Why though, we already have Nuclear energy, we could easily build enough that it could power the world's energy needs. The issue is storage, until we have a revolutionary storage solution very little will change with fossil fuel usage.
Storage is an issue for the other renewables due to intermittent peak power. Fusion should be able to operate like a traditional power station.
However! fusion plants are much larger than fission cores, and the neutrons are an order of magnitude more energetic, so you wind up with both way more mass and way higher activation.