It is obvious that physical growth (e.g., in energy) cannot continue
indefinitely on a finite planet.  We'll step through the amusing physical
conclusion that continued 2% energy growth consumes the entire Galaxy in
2500 years, putting Earth's surface at 100 C in just a few centuries from
now. We will then look at the less obvious conclusion that economic growth
must likewise cease.  In this context, we ask what level of existence may
be sustainable, and discuss the severe challenges that stand in the way
and put us at risk of failing to make a smooth transition away from fossil
fuels.