Hot in the city

Cities are urban heat islands that not only increase mortality, but also exacerbate climate change, but new roofing and pavement materials could help to cool things down, reports Anthony King

Urban heat islands occur where the temperature of a city is noticeably higher than the surrounding countryside. The phenomenon was first described by Luke Howard in his 1833 book The Climate of London. He had collected temperature records at Plaistow, Tottenham and Stratford and one site in the city, at the Royal Society.

The heat island situation in the 21st century is more extreme by virtue of larger cities, stoked by global warming. ‘The large majority of cities in the world have an urban heat island, and some cities are up to 10°C hotter than their surrounding countryside,’ says Elie Bou-Zeid, an environmental scientist at Princeton University, New Jersey, US. The average is closer to 1-4°C. Heat islands also exacerbate heat waves, which have major health impacts.

Cities are often hotter for several reasons. First, they tend to use dark materials such as asphalt on roofs and surfaces. These have low albedo (a measure of reflectance of solar radiation), and absorb heat and slowly release it, like a giant storage heater. Second, city surfaces do not hold much water, so less water evaporates from streets than the greener countryside around most conurbations. Third, tall buildings and narrow streets alter wind speeds, and create urban canyons that trap warm air. An added factor is anthropogenic heat from car exhausts and other activities. ‘The urban heat island is directly proportional to the size of the city, so that bigger cities with more people have a stronger heat island,’ says Bou-Zeid.

But while heat islands are bad news (see Box), steps have already been taken to reduce their effects. Hashem Akbari, an environmental engineer at Concordia University in Montreal, Canada, helped set up the Cool Roof Rating Council as a non-profit in the US in 1998 to develop credible ways of assessing the solar reflectance and thermal emittance of roofing materials. Having dark roofing materials that absorbs energy not only heats cities, but conducts energy inside, increasing air-con costs. One solution is to avoid dark colours, reducing air-con demand and cooling temperatures. Since 2009, New York city has painted or installed almost 10m ft2 of white roof. ‘The lion’s share of the [cool roof] market is basically existing materials that use lighter colours,’ says Akbari, ‘but there are newer materials coming to the market,’ with innovative companies spotting a business opportunity. Roofing manufacturers have developed asphalt shingle roofs with solar-reflecting granules to decrease heat transfer into houses.

The efforts are not confined to North America. The Global Cool Cities Alliance set up in 2012 aims to increase the solar reflectance of roofs and roads, as a cost-effective route to promote healthier cities. It is presently running a Cool Roofs Challenge, awarding up to ten $100,000 grants to support cool roof innovations, with a $1m prize for the winner.

The asphalt industry initially saw a focus on cooler materials as a threat, says Akbari, but some players are getting onboard with roadway innovations. Pavement comprises around one-third of an average city, while asphalt and dark concrete absorb 90-95% of the energy reaching them, instead of reflecting it into the atmosphere. ‘Concrete is very good at storing heat, but it also has large heat conductivity,’ says Li. ‘It heats up during the day and can conduct this heat into the deeper layers of concrete. At night, it releases this heat back to the surface, and back to the air.’ Asphalt and concrete can hit peak temperatures of 48-67°C during summer.

The correlation between heat and mortality rate is very strong. Heat is the number one weather-related killer. It kills more than flooding, hurricanes, cyclones etc.
Dan Li urban climatologist, Boston University, US

Unlike roofs, cool pavements have no official definition, as noted by Reducing Urban Heat Islands, developed in part by the EPA. The report lists strategies such as modified asphalt pavement mixed with lighter aggregate, which can reflect light and lower temperatures, reflective pavements such as those with clear tree resins in place of cement to bind aggregate, or coloured asphalt with pigments or coloured seals. Asphalt can also be made more reflective by topping with concrete. In Italy, engineer Anna Laura Pisello at the University of Perugia has tested phosphorescent-material based pavements in summer and shown they reduced average and absolute temperatures by 0.9°C and 3.3°C, respectively (Solar Energy, 2020, 540). Her glowing pavements substitute 10-20% of aggregates with glass, doped with light-emitting phosphorescent compounds.

Another option is to use plastic, metal or concrete lattices to provide support and allow grass or other vegetation to grow in-between. This is suitable for low-traffic areas such as trails, and best where there is enough moisture during the summer. It reduces temperatures through evapotranspiration. ‘To change water from the liquid phase into the vapour phase uses energy,’ says Li, and this allows vegetated landscapes to bring down air temperatures. ‘Impervious surfaces of cities do not hold water, and do not evaporate,’ whereas ‘rural vegetation is transpiring, and water evaporates from soil’.

