Those who argue that increasing carbon dioxide is good because it's "plant food" should consider this article from the WSJ about poison ivy. It says:

Poison ivy, the scourge of summer campers, hikers and gardeners, is getting worse.

New research shows the rash-inducing plant appears to be growing faster and producing more potent oil compared with earlier decades. The reason? Rising ambient carbon-dioxide levels create ideal conditions for the plant, producing bigger leaves, faster growth, hardier plants and oil that's even more irritating.

Although the data on poison ivy come from controlled studies, they suggest the vexing plant is more ubiquitous than ever. And the more-potent oil produced by the plants may result in itchier rashes. "If it's producing a more virulent form of the oil, then even a small or more casual contact will result in a rash," says Lewis Ziska, a plant physiologist for the U.S. Department of Agriculture in Beltsville, Md.