There have been wars fought over energy resources, like oil & gas. If countries use alternative energy like wind & solar, they won't need to go to war to provide energy.
The wind & sun will not run out before humanity dies out. Useable uranium, coal, oil & gas probably will - which will force the price up a lot and cause huge problems. Look up 'Peak Oil' for example.
Fossil fuels release huge amounts of air pollution and cause huge environmental damage as you have to dig up millions of tons of the stuff. Forests are cleared, mountains have their tops blown off, rivers are polluted by waste etc.
Nuclear power creates waste and by collecting uranium together, you can help it be turned into nuclear bombs (both of these are overstated fears IMO).
Fossil fuels release far more CO2 than alternative energies. This causes the Earth to warm, and this could potentially be one of the most expensive and dangerous threats human civilisation has ever faced.
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