As heat, humidity and populations rise by the end of the 21st century, so too will heat stress, researchers say, likely increasing the health risks of millions of people but also signaling another reason to reach important climate and emissions goals.

A paper published April 26 in Earth's Future draws on many heat-stress variables, where other studies tend to focus on only one aspect, such as duration or temperature. The researchers found that the potential impact of heat-stress events lasting from one day to one week will double from 2060-99 across the U.S., with population growth on the East and West coasts accounting for much of the added stress.

"The heat-stress events that we are considering to be extremes in the past are likely to be more normal in the future," said co-author Ashok Mishra, an associate professor of civil engineering at Clemson University. "Overall, we're likely to see more extremes in the future."

Heat stress is a major cause of mortality around the world, with low-income communities bearing much of the burden. The National Weather Service defines excessive heat exposure in terms of heat index, the "feels like" measurement that combines temperature and humidity. For the agency, excessive heat exposure, or heat stress, equals a heat index over 105 degrees Fahrenheit. 

A "stressful" heat index not only induces heat exhaustion and heat stroke but can also exacerbate cardiovascular disease and possibly lead to heart attacks and strokes, according to the Environmental Protection Agency. People age 65 or older are also more susceptible to the dangers of heat stress.

Mishra and lead author Sourav Mukherjee, a Ph.D. student in water resources engineering at Clemson, began looking at heat stress after witnessing record-breaking heat waves in the Southeastern U.S. over the summers of 2019 and 2020.

To determine the potential impact of heat stress, the pair and colleagues first collected climate data, accounting for temperature and humidity from 1980 to 2019, and calculated heat stress from this period. Then, they projected these values into the near and distant future using projected climate and population datasets, developing a framework to examine the present and projected potential impact of heat stress. Crucially, the calculations account for humidity, which promotes heat stress. 

"If we are only considering the temperature, it is fine, but if you include the humidity that is done in this study, that has a significant impact on the health, because humidity prevents the body from cooling off," Mishra said in an interview with The Academic Times. 

Population growth along the East and West coasts of the U.S. is expected to drive heat stress as urban populations grow and rural populations shrink. 

Heat stress is going to become even more dangerous because, since the 1960s, temperatures have not been rising in a linear fashion, according to Mishra. They're rising with more variability, which makes them harder to adapt to. The researchers also accounted for these fluctuating, rapid changes in heat stress that shock the body, which prior studies have not done.

"Suppose I have acclimated to a temperature of 70 degrees Fahrenheit, and suddenly, for the next couple of days, there is 100-degree-Fahrenheit heat wave," Mukherjee said. "That also gives the body a heat shock, and the body has to acclimate to that shock."

A recent study on tobacco hornworm larvae confirmed some negative effects of heat stress, because larvae subjected to long heat waves did not grow as well as those experiencing shorter heat waves or none at all.  

For policymakers, Mukherjee said, the takeaway is "how they need to understand the implications of pinpointing the geographical region, which is likely to see aggressive potential impact of heat stress during aggressive emission scenarios without mitigation." 

According to Mishra, effective mitigation could come through net-zero carbon initiatives that seek to drastically reduce and eventually eliminate greenhouse gas emissions that contribute to climate change. In April, the U.S. and Canadian governments jointly announced a new Greening Government Initiative, which seeks to make government facilities and activities emission-free by 2050. 

"We need to bring carbon emissions down globally by a factor of two within the next decade to avoid crossing the 1.5 Celsius warming threshold," said co-author Michael Mann, a distinguished professor of atmospheric science at Pennsylvania State University. "The Biden administration has pledged to bring the U.S. in line with that commitment, but they need to work with Congress now in advancing a specific policy agenda, which includes measures such as carbon pricing and subsidies for renewable energy, that can meet that commitment."

According to Mann, the 1.5 degrees Celsius threshold is significant because it is the point at which heat stress and other harmful climate impacts show a "marked" increase when exceeded. Limiting global temperature rise by that amount is also the more aspirational goal of the international Paris Agreement.

Mishra and Mukherjee are interested in expanding their heat-stress research to see what factors aside from temperature and humidity could contribute to heat stress. This could include soil moisture, because low-moisture soil absorbs less heat than high-moisture soil and contributes to higher temperature in the lower atmosphere, according to Mishra. 

The duo also believe the study opens new avenues to examine not just where heat stress is happening but also whom it hurts most. Access to air conditioning, for example, is less common in low-income communities. Mishra and Mukherjee envision future studies, potentially collaborating with social scientists to look at the disparity of heat stress within populations. 

The study, "Projected doubling of US heat stress by the late 21st century," published April 26 in Earth's Future, was authored by Sourav Mukherjee and Ashok Mishra, Clemson University; Michael E. Mann, Pennsylvania State University; and Colin Raymond, California Institute of Technology.