For a warm up I use the "What's the Word" Game created my ChrisKorea you can download all of them here:
http://www.waygook.org/index.php/topic,80445.0.html This is how I structured the lesson:
-What's the word warmup
-Ask students what Global Warming means
-Brainstorm ways people have destroyed/polluted the Earth on board
-Go through ppt
-Brainstorm ways we can help the Earth/stop global warming on board
-Continue ppt
-Split class into groups of four
-Handout Dialogue sheets, students write four ways they can help stop global warming
-While they write, set up the game on the board
-Repeat with second dialogue
See the notes in the ppt for further instructions on the game.