What is the most important engineering problem of our future?

[IMDWalrus]

The biggest problem we're going to have? People.

The world's population is somewhere around 6.5 million right now; in ten years, according to a quick and unscientific measurement, we could very well have another million on top of that.

Finding ways to provide food and water, to create shelter, to have medical care, to transport people and goods, and to keep our world and all of its inhabitants alive and in one piece? It's going to take a lot of work from all kinds of disciplines to solve some of the problems that come with more people.

Engineers are going to be key in solving some of these problems, transportation and housing being two of them.

It's a broad answer, yes, but why not focus on everything instead of just one piece of the puzzle?