Nothing about our energy usage is unsustainable, yet. Just our energy sources. We have the technological ability to replace the majority or our petroleum based fuels with electricity or bio-fuels, but we don't because the cost and supply of oil is very cheap and very abundant. When that is no-longer the case there will be a period of stagnated growth while various industries are bailed out and cannibalize each other and probably a few million people perish unnecessarily, but for the rich, they will be almost untouched.
Of course the above is just under-capitalism. A government that prioritized people over profit could make the transition off of oil with much less population fallout. Who knows if such a government will ever exist though. I guess China is close, but still not good enough.