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We moved to ATL in 1964.  I recall most of the houses we looked at had central AC, and it was a big factor even though the house we bought (new) did not.  We had a large window unit in the family room and a whole house fan.  My parents installed central air in 1970.  I think by 1970 every new middle class house had central air.

There is a residential area just north of me that was started in 1904.  Nearly every house there has a large front porch, which was their version of AC along with fans.  It's now a rather expensive area so they all have AC today.  Some of the houses have  been demo'd and replaced with new, most were just restored/updated.  They look really nice.

Living here without central air, even today in June, would be ... unpleasant.  I was just out running and I'm soaked.  There are other factors for population growth, in the 1950s there were a lot of cloth mills around, carpet production, etc., because labor was cheap, so was land.  The textile industry then moved to Asia leaving some towns here bereft.

But I think without AC,  the South would have half its current population.

Really?  June?  Realistically speaking, most of the time we have to crank our AC's up in late February or certainly March.  By June it's at least 90+ degrees, and as always the humidity is super high. 

I think a lot of the population increase of the southern states can be traced back to several different factors, not just necessarily AC.  For example, Union labor is much less common in the South, so companies have a much easier time getting things done.  There are lots of other things that I think are "pro-business" but also we're simply less regulated as an individual and also much of the time less taxed.  

There are just natural things that exist in much of the south that you simply cannot reproduce like oil, which leads to oil refineries, which needs shipping which needs ports.  Last I checked there is no ocean front property in Ohio and many of the more northern states.  

Ehhh. More than a few lake ports in those cities.

Also lots of railways connecting them... 

We also had reverse migration as blacks went north for jobs.  When I was a kid, the South was impoverished, mostly, the small towns were suffering, infrastructure was problematic.  The black folks had it worst of course because of the Jim Crow stuff.  There were low taxes and a small tax base.  The Interstate highway system helped a lot, I think.  Every exit on a new expressway soon attracted stuff, often not very pretty stuff, but stuff.  As noted, companies opened factories because labor was cheap, and now transportation by truck became practicable.  DoTs built bypasses around many towns that led to Walmarts which tended to ruin the downtown areas, some of which recovered a bit due to tourism (see freeways).  I think much of north Georgia exists because of Atlanta, it's an easy drive up into the mountains and the tourist towns have stuff to do.