3.1. Managing Ecological Processes and Functions as a Way of Reintegrating Cities with Nature
Dark, heat-absorbing, impervious surfaces—roofs, roads, and parking lots—are one iconic hallmark of urbanization. Such surfaces, often unmitigated, have a range of significant and cumulative adverse effects on the ecological and biogeochemical processes and functions that underwrite our cities, and so shape our inhabited world. Conventional building practices result in increased ambient temperatures due to the proliferation of heat-absorbing surfaces, increased urban storm-water runoff, reduced groundwater recharge, disruptions of local landscape ecologies, fragmentation of natural habitats, increased air pollution, increased water pollution, increased biological and mechanical heat stress, and exacerbate as well the separation of humans from nature.
This diverse range of changes to processes and functions can partly be captured by the concepts of urban heat islands, urban forestry, and xeriscape. We can dramatically change how our cities work, and how they sit in nature, by paying conscious attention to these ecological phenomena in land use planning. Although these various effects, as well as the measures that can effectively mitigate them have been known for some time, the ways in which we choose to plan and build have just barely begun to take these factors into account—perhaps because we continue to treat the urban world as mechanical, rather than embracing its essentially organic basis.
Such dark, under-shaded surfaces absorb in-coming solar radiation and then re-radiate this heat into the lower atmosphere, raising localized temperatures, often by 2.5 to 5.0 degrees Celsius. This increase in ambient temperatures usually results in greater expenditures of energy for cooling the structures we inhabit, particularly in the mid- to low-latitudes and in the summer afternoons, when energy demand is often at its highest.
Mitigation measures—the use of lighter colored and heat reflecting surfaces for roofs and paving, as well as the increased planting of ecologically suitable species of trees and vegetation—are capable of reducing ambient temperatures by 2.0 to 4.0 degrees Celsius. This reduction is achieved partly by physically altering the heat-absorbing properties of surfaces, partly by increasing localized cooling due to evaporative transpiration from plant and soil systems, and partly by morphologically inserting shade into the urban landscape, thus reducing energy consumption in the summer and in the afternoons when energy demand is highest.
It should be mentioned here that there is a converse “winter penalty” that is incurred, in some cases, by the wide-spread application of these heat island mitigation measures, in that the cost of heating buildings in the winter would be increased somewhat. But Rosenfeld et al
] (p. 54) find that this is a small penalty, and the cumulative summer-time benefits of reducing air conditioning costs by far outweigh the winter penalty. Elsewhere, Rosenfeld et al
] (p. 57) note that this net energy saving applies as far north as New York City, explaining that, in all mid-latitude locations, winter sun is lower in the sky, and thus the ratio of sunlight striking the roof to the walls is also smaller. In addition, winter days are shorter, and so they suggest that the summer benefits of lighter colored roofs may substantially outweigh their winter penalty.
Taken together, these mitigation measures have a number of other quite substantial benefits as well. Tropospheric ozone formation is a temperature sensitive photochemical reaction, in which precursor gases—volatile organic compounds (VOCs) and oxides of nitrogen (NOx)—react in the presence of sunlight to form smog. This reaction is temperature-sensitive. Thus, reductions in urban ambient temperatures carry the potential of reducing smog-formation, without physically reducing the volume of precursor gases exhausted into the lower atmosphere.
The extensive planting of ecologically suitable species of trees and shrubbery, besides increasing morphological shading and enhancing the locally cooling processes of evaporative transpiration in soil-plant systems, also greatly increase the surfaces available to capture ambient particulate matter (dust) generated by traffic and by urban activity, thus potentially benefiting respiratory health. In addition, the vegetation of the urban landscape increases the proportion of pervious to impervious surfaces, which, in turn, reduces storm-water run-off even as it increases ground-water recharge. A variety of habitat-enhancing ecological and community effects can also be ascribed to the increased native vegetation resulting from such measures. Not incidentally, these reductions in temperature also reduce the often considerable thermal stress on roofing and paving materials, measurably increasing their effective life span and reducing maintenance costs [15
] (p. 1).
