Far in the future, the sun will expand, turning into a red giant, engulfing the earth and boiling the oceans away. Ultimately, it will turn into a white dwarf.

Far in the future, the sun will expand, turning into a red giant, engulfing the earth and boiling the oceans away. Ultimately, it will turn into a white dwarf. This hardly counts as renewable energy; it's just really vast energy. Nuclear energy is far less vast, but still huge compared to what we're using today. If you think of easy energy sources as civilization's starting capital, nuclear can give us a very long runway.