This is the hidden reason for the wall. No politician could possibly admit it, but geopolitically, temperate climate countries are going to be net beneficiaries and will need to protect themselves from tropical and sub tropical countries. The crop famines and water shortages will displace a billion people and no northern country can handle that many refugees. No matter how horrific it will be, the US military knows they will need to be stopped at a strong border and is preparing for the next few decades. It is also the root cause behind Brexit, the UK is preparing for the EU to be overwhelmed across its enormous southern and eastern border and is cutting its losses now.

If you think there are not strategists taking the best available information an d making 30 year plans then you are quite mistaken.