This is the tip of the iceberg. Global warming will only get worse. If the US thinks it has a border problem now, just wait a few years. If only 5% of those thirsty 21 millions who live in Mexico City decide to head north, that's another million at the border. Compared to other places -- India, China -- the US is sparsely populated. There will be much migration from the unbearable heat and thirst of the South to the relative cool of the North.