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Wednesday, Aug. 18, 2004
Tokyo's 'heat island' keeps breezes at bay
Waterfront wall of high-rises insulates asphalt jungle from cooling effect
By ERIKO ARITA
Japan is unusually hot this summer. But Tokyo is hotter.
The capital's high temperatures have been blamed on the "heat island" effect -- the high concentration of concrete and asphalt and few areas of vegetation and open water. Experts are now also pointing to another culprit: the expanding wall of skyscrapers that line Tokyo Bay.
On July 20, temperatures in parts of the Kanto region hit record highs. The highest for the day, 40.2, was recorded in Ichihara, Chiba Prefecture, while in central Tokyo the mercury hit 39.5.
But as the temperature in Ichihara dropped to a low of 22.7 early the following morning, in central Tokyo the mercury only fell to 30.1, a common heat island trait because urban temperatures remain higher than the surrounding areas.
Experts say that tall buildings, especially those recently constructed in the Shiodome district of Minato Ward, are becoming a major problem as they block the bay breezes that could push some of the hot air out of central Tokyo.
The 31-hectare Shiodome area hosts 13 buildings more than 100 meters tall.
If the heat island phenomenon is to be curbed, "no more building complexes should be constructed along the bay" said Masakazu Moriyama, a professor at Kobe University's department of architecture and civil engineering.
For breezes to have clear paths through the city, it is necessary to have open spaces, including wide streets, he said, adding that building regulations to create wind paths are needed.
The wind theory has caught the attention of the Environment Ministry, which plans to research the effects of skyscrapers on urban warming, according to ministry officials.
Vegetation, such as that found in parks, and bodies of water also curb temperature rises by decreasing heat through evaporation, Meteorological Agency official Koji Ishihara said.
But a common feature of large cities is that green belts and lakes have been replaced by the concrete jungle, nurturing the heat island effect.
"The asphalt of streets and concrete of buildings retain the sun's heat and expel it at night," Ishihara said.
According to the Tokyo Metropolitan Government, 271.9 hectares of greenbelt and open water -- equivalent in size to 17 Hibiya Parks -- disappeared in the capital between 1991 and 1996.
Tokyo's annual average temperature has risen 3 degrees in the last century, while the average in small cities, including Mito, Ibaraki Prefecture, and Abashiri, Hokkaido, increased only about 1 degree in the same period, according to Meteorological Agency statistics.
Air conditioners and motor vehicles also release heat in the metropolis.
According to Environment Ministry studies, about 50 percent of the heat in Tokyo that comes from urbanization is from vehicle exhausts and the use of air conditioners, while the remainder is from asphalt and concrete.
Central and local government officials are currently trying to find solutions to the growing heat problem.
Research conducted by Tokyo Metropolitan University professor Takehiko Mikami has shown that large plots of vegetation can help lower temperatures.
Mikami found that nighttime temperatures at Shinjuku Gyoen National Garden in central Tokyo were on average 4 degrees lower than outside of the park and that this cooler air moved 80 to 90 meters outside the park's perimeter.
"It is important to increase green spaces, which have this cooling effect," he told a recent symposium.
In March, the Environment Ministry compiled measures to address the heat island problem that include a 10 percent nationwide increase in publicly managed greenbelts and water bodies over fiscal 2002 levels by fiscal 2007.
The metropolitan government for its part has been actively pushing for vegetation to be installed on building rooftops since 2000.
An experiment conducted by the Tokyo Metropolitan Research Institute for Environmental Protection found that the temperature emanating from the concrete portion of a building's roof reached 55 degrees, but the area with plants rose to just 30, according to Akira Yamamoto at the Urban and Global Environment Division.
The roof's greenbelt released just 25 percent of the heat compared with the concrete portion, he said.
In 2001, the Tokyo Metropolitan Government issued an ordinance requiring contractors building on plots of 1,000 sq. meters or more to cover at least 20 percent of their structures' roofs with vegetation.
By the end of fiscal 2003, about 450,000 sq. meters of roofs in the capital had greenery, a little less than 10 times the area of Tokyo Dome, according to Yamamoto.
Some Tokyoites are also turning to a more traditional way of cooling down.
Last August, citizens' groups coordinated an event in several parts of Tokyo to pour water on the streets, a practice called "uchimizu."
Tadahiro Katsuta, a member of the nonprofit group Peaceful Energy and one of the organizers, claimed about 340,000 people took part.
He explained that sprinkling water on the streets was an old way to ease the summer heat, as the water absorbs ground heat when it evaporates.
Temperatures measured at four event sites were down about 1 degree on average, he said.
Organizers will hold the same event for one week starting Wednesday in Tokyo and other cities, with hopes of getting 1 million people to participate. They are asking participants to use rainwater or recycled water.
"It's a way of cooling cities down that anyone can do without spending a lot of money," Katsuta said.