Economist: ASK anyone who has read a newspaper in the past few months what is the greatest threat faced by Mexico, and the answer will inevitably be the drug gangs whose violence resulted in over 6,000 deaths last year and is the main reason Barack Obama came to visit this week. Yet even though Felipe Calderón, the country’s president, has staked his job on his crackdown against the traffickers, he has a different answer to this question: global warming. “Climate change is the most important challenge …