Canada's The Nature of Things, episode on Climate Change, free on iTunes (other episodes cost $2)
If you want to navigate to it, set your iTunes to Canada (My Store: Canada at the bottom of the iTunes home window). Then TV Shows->Nonfiction->The Nature of Things->The Best of The Nature of Things. I don't know if it's geofenced.
UPDATE: Incidentally I notice that CBC documentaries has a Twitter feed,
but like some other shows, they're using it more like an RSS feed; it just has tweets about the latest documentaries, no discussion or individual voice.