A lot of the course books we use have a unit on the environment (to tell you the truth a subject that bores most students silly). So here is a a short exercise that make the the subject a little more palatable.
1 Ask students if the world will be better or worse in the future. The students then get into groups according to their answer.
2 They then write down reasons why.
3 Put the students into pairs/groups (one from the "pessimists" with one from the "optimists") and ask them to discuss their opinions.
4 Explain to students that they'll see part of a film that shows a possible version of the future. They have to say what the film maker is trying to tell us.
5 Show the first three minutes of Soylent Green.
(The first part of the film is a photo - montage which shows the world becoming more over-populated and polluted.)
6 Students then discuss their answers with each other. Ask them if they agree with this prediction and why/why not.
This can be used as a warm up exercise for any kind of environmentally orientated kind of lesson. Here is the whole film on Google Video.