From the BBC:
More than three billion people will be living in places with "near un-liveable" temperatures by 2070, according to a new study.
Unless greenhouse gas emissions fall, large numbers of people will experience average temperatures hotter than 29C. This is considered outside the climate "niche" in which humans have thrived for the past 6,000 years...
Researchers used data from United Nations population projections and a 3C warming scenario based on the expected global rise in temperature. A UN report found that even with countries keeping to the Paris climate agreement, the world was on course for a 3C rise.
According to the study, human populations are concentrated into narrow climate bands with most people residing in places where the average temperature is about 11-15C. A smaller number of people live in areas with an average temperature of 20-25C.
Ho hum.
From National Geographic:
The tropics are regions of the Earth that lie roughly in the middle of the globe. The tropics between [sic] the latitude lines of the Tropic of Cancer and the Tropic of Capricorn. The tropics include the Equator and parts of North America, South America, Africa, Asia, and Australia. The tropics account for 36 percent of the Earth's landmass and are home to about a third of the world's people.
So the hot areas between the tropics have much the same population density as north and south thereof. They are not concentrated into "narrow climate bands", they are at all latitudes (except the Poles and the far north/south) and everywhere that's not a desert. And half of the world's population (3.5 billion people) live within this circle, which is almost entirely between the tropics (the emptiest bit of that circle is north/central China, which is north of the Tropic of Cancer).
The tropics are warm all year, averaging 25 to 28 degrees Celsius (77 to 82 degrees Fahrenheit). This is because the tropics get more exposure to the sun. Because of all that sun, the tropics don't experience the kind of seasons the rest of the Earth does.
So one source says "A smaller number of people live in areas with an average temperature of 20-25C" and the other says that one-third of the world's population lives in areas with average temperatures of 25 - 28C. Which source looks more reliable?
If you add an arbitrary increase, like 3C, you can predict that average temperatures will be about 29C. Easy. I'm sure there are already plenty of inhabited areas with average temperatures of 29C, if it's so terrible, why do people live there?
And so on.