Air pollution/climate change/etc. That issue is going to get
   tougher as our technological needs increase. Got stuff that's
   powered? Don't want to use a foot pedal? Then you need an engine
   to do it for you. How do you get the engine to do it for you?
   Electricity is nice. Alternative current back and forth - great
   for motors. But then, how do you make electricity? You need a
   generator. How do you get the generator to generate the
   electricity? It's gotta move. Steam's nice for that. How do you
   make steam? Water and heat. How do you get the heat? and...
   that's where things get messy. But, I don't see an increase in
   the demands of foot powdered phone chargers, so I guess "less
   offensive" noxious burning residue is best. I read something
   about the debate between shale vs coal; whole things's a mess.