on earth was in jeopardy.
Will rising CO2 levels enhance the growth of many plant species existing today? The answer is clearly yes. A note of caution is warranted here in that the burning of coal, oil and wood have been responsible for most of the increase in CO2 concentration in the modern era, and the burning of these fuels also releases many other toxic substances and pollution into the atmosphere, such as soot, nitric oxides, sulfur dioxides, and trace amounts of radioactive material. However, it would appear that within certain limits the increase in atmospheric CO2 has been beneficial to date, and very likely will continue to be for some time into the future.
