[News release] Exposure of US population to extreme heat could quadruple by mid-century

From the 18 May 2015 EurkAlert

Interaction of warming climate with a growing, shifting population could subject more people to sweltering conditions

NATIONAL CENTER FOR ATMOSPHERIC RESEARCH/UNIVERSITY CORPORATION FOR ATMOSPHERIC RESEARCH

IMAGE: THIS GRAPHIC ILLUSTRATES THE EXPECTED INCREASE IN AVERAGE ANNUAL PERSON-DAYS OF EXPOSURE TO EXTREME HEAT FOR EACH US CENSUS DIVISION WHEN COMPARING THE PERIOD 1971-2000 TO THE PERIOD 2041-2070. PERSON-DAYS… view more

CREDIT: ©UCAR.

BOULDER – U.S. residents’ exposure to extreme heat could increase four- to six-fold by mid-century, due to both a warming climate and a population that’s growing especially fast in the hottest regions of the country, according to new research.

The study, by researchers at the National Center for Atmospheric Research (NCAR) and the City University of New York (CUNY), highlights the importance of considering societal changes when trying to determine future climate impacts.

“Both population change and climate change matter,” said 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 United States than any other weather-related event, and scientists generally expect the number of deadly heat waves to increase as the climate warms. The new study, published May 18 in the journal 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 were ignored.

The total number of people exposed to extreme heat is expected to increase the most in cities across the country’s southern reaches, including Atlanta, Charlotte, Dallas, Houston, Oklahoma City, Phoenix, Tampa, and San Antonio.

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he average annual exposure to extreme heat in the United States during the study period is expected to be between 10 and 14 billion person-days, compared to an annual average of 2.3 billion person-days between 1971 and 2000.

Of that increase, roughly a third is due solely to the warming climate (the increase in exposure to extreme heat that would be expected even if the population remained unchanged). Another third is due solely to population change (the increase in exposure that would be expected if climate remained unchanged but the population continued to grow and people continued to moved to warmer places). The final third is due to the interaction between the two (the increase in exposure expected because the population is growing fastest in places that are also getting hotter).

“We asked, ‘Where are the people moving? Where are the climate hot spots? How do those two things interact?'” said NCAR scientist Linda Mearns, also a study co-author. “When we looked at the country as a whole, we found that each factor had relatively equal effect.”

At a regional scale, the picture is different. In some areas of the country, climate change packs a bigger punch than population growth and vice versa.

For example, in the U.S. Mountain region–defined by the Census Bureau as the area stretching from Montana and Idaho south to Arizona and New Mexico–the impact of a growing population significantly outstrips the impact of a warming climate. But the opposite is true in the South Atlantic region, which encompasses the area from West Virginia and Maryland south through Florida.

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Exposure vs. vulnerability

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 United States. 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.”