What could be hotter than the desert? The answer is a city in the desert. And what could be even hotter than a city in the desert? The dark, dry, exposed surfaces that surround us every day.
We've all felt that unbearable blast from the asphalt while walking the final few metres to our cars; a heat that, if anything, feels even more intense after the sun has gone down, but few realise how damaging the effect of this can be. Like all extreme weather events, the heat that cities such as Abu Dhabi radiate can cause more than discomfort to their inhabitants.
According to the US Environmental Protection Agency, roads, roofs, and pavements can become anything from 27°C to 50°C hotter than the surrounding air temperature on a clear, sunny day. Given that the average summer temperature often hovers in the mid-40s, this is serious heat.
The result of our overheating cities is a peculiarly localised form of climate change known as the Urban Heat Island Effect (UHIE), a curious but very serious phenomenon in which cities become hotter than the natural landscape that surrounds them. Average temperatures might only differ by 1°C to 3°C during the day, but this can increase by up to 12° centigrade at night. Indeed, a 2011 study conducted by Columbia University compared the temperatures of traditional black tar and asphalt covered roofs with those covered in a new generation of white, EPDM (ethylene propylene diene monomer) membrane. Some of the dark, sunlight-absorbing roofs reached 77°C while some of the roofs clad in the white EPDM achieved temperatures that were 24°C cooler.
Heat islands can have a profound effect on the sustainability of urban communities and contribute significantly to their carbon footprint, not least through an increased demand for air conditioning at peak hours that can often push electrical grids to capacity. And in the long term, UHIE is also responsible for increased greenhouse gas emissions.
Peter Stair, a sustainability expert and Estidama assessor with Abu Dhabi's Urban Planning Council, says the simple effect of all this increased heat on daily life and the risks it poses to public health are enough to make UHIE a matter for genuine concern. "It's a significant issue here in Abu Dhabi. Just from the perspective of pedestrian comfort and the issue of air pollutants … The increased incidence of lung, throat, and other respiratory illnesses like asthma is reason enough to take the issue seriously."
So what causes the UHIE and what can be done to combat it? For Prof Ahmad Okeil, an expert in the relationship between urban climate and urban form, and chairman of architecture and design at Abu Dhabi University, there are three key issues that any solution has to address: urban infrastructure, building materials, and the very structure of city itself.
Cities are full of industrial processes, including transport, air-conditioning, and waste disposal systems that contribute to urban temperatures and these must become more energy efficient or we must reduce our reliance on them completely. Encouraging a shift from private car use to public transport is a key example of the kind of behavioural change that is required, and is already one of the tenets of Estidama, Abu Dhabi's sustainable development code.
Internationally, the use of green, vegetated roofs and walls is seen as a key weapon in the war against UHIE, as plants reflect light and heat, provide shade, and also help keep streets and cities cool through a process of natural evaporative cooling called evapotranspiration. Unfortunately this is not a sustainable option for cities such as Abu Dhabi.
As Mr Stair explains, "This is a water-scarce region, so Estidama focuses instead on shading and the reflectivity of surfaces."
As the Columbia University study shows, materials have a massive role to play in combating the UHIE. Prof Okeil agrees, "There is a huge difference between the countryside and the city. The countryside contains materials that are porous, reflective, and that breathe. The city contains darker materials like asphalt that absorb more light, convert this to thermal energy, and emit this back into the surrounding air as heat, warming the city."
Desert sand is 10 times more reflective than fresh asphalt, and it's this quality that allows it to release and reflect heat quickly. When this reflectivity combines with open skies that allow solar radiation and heat to escape easily to the upper reaches of the atmosphere, you have an environment that cools relatively easily once the sun has set. Unfortunately, the situation in dense cities like Abu Dhabi could not be more different.
3.The shape of the city
As Prof Okeil explains, "The solar energy that enters the city ricochets off different surfaces, and each time it hits a surface, part of that energy is absorbed and converted into heat. Unless they are carefully designed, dense cities actually absorb more heat and make it more difficult for energy to escape into the sky."
This situation effectively creates urban microclimates that become trapped in a zone called the urban canopy layer that is defined by their skylines. A city with a very even skyline, in which many of the buildings are the same height, effectively becomes aerodynamic and cooling breezes skim over its surface, leaving all the heat and hot air trapped in its streets and open spaces below.
Traditionally cities in arid climates were always dense and compact as this was the best way to provide the shade necessary to keep buildings and open spaces cool. Higher densities are also now championed by contemporary architects and planners as the answer to sustainable urban development. The irony is not lost on Prof Okeil. "Architecture is all about paradoxes. You try and fix one thing and then something else goes wrong! If you increase density, you make a positive contribution by reducing the need for transport and processes that create heat, but at the same time, you decrease natural ventilation."
For Prof Okeil, the answer is to balance density with an urban form that also maximises natural ventilation, something he would achieve by making the skylines and façades of cities more textured and varied, with tall buildings located at key points across the city and streets with greater variety in their shape and design. This would create key points around which turbulence would develop that would force cooling breezes down and through the urban canopy layer, creating a form of natural ventilation that will flush heat from the city like smoke from an exhaust.
There is a beguiling simplicity to Prof Okeil's solution, not least because it can be retrofitted to existing city blocks and does not require any power, but even he admits that there is still a lot of research that needs to be done. "We cannot assume that the air flowing above the city will be cooler than the air flowing inside and not every measure that may work in a cool climate may work in an arid one. The theory for cities in hot climates is not yet clear."
* The National