Science Fair Help (Environmental Science)?

Hey guys, i submitted my topic for the science fair (High School level), but my teacher rejected it. My topic was:

"The Effect of Various Pollutants on the Melting Time of Ice"

The idea was that i'd get a transparent container, and put an icecube in it, then time how long it takes to melt. Then i was going to put a new icecube in, and fill it with one specific automobile's exhaust (SUV, truck, hybrid, motocycle, etc) and see which had the least and greatest effect on melting time.

My teacher rejected it because she said the heat of the exhaust would cause the ice to melt faster, not the trapped light (like global warming/green house effect.) Does anyone have any ideas on how to improve this experiment, or make it easier, etc.

I was thinking of taking a "double boiler" approach where i'd have one transparent container inside of a transparent container. The inside container would have the icecube, and the outside would have the car exhaust. This way, the heat of the exhaust couldn't interfere with the ice, and would be a more realistic model of the Earth.

Comments, suggestions, concerns?

Thanks in advance,

Brendan