You need a schedulable power source if you want to fully replace carbon. A lot of batteries with super high capacity (that may exists in the future) could stabilize solar enough, and a lot of solar arrays may give enough power, but in summers you you will be forced to throw away some of the energy, which is a big waste. And this is an hypothetical scenario, nuclear is a technology that already exists, we could have decarbonised decades ago using nuclear. -> Don’t get me wrong, I don’t mean that we should rely on nuclear power alone, but we should first cover the base energy load with nuclear, then use solar and wind for the rest