Unfortunately it would have to be a myriad of similar innovations --

1. Insanely inexpensive ocean water desalination and purification
2. Crazy cheap home water recycling
3. National reasonably priced irrigation systems for #1
4. A way to send areas with too much water over to those who need it

We'll have the emissions thing solved in a reasonable amount of time. We'll have ample power for almost anything. But what good is all that without water?