Urban trees are another attractive strategy for cooling cities. They intercept sunlight, reduce ground temperatures by their shade and cool the air when water evaporates from their leaf surfaces. ‘You can have trees that shade buildings and parking lots, and stop the sun’s energy, so surfaces and buildings do not get so hot,’ says Akbari. ‘Transpiration transfers energy into latent heat, and particularly in drier climates that can reduce city heat and demand for air conditioning.’ Planting deciduous trees or vines to the west is usually most effective for cooling buildings, especially if they shade windows. According to the EPA, trees can also slow the deterioration of pavements, improve rainwater runoff and remove air pollutants. A 2005 study of five cities in the US suggested that cities gained $1.50 to $3 for every dollar invested in trees.

The city of Sydney aims to expand its urban forest by 50% by 2030. In many cities, though, tree counts vary greatly from one neighbourhood to the next. Environmental scientist Carly Ziter decided to investigate temperature variations within a city. Many studies use surface temperatures from satellite data, but she wanted to obtain air temperatures. She considered placing sensors around the city of Madison, Wisconsin, but instead decided to cycle ten transects through the city with a mobile weather station. ‘I was able to go through areas of the city that really differ in how much pavement they had, and how much tree cover they had,’ says Ziter, now at Concordia University in Canada. She obtained hundreds of thousands of real-time measurements over the course of the summer of 2016 and found that daytime temperature varied substantially (Proc. Nat. Acad. Sci., 2019, 15, 7575).

Daytime temperature was greatly reduced in Madison when canopy cover was greater than 40% at the scale of a typical city block (60-90m), especially on the hottest days. ‘If you lived on a street with lots of trees, compared with one neighbourhood over with lots of buildings and pavements and low canopy cover, the difference could be 4-5°C,’ recalls Ziter, who concluded: ‘You really need to pass a certain threshold of canopy cover to see the target effect.’ She hopes quantifying benefits and specifying how much canopy cover is necessary will assist changes in city policy or planning. Her study highlights the complexity of urban heat island effects, even within a city. ‘We found temperatures can vary just as much within a city as between the city and surrounding countryside,’ says Ziter. ‘It is not just one heat island. We’re seeing more of an urban heat archipelago, with differences of 4-5°C on hot days between hot and cool areas of the city at the same time of day.’

There is also variation between cities. Bou-Zeid in Princeton, with colleagues at ETH Zurich in Switzerland, found that the heat island phenomenon is more pronounced the bigger the city and the more rainfall in a region (Nature, 2019, 573, 55). More rain encourages growth of vegetation surrounding a city, making it cooler than urban areas. In wet climates, ‘the cities are comparatively dry, evaporate less water, and so you see a big difference in temperatures,’ Bou-Zeid explains. The opposite can happen in cities such as Phoenix in Arizona or Madrid in Spain, which are an oasis of water, compared with the rural environs. In cities in tropical areas, such as Singapore, more green spaces would be required to supress temperatures, which would increase humidity, and use of shade, heat-dispersing materials and wind circulation is expected to be most effective.

Chinese scientists, meanwhile, have explored the concept of a ‘sponge city.’ Bao-Jie He at the University of New South Wales, Australia, and colleagues at South China University of Technology, Guangdong, noted that traditional drainage systems for flood protection were failing (Land Use Policy, 2019, 86, 147). They cited research showing that 214 of 351 cities in China experienced waterlogging between 2008 and 2010. The Chinese government is now supporting the idea of sponge cities, where urban surfaces would absorb, store, seep and purify water. Forests, grasslands, lakes and wetlands would be conserved, and urban surfaces would be made permeable, with porous brick, and concrete and asphalt pavement. These surfaces allow transpiration, reducing surface and air temperature via latent heat release.

China is not alone in focusing attention on heat island mitigation. ‘New York, Chicago and Paris – and lots of other cities – are taking aggressive actions to cut urban heat islands, but these efforts are new,’ says Bou-Zeid. ‘They haven’t scaled up to the point where you can take a measurement and say, look, we were able to reduce our urban heat island at the city-scale.’

In Europe, researchers in the SaferUp project are working on sustainable, smart pavements, including a ‘hydronic asphalt pavement’ that would reduce surface temperature during warm periods. This is structured like a regular asphalt pavement but contains pipes under the surface layer. ‘The pipes can be metallic such as copper or a composite material, which makes it possible for the pipes to support the compaction procedure,’ says Arsel Inestroza, a PhD student at Durth Roos Consulting and part of the SaferUp project. The pavement captures heat during hot weather through a built-in heat exchanger.

Akbari says tackling urban heat islands can help address global warming. Using more reflective materials on roofs and pavements could raise urban albedo by 0.1/m2. That would reduce temperatures corresponding to an equivalent reduction of ca 7kg of CO2 emissions, based on estimates of global warming per unit CO2 emission (Environm. Res. Lett., 2012, 7, 024004).

Measures such as more reflective surfaces and greener cities are being adopted in cities like Singapore, Los Angeles, New York and Athens. ‘The cities at the centre of this are responsible for 90% of the energy used in the world,’ says Akbari, ‘and city leaders recognise that they not only have the power but also the responsibility to do things that will benefit their communities and at the same time help with global warming.’ 

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