Does anyone know a good global warming experiment to do for a science fair?

If it's gotta be a hands-on project with props you can display in a demonstration, the only thing I can think of is a demonstration of the greenhouse effect.

You would need a heat source to serve as the sun -- maybe a high wattage incandescent lamp with a reflector -- and one or more fairly large clear containers -- shallow boxes with a glass or clear plastic top. Greenhouse gasses (carbon dioxide, methane, and a number of organic compounds) act the same way as a clear solid sheet acts: they let light pass through, the light is absorbed by the ground and buildings, the energy is transformed into heat (entropy), and the greenhouse gasses trap the heat (just as a clear cover over a box would do).

Have one box with no cover (no green house gases) and one box with a cover. Put in some kind of diorama (miniture buildings, people etc.) if you have something like that. And have a thermometer for each box, but don't let the light shine directly on it. The temperature will rise much faster in the box with the clear cover.

You could also have two diorama boxes to demonstrate the increased effect of global warming as we lose all the snow pack (glaciers, artic ice cap). One box would have a white glacier in it (you don't have to use real ice) and the other would have blue ocean and or green trees -- the darker the better. The point is: As the polar ice caps melt, the light they have been reflecting will be absorbed by the darker surface whether it's land or water, thus raising the temperature. Shine your heat source on the dioramas and the thermometers will show the difference in temperature change. You can do these two boxes with or without covers.

If you have closed containers, you might be able to add some humidity to one and demonstrate how more heat creates more humidity which traps even more heat ("positive feedback" -- the runaway train effect). Definitely experiment with this first to be sure it works. I've never tried it.

A couple of attractive posters explaining the theory of global warming and your apparatus and, you may not win, but you oughta at least pass.

Good luck!