For saving the planet? No problem
Although in the long run only the colonisation of space solves the problem simply because we'll run out of room.
I used to be 100% for a manhatten project for fusion power, but it seems it's always a little out of reach. I now believe we should have the project on almost immediately attainable things, to start harvesting the value of space like solar collectors and mining, once we're up there fission bombs are a perfectly acceptable method of drive. Become much wealthier as a race, we'll figure out fusion eventually. Probably. Possibly. Well maybe.
Pete