Most plants respond very favorably to increased CO₂ concentration of surrounding air. Yields of agricultural crops and trees increase substantially when they are supplied with additional CO₂ enhancement.

Corn yields typically increase by 20% in response to a doubling of the atmospheric CO2 concentration. Grain production increases 40% in high CO2 conditions, trees show 50% yield increases, and vegetables can have up to a 40% yield increase.

CO₂ enrichment has many other benefits to crops, directly and indirectly, like increased soil fertility and improved soil structure.

http://www.greeningearthsociety.org/Articles/1999/averts/averts1.htm

http://collection.nlc-bnc.ca/100/200/300/fraser/g_warming/consequences.html