Cities are complicated, with densely built urban centers and sprawling suburbs. Because of this complexity, cities can become hotspots for temperature, with some neighborhoods and their residents experiencing more heat than others.
New research under the direction of researchers at the Department of Energy’s Pacific Northwest National Laboratory aims to gain an understanding of this gap in the environment. The average Black resident in all major U.S. cities is exposed to air that is 0.28 degrees Celsius warmer than the city average, according to a new study. On the other hand, the typical white urban resident lives in an area where the temperature of the air is 0.22 degrees Celsius lower than the average.
There were two parts to the new work, which was published last week in the journal One Earth. The authors of the study wanted to come up with a better national estimate of urban heat stress—a more accurate description of how our bodies react to heat outside. They also attempted to gain a deeper understanding of which populations are most vulnerable to urban heat stress by developing these estimates and contrasting them with demographic data.
“You’re not really feeling it unless you’re walking around barefoot or lying naked on the ground. At best, land surface temperature is a rough proxy for urban heat stress.”
Earth scientist TC Chakraborty
The results show that racial and income disparities are widespread in cities across the United States. 94%, or roughly 228 million people, of the urban population in the United States lives in cities where the poor are disproportionately affected by summertime peak heat stress exposure.
The authors of the study also find that people who now live in historically redlined neighborhoods, where loan applicants were once turned down for racial reasons, are more likely to experience heat stress outside than their neighbors who lived in areas of the city that were never redlined.
The study also reveals flaws in the standard method used by scientists to estimate urban heat stress at these scales, which typically relies on satellite data. According to the new work, this conventional satellite-based method can overestimate such disparities. The findings have the potential to inform urban heat response plans proposed by local governments seeking to assist vulnerable groups as the world warms.
How does heat stress work?
The human body has developed the ability to function within a relatively limited temperature range. When your core body temperature rises above six or seven degrees, you quickly experience significant physiological effects. The heart is stretched to its limits, and organ failure begins.
The sweat helps. However, the level of humidity in the surrounding area has an impact on sweat’s ability to cool down. The body has a hard time adapting when both heat and humidity are constant and difficult to escape.
How can heat stress be evaluated?
Scientists use a few indicators to measure heat stress, many of which are influenced by air temperature and humidity. Weather stations give such information. However, because the majority of weather stations are located outside of cities, scientists frequently rely on other methods, such as satellite sensors, to gain an understanding of urban heat stress.
From measurements of thermal radiation, those sensors deduce the land surface temperature. According to lead author and Earth scientist TC Chakraborty, however, such measurements do not provide a complete picture of heat stress. According to Chakraborty, measuring only the skin of the Earth, such as the sidewalk or grass surface, provides only an idea of what it’s like to lie flat on that surface.
“You’re not really feeling that,” Chakraborty said, “unless you’re walking around barefoot or lying naked on the ground.” At best, land surface temperature is a crude indicator of urban heat stress.
In fact, the majority of us walk upright in a world where the temperature of the air and the moisture in the air determine how hot we actually feel. Another limitation is that these satellite data are only available on clear days. According to Chakraborty, models can provide more complete and physiologically relevant estimates of heat stress that incorporate a variety of factors.
Chakraborty’s team used satellites and model simulations to look at 481 urbanized areas across the continental United States to get a better understanding of the differences between the land surface temperature that was derived from satellites and the heat exposure to the air in cities.
The land surface temperature was provided by NASA’s Aqua satellite. The authors also produced nationwide estimates of all of the variables needed to calculate moist heat stress through model simulations that took urban areas into account. The scientists were able to measure the combined effects of air temperature and humidity on the human body using two such metrics of heat stress: the National Weather Service’s heat index and the Humidex, which Canadian meteorologists frequently employ.
They then distinguished heat pressure areas of interest across the nation for mid-year days somewhere in the range of 2014 and 2018. The team discovered connections between communities and heat exposure by overlaying maps of both census tracts and historically redlined neighborhoods.
What is the distribution of heat in cities?
Heat stress is frequently more common among poorer residents. Additionally, poorer residents of a city are frequently more susceptible to heat stress when income inequality is higher.
This disparity is evident in the majority of American cities, including heavily populated ones like New York, Los Angeles, Chicago, and Philadelphia. However, the connection between heat stress and residential segregation based on race is even more striking.
Black people live in areas of 87.5 percent of the studied cities that have higher land surface temperatures, warmer air, and greater moist heat stress. Additionally, according to Chakraborty, the correlation between the degree of heat stress disparity and the degree of segregation between white and non-white populations across cities is especially striking.
“Further, higher percentages of all races other than white are positively correlated with greater heat exposure, no matter which variable you use to assess it,” Chakraborty stated. “The majority—83 percent—of non-white U.S. urban residents live in cities where outdoor moist heat stress disproportionately burdens them.”
In an effort to determine the suitability of real estate investments, the Home Owners’ Loan Corporation of the United States of America graded neighborhoods in the 1930s. “Redlining” is the practice of giving poorer and minority-populated neighborhoods lower grades (and, as a result, fewer loans). The authors discover that the environmental conditions remain worse in these red-lined neighborhoods.
Heat exposure is higher in neighborhoods with lower ratings than in those with no redlines. Neighborhoods with higher evaluations, interestingly, by and large get less intensity and openness.
This is in line with previous studies that found lower tree cover and higher surface temperatures in urban neighborhoods that were initially redlined. Chakraborty, notwithstanding, takes note of the fact that utilizing land surface temperature would by and large misjudge these differences across neighborhood grades compared with utilizing air temperature or intensity files.
According to Chakraborty, “Satellites give us estimates of land surface temperature, which is a different variable from the temperature we feel outside, especially within cities.” Additionally, urbanization alters humidity, which satellites cannot directly provide, and the physiological response to heat.”
The authors added, “The findings are not without some uncertainty.” ” Co-author Andrew Newman of the National Center for Atmospheric Research, who generated the model simulations, stated that ground-based weather stations helped to reduce but not eliminate model bias.” However, both the theory and previous large-scale observational evidence are still supported by the findings.
What options exist?
According to Chakraborty, increasing the number of trees planted frequently is a potential remedy for heat stress. However, there is little room for trees in densely built urban cores, which are frequently home to poorer and minority populations in the United States. Furthermore, numerous past assessments of vegetation’s capability to cool city environmental elements are additionally founded exclusively ashore surface temperature — they are maybe inclined to comparative misjudgment, the creators propose.
They added that more robust measurements of urban heat stress would be beneficial. How heat actually affects the human body is influenced by factors like wind speed and solar radiation. However, due to their difficulty in measuring or modeling at neighborhood scales, these factors are not included in the majority of scientific assessments of urban heat stress.
Notwithstanding Chakraborty, the PNNL creators of the new work incorporate Yun Qian. Authors include Arizona State University’s Glenn Sheriff, the University of North Carolina-Chapel Hill’s Angel Hsu, and the National Center for Atmospheric Research’s Andrew Newman. The National Institutes of Health and the DOE Office of Science supported this work.
More information: TC Chakraborty et al, Residential segregation and outdoor urban moist heat stress disparities in the United States, One Earth (2023). DOI: 10.1016/j.oneear.2023.05.016
