Many will dig in, suffering through heat, hunger and political chaos, but others will be forced to move on. By 2070, the kind of extremely hot zones, like in the Sahara, that now cover less than 1 percent of the earth’s land surface could cover nearly a fifth of the land, potentially placing one of every three people alive outside the climate niche where humans have thrived for thousands of years. According to a pathbreaking recent study in the journal Proceedings of the National Academy of Sciences, the planet could see a greater temperature increase in the next 50 years than it did in the last 6,000 years combined. But as the planet warms, that band is suddenly shifting north. For most of human history, people have lived within a surprisingly narrow range of temperatures, in the places where the climate supported abundant food production.