ON THE ONE HAND . . .

As climate change cranks up the thermostat to broil, especially in the world's poorest countries, economists have been studying how we can adapt. A recent working paper for MIT's Department of Economics finds promise in turning up the AC. The researchers compared U.S. heat-related deaths between 1900 and 1959, when air-conditioning was rare, to those between 1960 and 2004, when it was widespread. They found that AC reduced hot weather's mortality effect by 80 percent. Although its proliferation will speed the rate of climate change, the authors conclude, "residential AC appears to be the most promising technology to help poor countries mitigate the temperature-related mortality impacts of climate change."

ON THE OTHER . . .

The coolants used in modern air conditioners, while ozone friendly, can have a global warming effect thousands of times greater than carbon dioxide. Known as hydrofluorocarbons, these coolants currently account for less than 1 percent of the world's warming. That figure is climbing rapidly as the use of air-conditioning spreads in the developing world. In China and India, sales of air conditioners are growing by 20 percent a year. The journal Science recently published a chilling statistic: If current trends continue, gases from air-conditioning and refrigeration could be responsible for up to 27 percent of all global warming by 2050, and the amount of electricity required to run all those coolers could rise eightfold.