Technologies that automatically reduce emissions (and increase efficiency) are being created every day, with or without help. In 100 years cars will be incredibly cheap, clean, and likely use alternate power sources, like electricity or nuclear power.

Currently the US Navy powers its submarines with a nuclear power pack that requires no maintenance or fuel for 40 years and is the size of a human being.

100 years ago we feared we would run out of hay for horses. Today we worry about fuel for cars. Exactly the same ridiculous fear -- why do we assume that gasoline power and coal power is what we'll use in 100 years?