A better question is:
How do you know that in 50 years this will in fact happen? Not to be rude, but do you know the history of such doomsday sayings regarding oil? It was supposed to "run out" by the 1990's or 2000...every year it gets pushed back some decades.
This, along with the question of, "When we run out of food in 20 years..." etc. -- it just shows that the propaganda we are spoon and shovel-fed out-paces the facts.
Look: I'm all for "clean" energy sources - I'm not some oil tycoon - but for as long as I have been alive, we have been warned that:
oil or "fossil fuels"
etc. will run out by "30 years' time" or "the 21st century."
After almost 40 years of this propaganda - with the only thing changing being the players in the propaganda positions of power or the actual prophetic dates - I want to know:
In 50 years, when we're all running our skateboards on gasoline or diesel, will we actually still be asking questions about what to do when oil runs out?
(No offense to you, Daniel Neumann, I love the environment, but do not like the assumption that oil will run out - I 100% do not believe it will in our time, or my great great great great grandkids' time.)
Having said all that - the irony is that I don't frankly think we need another energy source (see your other answers) - we just need better people in power to make the changes to use what we have already shown to work.
When will THAT happen?