We have known about these processes and phenomena for some time, but the shape of how we plan and build has only just begun to take these factors into account in transformative ways. No doubt this lag in adoptive action is shaped most by disciplinary fragmentation in research, and by the professional segmentation of environmental planning into functional and siloed typologies such as land use planning, air quality planning, water quality planning, storm-water management, urban forestry, and so on. But it may as well be the case that conventional descriptions of the world are traditionally biased toward the morphological—in that, it is easier to mobilize action against pollution processes that are directly sensible to us, based on sight and smell, and harder to do so against pollution processes that can only be indirectly measured, using instruments and models, such as climate change.
3.2. Albedo Modification, Vegetation and Urban Forestry as Heat Island Mitigation
Impervious surfaces are a hallmark of urbanization. Vitousek [16
] argues that land use and land cover change, taken together, are one of the three most significant global change processes that ecologists must take into account. From within an ecological perspective, roofs, roads and paving are perhaps the single most critical factor that set cities apart from the countryside [17
]. The consequences of such a concentration of impervious surfaces, usually in the form of dark asphalt and roofing materials, extend to influencing the local climate and the local hydrology in varying degrees, depending upon the particulars of locational and ecological context. Taha [24
] (p. 99) notes that “northern hemisphere urban areas annually have an average of 12% less solar radiation, 8% more clouds, 14% more rainfall, 10% more snowfall, and 15% more thunderstorms than their rural counterparts”.
However, urban heat islands, like most ecological phenomena, are not a singularity. In general, urban areas are 2.5 to 5.0 degrees Celsius warmer than their surrounding countryside. Depending upon latitude, the surrounding ecology, and meso-scale climate, a heat island effect may show itself most either in the summer or in the winter, during the day or at night, and cause increases in heating, smog formation or rainfall. In the higher latitudes, urban heat islands may most markedly increase temperatures in the winter, thus reducing building heating costs. In lower latitudes, the effect may be most pronounced in the summer months, resulting in higher air-conditioning costs and increased smog formation. In coastal arid climates such as Los Angeles, the heat island effect may be most relevant in the afternoons, causing increased smog formation and energy consumption. Along the more humid Atlantic seaboard, heat islands may generate increased rainfall and thunderstorms [25
]. While in desert locations, and with depressed topographies such as Phoenix, the effect may most show itself most at night, keeping the urban core hotter for hours after the sun has set [29
] and thus increasing energy consumption long into the otherwise-cooler nights.
In Southern California, in the case of the urbanization in the region surrounding Los Angeles, the Mediterranean climate is influenced by its coastal location, juxtaposed with an inland desert ecology, and capped by a tropospheric inversion layer that tends to trap smog-forming precursor gases—namely, volatile organic compounds and oxides of nitrogen. In such a case, and broadly speaking, the urban heat island effect is most markedly manifest at about 2 p.m. in the afternoon, when increased ambient temperatures most severely affect peak demands for electricity, and when the temperature-sensitive photochemical smog-forming reactions most manifest their pollution effects.
Here, three sorts of strategies are available to mitigate the heat island effect. We can physically increase the albedo, or heat reflecting properties, of sunward oriented surfaces such as roofs, roads and paving, by using lighter colored or otherwise more heat-reflecting materials. We can increase the proportion of vegetation and shrubbery to hard landscapes, and promote the adoption of roof-top gardens or green roofs, thus increasing opportunities for the plant-soil based processes of evapo-transpirative cooling to find play. And we can use urban forestry programs to extensively plant strategically sited and ecologically suitable tree species throughout the urbanized area, increasing both evapo-transpiration and physical shading.
Rosenfeld et al
] describe a “cool communities” strategy for the inland urbanized Los Angeles area, in which they assess the energy conservation and tropospheric ozone (smog) air pollution reduction benefits of a two-pronged strategy that focuses on increasing the albedo of roofing and paving materials by an average of 0.30, and on the strategic planting of 11 million trees in the more densely inhabited parts of the region. Their analysis shows a 12% reduction in the number of days per year on which tropospheric ozone exceeds the National Ambient Air Quality Standards (NAAQS), and a 10% reduction in air-conditioning loads during peak early afternoon demand. They found that, at peak temperatures, around 2 p.m., an approximately 2.5 to 3.0 degrees Celsius reduction in ambient temperatures would be effected by their “cool communities” strategy.
