It’s a sad irony that the air conditioners that cool us on hot days also make the days hotter. ACs already account for nearly 20% of world electricity use in buildings, and the emissions of heat-trapping greenhouse gases associated with that use. Demand for AC is slated to skyrocket, driven by rapidly growing economies in warm climate zones. Indeed, the number of air conditioners is projected to rise from 1.6 billion now to 5.6 billion in 2050, the equivalent of 10 new units every second. The additional electric generating capacity needed to meet this demand would match the combined capacity of the European Union, the US, and Japan today. What can be done about this environmental disaster in the making? For starters, mandating more efficient AC units could cut their energy demand in half, and smart design of buildings and urban landscapes can also reduce cooling requirements. Beyond that, we may need to raise the thermostats a bit in summer and live with a little more heat indoors.