This was the first time I have ever seen the movie, and it shocked even me. The absolute clarity of his core argument is amazing and indisputable. I was also compelled to watch it after reading this Guardian article entitled, "Global warming: the final verdict". The short version is that the 1000's of climate scientists are in 100% agreement that the earth's temperatures are rising dramatically and it is caused by human activity. The debate now is what will happen. Keep in mind that this article is thus a conservative look at scientific debate, because the stuff they are arguing about is not even in the report.
This graph was the main point of the movie in my opinion. It is from the journal "Science" and it is the CO2 concentrations in core samples of Antarctic ice going back 650,000 years. (top graph) Below that is the corresponding temperature that the ice was formed at, taken from the same samples. He points out that the ups and downs are normal, and each one equals an ice age. Or more personally, a mile of ice over Minneapolis. But the CO2 concentration is now are higher than they have ever been in 650,000 years, and the red line is the projection for the next
I have also been frustrated by the idea that the earth is so big, how can we be messing with it? He pointed out that on a normal globe, the atmosphere is thinner than its coat of varnish. So all these people living under a strip of varnish can and are changing its chemical make up. That doesn't seem to hard to picture in your mind.
So, either go rent it, or watch it for free on your computer. I watched it in two parts even.