Large parts of the world's most populous countries could become uninhabitable.

That's the stark warning behind Gaia Vince's new book, Nomad Century, which looks at what could happen if temperatures rise to four degrees above pre-industrial levels, a possibility that she says is far more likely than we want to believe.

That would mean billions of people affected, and many of them on the move — an era of climate refugees.

But Vince believes that migration can be a solution instead of a problem, and that beginning to plan now will prevent much worse upheaval in the long term.