Via BBC, a report on how US cities are fighting extreme heat:

In 2013, Los Angeles became the first large city to pass a law requiring all new homes to have a cool roof. Since then, the city – where the number of days at 35C (95F) is expected to soar by 2050 – has rolled out numerous other keep-cool initiatives, including painting pavements white and expanding its Green Standards Building Code to include cool roofs on non-residential buildings and retrofits.

Despite the mitigation efforts, however, the population is still suffering from issues linked to excess heat. A study found the number of extreme heat-related emergency calls in Los Angeles between 2018 and 2022 directly correlated with the number of days that were 32C (90F) and above.

“The opportunities for heat mitigation in the US are huge,” says Steffen Lehmann, director of the University of Nevada’s Urban Futures Lab. “The knowledge is there, but the things that need to be done are not being done. It is extremely frustrating.”

In June, an ongoing heat dome brought dangerous temperatures to the western US. Residents have experienced temperatures of up to 46.1C (115F) in California, Nevada, Utah and Arizona. The National Weather Service (NWS) warned temperature records would be “tied or broken” with “little to no overnight relief from the heat”.

This latest heatwave is making it even more vital for cities to ready themselves for hot weather. But some experts say that cities are inadequately prepared – even though the science behind how to cool cities down is well known.

Around 80% of America’s population lives in urban areas – where the heat island effect can drastically worsen the impact of hot weather. Urban heat islands are densely populated, built up areas with few trees and large areas of dark concrete and asphalt that absorb the sun’s energy. As the sun goes down, the manmade materials release the stored heat – ensuring that the city stays hot, even at night. Urban heat island temperatures can be up to 11C (20F) hotter than less populated areas.

Vulnerable communities, such as the elderly, children, and low-income populations, are disproportionately impacted by the heat – with hotter temperatures even taking a toll on newborn babies.

As US cities continue to experience record-breaking, life-threatening triple-digit days, researchers like Lehmann say many areas are still underprepared. In 2022, a group of scientists examined 175 municipal city plans from the 50 most populous cities in the US. Although the majority mentioned heat, “few included” strategies to address it, the report found.

“If cities are not painting a complete picture of heat — how chronic it is, and its disparate impacts on the ground — we’re not going to be able to fully protect residents, and we could end up exacerbating existing social and environmental injustices,” says co-author Emma French, a doctoral student in urban planning at University College of Los Angeles.

Another study found 41 million people live in areas with extreme urban heat island temperatures. The report, by Climate Central, found 14 cities had an extreme contrast between temperatures in urban areas versus surrounding less-developed areas. These included Albuquerque, Bakersfield, Fresno and Las Vegas.

The problem is “too much talking and not enough action”, Lehmann says. “It’s extremely difficult to effect change.” It is hard to hold one city up as a model case study when it comes to heat mitigation, Lehmann continues, because there’s nowhere in the US doing it well.

“But”, he continues, “I am optimistic. Because there are things cities can do to cool down. And I do think there is a mindset-change happening.”

Green over grey

Tree-planting is widely known to lower surface and air temperatures by providing shade, and cooling by evaporation and transpiration. Research has shown that urban forests have temperatures that are on average 1.6C (2.9F) lower than unforested urban areas.

As a result, cities across the US have rolled out their own greening initiatives. Austin, Texas, requires 50% tree canopy coverage in the city by 2050. In Phoenix, Arizona – a place with the reputation for being the hottest city in the US – a tree-planting drive is bringing shade to some of the city’s warmest neighbourhoods. More than $1.4m (£1m) has been approved to plant up to 1,800 trees across the city to provide cool corridors.

In Tuscon, Arizona, drought makes tree-planting even harder in low-income neighbourhoods, where residents often can’t afford to plant or maintain trees in their gardens. The city runs a rebate programme to reimburse residents up to $2,000 (£1,563) for installing rainwater collection systems, where the water could be used for trees and green areas. This includes zero-interest loans and grants for economically challenged communities, as well as providing workshops in Spanish.

Plants are not just being planted on the ground, though, but also on roofs. In 2017, San Francisco mandated that at least 15% surface area of roofs on new buildings bigger than 2,000 sq ft (1,858 metres) must be covered by either solar panels or vegetation. A number of large buildings in the city have already installed green roofs, which not only remove heat from the air through evapotranspiration, but reduce surface temperatures of the roof surface. On hot summer days, a green roof’s temperature can be cooler than the surrounding air, whereas a conventional rooftop can be over 40C (72F) warmer.

Paint it white

A recent study found that a clean white roof that reflects 80% of sunlight will stay about 31C (56F) cooler on a summer afternoon.

The idea isn’t new – cities in North Africa and southern Europe have been doing this for centuries. Lehmann helped spearhead painting white roofs in Australia back in 2012, where community projects have been found to cool the insides of buildings by up to 2.5C (4.5F). Now, he says, the movement is finally making its way over to the US.