Their research concludes that the proposed albedo modification component and the tree planting component of their “cool communities” strategy generate roughly equal amounts of ambient cooling in the lower atmosphere of the Los Angeles urbanized area. That is to say, if about one-third of the rooftops within the region, and if the paved surfaces concentrated within 25% of the inland urbanized area, were treated so as to increase the albedo of treated roofs by about 0.35 and the albedo of modified paving by about 0.25, this would generate an average increase in the albedo of sunward oriented surfaces in the order of about 0.30. And if, in addition, about 11 million ecologically suitable species of trees were to be planted strategically across the region, then about half the cooling in ambient temperatures would be attributable to each of these two strategies. “The cooling for ‘albedo only’ turns out to be equal to that of ‘trees only,’ and is additive” [13
] (p. 53).
Estimating smog reduction benefits on the basis of the reduction in the number of days in the year that smog concentrations exceed the California ambient air quality standard of 90 parts per billion by volume (ppbv), their simulation shows that the combined benefits of the tree planting and albedo modification strategies result in a 12% reduction in the number of days in a year on which the air quality standards for tropospheric ozone are exceeded. “In apportioning how much of the benefits we calculated could be attributed to the three separate strategies (trees, roofs, and pavements), we found 50% of the temperature decrease (and thus 50% of the smog reduction) arises from tree planting. The remaining 50% was proportionally attributed to albedo changes resulting from light-colored roofs (0.35) and pavements (0.25), which translates to 29% of the benefits from light-colored roofs and 21% from light-colored pavements” [13
] (p. 53–54).
Smog, or tropospheric ozone, is not a directly emitted pollutant, but rather is the product of a complex reaction involving two sets of precursor gases—oxides of Nitrogen (NOx) and volatile organic compounds (VOC)—in the presence of sunlight. The photochemical reaction may be either NOx-constrained or VOC-constrained, depending on the relative proportion of the gases present in the troposphere. In the case of Southern California, Rosenfeld et al. take the reaction to be NOx-constrained. In assessing the smog-reduction benefits of their proposed heat island mitigation measures of shade tree planting and a change to lighter colored paving and roof surfaces, they consider two components in the reduction of NOx gases—the direct reductions in NOx emissions by power plants, due to reductions in peak-time electric power consumption, and the effective or “equivalent” reductions in NOx, due to reductions in ambient temperatures.
In the base case for Southern California, they assume that 1225 t of NOx
and 1350 t of VOCs are present and available to the photochemical smog-formation reaction by the early afternoon peak reaction time. They find that the reductions in electricity consumption result in a small reduction in NOx
emissions by power plants, in the order of 6.35 t, or a direct reduction of 0.5% in NOx
. However, as they point out, “(r)educing smog by citywide cooling can be considered equivalent to reducing the formation of smog precursors at constant temperatures”. Relying on research by Taha [31
], Rosenfeld et al
. conclude that the two strategies of shade trees and lighter colored or higher albedo surfaces, together result in a 10% reduction in smog. They conclude that this 10% reduction in smog is equivalent to a 25% reduction in precursor gases, with the tropospheric system behaving as though there had been a 317 t reduction in NOx
emissions within the air basin.
Albedo modification strategies, cool roofs and cool paving interventions that cumulatively increase regional albedo from 0.25 to 0.40, have been modeled to effectively reduce localized ambient temperatures by as much as 4.0 degrees Celsius in Southern California’s mid-latitude climate [24
] (p. 101). Taha concludes, “temperature decreases of this magnitude could reduce the electricity load from air conditioning by 10% and smog (ozone concentrations) by up to 20% during hot summer days”. Elsewhere, Taha [32
] (p. 1668) has found that the average albedo for sunward oriented land surfaces in Southern California is 0.14, and has concluded that the theoretical “maximum increase in albedo will probably never exceed 0.30”, and that this should be established as the extreme upper bound for modeling purposes, while an albedo increase of 0.15 for sunward oriented surfaces is a reasonable moderate increase.
