Japan is most widely known for islands of the geological sort, but unfortunately the country is also the world’s premier spot for a completely different sort of island: Urban Heat Islands. Even if you’ve never heard of an Urban Heat Island, chances are you live on one if you reside in Japan. Technically known as the “Urban Heat Island effect,” the term describes how temperatures in inner city cores are abnormally higher than temperatures in surrounding, less densely developed areas. The underlying mechanisms behind Heat Island formation are rather complex, but they are formed by excessive waste heat emissions from human activity, heat-trapping geometries of concrete city blocks, and a shortage of the natural cooling-effects from winds, waterways and greenery inside urban space. In Japan, a near dearth of large scale urban greenery combined with high building densities in major metropolitan regions have given the country some of the worst Urban Heat Island conditions found on the planet. While urban heat islands are not unique to Japan, the Urban Heat Island intensity (which describes the temperature gradient between urban centers and outlying areas), tends to be highest here in comparison with other metropolitan regions around the world. For example, the average annual temperature in Tokyo has increased 3 degrees centigrade (5.4 F) over the last 100 years, which is over three times faster than the less-than-1 degree rise in global average temperatures over the same period.

...hotter urban temperatures simply make city-life miserable in a range of ways. Click To Tweet

So what’s the big deal about a few extra degrees? Those hotter temperatures in Japanese cities are behind a host of environmental problems and ecological changes with serious implications for the health of the planet, and for the various economic and social functions of our cities. Recent years have witnessed an explosion in research and innovation to develop architectural products and techniques designed to mitigate urban heat islands, but truth be told, the effects are rather paltry. The true solution lies in cultivating a range of financial and legislative incentives designed to give Japanese cities an eco-groovy face lift that restores rivers and streams, reduces buildings, and provides for more greenery in Japanese urban landscapes.

Urban Heat Islands – Heating Up Japan

Urban Heat Islands are influenced by a combination of natural conditions (i.e. amount of sunlight, elevation, cloud cover, wind conditions, soil moisture content, etc.), as well as manmade conditions like the types and distributions of manmade structures, or the nature of energy supply and consumption. A useful way of understanding urban heat island formation is to look at them in terms of three general categories: 1) Urban metabolism, which describes how energy, heat and water circulate in urban systems; 2) Urban fabric, which describes the thermal properties of materials in urban surfaces, and 3) Urban form, which describes how the geometric properties of buildings and roads contribute to heat retention in the city. In big, energy gobbling, concrete-covered, densely-packed places like Tokyo, these three categories combine to create a nasty heat island, especially in summer months.

The excessive heat that fuels heat island formation is caused by higher concentrations of both anthropogenic heat emissions, as well as heat-absorbing thermal mass in urban cores. “Anthropogenic heat emissions” are sources of heat caused by human activity, and refer to the heat given off by car engines, air-conditioning units, various industrial processes like waste incineration, your local taco truck, or anything else that consumes energy and gives off heat. Consequently, the waste heat emissions of a certain city are closely linked to economic and cultural issues as well. Higher urban temperatures in Los Angeles are more likely to be caused by car emissions than, say, air conditioning units, as in the case of Tokyo. In addition to sources of waste heat, Urban Islands also require lots of “thermal-mass” to properly form. This refers to the extensive volumes of concrete buildings and asphalt roads that soak up heat from the sun and the surrounding vicinity. Despite massive amounts of waste heat generated from air conditioning in Tokyo, it is solar heat energy absorbed by exposed road and building surface area that is responsible for over half of the abnormal temperature increase in Japanese cities. In Tokyo, the vast expanses of manmade surface area combined with the grid geometry of city blocks serves to soak up and trap heat. Urban elements like walls and roads become heat sinks while the sun is out, and then become heat sources after the sun goes down, reemitting heat absorbed during the day as thermal radiation at night. In Japanese cities, this is behind a phenomenon called “Tropical Nights,” where night temperatures fail to fall below 25 degrees centigrade.

