Living in Tokyo

Urban Heat Island Effect: Why It’s So Hot in Tokyo & What’s Being Done About It

As Japan continues to broil in the midst of a record-breaking heat wave this summer, we wanted to highlight a phenomenon that has been causing sustained long-term temperature increases in Tokyo for the past century: the urban heat island (UHI) effect.

With the mercury hitting unprecedented levels across Japan, many people have pointed to global warning as the chief culprit. But is this the case? Japanese climate researchers at the Tokyo Metropolitan Institute for Environmental Sciences have found that in the last one hundred years, average temperatures in Tokyo have risen three degrees (Celcius), versus one degree for Japan as a whole. Of these three degrees, two are attributed to the urban heat island effect and one to global warming.

Average annual temperature (Celsius) in Tokyo versus selected cities from 1900 to 2010. Tokyo (purple), New York (yellow), Paris (green), Berlin (blue), World (red). Source: Japan Meteorological Agency (JMA)

What is the Urban Heat Island (UFI) Effect?

UHI is a phenomenon in  which temperatures in urban areas are markedly higher than those in surrounding areas.

Isotherm map of the five year average of the total number of hours in the Kanto Region where the temperature was 30 degrees Celsius or above. Left map 1980 to 1984. Right map: 2003 to 2007. Source: JMA

When isotherms on a map show an urban area with higher temperatures emerging like a hot “island” surrounded by a cooler “sea” of surrounding areas, you have an urban heat island.

The UHI effect results in higher average temperatures and a greater number of sweltering summer nights, defined in Japan as nights when the temperature stays above 25 degrees. This obviously affects peoples’ daily lives and health and has also been blamed for localized torrential rains (called “guerilla rain” in Japan).

What causes the UHI effect?

Basically, UHI is caused by urbanization. Of course, it is not a phenomenon isolated to Tokyo. Modern transportation, business activities and manufacturing, and air conditioning all lead to an increase in heat emissions, while high-density urban living tends to decrease the factors that would lower temperatures: bodies of water, wind, and green space.

Asphalt roads and concrete buildings also absorb and retain heat, rather than reflecting it. The many high-rise buildings in Tokyo also block or weaken wind and air flow, which would otherwise naturally lower the air temperature. Tokyo does have some sizable green spaces in the city center, including Yoyogi, Shinjuku National Garden, and Ueno, but it is the relatively small amount of green space compared to the degree of urbanization that works to increase the heat island effect.

There is also, unfortunately, a vicious cycle that operates between the UHI effect and global warming. As well explained by Japan for Sustainability: “Rising temperatures due to the UHI effect create increased demand for air conditioning, which increases the amount of exhaust heat vented, which in turn leads to further temperature rise. In addition, the more electricity is consumed with the increasing use of air conditioners, the more carbon dioxide (CO2) is emitted and temperatures continue to rise further as global warming worsens.”

How hot _is it_ in Tokyo? Where is it hottest?

According to data from the JMA and  Tokyo Bureau of Environment, unsurprisingly, points nearest the coast tend to be coolest. The ranking below is based on data from 2002 to 2015, for days between July 20th and September 30th. The “number of hours” in the ranking refers to the number of hours where the temperature was 30 degrees Celsius or higher.

Hottest Spots in Tokyo 23 Wards

  1. Chiyoda, Chiyoda Ward (466 hours)
  2. Nerima Nerima Ward (410 hours)
  3. Nerima Kitamachi, Nerima Ward (408 hours)
  4. Nishi Arai, Adachi Ward (390 hours)
  5. Setagaya, Setagaya Ward (377 hours)

Coolest Spots in Tokyo 23 Wards

  1. Harue, Edogawa Ward (237 hours)
  2. Seijo, Setagaya Ward (251 hours)
  3. Minami Kasai, Edogawa Ward (254 hours)
  4. Daiba, Minato Ward (275 hours)
  5. Shishibone, Edogawa Ward (333 hours)

The JMA found that thus far in 2018, there has been an average of a 5 degree Celsius temperature difference between the highest average temperature in Nerima Ward (33.6 degrees C) versus Edogawa Ward (30.1 degrees C).

Efforts and Policies to Combat the UHI Effect

The Japanese government outlined a basic policy to mitigate the the UHI effect in 2004 centered on reducing artificial (human-caused) heat emissions, “greening” and creating green belts (the planting of more trees and creation of more green space), improving road surfaces, and creating wind paths (strategic placement of skyscrapers in line with urban renewal projects).

“Greening” and “Green Belts”

According to surveys by the Tokyo Metropolitan Government, when the temperature of concrete surfaces rise to 55 degrees C in mid-summer, the surface in green areas is as low as about 30 degrees C. The Japanese national government and local governments have promoted greening through subsidies, reductions in fixed property taxes, bonus plot ratios and other measures.

In 2001, the Tokyo Metropolitan Government passed an ordinance requiring rooftop greening on newly constructed buildings. Under this rule, over 20 percent of the total site must be set aside for “greening” in cases where the facility is over 1,000 square meters. A well-known example of this is the rooftop garden in Roppongi Hills, where residents cultivate their own rice! For more examples of “green roof” buildings around Japan, please see: Japan’s Amazing Green Roof Buildings.

Other initiatives by the Tokyo Metropolitan Government include improving existing parks, greening the tops of local government buildings and high schools, planting grass on the grounds of elementary and junior high schools in Tokyo, and planting grass along tram lines.

Improving road surfaces

To reduce the amount of heat absorbed by asphalt, the city has also adopted water-retentive pavement, experimenting with insulating pavement and even conducting a feasibility study on spraying treated sewage water over pavement.

Creating wind paths

One of the biggest (literally!) causes of the UHI effect in Tokyo are the very things that contribute to making Tokyo the dynamic metropolis that it is: densely packed skyscrapers, especially those nearer the coast, which block the wind blowing in from the ocean.

The most well-known example of this effort is included in the renovation plan for the Tokyo Station area, where there is a project to construct twin skyscrapers located about 246 meters apart and connected by a pedestrian deck. After these buildings are completed, an existing old 12-story building that now blocks the wind from Tokyo Bay will be demolished. It is expected that creating this wind path will significantly lower temperatures in the area.

Uchimizu (Water Sprinkling)

There are also a number of citizen-based initiatives, which you may have seen for yourself while walking down the street in Tokyo or any other city in Japan, called Uchimizu (打ち水 or “water sprinkling”). In this campaign, people are being asked to used leftover water (such as used bath water) to sprinkle on the streets on hot summer days to create a cooling effect from evaporation.

People participating in a Uchimizu ceremony in Ginza. Image: Wikipedia

Sources: Japan for Sustainability, Nikkei Shinbun, July 19, 2018, Tokyo Metropolitan Environment Bureau, Japan Meteorological Agency