It's difficult because most of the evidence of global warming is not easy for non-scientists to see. For example, if you've lived by the sea your whole life - even if you were 100 years old - you'd not have seen any noticeable rise in sea levels. Similarly, the weather seems pretty erratic and always has done, so changeable weather is hardly rock-solid proof. Warm summers are followed by cold winters - look at the European winter of 2009/2010 - which is confusing for laymen.
One thing that no young scholars should be required to do in a science lesson, is believe "facts" without having them demonstrated, proven or explained. How can they become scientists in the future if they are not encouraged to doubt, to question, to test and, ultimately, to understand?
I would approach it from the point of view of pollution. "Here's the nice clean world that humans found themselves living on. Is it right that we should violate it with our waste, or are we responsible for keeping it in a condition suitable for human life in the time of our children, and grandchildren?"