Cleaner city design can cool Barcelona by up to 1.75°C, but new white roofs and parks still cannot offset the more than 6°C warming scientists project by 2100, leaving heat risk high in the neighborhoods that need help most

American cities are getting hotter every summer, and people are dying because of it. According to a 2024 study, ‘Trends of Heat-Related Deaths in the US, 1999-2023,’ published in JAMA, by Howard et al. , heat-related deaths in the US rose from 1,069 in 1999 to 2,325 in 2023, a 117% increase in raw numbers over that period. The average number of heat waves that major US cities experience each year has doubled since the 1980s, according to ABC News citing the federal government's Fifth National Climate Assessment. Cities built on concrete and asphalt were not designed for what is coming. Now, scientists are testing whether the proposed fixes can actually keep people safe.

A new study titled ‘Adapting urban areas to rising temperatures: Strategies to reduce heat and vulnerability in a warming world,’ published in the journal Urban Climate, led by researchers at the Institute of Environmental Science and Technology at the Universitat Autònoma de Barcelona (ICTA-UAB), put three of the most discussed urban cooling strategies through rigorous climate simulations to find out. The findings are sobering, practical, and broadly relevant to every American city suffering from ever-hotter summers.

What the researchers actually tested
According to the ICTA-UAB study, the team used high-resolution meteorological simulations and the Pseudo Global Warming approach to project future heat waves in the Barcelona metropolitan area, a dense, sunbaked city whose climate patterns resemble parts of the American Southwest and Southeast. They evaluated three strategies: painting roofs white to reflect sunlight, installing irrigated green roofs, and expanding urban parks and peri-urban agricultural areas.


According to the ICTA-UAB study, white roofs were the most effective daytime solution and could cut temperatures by up to 1.75°C in the most vulnerable neighborhoods. The mechanism is simple: white surfaces reflect much of the incoming solar radiation, rather than absorbing and storing it. But the same study notes that this benefit disappears and may even backfire when reflective paint is used on building facades rather than rooftops, because the reflected heat can raise street-level temperatures.

According to the ICTA-UAB study, urban parks and agricultural areas in peri-urban areas provide more moderate cooling, about 0.26°C during the day, and green roofs reduce daytime heat by about 0.37°C. But both nature-based options had a key disadvantage at night. Green roofs increased nighttime temperature by 0.24°C. As lead author Sergi Ventura explained, vegetation slowly releases the heat stored during the day and limits heat loss through radiative cooling to the atmosphere, meaning the very things meant to cool cities can subtly warm the hours when the body most needs to recover.

The study finds that the best strategy for protecting neighborhoods most vulnerable to heat is to pair white roofs with urban parks.

Who gets hurt most, and the numbers are alarming
According to the ICTA-UAB study, heat vulnerability in densely populated, low-income regions could double by 2100, when temperatures are expected to rise by over 6°C. Heat vulnerability can be reduced by 43% to 47% in the most vulnerable areas, under present-day conditions, through a combination of strategies. But at the temperature levels projected for the end of the century, the same tactics fall to just 16% effectiveness, a telling case of adaptation tools being outstripped by the problem.

This maps directly onto what is already happening in the US. According to JAMA, the age-adjusted mortality rate for heat-related deaths increased by 63% between 1999 and 2023, and the pace of growth has accelerated sharply since 2016. There is no even distribution of heat-related deaths. Lower-income neighborhoods often have less tree cover, fewer parks, and older buildings that trap heat much more than wealthier areas.

Green spaces are not equal everywhere
The inequality dimension makes this urgent in a peculiarly American way. According to a 2024 study in Nature Communications, titled ‘Green spaces provide substantial but unequal urban cooling globally,’ cities in the Global South have only about 70% of the cooling capacity of cities in the Global North, a gap shaped by differences in the quantity and quality of green infrastructure, and by socioeconomic and natural factors. It’s also true of American cities, where lower-income neighborhoods often have less tree cover, fewer parks, and an older building stock that traps heat.

The fix is targeted, not universal
The ICTA-UAB researchers are clear that extreme warming on the scale now projected cannot be compensated by a single intervention. But the ICTA-UAB study shows that even if effectiveness declines over time, focusing strategies on the most vulnerable neighborhoods can still significantly reduce health risks. The best and most cost-effective combination is white roofs plus new urban parks strategically located in the hottest and densest low-income neighborhoods.

These are not far-off policy discussions for American cities like Phoenix, Houston, Chicago and Miami, where summer temperatures are already reaching record levels. These are decisions that should be made at the neighborhood level, now, while interventions can still make a difference.