I am lecturing the introductory level Physical Geography this fall and the last week of the class (3 lectures) is devoted to climate change. I have had several discussion with my teaching colleagues about how to teach it. Of all the things the students will learn, this may be the most important. Of course, I would like them to remember everything, but that seems unlikely. The biggest thing I would want them to take away is not the finer points of climate change, but why we know them. Those of us that are daily involved in doing science often forget that most of the public, including college students, don't have a good understanding about how science is done. The idea of the lone person in a lab coat still reigns. A lot of the popular opposition to anthropogenic climate change has to do with powerful misunderstandings about this.
All of My Faults Are Stress Related did a couple posts about how to teach climate change to students. Sadly given the level of my students (very few are science majors) I don't think a discussion would work. My idea is to talk about the past climate and climate change first. Show them the various types of evidence used from many different techniques and types of scientists. The evidence doesn't point different directions, but converges on an accept view of what the climate was like in the past. Only after this will I talk about present climate change. Then using the same lines of evidence that were discussed before, show them that the climate is now changing primarily because of the actions of us humans. Then end with some predictions about how the world will be different in the future.
There are some excellent resources available, including the IPCC reports, real climate, and nice images at global warming art (see image below) which should make my job easier.