We need a much more deliberate Plan Z, with detailed scenarios of plausible climate shocks; close analyses of options for emergency response by governments, corporations and nongovernmental groups; and clear specifics about what resources — financial, technological and organizational — we will need to cope with different types of crises.

In the most likely scenarios, climate change would cause some kind of regional or continental disruption, like a major crop failure; this disruption would cascade through the world’s tightly connected economic and political systems to produce a global effect. Severe floods dislocating millions of people in a key poor country — as we’re seeing right now in Pakistan — could allow radicals to seize power and tip a geopolitically vital region into war. Or drought could cause an economically critical region like the North China plain to exhaust its water reserves, forcing people to leave en masse and precipitating a crisis that reverberates through the world economy.

A climate shock in North America is easy to imagine. Say a prolonged drought causes major cities in the American Southeast or Southwest to run out of water; both regions have large urban populations pushing against upper limits of water supply. The news clips of cars streaming out of Atlanta or Phoenix might finally push our leaders to do something serious about climate change.