Renewable energy sources, energy efficient design, fossil fuel purification, electric vehicles and carbon reduction are all necessary measures when talking about environmental sustainability.
However, these measures are all focused on reducing greenhouse gas concentrations. They do nothing to address the massive amount of heat stored in, and emitted from, our urbanized lifestyles. This nefarious process occurs in every city on the planet – inevitably impacting on regional climates and subsequently on the global climate. But solutions to it have largely been absent from conversations about climate change.
If we can figure out how to remove this thermal pollution, then the climate-changing toxicity of greenhouse gases is rendered relatively innocuous.
Huge concentrations of these gases are already replete in the atmosphere and are very long-lived. So even if we miraculously stopped emitting all thermally sensitive gases, the global climate will continue to warm and be increasingly disrupted for at least the next 100 years.
Get the heat out
The only reasonable and timely remedy is to take the heat out of the equation.
A range of challenging consequences emerge from not moderating thermal pollution. Hundreds of thousands of massive hotspots are generated in the global climate system with extreme urban-climate weather events visited upon cities. Excess heat also impacts negatively on the general health, wellbeing and comfort of citizens. It is lack of relief at night, rather than high daytime temperatures, that puts people at most risk from heat stress.
Moreover, a significant quantity of fossil-fuelled energy is expended to cool buildings and vehicles to mitigate the negative effects of this thermal effect.
The major culprit in this thermal pollution equation is the “designed environment” – or the way a city is built.
In principle, medium-density, mid-rise cities with good sky-views set on narrow labyrinth street grids that shade and ventilate more naturally, help moderate heat excess. Massive high-rise canyon-cities, set on wide, dark, right-angle roads and concrete sidewalks, excessively trap and emit heat.
Whatever the form of a particular setting, there are hundreds of elements in the designed environment that are involved, each absorbing and emitting heat, in a never-ending cycle, from dawn to dusk to dawn.
You can download a study conducted in Victoria Park, Sydney using an infra-red thermal camera, rendering these otherwise invisible elements visible, here [The link will download it]: https://cityfutures.be.unsw.edu.au/documents/36/Final_Report_Hassell.pdf
The principle is simple. When the atmosphere is warmer than the elements in the designed environment, they absorb some of that warmth. When they are radiating at higher temperatures than the ambient air, they transfer some of their heat to it. From the measured infrared imagery from a study in Sydney, you can see the differences between elements and their maximum and minimum temperatures over 24-hours.
Coolers are simple and effective
An index in the research document is scaled from elements contributing the most heat , known as ‘radiators’, to those providing the most effective cooling, known as ‘coolers’.
Some of the best coolers include white and south-facing façades (in the Southern Hemisphere), tree shade in a grassy park and on light-coloured paving and on white walls. However, unshaded grass in a park on a sunny day emits heat at levels similar to air-conditioner waste.
Some major radiators include exposed truck engines, car bodies, engines, tyres, exhausts, roads and black walls on a western façades.
These micro-climatic insights can assist in designing for cooler cities and climates.
Citing air quality concerns as the rationale, France and Britain have recently announced the phase-out of new petrol and diesel vehicles from 2040. However they have not yet declared that fossil-fuelled vehicles also emit greenhouse gases. Clean air in cities is, of course, vital for quality of life. However, only if electric vehicles are charged from renewable energy sources will this make any global warming difference.
Besides this, there is the undetected thermal signature of all vehicles, silently emitting heat into the air.
Only by designing humane-scale cities where streets are amenable to pedestrians so cars are needed much less, can this invisible thermal dilemma ever begin to be solved.
These night time thermal images indicate the heat signatures of buildings, roads and cars. There are still heat shadows as hot as the road under the parked vehicles. These shadows and the vehicles then emit all their accumulated heat to the air. The roads are still radiating at 25C, although the sun has set many hours earlier. Note the cool green of the trees.
Standardized responses to the cooling of cities include the evaporative cooling capacity of the natural, green environment, and increasing whitening and reflectivity.
Greening is the best solution. Vegetation on building roofs, terraces and façades, inside buildings in atriums, trees in streets, urban squares and courtyards, and rooftop farming and aquaculture are all feasible ways to help cool cities.
