A new study predicts that major U.S. population centers will experience the most extreme increases in heat over the next 20 or more years.Some parts of the United States could experience up to 30 times more extreme heat than scientists had predicted, the study found.The study was a project of researchers at Arizona State University. Their findings were reported in Proceedings of the National Academy of Sciences.The researchers made predictions for extreme heat and cold exposure in 47 major U.S. cities.To break down the local effects of rising temperatures worldwide, the research team measured the effects of heat in "person-hours."This was defined as a human being exposed to extreme heat for one hour.The researchers considered three areas. The first was local definitions of what an "extreme" temperature is.Another was how city environments change the effects of extreme heat. The researchers also studied the effects of population migration and growth.