Greenhouse in a Greenhouse: A Closer Look at Urban Heat Islands
Humans have effectively adapted to diverse climate regions throughout the world. Especially now, it’s clear that we can build homes, productive centers, and sustainable infrastructure almost anywhere.
Comfortable habitation in otherwise hostile natural regions like deserts has become a norm. However, since the emergence of industrialized nations and dense urbanization over the last century, certain populated areas of the world have begun to exhibit unforeseen consequences of civilization; as a direct result of the built environment, cold regions are loosing their cool while warm regions are becoming even warmer.
The foundation of the urban heat island effect is the dichotomy between man-built areas and their surrounding green, rural landscapes.
The urban heat island effect, first identified in the early nineteenth century, occurs when buildings, sidewalks, pavement, and other built structures absorb the sun’s radiation and slowly release it throughout the day and night. Urban areas, especially those in non-windy geographies, experience more warming than surrounding rural areas.
The urban heat island effect is a larger problem for already warm areas of the world. The average temperature for a mid-sized city can be as much as 5°F warmer than its surroundings. During the evenings, this temperature difference can be as high as 22°F. For cities like Death Valley, California, where summer temperatures regularly boil into triple digits, the potential threat of this effect is exacerbated.
The effect is most prevalent in more humid areas. Because water vapor acts as a greenhouse gas, and warmer air can hold more water vapor, a positive feedback loop is created between urban warming and urban humidity, with each factor causing increases in the other.
Urban heat islands all have one thing in common: an over-absorbing built environment. Sidewalks and buildings made from thick, dense concrete and roads from black asphalt agitate many of these urbanized areas to consistent overheating. This thick, absorbent infrastructure gathers UV heat during the day and into the night, when it continues to release the gathered heat, but more slowly, in the form of infrared radiation.
Another contributing factor to urban heat islands is the average increase in global temperatures as a result of climate change. Rising global temperatures further magnify the urban heat island effect, which coupled with more humidity, cooks up a recipe for a hot, muggy disaster.
Naturally, urban farmers and landscapers are as concerned as city planners and climate scientists in mitigating the urban heat island effect, for which the principal cause is building cities with materials that absorb heat.
The idea of rebuilding entire cities with less absorbent materials sounds ludicrous. Indeed, it is. Yet there are manageable steps we can take – individuals, governments, and private developers alike – to reduce urban heat absorption without removing any existing infrastructure.
1. Green Infrastructure
The phenomenon of green infrastructure – rooftop gardens, urban greenery, and vegetation – has thoroughly permeated urban design discussions, the general news, and PowerHouse Growers. The benefits of green infrastructure in urban areas are (at least) three-fold: environmental cooling, carbon dioxide sequestration, and sustainable food production – plus, it’s easier on the eyes than mile-long slabs of concrete.
Vegetation and other greenery absorb far less heat than traditional building materials. Since Because grass and trees transpire and are much less dense than concrete and asphalt, they do a much better job of reflecting and “breathing off” the sun’s heat during the day.
More vegetation could potentially reduce average daytime temperatures in urban areas by 4°F. Rooftop gardens and other green roofs, for example, are often cooler at their surface than the surrounding air, whereas concrete and shingle roofs can raise temperatures up to 90°F warmer than the surrounding air.
Large bushes and trees also provide shade over concrete structures, thereby effectively preventing them from warming throughout the day. Moreover, the presence of rougher, jagged natural surfaces like trees actually contributes to more wind flow, and thus more cooling, than areas with flat, grassy vegetation. Although research is currently underway for large-scale reversals of the urban heat island effect, the Yale School of Forestry & Environmental Studies has produced studies showing that these surfaces, even in the middle of a dense city, actually lead to more cooling than the flat surfaces in rural areas.
2. Cool Architecture
“Cool architecture” involves designing buildings with the goal of maximizing heat reflectivity and minimizing absorption. Besides having an awesome name, cool architecture can greatly benefit the surrounding temperatures of urban areas through considering two distinctive aspects of a building’s material: thermal mass and color.
The term “thermal mass” may sound intimidating, but it’s essentially the same principle discussed when comparing green roofs to conventional roofs. Green roofs have a lower thermal mass, therefore absorbing and conducting less heat than conventional roofs. When green roofs are considered not viable or impractical, environmentally responsible architects employ other materials with generally lower thermal masses. These materials can include metal roofing, light ceramic shingles, and lightweight walling. As a rule of thumb, thinner, more sheet-like materials are more reflective of heat than thick materials.
Color is also a strong determinant of heat absorption (or lack thereof). Most are probably familiar with the notion that lighter objects reflect more heat – architects and designers are well aware of this fact. White roofs can reflect up to 65% more sunlight and consequently lead to peak temperatures that are more than 50°F cooler than black roofs. Ultimately, light-colored infrastructure such as white roofs can reduce the urban heat island effect by 1°F to 2°F.
3. Cool Pavements and Smart Pavements
As mentioned earlier, another main contributor to higher urban temperatures in urban environments is black asphalt. This material pervades every major (and minor) city and the surrounding suburbs. But civil contractors and manufacturers are developing ways to turn the overwhelming and multifaceted effects of asphalt around.
After all, not only does black asphalt contribute to urban warming – it also blocks the natural pathways of water back into the deep underground aquifers from which cities source their water. When black asphalt goes unchecked (and it has in many areas of the world), it leads to major problems in water access and irrigation for agriculture.
Cool pavements present a similar idea to that of cool roofing: If you reduce the thermal mass of an urban surface, you allow less heat absorption, which leads to a reduced urban heat island effect. In many cases, it’s not necessary to completely remove and overhaul the existing pavement and replace it with a new material. The video below, created by Lawrence Berkley Lab and UC Davis, explains one of the simplest and most effective ways to achieve this goal through the use of a cool pavement coating.
Another avenue through which to improve pavement performance and reduce urban warming is by implementing smart pavements. Also known as permeable pavements, this development tackles both the issue of urban warming as well as storm water runoff by being both low in thermal mass and porous enough to let water be transferred through the pavement and into the ground below. Permeable pavements effectively reduce heat absorbed by the sun and are expected to help contribute to the reduction of the urban heat island effect.Featured Image: Temperature gradient between the city of Providence and surrounding regions. Image via NASA Earth Observatory.
Copyediting by Scott Lindquist