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The combination of global warming and shifting population means that by mid-century, there will be a huge increase in the number of Americans sweating through days that are extremely hot, a new study says.

People are migrating into areas — especially in the South — where the heat is likely to increase more, said the authors of a study published Monday by the journal Nature Climate Change. The study highlighted the Houston-Dallas-San Antonio and Atlanta-Charlotte-Raleigh corridors as the places where the double whammy looks to be the biggest.

"It's not just the climate that is changing in the future," said study co-author Linda Mearns, a senior scientist at the National Center for Atmospheric Research in Boulder, Colorado. "It is many things: how many people and where people are that affects their exposure to climate changes."

In a unique study looking at the interplay of projected changes in climate and population, scientists tried to characterize the number of people who will feel temperatures of 95 degrees or higher and how often they will feel it.

Between 1970 and 2000, the U.S. averaged about 2.3 billion person days of extreme heat each year. But between 2040 and 2070 that number will be between 10 and 14 billion person days a year, according to the study.

IN-DEPTH

--- The Associated Press