The US has been responsible for a large plurality of the emissions of CO2 into the atmosphere. China is now the largest emitter, but it's not particularly close if you integrate that function over time.

Looking at the data, it doesn't seem like the US transitioning to nuclear would have made climate change a "minor" problem: https://ourworldindata.org/grapher/annual-co-emissions-by-re...

For nearly all of the time before China outstripped the US, the EU (if you include EU-27) was a larger contributor than the US anyway. And much of the early US emissions edge came before nuclear power was invented. Global warming itself was poorly understood prior to the 1960s: https://www.scientificamerican.com/article/discovery-of-glob...

A transition to nuclear would have helped, certainly! But climate change would not be "minor."

People seem to be missing that this was my entire point... The OP made too strong of a statement.