The combination of global warming and a shifting US population will by mid-century deliver a “double whammy” that greatly increases the number of Americans exposed to extremely hot days, a new study says.

People are migrating into areas where the heat is likely to increase more, said the authors of a study published on 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 effect looks to be greatest.

“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 95F (35C) or higher and how often they will feel it. They used a figure called person days for the extreme heat to reflect both the length of time heat waves continued and how many people felt it by multiplying people affected by how many days they felt the heat.

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

The scientists used 11 different climate models based on current trends of heat-trapping carbon dioxide emissions and matched those with demographic trends. Both the increased heat and population shifts had about equal effects but together they made matters even worse, said lead author Bryan Jones, a population geographer at the City University of New York.

The question is whether people will be able to adapt by changing their lifestyle, such as staying indoors and using more air conditioning, or will they move to cooler climates. Jones said it was unlikely that people would move to cooler areas to escape the heat, saying historically people tended to move away from colder areas and into warmer ones like Florida, Arizona and North Carolina.

Several outside scientists praised the study. University of Georgia meteorology professor Marshall Shepherd said the problem was likely even worse when taking into consideration the fact that cities get hotter than rural areas.