The Hidden Cost of Urban Heat Loading – Part 2

The Triumph of Symbolism over Science - Peter j. Brennan Ph.D.

In Part 1 of these essays sub-titled ‘Facing Facts’, I wrote on the impacts of non-optimal temperatures on urban communities, and in particular on how higher temperatures are a factor in the deaths of over half a million people annually[1]; a figure that is on the increase due to the added burden of climate warming. We also reviewed research documenting the link between higher temperatures and increased rates of murder, suicide, domestic violence, public affray, social disorder, stresses on emergency services, and economic costs.[i]

In Part 2, we examine how existing approaches to the planning and design of our cities, can exacerbate heat loading, and provide a brief overview of the tools of science available to us in combating excess urban temperatures.

Advances in satellite technologies over the past 3-decades, have provided us with evermore reliable remote sourced data, essential for the formulation of strategies that not only target existing urban areas impacted by excess heat loads, but enable us to identify those communities that are most susceptible to increasing heat loads in the future. 

Images of a number of European cities (in this example it is Milan, Italy) were taken by the European Space Agency in June of 2022.  This image reveals urban hotspots across Milan, where despite an ambient temperature of 33°C at the time the image was taken, surface temperatures of 48°C and above are registered across multiple locations. 

As is evident, the only real respite comes in the form of the parks and gardens of the city, and even here we see that smaller green spaces that are of insufficient scale, lack the optimum shade densities, or have insufficient available moisture, are simply unable to absorb the levels of heat energy, necessary to provide any meaningful localised cooling benefit. 

In 2018, Dr James Cheshire undertook a similar satellite study of London using thermal imaging.  That study provides further evidence of the different rates and intensities of heat gain across Greater London, dependant on the extent and density of built form in different urban settings. The image also provides even further evidence of the significant influence that shade trees, parks and water bodies have on local temperatures. 

Dr Cheshire’s research reveals wide variations in temperatures across relatively short distances throughout metropolitan London, ranging from 38°C at the Excel Centre in East London, to 20°C recorded at Epping Forest, and 22°C in Walthamstow Wetlands. Even the London CBD registered a temperature 10°C cooler than the Excel Centre.

The ability to identify urban hotspots at a city wide level is important in pinpointing areas of highest public risk, but understanding the contribution of individual urban elements in generating those hotspots, requires a greater level of forensic investigation. To understand how different urban elements, respond to heat loads, and either exacerbate or negate the rate and magnitude of urban heating, requires not just an understanding of the complex physics involved, but a knowledge of the physical properties of the materials themselves. One of the more interesting data gathering tools that assists us in better understanding our urban microclimates, is MaRTy.

MaRTy is the name given to a ‘mobile human bio-meteorological instrument platform’ designed by researchers at Arizona State University (ASU)[ii]. Between 2017 and 2019, those researchers sought to provide landscape architects and policy makers in Tempe, Arizona (a suburb of Phoenix)[2], empirical evidence on the effectiveness of the various shade and landscape treatments, commonly employed across the downtown area of that city.

With the aid of MaRTy, researchers were able to collect detailed evidence on the extent and depth of the shade cast by buildings, engineered shade solutions (shade sales and market umbrellas), and a variety of common tree species planted across the urbanscape of downtown Tempe.  Data was also collected on the various landscape treatments commonly used across the city, including synthetic and irrigated turfs, concrete, asphalt, brick and masonry pavements, and garden bed ‘mulches’ such as river stone and gravel.  From the data collected, researchers were able to provide empirical evidence of the impact those shade and landscape treatments had on the Mean Radiant Temperature (MRT) of individual locations, i.e. what temperature we actually feel.[iii]

At its most extreme, a 50°C temperature difference was recorded between a reading taken beneath a large shade tree set within an area of irrigated turf, and a nearby hard surfaced public space.

While the research found that all shade sources provided at least some protection from the harmful effects of direct sunlight, ‘engineered solutions’ including shade sails and market umbrellas, proved to be the least effective at lowering MRT. Equally, the planting of trees for shade that were naturally evolved for hotter and more arid conditions, yielded poorer shade protection outcomes, than larger leaf species that had evolved to compete for sunlight.

