We fire up coal/gas/biomass plants. Nature doesn't care about the ideological purity of the electricity grid - all that matters is total cumulative emissions

People treat this question with religious mindset, as if burning fossil fuels is a sin that must be banished. In reality, there's nothing wrong with powering countries ~80% of the time with renewables + storage, and ~20% of the time with fossils - that's still a decently decarbonized grid