Cooling Potential of Urban Green Spaces in Summer

Kochi Tonosaki, Shiro Kawai, and Koji Tokoro

Abstract The urban heat island phenomenon has recently become a serious subject of public concern. This chapter aims to clarify the cooling potential of urban green spaces in summer. At first, it shows that green space and anthropogenic heat emission have a great effect on the temperature in downtown areas from the various data collected from 27 observation points in Minato-ku, Tokyo. Then, it clarifies the cooling potential of green spaces. The results of multiple regression analysis, using the mean daily maximum temperatures in August as dependent variables and the size of woodland area and the amount of anthropogenic heat emissions as explanatory variables, showed that an increase in trees contributes to a reduction in temperature in urban areas, and that an increase in the amount of anthropogenic heat emissions causes a rise in temperature. A multiple regression model in equations and a coefficient of correlation among mean daily maximum temperature, woodland area, and amount of anthropogenic heat emissions were obtained as follows: Y = 32.0011 — 0.001(X1) + 0.0033(X2), r = 0.7276, where Y = the mean daily maximum temperature, X1 = woodland area, and X2 = the amount of anthropo­genic heat emissions. From this regression analysis, it can be said that the cooling influence by green spaces of 22,500 m2 is equivalent to the heating influence by the anthropogenic heat released from 70 offices of average size in Minato-ku, having a total floor area of about 211,726 m2. Furthermore, the cooling potential of a green space of 22,500 m2 during July to September can be expected to reduce about 236 times as much carbon dioxide as the same green space absorbs for 1 year. In conclusion, green spaces in urban downtown areas have the function of air conditioning provided by nature.

K. Tonosaki (*)

Organization for Landscape and Urban Green Infrastructure, Kandajimbou-cho 3-2-4, Tamura Bld 2F Chiyoda-ku, Tokyo 101-0051, Japan e-mail: tonosaki@urbangreen. or. jp

S. Kawai • K. Tokoro

Soken, Inc., Nagoya, Japan

e-mail: kawai@soken. co. jp; tokoro@soken. co. jp

N. Nakagoshi and J. A. Mabuhay (eds.), Designing Low Carbon Societies in Landscapes, Ecological Research Monographs, DOI 10.1007/978-4-431-54819-5_2, © Springer Japan 2014

Keywords Anthropogenic heat emission • Carbon dioxide • Cooling potential • Green coverage ratio • Green space • Heat island phenomena • Minimum air temperature • Tokyo • Woodland

2.1 Introduction

The heat island effect is acknowledged by the Japanese government as a pollution issue: heat islands were identified in 2003 by the Ministry of the Environment as causing thermal pollution of urban air and were noted in the Third Basic Environ­ment Plan drawn up in April 2006 as an air pollution problem in Japanese cities. With the emergence of the heat island phenomenon during the past few years as a major social issue affecting urban areas, there has been a growing move to promote urban greening in view of its potential benefits in a variety of areas—not only in heat island mitigation but also in absorbing the greenhouse gases that cause global warming and in enhancing the cityscape. In particular, from the perspective of heat island mitigation, interest in the cooling effects of urban greening is mounting, and a number of studies have been conducted in this area.

From the correlation between green coverage ratio and the minimum air tem­perature during days on which “tropical nights” occur, Owada et al. (2007) showed that the air temperature tends to be lower the higher the green coverage ratio. By analyzing land-use data derived from Landsat data, and air temperatures based on AMeDAS data, Irie (2003) demonstrated that the effect of green space is most apparent at times when the air temperature is lowest. Ando et al. (2008) examined actual measurements of the cool air drifting out of the Imperial Palace gardens as a green effect during the hottest times of day. Yamada and Maruta (1991) took the highest and lowest thermometer readings at 57 sites in Tokyo’s Suginami-ku and found that wooded areas had an effect in reducing both maximum and minimum temperatures.

On the other hand, in April 2007, the U. S. Supreme Court handed down a ruling requiring regulation of CO2 and other greenhouse gases that cause global warming to the federal Environmental Protection Agency. In March 2010, the Climate Action Reserve (2010) provided guidance for calculating, reporting, and verifying greenhouse gas (GHG) emission reductions associated with a planned set of tree planting and maintenance activities to permanently increase carbon storage in trees. The appendix, which was included in the Protocol, introduced previous studies on the effect of air conditioning: for example, McPherson and Simpson (2003), EIA (2002), CARB (2007), Nowak and Crane (2002), Sampson et al. (1992), Trexler (1991), Moulton and Richards (1990), and McHale et al. (2007). Akibari (2002) noted, “Urban tree planting can reduce net cooling and heating energy consumption in urban landscapes by 25 %.”

All these previous studies evaluated the effects on air temperature of the different types of ground cover that make up green space (woodland, grassland, bare ground, etc.). However, there appear to be no previous studies that evaluate the effects of the floor area of buildings, amount of vegetation cover, or amount of anthropogenic heat emissions on air temperature.

Against this background, our study considers at the effect of woodlands in reducing air temperature, based on air temperature data obtained in August 2007 from digital weather stations set up at 27 locations in elementary schools and parks in Minato-ku, Tokyo. In addition, by showing that the temperature at the observa­tion points can largely be explained in terms of the size of the woodland and the amount of anthropogenic heat emissions, we ascertained the ratio of these two factors (negative and positive, respectively) in contributing to air temperature. Furthermore, we tried converting the reduction in air temperature per unit area of woodland into the cooling potential of air conditioners and other such equipment.