Researchers at the National Center for Atmospheric Research (NCAR) and the City University of New York (CUNY) warn a warming climate and fast-growing population point to a six-fold increase in exposure to extreme heat.
Their new study, published 18 May in Nature Climate Change, finds that the overall exposure of Americans to these future heat waves would be vastly underestimated if the role of population changes was ignored. The research was funded by the US National Science Foundation (NSF), and the US Department of Energy (DOE).
"Both population change and climate change matter," says NCAR scientist Brian O'Neill, one of the study's co-authors. "If you want to know how heat waves will affect health in the future, you have to consider both."
Extreme heat kills more people in the US than any other weather-related event. Exposure is expected to increase the most for residents in cities across the country's southern reaches.
To forecast US temperatures, the research team used 11 different high-resolution simulations produced by the North American Regional Climate Change Assessment Program.
Total exposure is measured in 'person-days,' calculated by multiplying the number of days when the temperature is expected to reach 95˚ (35˚ C) by the number of people projected to live in areas where extreme heat is occurring.
Between 1971 and 2000, the US had about 2.3 billion person-days; person-days may increase to between 10 and 14 billion between 2041 and 2070, researchers concluded. Of that increase, roughly a third is due solely to the warming climate, a third is due solely to population change, and a third is due to the interaction between the two factors.
"We asked, 'Where are the people moving? Where are the climate hot spots? How do those two things interact?'" says NCAR scientist and co-author Linda Mearns. "When we looked at the country as a whole, we found that each factor had relatively equal effect."
At a regional scale, the cause of extreme heat exposure varies. In some areas of the US, climate change and population growth swap roles. For example, in the US mountain region - from Montana and Idaho south to Arizona and New Mexico - the impact of a growing population outstrips the impact of a warming climate. But the opposite is true in the South Atlantic region from West Virginia and Maryland south through Florida.
Regardless of the relative role that population or climate plays, some increase in total exposure to extreme heat is expected in every region of the continental US. Even so, the study authors caution that exposure is not necessarily the same thing as vulnerability.
"Our study does not say how vulnerable or not people might be in the future," O'Neill said. "We show that heat exposure will go up, but we don't know how many of the people exposed will or won't have air conditioners or easy access to public health centers, for example."
The authors also hope the study will inspire other researchers to more frequently incorporate social factors, such as population change, into studies of climate change impacts.
"There has been so much written regarding the potential impacts of climate change, particularly as they relate to physical climate extremes," said Bryan Jones, a postdoctoral researcher at the CUNY Institute for Demographic Research and lead author of the study.
"However, it is how people experience these extremes that will ultimately shape the broader public perception of climate change."
-- Laura Snider, senior science writer, NCAR/UCAR Communications
Read more about the increasing exposure to extreme heat here.