One analysis appearing in the journal Weed Science (who knew?) compared poison ivy plants exposed to an environment replicating 1950 carbon dioxide levels -- about 300 parter per million -- with plants grown in today's atmosphere, where CO2 concentrations are about one third higher. 

Authors found that the modern plants were bigger, badder and better able to bounce back from grazing or other disturbances. They contained 50 to 75 percent more of the oil that causes the famous alergic reaction. 

Researchers at Duke University, meanwhile, say that higher CO2 is also making that oil more potent.

This is the sort of thing that brings global warming closer to home for most Americans. Probably more than news about malaria or dengue fever, however unpleasant they may be. 

By the way, that whole leaves-of-three thing? Apparently not. Here's a tutorial on the many flavors of poison ivy waiting in the woods near you.