In Chester’s view, a Phoenix heat catastrophe begins with a blackout. It could be triggered any number of ways. During periods of extreme heat, power demand surges, straining the system. Inevitably, something will fail. A wildfire will knock out a power line. A substation will blow. A hacker might crash the grid. In 2011, a utility worker doing routine maintenance near Yuma knocked out a 500-kilovolt power line that shut off power to millions of people for up to 12 hours, including virtually the entire city of San Diego, causing economic losses of $100 million. A major blackout in Phoenix could easily cost much more, says Chester.

But it’s not just about money. When the city goes dark, the order and convenience of modern life begin to fray. Without air conditioning, temperatures in homes and office buildings soar. (Ironically, new, energy-efficient buildings are tightly sealed, making them dangerous heat traps.) Traffic signals go out. Highways gridlock with people fleeing the city. Without power, gas pumps don’t work, leaving vehicles stranded with empty tanks. Water pipes crack from the heat, and water pumps fail, leaving people scrounging for fresh water. Hospitals overflow with people suffering from heat exhaustion and heatstroke. If there are wildfires, the air will become hazy and difficult to breathe. If a blackout during extreme heat continues for long, rioting, looting, and arson could begin.