Cool solutions for urban heat islands

Cities are, on average, one to seven degrees Fahrenheit warmer than rural areas and can reach temperatures of up to 20 degrees higher. These “heat islands” are unique to urban areas, where the combination of too much concrete and too little tree canopy drives up temperatures. With nearly 80% of the U.S. population residing in cities amid a warming climate, the heat island effect poses a health threat to tens of millions of Americans. On October 5, 2023, the Stanford Woods Institute for the Environment hosted a webinar to explore a range of options for addressing heat challenges in cities. The discussion was the latest in a series of policy-focused briefings exploring the intersection of environmental and energy concerns. 

Designing to beat the heat 

It’s easy to believe that the solution to increasing heat is simple - just expand the use of air conditioning. However, the more we run AC, the more electricity we use; electricity that is generated through the burning of fossil fuels releases greenhouse gasses, heating the planet further. The existing technology in cooling equipment also uses potent greenhouse gasses called hydrofluorocarbon refrigerants that can speed the warming process. Although AC is a cooling solution that may work in the short term, it contributes to climate change in the long term; the more we cool ourselves in this way, the more we heat our planet. “Every study we know of says that if the solution to the heating of the globe is air conditioning in every building, that is a path to disaster,” said Dian Grueneich, Stanford Affiliate at the Bill Lane Center for the American West. “The world will need other solutions to avoid the doom-cycle of heat, air conditioning, and worse global warming”.

While the physical structure of buildings, roads, and other infrastructure can drive up temperatures in cities, changes to the physical environment can also bring temperatures back down. Grueneich noted that if a building has a dark roof, heat absorption drives up the use of AC, but switching over to a white roof can reduce air conditioning load by as much as 20%, also reducing costs and stress on the electricity grid. 

Anne Guerry, Chief Strategy Officer at Stanford’s Natural Capital Project, explained how incorporating “green and blue” urban spaces - large urban parks, street trees, green roofs, and water bodies - provides shade, modifies thermal properties of the urban fabric, and increases cooling through evapotranspiration. These natural climate solutions create win-win scenarios by addressing multiple problems at once. By adding green spaces to cities, we create a cooler environment, but we also increase our capacity for carbon storage and improve people’s physical and mental health. If green spaces include urban agriculture, they can also serve as a source of healthy food for a community.

“Nature based solutions are affordable and are not a luxury,” said Guerry. “People all over the world need their benefits.” 

Equal access to cool solutions

While green and blue spaces offer an approach to mitigating urban heat, they are not always distributed in an equitable way. “When you compare a low income community in an urban heat island to one in west Los Angeles…the big difference is nature,” says Marta Segura, Chief Heat Officer for the city of Los Angeles. Los Angeles is one of the first US cities to proactively address the problem of urban heat islands through the development of a Heat Action Plan. The plan will include a climate vulnerability assessment, offering more granular recommendations across the city than what is provided by the state of California. Segura hopes that the city assessment will inform state and federal maps of climate hazards, which do not accurately reflect climate hazards at the city level.

To ensure that climate action is benefitting city residents, Segura stressed the importance of an accountability tool to compare climate metrics with health disparity data. Los Angeles uses the LA Equity Index to ensure that the outcomes of the city’s carbon reduction initiatives also include improved, healthy, thriving communities across the region. Segura said that we ultimately need to go beyond reshaping our cities with infrastructure and climate adaptation technologies. “We also need to reshape the way that we think about extreme heat and how we prepare for (it) as a disaster, as an emergency.”

Live in the Los Angeles area? Check out the Cool Spots LA app for a map of cooling centers and hydration stations across the city.

While green and blue approaches offer many benefits, they do require long-term financial planning in order to be successful. “You could plant a beautiful tree, but if there's no maintenance funds, unfortunately those trees will not survive or thrive,” noted Segura. Grueneich pointed out that when money is allocated to addressing urban heat islands, there can also be conflicts between city departments on where that money should be spent. “Transportation and paving have one view of things, while builders will require different materials for cool roofs, while landscaping and planting will require different things,” Grueneich said. All of these areas are managed separately, which can create contradictions. Tensions can also emerge between the need for more housing in cities and the prioritization of open spaces to help with urban cooling.   

Could technology save us?

The mechanics of heat involve two types of radiation. The first is sunlight and its reflection - whether a material reflects or absorbs light. The other is thermal radiation, an intrinsic heat dissipation mechanism. Shanghui Fan, the Joseph and Hon Mai Goodman Professor in the School of Engineering at Stanford, explained that many cutting-edge science and engineering projects are exploring how to make surfaces cooler by controlling the solar side and the thermal side of radiation. 

Fan’s research group has designed materials that are strongly reflective of sunlight while generating strong thermal radiation. Under direct sunlight, these materials can reach sub-ambient temperatures without any electricity input. Potential use cases for the new technology are diverse. “Inside a building, if the environment heats up, so does the load that you put on the air conditioning,” details Fan. “One of the things we pursued (to address this) is to develop paintings on the wall that are strongly reflective for thermal radiation, therefore getting better insulation of the inside of the building from the outside and reducing electricity consumption.” The materials developed in Fan’s lab could even be added into clothing to enhance heat dissipation from human bodies and keep wearers more cool. 

While neither nature-based solutions nor technological advancements are silver bullets for rising global temperatures, it’s clear that solutions do exist to help reduce the worst impacts of the urban heat island effect. So, why are so many city residents still suffering through the record heat? 

“As a policymaker, it’s not necessarily the technology that holds us back,” said Grueneich. “It’s really getting good facts - a good understanding - and then making a rapid enough shift in terms what our rules are.”

Watch the full conversation and see past Woods Webinars on our YouTube channel