Ummh, no. What's happened is two things - first of all the massive growth of Sunbelt cities relative to the old urban centers of the northeast and midwest, something which wouldn't have happened without the invention of AC.  There's also the ability of people to avoid extreme discomfort courtesy of technology and affluence. AC use is skyrocketing in China not because of global warming but because of rising urban incomes.  Americans take it way overboard but that's nothing to do with climate change. (My basic criteria for using heat and AC at home are can I be comfortable dressed in an undershirt, shirt, sweater, long pants and warm slippers; can I be comfortable dressed in shorts, sandals and shirtless or in a t-shirt depending on whether I've got company - if not, on it goes)

"Either the first settlers were very irresponsible, or global warming (at least in the cities) crept quite a distance... Are we really to suppose that summer temperatures in the biggest cities (like LA, Tokyo, etc.) were just as unbearable 100 years ago, without any air conditioning?"

Most of the southwestern U.S. was settled after A/C was invented. LA only passed the 1 million mark in the 1920s, and places like Tucson and Denver were quite small until the 1950s.