The combination of climate stress and a population surge in Africa, the Middle East and Central Asia (heading to ~3 billion, with 1.5-2 billion of working age) means even a fraction migrating would dwarf historical movements like the Mongols or barbarians, so the only humane solution is an 'enlightened Marshall Plan' (~10-15% of GDP, $5tn+) to raise local productivity and prevent mass migration, pandemics and war.