40C heatwaves are now reality. It's time to transform our cities

In describing the Rumble in the Jungle world heavyweight title contest between Muhammad Ali and George Foreman, celebrated journalist Norman Mailer once said of the ruthless early punishment Ali suffered: “The nightmare he had been awaiting…had finally come to visit him.” If Mailer had pulled up a ringside seat as I watched in horror the UK experience its first ever 40C day, smashing the 2018 record by a full 1.6C, he might well have drawn some parallels.

Yes, there have been other warning signals that the global warming is accelerating – in the past four years alone, the UK has experienced its first winter day above 20C, largest number of ‘tropical nights’ above 20C, and the hottest day on record has been broken three times – but somehow broaching 40C was a real ‘crossing the Rubicon’ moment. Even the usually unflappable climate scientists looked on in horror as one of an unprecedented number of simultaneously active wildfires destroyed 40 families’ homes and the London Fire Brigade announced it had experienced its busiest day since the Second World War.

So, the urgent question is, what can we do to reduce the frequency, intensity, and duration of human-influence heat that is already ‘baked in’ by historical emissions? It turns out, quite a lot.

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It will not have escaped many people’s attention that London can often be around 10C hotter than surrounding areas. This is primarily due to a phenomenon call the urban heat island effect. Essentially, this additional heat is generated through the everyday functions and activities of cities and their citizens: commuting, delivery of goods and services, operation of buildings, aviation. In fact, it is estimated that every million of the capital’s residents add around 1C to local temperatures, with London’s 2.6 million passenger vehicles adding around another 1C.

That’s how 9.5 million residents and 2.6 million cars turn a 30C heatwave into a 40C heatwave.

Since around a quarter of the extreme temperatures we witnessed on and around July 19 relate to local human activity, we can exercise some influence over the magnitude of that contribution. This is an approach to urban cooling I refer to as “cover, remove, and replace”, and it involves radically reimagining how our cities look and function.

One of the biggest contributors to both daytime and night-time temperatures in our cities is the built environment, which we have regrettably constructed out of materials and in ways that could almost have been specifically designed to trap and magnify heat. I’m talking concrete, steel, glass, and tarmac. London alone has 15,000km of roads.

In the short-run, removal of such materials en masse is impractical, so we need to shade as much of it as possible during daylight hours. That means investing heavily in greenery, or green infrastructure. Street trees in particular have the potential to deliver an enormous amount of cooling through shading – one Manchester study, for example, found that shade from street trees reduced surface temperatures by an average of 12C – and via the transpiration and evaporation of water through leaves. To illustrate just how pronounced this cooling effect can be, researchers in the US have discovered that a single, young, healthy street tree produces the same atmospheric cooling effect as five room-sized air conditioning units operating 20 hours a day.

But it’s not simply a case of planting a few trees here and there. The research in this emerging field is clear – we need to achieve a minimum of 40 per cent on-street canopy cover, and preferably more, before we start to really feel the benefits of nature’s most advanced air conditioning technology. And the best thing about it? Upfront investment in green infrastructure is not only relatively cheap, it also delivers wider cash benefits through reduced levels or crime and lower NHS operating costs.

Green infrastructure more generally, such as rainwater gardens, grasses, natural play space, and new neighbourhood parks also provide direct cooling functions. Not only that, but by ‘removing’ heat-trapping concrete, stone, and tarmac to accommodate this greening, we are also able to address the ‘storage heater effect’ created by these materials, which slowly release heat accumulated during the day throughout the night, elevating temperatures by up to 25 per cent and increasing both the intensity and longevity of extreme heat events.

But even the radical “cover and remove” measures outlined here will be insufficient adaptation measures in the long-term, so we will also need to radically reimage the future development of our cities, and London in particular, to reflect this new reality. This means taking architectural lessons from far better heat-adapted cities, by systematically replacing concrete, steel, and glass structures with sustainable, naturally-insulating materials such as cross-laminated timber, which also acts as a long-term store of carbon.

The new structures should also be dense, to allow for more efficient land use, of medium height, and characterized by narrow, shaded walkways, into which is integrated new ground-level green infrastructure, green roofs, and innovative, playful-even, interventions such as routine use of screens, shades, and shop awnings to reduce thermal gain and reflected heat.

The campaign group Create Streets refers to this kind of urban development as “gentle density”, which has strong links to the “15 minute city” concept, in which many of our daily functions are within a quarter of an hour’s reach by foot, bike, or public transport. In short, the opposite of the way we’ve been developing cities and building sprawling, car-dependent housing for decades.

It has often been remarked that our failure to address the genuinely unprecedented challenge of global warming has left us ‘teetering on the precipice’. Generations to come may look back on Tuesday July 19 2022 as the day we went over into the void. But it doesn’t have to be like this. Radically reimagining our relationship with urban green infrastructure, removing and undoing the heat-trapping architectural errors of the past, and replacing them with human-scale development that prioritise the health and happiness of our citizens can help to stave off, if not entirely eliminate, a 40C UK.

You never know, it might even make for a happier, healthier, and more stable society – that’s certainly what the research shows.

Jon Burke works with cities to help them meet the decarbonisation and environmental aspirations set out in their climate emergency declarations. He tweets about getting to net zero @jonburkeUK.