Why we [might] need fusion power:
a) Shipping. Big container ships *can't* be sail-powered (the largest sailing ships ever are about 1% of the size we'd need). But existing marine nuclear power is a proliferation and/or meltdown risk. If we can make a small fusion reactor, we could make our existing global supply chains carbon-neutral.
b) Continuous industrial processes that can't survive a power glitch, much less nighttime on a windless night (no renewables). There are a lot of those.
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@cstross Fusion power actually worries me. Because it comes with the implicit assumption that when our power is "clean" we can use as much of it as we like. And we can't: Earth can only shed waste heat via well-understood radiation into space. Which isn't enough if our power usage continues to increase at its current rate, even "clean" we cook ourselves within a couple of centuries.