The analysis of the various ground surface treatments, also revealed that many of the commonly used urban surface and groundcover treatments, were not only ineffective in lowering MRT, but in some cases actually increased temperatures and urban heat loads.   

That ASU research is supported by 2010 research in Massachusetts, where infrared readings were taken of an urban sports precinct at 4PM on a July afternoon, when the ambient temperature was 37.8°C. At that time, a synthetic turf sports field recorded a surface temperature of 69.4°C[iv], a nearby asphalt carpark registered a temperature of 60°C, and an adjoining natural turf sports field a temperature of 34.4°C; a full 35°C (or 95°F) cooler than the synthetic turf field. 

The physics of urban heating and cooling follow a few long understood laws of thermodynamics, and, while the equations for calculating those rates of heat exchange are complex, they are none-the-less predictable and replicable.

Quite clearly, it is time to embrace the natural sciences in the framing of strategies for sustainable urban futures; and that all starts with the scientific basics.  The first law of thermodynamics states that energy cannot be created or destroyed, but it can be transferred. In other words, heat energy does not simply disappear, heat can only exchange that energy with something that is physically cooler than itself, be that ‘something’ solid, liquid or gas, and the most readily available and effective absorber of heat energy on Earth, is water. 

Water has the highest Specific Heat Capacity[v] (aside from liquid hydrogen) known to science, at 4184 J/kg°C. So while so many urban heat adaptation strategies focus on initiatives like tree planting and water savings (by which I refer to treated water for human consumption) as the foundations to their urban heat mitigation and sustainability strategies, it is in improving urban water capture, filtration and reuse for landscape, environmental, industrial, and some domestic uses, that the greatest opportunities for improved urban cooling reside.  

Currently, the majority of all the precipitation that falls on the rooves, pavements and roadways of our global urbanscapes, is lost to those urban environments. Billions of megalitres of water drain off urban catchments annually, depriving parks, public and private gardens, street trees, urban forests, local river and stream environments, and ecological corridors, of the water that is so vital to ensuring healthier, safer, and more sustainable urban environments overall.[vi]

Historically urban authorities have prioritised the rapid draining of rainwater and snowmelt from urban areas, and advances in engineering and construction technologies across the 20th-century, enabled those drainage works to remove larger volumes of water more quickly; but that has come at a cost. 

Greater volumes of water, moving at high flow velocities, increase the capacity of those waters to carry urban litter, pollutants and sediments, that are harmful to the riparian, aquatic and marine environments into which they are inevitably discharged.  Equally, the discharge of such large volumes of water from cities into rivers and streams, exacerbates flooding at both the discharge point, and commonly, for communities immediately downstream from those discharge points. 

Whether we are looking at Tempe covering an area of 104km2 and receiving 279mm[vii] average rainfall, Greater London covering 1,569 km2 and receiving 615mm average rainfall, or a new sub-division in Melbourne covering 20 km2 and receiving on average 577mm of rainfall annually, the unrealised water catchment potential of urban areas, represents an environmental opportunity for urban heat mitigation, that remains substantially untapped (no pun intended). It is estimated that in excess of 90% of all of the precipitation that falls on the rooves, roadways, and pavements of the worlds cities, goes uncaptured and is therefore largely wasted.[3]

Within this paper I have touched on a few of areas of science, and written on the importance of those sciences in the heating and cooling of our cities. It is perhaps tempting to assume that these are ‘new’ discoveries, or principles of science that are not easily applied to the strategic planning and design of our urbanscapes. Even a cursory review the history of science and the history of some of the world’s most iconic cities, reveals that these scientific principles have not only been long acknowledged, but historically applied. 

Consider this potted history of both the science, and the application of scientific knowledge:

Specific Heat Capacity:

At the conceptual level, we have known that materials have Specific Heat Capacity since the 18th-century, when Scottish scientist Joseph Black first wrote of his observations on the topic. It is only our ability to better measure SHC that has advanced since that time. 

Heat Exchange: Convection, Conduction and Radiation.

