So let’s take it from the start. It’s the 24th century, and things aren’t, for the Earth, going well.
Because global warming has, of course, managed to eliminate a whole lot of the planet’s population. (What’s a few billion between friends?) Between increasing heat and rising sea levels, a whole load of the planet is now uninhabitable, and what’s left of humanity keeps a brave face on while moving towards the poles, in the hope that the areas of declining iciness might provide a place to live, at least for a time.