Urban Heat Islands - Kobe Rokko Island Landfill

Urban Heat Islands – Kobe Rokko Island Landfill

Urban Heat Islands are implicated in a wide range of environmental problems. For starters, hotter urban temperatures simply make city-life miserable in a range of ways. People have trouble sleeping, become lethargic and can loose their wits when exposed to excessive, humid heat environments. While the relationships are fuzzy, studies have shown links between higher rates of crime, suicide and poorer economic performance under higher temperatures. Additionally, Urban Heat Islands alter ecological systems and diminish air quality in cities by affecting plant growing seasons, skewing the sexual distributions of insects, and triggering the chemical reactions that cause smog. However, the biggest concern is that they contribute to global warming by increasing the demand for air-conditioning. In Tokyo, it has been shown that daily energy consumption rises about 1.6 Gigawatts for every 1 degree temperature increase during summer.

Recently, much research and innovation has gone into developing new technologies for mitigating the Urban Heat Island effect and cooling urban structures, but much remains to be done in order to implement such technologies on an appropriately sized scale capable of really making an impact. Some well-known examples include rooftop garden systems, heat reflective paint, or water absorbing concrete. While many of these cooling materials and techniques seem promising, they are not silver-bullets, and actually offer limited hope for making a significant impact on Urban Heat Islands. With rooftop gardens, it has been shown that the surface temperature of a concrete rooftop can be reduced by upwards of 25 degrees. While such numbers might sound impressive, the ability for rooftop gardens to make a large impact on urban temperatures is rather limited. Higher outside air temperatures, not surface temperatures, are behind most of the energy consumption and environmental issues caused by Urban Heat Islands. Lowering the surface temperature of the roof will have little impact on the energy consumption of a multi-story office building. Furthermore, rooftops in Japanese buildings are filled with elevator service units, water storage tanks (this is because the government will only provide water pressure up to three stories; anything above that requires onsite pumps and rooftop water tanks), railings and other sorts of structural bric-a-brac, which limits the amount of space available for rooftop garden systems. While introducing greenery onto Tokyo rooftops has other positive externalities like reducing storm run-off through rainwater retention, providing a natural habitat for birds and insects, and generally making cities more esthetically pleasing, a garden on even half the roofs in the city would reduce temperatures by only a small fraction.

It’s a similar story with other heat Island mitigation technologies done on the building-scale as well. Some impressive surface temperature reductions can also be achieved using reflective paint and water-absorbing concrete, but neither of these technologies, which address the fabric of the city, solve the real culprits: a lack of large-scale natural vegetation and urban forms conducive to heat retention. In fact, studies of temperatures in Japan reveal that “Urban Heat Islands” may be a misnomer, as persistent Urban Heat Islands have been observed on small collections of buildings and roads located in rural areas throughout Japan. It seems the barren, concrete-block-and-asphalt-road nature of built-up space in Japan encourages excessive temperatures.

Tokyo was once crisscrossed by many streams and rivers that provided temperature-cooling wind paths through the heart of the city, but most of these waterways were aggregated into a handful of unsightly concrete culverts, or simply paved over. While much of modern-day Tokyo was laid out in the post-war era, the Japanese actually started engineering local inlets, streams and other waterways to exist on the periphery of urban settlements for several reasons. It may be tempting to blame all the concreted rivers (and by extension, nasty heat islands) on the general construction boom and construction pork barrel politics that has gripped Japan in the last 50 years, but there are actually some very fundamental cultural issues at play that have affected the current river-less state of Japanese cities. In feudal Japan, rivers were not only the places to discard trash, but they were also the location of Buraku communities that were typically engaged in spiritually “tainted” industries like leather working, undertaking, and butchering. Consequently, as Japanese cities modernized around the turn of the century, rivers and all their dirty work were often brushed aside to make way for more illustrious and pleasant things, like roads and buildings. However, with average summer temperatures on pace to rival the likes of Calcutta within the next several decades, Japanese urban planners are faced with little choice but to ease restrictions of building taller buildings (to allow for more inhabitable space on less land), free up more space for parks and natural vegetation, and look into unearthing a few of those subterranean rivers that currently zigzag beneath Tokyo roadways.

Troy Fowler
PhD Environmental Systems,
Yokohama National University