What is the most important engineering problem of our future?

[KenWittlief]

there is enough space and room for people to survive (at this point), but there are not enough resources for every person on the planet to have a 4 bedroom house, 2 cars, 4 computers, 5 TVs, all the food they can eat...

there are only so many acres of land that can be farmed. Once the population gets above 1 person per acre, or 2 people per acre of farmland, people will starve.

but why do we need to let our population grow that large? what benefit is there to having 6 billion, or 10 billion people, instead of only 1 billion, or 100 million...?

I cant see any benefit to having the surface of the earth covered with humans.

Maybe this is part of the engineering question - how can we prosper without the need for a constantly increasing population?

What science and technology advancements would allow us to turn the size of our population back, way back? To allow our standard of living to continue to improve, with less and less people?