The not-so-temperate zone

Posted 9/6/22

Hot weather is in the news, partly because so many areas of the U.S. are experiencing record and prolonged heat, and also because we are learning more about just how dangerous extremely hot weather can be. 

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The not-so-temperate zone


Hot weather is in the news, partly because so many areas of the U.S. are experiencing record and prolonged heat, and also because we are learning more about just how dangerous extremely hot weather can be. 

For our region, climatologists expect steady increases over the coming decades in the annual number of days over 95 degrees Fahrenheit, to as many as 39 days per year by the 2080s. 

Heat waves, defined as three or more consecutive days with temperatures above 90 degrees Fahrenheit, are expected to be five times as frequent, and as many as eight days long per event. 

Researchers from Harvard and the University of Washington have projected a three- to 10-fold increase in heat index levels in the mid-latitudes, which includes most of North America, by 2100, with “highly consequential” impacts on health, agriculture and even the habitability of large parts of the country.

Meanwhile, scientists at the University of California at Berkeley and at Lawrence Berkeley Laboratory have developed a more accurate calculation for the heat index. It combines temperature and relative humidity to convey the apparent or “felt” temperature. 

This is significant because high heat combined with high humidity can cause severe illness and even death; the “felt” temperature isn’t just an indicator of comfort, it’s a life-and-death predictor of how hard our bodies will have to work to maintain a core temperature around 98.6 degrees Fahrenheit. 

When the weather turns hot, our bodies perspire to cool the skin’s surface through evaporation, but high humidity suppresses evaporation. As the body works harder to pump blood to the skin’s surface to be cooled, the cardiovascular system becomes strained, which can lead to heat exhaustion, heat stroke and even death. 

According to the Centers for Disease Control, 658 Americans die from heat-related illness each year. People over 65, young children and pregnant women are particularly vulnerable, as well as people with cardiovascular disease, COPD and asthma, and athletes, military personnel and anyone who works outdoors.  

The Harvard T.H. Chan School of Public Health has reported that a range of commonly prescribed medications can also make people more vulnerable to heat-related illnesses, including antidepressants and antipsychotic medications, antihistamines, diuretics, blood pressure drugs and overactive-bladder treatments. 

Mental health is also affected.  Researchers at Boston University School of Public Health have documented an increase in emergency room visits for substance abuse, mood and anxiety disorders, schizophrenia and dementia during periods of high heat. In a separate study, researchers at Boston University reviewed test scores for millions of students in the U.S. and 60 countries around the world, finding that students who attended school during periods of higher heat scored lower on standardized tests that measure educational achievement and cognitive ability.  

In a paper published in Environmental Research Letters last month, UC Berkeley researchers David M. Romps and Yi-Chuan Lu shared new calculations that revised the original heat index developed in 1979 by physicist Robert Steadman. The new calculations provided more accurate estimates of the “feels like” temperature of high-heat events that are increasing due to climate change. 

Based on the then-known temperature record, Steadman assumed an upper temperature limit of 88 degrees Fahrenheit when relative humidity reaches 80 percent. In their update, Romps and Lu developed more accurate calculations for the human body’s real-world physiological response to the unprecedented high humidity/high temperatures events we have been experiencing with global warming. Using their methodology, a temperature of 97 degrees Fahrenheit, combined with 80 percent humidity, produces a heat index of 161 degrees, a danger zone in which the human body runs out of strategies to regulate core temperature.     

Romps and Lu concluded that the calculation used by the U.S. National Weather Service underestimates the apparent temperature (the “feels like” temperature) in extreme heat waves by as much as 20 degrees Fahrenheit, with real consequences for our understanding of physiological impacts of extreme heat events, which are going to become increasingly common. 

An effective health response to our increasingly warming planet will need to go beyond the establishment of temporary cooling centers, installing air conditioning and learning the warning signs and emergency procedures for heat exhaustion and heat stroke—as important as those actions are.  We need to address the underlying conditions that contribute to heat vulnerability, including health issues like cardiovascular disease, asthma and COPD, as well as mental health and substance abuse; lack of equity in access to health care and good nutrition; substandard housing, poor ventilation and toxic building materials. We need to understand the importance of conserving green space, and make sure all residents have equitable access. 

Extreme heat threatens forests, agriculture, food safety and water quality, so we need to incorporate heat issues more robustly in our emergency planning efforts, and we need to understand that heat will eventually change how—and where—we live. 

climate change, rising temperatures, heat waves, heat index


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