With heatwaves in the news again, there’s been a bit of a wake-up call on the growing risk of heat. Cities will want to consider how resilient they are to the increasing threat of heatwaves as the climate warms – especially as exposure to heat is a matter of justice. Those most vulnerable to heatwaves include the elderly, those on lower incomes who can’t afford air conditioning, and residents in parts of town with less green space. The latter category often includes a higher percentage of people of colour, adding a racial dimension to the injustice.
Built-up areas are hotter than the surrounding countryside, due to the concentration of buildings and tarmac, which absorb heat during the day. Reducing and mitigating this ‘urban heat island effect’ is going to have to be part of cities’ and towns’ climate adaptation plans.
What can cities do to reduce their temperatures? Here are a handful of strategies.
Trees. The best time to plant a tree, as they say, is twenty years ago. Failing that, do it now. Trees produce shade, but they also cool the air through transpiration. A building shaded by trees has lower air conditioning costs, which also reduces both direct heat and carbon emissions. Some parts of the UK have been removing street trees recently in order to save money, which is an epic false economy. I’ve never seen a city that would benefit from fewer trees, and they get more important every year. Trees remain the best place to start for cooling the city.
Shade structures – traditional architecture in hot countries has often made use of arcades, collonades, pergolas and awnings, and I suspect we shall see more of these added. Some of this has to be designed in from the start of course, but not always. Consider the slatted shades or hung fabric across the souks of Marrakech, or the canopy built over the top of an entire street in Darwin, Australia. The two pictures below are both in Spain – vines grown over a street in Jerez, and umbrellas in Barcelona. Don’t you want to walk down those streets?
Green roofs and walls. When Chicago was hit by a serious heatwave in 1995, the city responded with a series of measures to reduce the risk in future. Among them was a programme to add green roofs, and Chicago has more than any other US city. Green roofs insulate buildings from heat, and also cool the air through evapotranspiration. Having looked into projects locally, this kind of green infrastructure can be prohibitively expensive, so it’s worth local authorities pioneering their development, building expertise locally and reducing costs. Singapore is a leader in green walls and has some magnificently verdant buildings.
Painting roofs white – darker colours absorb more heat, so increasing the reflectivity of buildings can reduce heat. Some hot parts of the world have always known this, which is why there are so many white cars in tropical countries, and white buildings in Greece. The easiest way to retrofit that logic is to paint rooftops white, and New York is one of the leaders on that front. Its volunteer-led NYC Coolroofs programme has painted over 10 million square feet of roof over the last ten years.
Cooler streets and pavements – inspired by white roofs, some municipalities have experimented with painting whole streets white in similar fashion. Some say this doesn’t work, because people walking the streets feel the heat reflected back at them off the ground – a problem that doesn’t occur on rooftops. Other studies suggest it can make a significant difference to heat overall, so perhaps it’s best reserved for parking lots or streets that don’t have high foot traffic. Others are trying a spray-on treatment that is supposed to reduce heat and pollution. Eco-architect Bill Dunster suggests rainwater storing pavements as a way of cooling the city. If nothing else, reducing the extent of asphalt and planting grass can make a difference.
Water features – everyone wants to be by the water when it’s hot, and water features can help to cool a city. Canals and ponds have a limited effect, but moving water creates spray and has a greater cooling power – it’s why some traditional building forms place a fountain at the centre of a shaded courtyard. This same logic can be applied in other ways, such as the mist-spraying bus-stops in China. As part of its Cool Strasse programme, Vienna fitted streets across the city with arches. They serve as drinking fountains, but if you stand under them and press the button, they also dispense a cooling mist – as demonstrated by this guy and his tiny dog.
Design out heat – Finally, there is plenty of research into the whole subject of urban heat to inform new development. For example, a variety of building heights encourages better air flow, which moves heat away more efficiently. Passive cooling techniques can be incorporated into new buildings to minimise the use of air conditioning. Shade can be planned in from the start. Many cities will need to tweak planning regulations to encourage cooler buildings and streets, and avoid the need for retrofitting measures later. With the long view in mind, one of the most important things that cities can do is to apply best practice and stop making it worse.