The combination of global warming and shifting population means that by midcentury, there will be a huge increase in the number of US residents sweating through extremely hot days, a new study says.

People are migrating into areas, especially in the Southeast, where the heat is likely to increase, 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 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 35 C or higher and how often they will feel them. 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 them, multiplying people affected by how many days they felt the heat.

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

(China Daily 05/20/2015 page10)