Climate change is remapping where humans can exist on the planet. As optimum conditions shift away from the equator and toward the poles, more than 600 million people have already been stranded outside of a crucial environmental niche that scientists say best supports life. By late this century 3 to 6 billion people, or between a third and a half of humanity, could be trapped outside of that zone, facing extreme heat, food scarcity and higher death rates, unless emissions are sharply curtailed or mass migration is accommodated.