The results of Taha’s simulation of such changes in albedo, for a clear and warm day in August, at 3 p.m., indicate that the urban core might see a decrease in temperature of about 1.5 to 2.0 degrees Celsius in the case of moderate (0.15) increase in albedo, and up to 4.0 degrees Celsius in the case of an extreme (0.30) increase in albedo, with outlying areas showing a more modest decrease of about 1.0 and 2.0 degrees Celsius [32
] (p. 1670). The estimated effect of such a temperature reduction on tropospheric ozone formation was considered to account for “(1) a decrease in some photochemical reaction rates; (2) a decrease in temperature-dependent biogenic hydrocarbon emissions; (3) a decrease in evaporative losses of organic compounds from mobile and stationary sources; and (4) a decreased need for cooling energy, generating capacity, and, thus, emissions from power plants” [32
] (p. 1667).
3.3. Changing the Albedo of Roofing and Paving Materials
Vernacular architecture, in a cross-cultural context, is defined as the traditional, native, locally prevalent mode of building, using locally available materials and construction techniques, and based on a traditional and historically tested knowledge-base. Many “traditional”, and hence by implication “primitive”, modes of knowing may actually be more effective than modern-day beliefs and practices. Take, for instance, the traditional architectures of places that fall within desert climates. In most cases, structures in such places are regularly white-washed, including rooftops. For instance, “building owners in hot cities like Haifa and Tel Aviv are required to whitewash their roofs each spring, after the rains stop” [14
] (p. 55). Modern day building practices are driven far more by the contemporary economics of air conditioning, which routinely fail to internalize many of the costs of not using such traditional building techniques.
An ecological approach to building would require attention to such knowledge processes. One key insight from process-function ecology is that direct human sensory perception is at best a limited means of “getting at” the processes and functions that actually shape our world. Conventional empiricism, being based on a reliance on our senses of sight, smell, hearing, taste and touch, has only limited value in an ecosystem approach. Processes and functions outside the scope of our senses drive many of the phenomena that matter most to us.
Albedo is one such phenomenon. In general, and very incompletely, the gradient from light to dark colors does approximate the gradient from high to low albedo—that is to say, from highly heat reflecting properties to highly heat absorbing properties. But a substantial part of the heating that occurs due to incoming solar radiation is in the near-infrared range of the spectrum, and so hidden from our direct sensory abilities. This explains why, for instance, “dark” terracotta roofing tiles may be measurably cooler than “white” asphalt-fiberglass shingles [14
] (p. 57), and why old “white” shingles may be more heat reflective (by up to 10 degrees Celsius) than modern “white” shingles, which use one-sixth the thickness of white pigment than they did in 1960 [14
] (p. 55).
What this means, of course, is that we are not strictly constrained to the aesthetic of “white”, in our urban landscapes. The use of, for instance, titanium dioxide (TiO2
) as an additive to paints used to coat roof surfaces, allows us to apply a range of pastel shades which still have the high albedo properties in which we are most interested. Recent developments in building materials, particularly some very interesting contemporary research about the dirt-repelling properties of TiO2
-coated materials, for instance, raises interesting prospects for longer-lasting albedo-increasing effects in a variety of building materials [33
]. Another facet of such an albedo-modification approach would focus on roads and pavements, where direct experiments show substantial heat reduction benefits as well.
3.4. Tree Planting and Vegetation Change as Integrative Regional Environmental Interventions
Landscape level land use change is one of the most significant ways in which we shape, and by which we can reshape, our lived environments. The displacement of native vegetative cover, first by small-scale agriculture, then by the more extensive irrigated agricultural systems that mark our recent industrializing history, resulted in a host of ecological changes upon the land.
Just as one example, Southern California saw a significant decreasing trend in ambient temperatures as large-scale agriculture and orchard cultivation took hold at the turn of the previous century, with yearly high temperatures dropping almost as low as 35 degrees Celsius by about 1930. Then, urbanization became the ecologically dominant force in land cover and land use change, and the yearly high temperatures began a fairly steady increase, which has continued into the present [14
] (p. 56).
The insertion of ecologically appropriate species of trees and vegetative cover into the urban fabric can be at least as powerful a transformation of the ecosystem processes and functions that support the city, as was their displacement by impervious surfaces. In the particular context of urban heat island mitigation, the most obvious way in which trees help is by physically interjecting shade into our built landscape, thus reducing the heat loads on the walls and immediate surroundings of our urban environment. Shade alone may provide a significant reduction in heat flux, reducing the amount of heat transferred through walls and roofs into the interior spaces by as much as 16 to 27 degrees Celsius, and thus directly reducing the amount of cooling work needed to be done by our air-conditioning systems.
