Is it truly a problem if families move away from areas of high-risk natural disasters?
"Hypothetically, if we were to jam every human on earth into a space the size of Texas; it would just have the same pop. density as Manhattan" is not remotely the same as "in practice, all of humanity could comfortably live in a space the size of Texas with the density of Manhattan."
In the current landscape I think very few people on a global scale has any sympathy with Americans being affected by the changing climate.
(I don't know your personal political opinions here, this irony is blurred over the entire political landscape).
* not just the current noise from Trump, the 20 years in the UK before I moved to Germany
(the Chinese system of internal migration control that prevented everyone from migrating to the cities immediately)