Yes the world would be free of climate change if there were again only 100 million people. However that isn't a solution I would advocate.

> Perhaps the most cost efficient way to combat global warming is lifting the world's poorest out of poverty. It's been shown as populations become richer/educated their growth slows and shrink over time.

Exactly. And we do that by more technology development into superior technologies that don't suffer the inefficiencies of carbon sources. And we will get there faster with more people, more people working on these issues.

Btw more population also usually results in more urbanization and more efficient energy use per person.

I also believe that the effects are not gone be as bad as some people predict, as was the case with most of the 'this is gone kill us all predictions'. Slow adjustment over decades is gone mitigate most of the issues.