But greening is a complex issue. While urban forests generate park breezes, dense clusters of trees can also reduce air movement and increase humidity. This, in itself, is a potent health issue. Additionally, all green elements require watering, which might be compromised where urban and global warming induce droughts and water restrictions. Structural issues relating to green roofs must also be considered. Maintenance of street trees is a constant requirement.
Evaporate the heat away
Despite this, evaporative cooling is the best natural remedy for city and climate cooling. This includes the seminal roles played by water which also absorbs and stores heat. Moving water, fountains, mist fan sprays all play a role, while river frontage and sea breezes are well known coolers.
Evaporative cooling through permeable streets and porous paving is also an important element. This diminishes urban flooding and runoff issues. Natural cooling also occurs where streets are cobbled, and grass grows between the stones.
Carefully calculated shading on building façades or shade structures can also have an impact.
In the image above, the ambient air temperature was measured at 35C while the cooling effect of trees means air around them is radiating at around 27C. Tree shade, even on a paved pathway, is readily visible.
On the other hand, the effect of sunshine falling on the unshaded path is substantial. Cool materials with low heat storage and high moisture absorbance capacity can help moderate such situations.
Increasing a surface’s whiteness, known as albedo, functions by adding solar reflectance to architectural elements. This can include reflective or white roofs, walls or roads and leads to a lowering of temperatures.
However, there are several complicating factors to consider, including dust and weathering, density and location. There is also issues of glare discomfort and the risk of blinding motorists.
There also does not appear to be a comprehensive discussion of where the reflected heat ends up. It is not eliminated or transformed as in the process of natural evaporative cooling.
Although reflectivity is favourable for specifically treated or whitened buildings, the rejected heat is likely to bounce around, particularly in tall urban canyons. It is then absorbed by nearby buildings that are not albedo-treated, or transmitted above the building rooftops. This climate zone is already replete with heat absorbing greenhouse gases emitted from large urban areas. In this way, reflected heat will continue to impact on urban and regional climates, and ultimately the global climate.
An even more salient issue, almost entirely unrecognized, is the presence of an “infrared sky window” in the atmosphere. Only outgoing radiation in the narrow long-wavelength range passes through unimpeded, directly to space, without intermediate absorption.
But the infrared absorption of the principal greenhouse gases is mostly in two ranges which fall outside of this window. Carbon dioxide, methane and even the main greenhouse gas, water vapour, all absorb heat in these ranges. In other words, almost all greenhouse gases distributed in the urban climate and global atmosphere will absorb both emitted heat and reflected heat.
It is much more complex to ensure long-wave infrared radiation gets to space through and beyond the 11-kilometres of tropospheric atmosphere. Painting everything white is unlikely to solve the problem. Sophisticated reflective treatments combining both high reflectivity and high emissivity elements are better solutions. But these will still need to be engineered to function in that infrared window gap.
One innovative and possibly feasible way to help cool cities would be to harvest heat by trapping the emissions released at the exhaust outlets of millions of air-conditioners. Rather than simply ejecting this disruptive thermal load back into the urban climate, where it also continues to impinge on buildings, it could be captured and redirected to insulated water tanks which act as thermal mass. This could then be recycled as energy equivalent by pre-heating hot water used in buildings.
It’s even conceivable that advanced thermo-electric cells could convert this waste into zero-emission renewable energy. This could either be used to run the air-conditioners or fed back into the electricity grid.
The majority of global citizens are already urban dwellers. The 21st century urbanization trend will accelerate substantially as populations surge towards 10 billion. By then some 75% of humans will live in cities. Hundreds of new metropolises will be required to house them. Their design is absolutely essential. Persisting with the belief that only high-density heat-trapping cities can accommodate these millions, makes it even more likely that urbanites will be doomed to a hot future.
Urban climatologists recognize that cities are potent and toxic crucibles of both greenhouse gases and thermal pollution. However, this appreciation is rarely transmitted to urban designers, architects and engineers, not to mention city regulators and decision-makers. In practice, this group remains largely unaware of this phenomenon.