Some of the most effective aspects of Al Gore's Oscar-winning slideshow involved the photographic evidence that global warming was altering our world RIGHT NOW. This should help, too. Although it would be nice if it traveled to more cities - why should Boston, Greenwich CT, Vegas and a couple cities in California have all the fun?
Here's the Matterhorn, first in 1960(!) and then in 2006.
The ultimate before and after, no?