Teens, in general, know more about things that we give credit to. They may appear preoccupied with their cell phone or on social media but many pay keen attention to what is going on worldwide especially if it impacts their lives. They are faster at finding out answers (perhaps via Google) and many know what they are talking about including understanding global warming.
I think not all teenagers are equally aware of the problems of global warming. It depends on a number of factors: school and family environment, interest in political change and world news, friendship environment and more. In my opinion, the most important element in shaping the ecological culture of the younger generation remains the school, which has to work a lot in this direction.
The positive is that more and more young people and their organization are firmly behind the idea of nature conservation - this gives confidence to the future of the world.