3.4.1. Soil-Vegetation Evaporation and Transpiration as Cooling Processes
However, there is a subtler, though at least as effective, process of cooling that is a by-product of tree and plant growth. Vegetation draws up water from the soil below, through its root structures, and some of this water is released in the form of moisture by the foliage (transpiration) and by the soil itself (evaporation), so cooling the lower atmosphere. The soil-vegetation complex acts to enhance this natural process of evaporative transpiration, or evapo-transpiration. This process can be a major influence in micro-climate cooling, as walking under a broad, leafy tree on any hot, dry summer afternoon will directly demonstrate. Evapo-transpiration processes can generate estimated reductions in local ambient temperatures of 5.0 to 7.5 degrees Celsius, on a typical summer afternoon [35
]. This cooling effect is more pronounced in dry, semi-arid climates such as Southern California.
3.4.2. Green Roofs for Heat Insulation and Storm-Water Retention
A different, but equally effective and promising strategy is the widespread introduction of what are coming to be called “green roofs”, or roof-top gardens. As Oberndorfer et al
] (p. 823) point out, green roofs provide multiple ecosystem services in urban ecosystems, “including improved storm-water management, better regulation of building temperatures, reduced urban heat island effects, and increased urban wildlife habitat”.
Both through extensive experimentation and through materials innovation, green roofs are now poised to significantly help restore nature and natural processes back into the built urban environment. Broadly speaking, there are two sorts of green roofs—extensive and intensive. Extensive green roofs are usually thin layers of vegetative growing media, typically six inches or less, spread over large expanses of roofing, with some suitably durable and hardy species of ground cover, such as one of the many varieties of sedum. “The challenge in designing extensive green roofs is to replicate many of the benefits of green open space, while keeping them light in weight and affordable. Thus, the new generation of green roofs relies on a marriage of the sciences of horticulture, waterproofing, and engineering” [39
Green roofs have evolved, in recent years, from being thought of as an additional burden to be placed on roof structures to being seen now as an additional protective covering that helps shield the waterproofing membranes of conventional flat or very low slope roofs from heat stress. Experimental tests seem to indicate that well-designed and properly constructed extensive green roofs may help extend the life of the waterproofing membrane and of the roof structure itself, even as they insulate the enclosed spaces from the worst ravages of the summer sun [15
As a heat island mitigation strategy, green roofs are different from albedo modification and urban forestry in that their primary functional action is to physically insulate the roof membrane. Certainly the albedo of such green roofs is likely to be higher than that of conventional (particularly normal asphalt) shingles. But, when compared to the albedo of most materials normally used for their heat-reflective properties (titanium-dioxide treated white shingles or some of the more contemporary membrane materials), the benefits are likely to be nominal. There is certainly an evapo-transpirative effect, but since it plays out in the rather narrow zone immediately above the ground cover, its heat-reducing actions, either locally or regionally, are again likely to be nominal at best.
However, extensive green roofs do have one additional advantage, in that they can be designed to deliver, at little increase in cost and performance, virtually any desired level of storm-water retention. A 50% reduction in runoff is almost the default setting, and additional gains are easily made. Designers across the world have worked very extensively with green roofs, and case studies are available across a very wide range of siting conditions and using different technologies, making comparative analysis possible. Most researchers who have worked with green roof technologies seem to be clear that these technologies, with some little care and attention in execution, are consistently reliable and do, indeed, deliver the range of benefits that theoretical calculations suggest.
3.4.3. Green Façades and Living Wall Systems for Heat Island Mitigation and Air Pollution Control
Extending the discussion of green roofs to the remaining skin of the building envelope, it is worth noting recent developments in our understanding of green facades and living wall systems [40
]. Essentially, vertical panes of vegetation are used to envelope either exterior or interior walls of buildings. These provide multiple benefits—reducing energy consumption by improving the thermal performance of the building, mitigating the heat island effect, mitigating noise pollution, improving indoor air quality, improving health and well-being, and more generally, enhancing urban biodiversity [40
] (p. 2).
