William Brangham:

As we can see, these protests today are easily the biggest climate change demonstrations in history.

As we heard, this movement is calling for a fundamental remaking of how we power our modern world.

I'm joined now by our science correspondent, Miles O'Brien.

Miles, walk us through some of the practicalities. If these youth activists get their way, they would like us to be, by 2030, 100 percent green renewable energy. How tough is that?