A new study predicts major U.S. population areas will be seeing the most extreme increases in heat in the coming decades, some of which could experience 30 times more extreme heat than previously predicted.The study was conducted by researchers at Arizona State University and was published Monday in the Proceedings of the National Academy of Sciences.Researchers made projections for extreme heat and cold exposure in 47 major cities in the U.S., accounting for population size, warming caused by urban development, and local standards for extreme heat.To break down the local effects of global temperature rise, the research team measured the effect of heat in “person-hours” — defined as a human being exposed to extreme heat for one hour. They incorporated three metrics:1. Local definitions of what an “extreme” temperature is;2. How city environments change the effects of extreme heat;3. the effect of population migration and growth.The researchers considered local standards for extreme heat when making their calculations because, for example, a 100-degree day in locations like Pheonix or Austin, Texas, would feel not feel extreme to locals. The same heat in New York City could result in fatalities — especially among more vulnerable populations, like people who are elderly and homeless.With that in mind, the researchers came up with two categories — absolute increases and relative increases in extreme heat.Their top three cities for absolute increases in people affected by extreme heat include New York, Los Angeles, and Washington. The most intense relative increase — calculated by combining temperature factors with population growth — will be in places like Orlando, Miami, and Austin.The researchers say because those cities are growing rapidly, more residents inevitably will be exposed to extreme heat, creating a bigger change relative to the beginning of this century. Atlanta, Georgia, made both lists.The study team argues that cities must prepare at the local level to avoid the most serious illness and death resulting from extreme heat. They say key safety measures include having cooling centers and water available, as well as reliable sources of energy, as extreme heat can contribute to power outages. 
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