Seems unlikely given the expansion in standard of living in developing countries, the use of fossil fuels in ICE's .. and elsewhere outside of power generation in the EU.
It's a factor, sure, but "biggest" .. not so much.
The rate of growth in nuclear power was putting on a trajectory to replace all or nearly all hydrocarbon based base load power sources. And it was political opposition to nuclear power that put a stop to that. No other factor comes close as a contributor to climate change in my estimation.
The transition from ICEs to electric battery cars is largely orthogonal to base load power, but even electric battery cars depend on a base load source, so the extent that CO2 emitting energy sources have been replaced by non-emitting ones is highly dependent on the base load sources.
The transition from ICEs to electric battery cars is largely orthogonal to base load power, but even electric battery cars depend on a base load source, so the extent that CO2 emitting energy sources have been replaced by non-emitting ones is highly dependent on the base load sources.