As the planet warms in the coming decades, many parts of the planet that millions now call home will become uninhabitable. At first, people in these areas will move to the cities, then across international borders. This mass migration is already underway in the hottest parts of the world, and it is likely to accelerate in coming years. Just how many people will be forced to move? And where will they go? Guest: Abrahm Lustgarten, senior reporter at ProPublica Host Celeste Headlee   Learn more about your ad choices. Visit megaphone.fm/adchoices