There's a reason you don't find polar bears in the tropics or sloths in the Arctic. Animals are adapted to climatic niches based on temperature and rainfall. Mastering fire and air-conditioning allowed humans to expand beyond our original niche, but a study of population distributions over the last 6,000 years suggests more stability than expected for our preferred climate zones. Unfortunately, the authors add, global warming is moving many areas outside this niche, calling into question the continued habitability of up to 3 billion people's homes.

Dr Chi Xu of Nanjing University used two independent reconstructions of the distribution of human populations 6,000 years ago and compared them with the mean annual temperature (MAT) and annual rainfall of those locations, including nights and days. A few brave souls inhabited the Sahara and Greenland, Xu reports in Proceedings of the National Academy of Sciences. However, during the early development of agriculture, the vast bulk of the people on Earth lived in areas with MATs of 11-15°C (52-59°F) and 30-100 centimeters (12-40 inches) rainfall, something that remains true today.

The intervening period has seen some shifts – 300 years ago, the booming population of Europe meant the most common MAT for the world's population was around 9°C (48°F) but this was the exception. Both in ancient times and today, people living outside the 11-15 niche were more likely to be in places with a MAT of 25°C (77°F) than in the 17-22°C zone (62-77°F). The development of more advanced heating and cooling mechanisms has broadened the human niche, but only for a small minority.

Global warming is turning this from a curiosity to a potential disaster. Using models of climate in 2070 with unabated greenhouse gas emissions, Xu found plenty of the world will still lie in the 11-15°C (52-59°F) niche, but it won't be the same places as today. Indeed, the paper observes that "the geographical position of this temperature niche is projected to shift more over the coming 50 years than it has [in the last 6,000].”

Up to 3.5 billion people will have to adapt or move. For those living with an MAT of 13°C (55°F), a shift to the high teens may be manageable, but population growth is highest in the tropical regions with MATs 25°C (77°F).

“In the absence of migration, one-third of the global population is projected to experience a MAT >29°C (84°F) currently found in only 0.8 percent of the Earth’s land surface, mostly concentrated in the Sahara,” the authors note.

The possibility of rising sea levels forcing mass migrations, and which nations might take these climate refugees, gets some media attention. However, Xu's work suggests the numbers flooded from their homes could be dwarfed by those driven out by unbearable heat.