I watched a really interesting documentary on TV a few months ago about Global Dimming. It's a very controversial theory, in that many environmental scientists are scoffing at its existence, but it makes a lot of sense. Basically, the amount of light reaching the earth's surface is decreasing because of pollution, causing decreased evaporation, change in climate and crop failures. The scary thing is, global dimming is slowing global warming, so if we clean up our skies then the planet will get hotter. It's a bit of a Catch 22.

Tried to link to Google results for Global Dimming, comments won't accept the href link. Argh, just Google it yourself.