The main argument Gaia Vince makes in her book Nomad Century is that in order for three to five billion people on Earth to survive, it will require a planned and deliberate migration of the kind humanity has never before undertaken. NPR's Scott Simon discusses this possibility with Vince as she explains how human kind has hampered the success of migration through "artificial bordering of nation states," and as she talks of the need to "rethink how we decide where someone is allowed to live" in order to have a chance of survival in a warming climate with extreme temperatures.