The city of New York has recently coated more than 10 million sq ft (930,000 sq metres) of rooftops white, which is reducing internal building temperatures by 30%. California, meanwhile, has updated building codes to promote cool roofs, which are seen as an important way to save energy.

Scientists around the world have been developing cool paint coatings for pavements, roofs and walls that contain special additives to reflect the sun’s heat. White paint can reflect sunlight, but paint coating contains high concentrations of pigments which means the paint also has water-resistant and reflective properties. The coatings have been proven to help pedestrians feel 1.5C (2.7F) cooler.

The paint reduces surface heat absorption, meaning that at night, the lighter surfaces are not releasing heat they would have stored during the day.  Los Angeles has been trialling cool paint, but there are drawbacks; the paint the city is using costs $40,000 per mile (£31,268) and lasts seven years. Also, as Lehmann points out, “white roads don’t stay very white for long”.

Cooling a city down at night is extremely important because it gives residents a chance to cool down too – staying hot at night can lead to serious health problems. Night time is also when the urban heat island effect can be at its most severe.

Painting pavements with cool coatings is being trialled at various locations in the country. In 2020, a study in Phoenix found coating pavements with cool paint lowered the surface temperature of the streets. After the study’s findings, the city made it a permanent programme.

But it’s a complicated solution; reports also show white pavements can actually make people feel hotter, as the sunlight is reflected off the white ground, and is instead absorbed by the people walking on it. And although the surface temperature of the pavement in Phoenix was 5.5C (10F) cooler, the air temperature 6ft (1.8 metre) away from the pavement measured just 0.16C (0.3F) cooler.

Community participation

Data-gathering plays an important role in planning for the future when it comes to heat.

In 2022, 60 volunteers measured the morning, afternoon and evening temperatures across Clark County, where Las Vegas sits, as part of a National Oceanic and Atmospheric Administration-funded heat mapping study. The map produced from that data shows that elevated temperatures are worst in North Las Vegas, East Las Vegas and downtown, which can get up to 6C (11F) hotter than other parts of the city. The county is now using the data to inform heat mitigation policies which include community cooling centres and tree-planting initiatives.

In Albuquerque, New Mexico, the city worked with local volunteers to map temperature and humidity by handing out specially-designed thermal sensors. Residents drove or bicycled around prescribed routes twice daily to record more than 67,000 temperature points. The maps showed differences of up to almost 9.4C (17F) in different portions of the city, with hottest temperatures in downtown and neighbourhoods adjacent to highways. Again, the neighbourhoods worst affected were low-income communities. The city then teamed up with Nasa to take satellite images, which were overlaid with social vulnerability data to target frontline communities.

The city had never done something like this before, and the data allowed the city government to plan for heat accordingly, according to who needed help the most. “It led to two important strategies we’re implementing right now,” says Albuquerque’s sustainability officer Ann Simon. One is a community energy-efficiency programme, where low-income families are helped to maximise their energy efficiency in their home and lower their energy bills by around £233 ($300) a year. To date, the city has made improvements in 104 homes.

“We’re a small programme but we did just receive a large $2 million grant to help more families so we can help the number of families we serve by sixfold,” says Simon.

Nasa will also be imaging the neighbourhoods again, which Simon hopes will inform future planning.

Planning for the future

Planning is also crucial for cities like Las Vegas, which is the second fastest-warming city in the US behind Reno, Nevada, and where temperatures this June hit around 46.1C (115F).

“Just a couple of years ago, very few cities were talking about preparing for rising temperatures, so it’s an important step that heat is becoming a larger part of the conversation,” says V. Kelly Turner, professor of urban planning and co-director of the University of California’s Los Angeles Luskin Centre for Innovation.

Lehmann been working with Las Vegas city officials for the past six years developing a master plan for how to cool the city by 2050. In 2021, the city’s Clark County released its 2050 Master Plan, which features plans to mitigate heat. Planting low maintenance and drought-tolerant plants to provide shade, reducing hardscaped areas, and designing buildings that provide shade are just some of the policies laid out. “But I believe we’ll see change,” says Lehmann, “now that the city wants it.”

Las Vegas has already started on other initiatives, such as opening public cooling stations for the homeless during heatwaves, and beginning a project to plant 60,000 trees by 2050 to provide shade. At the parking lot of a large basketball stadium, 1,000 parking spaces are being removed in order to plant 1,000 trees. It’s a controversial move though, says Lehmann, in a city that is so car-dependent. The city is also working with engineers at the University of Vegas to develop a reflective roof coating for the hundreds of casinos and hotels in Las Vegas – but its application will be voluntary, Lehmann notes.

There is certainly a plethora of science and research on how cities can cool down. However, as with most things, the science is complex; trees can make the air feel more humid­ ­– which can also lead to dangerous health impacts – and white paint on pavements can still leave streets feeling hot.

As always, it comes down to implementing a multitude of solutions, and thinking outside the box. Despite his frustrations, Lehmann remains positive. “As architects, our job is to reimagine the future,” he says. “I don’t need to see what Las Vegas looks like now, I need to see what it looks like in 20 years time.”