The following three video clip series is from the "Friends of Science" and it does a good job of explaining why we should have some big questions about man-caused global warming.

Friends of Science Global Warming Part 1 of 3

Friends of Global Warming Part 2 of 3

Friends of Science Global Warming Part 3 of 3

I agree completely with the conclusions of this series.  We are arrogant beyond words if we think we can have any meaningful impact on our planet's climate.  As pointed out, the Vikings once farmed in a much warmer Greenland than we have today!  Yet Greenland obviously got a lot colder and now farming is all but non-existent there!  What did the Vikings do?  Simply, some of them left and found other lands to farm and some of them probably adapted and took up other means of existence, like fishing and hunting!  That is what we should be doing – adapting!  We are going to spend $trillions trying to stop something that we have little influence over, when our money would be far better spent simply adapting.