Oil, or lack of it. Production of world oil supplies are expected
to peak sometime between now and 2025. As oil is a gift deeply
entwined in our lives, the future without it will be quite
traumatic. Depending on who you listen to, the effects range from
the death of a growth based economy, to a population die off.

Fortunately, the US has lots of coal to make electricity with. Electric
motors with batteries will work just as well as combustion motors. It
just costs more to scrub the pollution from the coal emissions. Nuclear
power will look pretty attractive at that point too.