Sir Isaac Newton wrote a formula to describe heat exchange through convection in 1701.  Proof of concept emerged in the period 1880 to1920, as advancements in the instruments for the accurate measurement of the phenomenon were developed and refined. 

French physicist Jean-Baptiste Bion proved the theory of heat exchange through conduction in 1804, and French mathematician Jean-Baptiste-Joseph Fourier developed the formula for radiant heat movement in 1816, which the afore mentioned Bionconfirmed through his experiments in radiant heat exchange that same year.   

Urban Water Harvesting:

Urban water harvesting was a critical component in the work of Frederick Law Olmsted Snr, who in collaboration with a number of others, designed many of the most iconic greenspaces in major cities in the United States of America, including Central Park, New York City which was completed in 1858.

Nearly 300-years earlier in 1585, Pope Sixtus V initiated an ambitious period of urban renewal, that changed the face of Rome, and still defines the Rome we know today.  Critical to that renewal was the capture of urban waters, both rainwater and the cities effluent, and either disposing or harvesting those urban waters to create a city with living green spaces.

Priest Angelo Grillo, upon returning to Rome in 1595 after an absence of many years, wrote: “I am in Rome, but I no longer find Rome here: there are so many new buildings, streets, piazzas, fountains, obelisks and other extraordinary marvels... that I can hardly find a trace of the old Rome I left behind.” 

Heat related Urban Mortalities: 

Scientific papers have been published the link between rising urban mortalities and higher urban temperatures, since at least 1938 and the work of Mary Grover.[4]  And the preliminary observations on the study quoted in Part 1 of these essays (Global, regional, and national burden of mortality associated with non-optimal ambient temperatures from?2000 to 2019: a three-stage modelling study) was published in 2015 under the title Non-optimal temperature mortalities in urban areas.[5]

The Role of Trees and Greenspace in Urban Health:

In the 17th-century, Sir John Evelyn was advocating for the planting of trees in London to improve urban amenity, air quality and to provide greater public access to greenspaces for the purposes of improved public health overall.  It would take another 200-years, and the undergrounding of London’s sewerage infrastructure between 1859 and 1875, for engineer Joseph Bazalgette to use the overburden from those works, to build many of the great greenspaces that added to the existing inventory of parks, that changed forever the aesthetics of London. 

Our knowledge of the natural sciences that govern the heating and cooling of urban environments is well established, and the history of the renewal of cities to improve public security and safety, clean up streets, and reduce disease, dates back centuries. But across much of the 20th-century, and particularly after World War II, when the rate of population urbanisation began to gain pace, those lessons were seemingly either abandoned, or simply ignored. 

The mapping of urban heat loads has never been easier. What was until relatively recently, an arduous and complex process that required many, many hours of on-ground data collection, and sufficient resources to take simultaneous temperature readings across multiple urban locations, can today be achieved through satellite thermal imagery, allowing then a more targeted, forensic investigation into those areas recording excess heat loads. What we know from that research, is that there are certain common characteristics of areas across multiple cities, where excessive heat signatures are found.  

Most commonly, they are, areas of higher density built form, heavily industrialised areas, new housing projects, large railway stations and rail infrastructure, some (but not all) Old city areas, and many 20th-century and more modern era developments.  What is most commonly absent from high heat signature areas, are large greenspaces, ample numbers and appropriate shade trees, and of course, available water.

In one of the world’s great contemporary ironies, research reveals multiple examples of large urbanscapes filled with new subdivisions, apartment complexes, retail centres and sporting infrastructure, that all have (or claim) 5 and 6-star energy efficiency ratings, but where on any day of elevated ambient temperature, record external heat signatures that are acknowledged as presenting extreme risks to public health and safety.

The evidence of the threat posed to urban communities by higher urban temperatures is irrefutable.  That we as a species, are susceptible to even relatively small increases in temperatures above Optimum, is equally beyond doubt.  And that elevated urban temperatures impact so many other aspects of urban life from crime rates to the economy, is equally well documented and beyond refute. While satellite imagery reveals that 25-30% reductions in urban heat loads are commonly recorded across urban precincts. The question is not if urban heat load reductions are achievable, it is if the political will exists to drive the necessary changes to the planning, design and funding of our urban environments to deliver those changes? 

