My parents recently got back from a two-week trip to Alaska.
While in Alaska, they saw some glaciers. I remembered that in An Inconvenient Truth, Al Gore mentioned how some glaciers in the Alps have retreated quite a bit within the historical record, showing pictures from the late 19th century and comparing them to modern-day photographs. It turned out that some of the glaciers there have receded within the memory of the people who live there, but others have actually advanced.
It seems to me that the only way to know for sure if global warming is causing glaciers to retreat is to analyze a large number of them for which there is a historical record. (Although I'm not a climatologist, the Alps seem like the best site for such a study simply because people have been living there for quite some time.) Anecdotal evidence just isn't valid here. However, it seems like there are better ways of tracking temperature than looking at glaciers (for example, just looking at actual temperature numbers!) and so the glaciers are more a convenient symbol than anything, because miles-long ice formations seem like something that humans just couldn't possibly melt! What one person knows is simply an inadequate sample, although it can make for a powerful story.
The news of the moment regarding Al Gore and global warming seems to be that the snows of Kilimanjaro are retreating, but that's not due to global warming but rather due to a comparatively dry weather period over the last century or so. The retreat began about a hundred years ago, whereas global warming didn't really become a big effect on climate until maybe forty or fifty years ago. The ice on Kilimanjaro is sublimating, not melting. Philip Mote of the University of Washington has said that Gore shouldn't use Kilimanjaro as a symbol for global warming, as he has in the past. He suggests that [t]here are dozens, if not hundreds, of photos of midlatitude glaciers you could show where there is absolutely no question that they are declining in response to the warming atmosphere,". I agree. Scientists are often blind to the power of good symbols in getting across their message to non-scientists.
Mathematicians, more so.