It is already happening. Climate change and unsustainable practices are forcing more than people to move. Indonesia is considering moving its capital because the land there is sinking up to nine inches a year in some places.
“Right now, about half the city is below sea level. By 2050, if emissions aren’t drastically cut, 95 percent of Northern Jakarta is expected to be submerged.” – Grist
Arguments about causes are moot when effects can’t be ignored. Regardless of whether natural geological processes are sinking the land, or sea levels are rising, or too many people are depleting the aquifer, or even that much concrete weighing down the land, something must be done. And, they’re going to do something. Whatever Indonesia decides to do, similar situations are occurring around the planet (including Washington DC). We’ve already entered the reactive mode, suggesting that we missed the opportunities to be pro-active.