Here's a good rebuttal for anyone still running into the C02 - They Call It Pollution, We Call It Life crowd.
A favorite argument of global warming deniers is that the carbon dioxide is harmless because it's "just plant food". It turns out that like any other "food", too much will only cause harm.
Both Grist and Environmental Defense are covering new research showing that more carbon dioxide in the atmosphere will mean less food value (i.e. decreased nitrogen and protein) in our fruits and vegetables. Think you'll get around that by boosting your meat diet? Once more, think again. Lower food value in plants, means less protein for grazing cows and pigs and therefore less meat produced by each animal. Add to this the exploding demand for meat from developing countries like China and you will need to tell me where we're meant to get all of these food calories from.
Maybe the new denier jingle can be "C02 - They Call It Pollution, We Call It A Cure for Obesity".