This is a really nicely designed site which helps you calculate the amount of carbon your family emits. The site looks great and is rich in vocabulary. It's probably more useful with more mature students as it asks for information like the amount you pay for your gas bill etc, but you do have the option to say you don't know.
The CO2 Calculator is aimed at a UK audience and asks for post code, but you can just type in a few letters and numbers at random and proceed.
Would be a useful tool for creating a Carbon league within your class, to see who produces the least carbon.
If you wanted to follow this up with some research. Wikipedia has some useful information on Greenhouse Gases.