I don’t think it’s at all accurate to say humans are a “tropical species” in the relevant sense. Someone who knows more about human evolution is welcome to correct me, but I thought the early evidence of homo sapiens is from high altitude regions like highland Ethiopia and inland Kenya. Which are, yes, in the tropics, but also have quite cool climates.
Also I’d like to see a cite that “the human boy is designed extremely well to survive in extreme heat”: compared to what species? I don’t think we’re well designed at all to survive temperatures over about 40 C, and it’s exactly that range where we’re going to be seeing a lot more hot days.
Statistically, a lot more people per year die from extreme heat than from extreme cold (although that’s not all that informative a statement, since part of that is just inherent to temperature response curves in general). Both in the United States and worldwide. You can always deal with extreme cold by wearing more clothing (and people have, for many millennia).
Boston and Montreal are quite pleasant enough already, and they certainly could not “use a little more heat”, whatever that means.
Food is the big problem. After 104F/40C plants rapidly lose the ability to photosynthesize. In places where it gets so hot that it stays in triple digits all night you will not be able to grow plants. The arable areas will move north and south into parts of the planet that have less land area and that don’t have a good basis to farm intensively–poor soil, lack of centuries of rich alluvium to make things easy and the northern tundra lands will, as they melt, contribute to ever higher temperatures. If we can move it all underground using hydroponic methods we’ll be able to produce food, but not at the rate or variety we do now. All those millions of acres of grassland won’t be there any more so forget that omnivore diet–chicken and rabbit will be about the biggest pieces of meat you’ll see.
Food and clean water will be the big limiters of what’s possible. We may end up living in seriously crowded conditions to maximize growing areas, keeping cities at the very edge of the too hot zone and keeping population density super high to minimize the need for long haul food transportation. We might survive it if we get woke and start planning for it, admitting that all coastal cities are unsustainable and moving inland and uphill and concentrating population into Tokyo levels. I have every anticipation that American exceptionalism will provide no end of justifications as to why we simply can’t take these measures, but what else would you expect of a country with 4.4% of the global population that gobbles up 24% of the world’s energy and a similar piggish slice of other resources as well.
I think the assertion, that some are making here, that high temperatures themselves are not a threat to humans is false.
A “wet bulb” temperature of 35 or more is lethal to humans within just a couple of hours, and the number of places with such temperatures may soon change from zero to “a few”.
That said, while some like to imagine mother earth fighting back against humans, and our population tumbling, that’s not a scenario on the cards. Humans adapt, or move. I’m not trivializing climate change; it will cause huge suffering, could decimate our economies and numerous ecosystems.
But there’ll still be an increasing number of hairless apes until economics makes our population stabilize.