There are a number of ways in which green façade and living wall systems can be implemented. Pre-planted panels can be attached structurally to the wall, with an integrated irrigation system. Alternatively, felt pockets with growing medium can be attached against a waterproof membrane, with nutrient-laced fluids being used to keep the system moist at all times. A third alternative involves the use of planter boxes and a system of trellis-work. Such systems can be used on both eternal walls and interior vertical surfaces. In the latter case, it is not uncommon to link the living wall with the air conditioning and circulation system of the building, to capture the air purification and humidification benefits of the vegetated system.
While the aesthetic, air quality and noise pollution mitigation benefits of such systems are quite clear, it is not at all obvious that—at a systems level—green façades and living wall systems are economically viable. But research has started to emerge that seeks to establish the comparative life cycle analysis and the cost-benefit analysis of such vertical vegetation systems [41
3.4.4. Urban Forestry and Landscape Ecology in Air Pollution Mitigation
Heat island mitigation measures that include strategic and intensive tree planting can cumulatively reduce local ambient temperatures by between 2.0 to 4.0 degrees Celsius. As discussed earlier, this reduction in local temperatures can potentially reduce the formation of tropospheric ozone (smog) by up to 20%.
An additional and not insignificant benefit to urban ecology derives in the case of Southern California, from the implementation of tree planting ordinances for downtown surface parking lots and car dealerships. This is particularly salient in the case of Los Angeles County, where little effort is currently made to implement or enforce any such minimum tree cover measure, and acres of cars can be seen sitting baking in the sun all day. A 50% tree cover ordinance would go a long way to mitigating the range of adverse environmental impacts from these typically treeless expanses of impervious surfaces [43
]. Not only are there measurable benefits to be realized from the reductions in evaporative emissions from such parked vehicles, but substantial storm-water and ground water benefits would accrue as well, both in terms of storm-water mitigation and in terms of ground water recharge. This is especially true if tree-planting ordinances are combined with land-cover management techniques such as the use of porous pavement and pervious concrete, implemented in appropriate ways [19
These reductions in local ambient temperature have the additional benefit of decreasing the need for air conditioning during peak demand periods—that is to say, in the summer and in the mid-afternoon. This decrease reduces the region’s need for cooling energy, particularly in the residential context, as Rosenfeld et al
] and Taha [32
] point out, in turn reducing the demand for electricity generating capacity, and so indirectly reducing emissions from power plants. Of course, power plants supplying electricity to a particular region, such as Southern California, may or may not be located in that region. And nuclear power plants are also an exception to this case. But, in most instances, some air pollution benefits can be expected to accrue from this reduced demand for air conditioning energy. Beside toxic ozone-precursor emission reductions, a substantial abatement of greenhouse gas emissions can also be attributed to such heat island and urban forestry sorts of interventions.
An additional and related air pollution control benefit accruing directly from increased use of ecologically appropriate species of trees and vegetation is the capture and sequestration of carbon dioxide (CO2
), a significant greenhouse gas, through the natural process of photosynthesis. Rosenfeld et al
] (p. 57) suggest that urban trees may provide three times the CO2
reduction benefits than the same trees planted in forests or in non-urban areas. This reduction occurs because, in urban environments and besides the direct sequestration of carbon into the biomass through photosynthesis (which might be in the order of about 5 kilograms of carbon), these urban trees may also reduce energy consumption for air conditioning if they are appropriately sited so as to provide direct shading to buildings, by as much as 15 kilograms each year. Nowak and Crane [51
] (p. 387) estimate that urban trees, through a combination of direct carbon sequestration and carbon dioxide emission avoidance, may provide four times the GHG reduction benefits of the same tree planted in a forest stand. As such, projects that seek to implement tree planting as a net carbon sequestration strategy should consider prioritizing the planting of trees in urban environments, particularly in cases where these trees might directly and indirectly shade air conditioned buildings, as their return on investment will be much higher than if they were to fund similar projects in forest or rural areas.
Besides direct local shading and local cooling through evapo-transpiration, another local air quality benefit accrues from the ability of leafy trees to trap fine and ultra-fine particulate matter onto their leaf surfaces. The dense planting of otherwise low-biogenic emission tree species [52
] downwind of dust pollution sources such as traffic corridors with high volumes of, for instance, truck traffic, would substantially reduce human and ecological exposures to toxic exhaust gases in strategically identified “hot spots”, generating potentially substantial environmental health benefits.