The reality is that we will continue to have seasons, and temperatures will continue to climb as a consequence of climate change.  However, medical research clearly demonstrates that for every 1°C we able slow the rate of urban heat gain, and for every degree by which we are able to reduce the maximum temperature in urban environments, lives are saved, crime rates are reduced, rates of violence diminish, and the overall economic hit to communities is lessened. The question is not can lives be saved, but does the political will exist, to embrace the natural sciences in reimaging the form and function of our cities?

Finally, on design philosophy, it is incumbent upon me to say that truly sustainable strategies are neither environmentally, nor economically determinist in nature, but informed by science and targeted to the needs, not wants, of individual communities.  Successful strategies into the future, will be founded in the simple truth that truly sustainable communities are culturally, socially and physically connected in space and time, giving them a shared sense of identity and bond; what was once called ‘A sense of Place”.[viii]

Footnotes:

[1] Qi Zhao, Yuming Guo, Tingting Ye, Antonio Gasparrini,?et.al., Global, regional, and national burden of mortality associated with non-optimal ambient temperatures from?2000 to 2019: a three-stage modelling study., https://www.thelancet.com/journals/lanplh/article/PIIS2542-5196(21)00081-4/fulltext

[2] Ariane Middel, Saud Al Khaled, Florian A. Schneider, Bjoern Hagen, and Paul Coseo., October 5 2021., Bulletin of the American Meteorological Society ’50 Grades of Shade’., Vol. 102: Issue 9.

[3] Martire, Jodie Lea., December 2018., Stormwater reuse for parks and whole cities., renew – Technology for a sustainable future magazine., Issue 145

[4]  Grover, Mary., 1938., Mortality during Periods of Excessive Temperature., Journal Article – Pubic Health Reports, Vol. 53 pp. 1122-43.

[5] Antonio Gasparrini, et.al., Mortality risk attributable to high and low ambient temperature: a multi country observational study., www.thelancet.com., Vol 386 July 25, 2015., p.36

Endnotes:

[i] For further information on this I recommend House of Commons Environmental Audit Committee., 18 July 2018., Heatwaves: adapting to climate Change – Ninth Report of Sessions 2017-19., P.3 The House of Commons Report cited identified that on each day that the temperature exceeds 26?C in Britain, the economy suffers ￡770m in lost productivity.

[ii] The mobile human-biometeorological instrument platform MaRTy (Middel and Krayenhoff 2019; Middel et al. 2020). (a) EE181 temperature/humidity probe, (b) Gill 2D WindSonic horizontal wind speed/direction sensor, (c) GPS16X Garmin GPS sensor, (d) three NR01 Hukseflux four-component net radiometers to measure shortwave and longwave radiation in six directions (up/down, left/right, back/front).

[iii] The scientific definition of Mean Radiant Temperature is a little more complex, but for our purposes ‘the temperature we feel’ is a more relatable description.

[iv]At 60°C, scolding of human skin can occur with as little as 6-seconds of exposure. 

[v] Specific Heat Capacity (SHC) is a measurement of the Joules of energy required to heat 1 kilogram of a material by 1°C.  For example, it takes 4184 Joules of energy to heat 1 kilogram of water by 1°C. Every material has SHC and its value is expressed (for example) as Water 4184 J/kg°C. 

[vi] Having examined the claims made by more than 100-cities, I am yet to find a city that harvests, filters and reticulates, more than 7% of its hard surface urban catchment. It is true to say that much of that water is drained off into nearby rivers, lakes and oceans, but it is not captured, filtered and reticulated to be used as an alternative to treated drinking water for trees, parks and gardens across urban areas, nor is it available to the soils, ecosystems, local water courses or aquifer recharge areas to support natural environment areas.

[vii] By way of comparison for Australians, the regional city of Mildura receives 1mm more at 280mm, and Alice Springs receives 283mm.

[viii] A concept of connection to place, landscape, architecture, sculpture and artefact, that is often more easily understood by first nation and historic cultures.