your most valuable neighbor might be green.
what’s in it for you?
• learn about the Urban Heat Island effect• understand how vegetation provides cooling• grasp how vegetation cooling affects your walletsecretly dragging you down – urban heat islands.
Imagine walking through the city on a hot summer day while the sun is beating down. The air stagnates and the pavement radiates waves of heat. Craving a cool place? Try your closest green space!
Scorching temperatures are a reality for many of us city dwellers, but neither the weather itself nor recent changes in climate are solely responsible. A phenomenon named urban heat islands (UHIs) causes cities to experience higher temperatures than surrounding rural areas [1], in some cases reaching up to a difference of 6.0°C [2]. Although UHIs appear negligible at first glance, they have far-reaching implications and pose a major challenge that needs to be addressed urgently.
Many people associate higher temperatures with increased sweating and sleep issues. Their solution? Turning on the air conditioner and opening up their wallets to soaring electricity bills! But, what happens to citizens who cannot utilize an air conditioner? They are doomed to suffer lower productivity [3], heat strokes and dehydration [4].
Vulnerable populations, namely children and the elderly, are particularly affected. The latter often live in old, poorly insulated buildings - coupled with impaired biological thermoregulation, the health consequences are enormous. Recall the heat waves in India and England in 2022 [5,6]. Merciless heat, reportedly amplified by urban heat islands, contributed to the premature death of 6700 citizens, equivalent to 4.3% of total summer deaths in 93 European cities [7]. Opposing urban heat islands is hence crucial to ensure the safety and well-being of our most vulnerable populations going forward, whether it is our offspring, our parents or our grandparents.
Yet urban heat islands are a growing and entirely man-made issue. More than 55% of the world's population already lives in cities and this figure is expected to rise to 68% by 2050 [8], further increasing UHI intensity in the future. The newly arrived urban residents require living space, however it is precisely their dwellings that alter the physical urban conditions and thus the urban climate [9]. Unlike plant material, concrete walls restrict the flow of hot air, ultimately trapping energy from sun rays while leading to higher temperatures [10].
Figure 1. The concept of Urban Heat Islands
Source: Based on Hiemstra et al. [11]
countermeasures.
Now imagine you are a caring city government, and you want to ease the burden of your citizens - what are your options? Spanish researchers [12] identified four available solutions, consisting of vegetation cover or green-space development [13], night stack ventilation [14], and albedo-driven surface optimization [15]. Translated into vivid examples - befriend trees and plants, use windcatchers and paint your city white, Santorini style.
Sounds intriguing? We will explore each approach within our upcoming asset.city article series.
Today, the focus lies on vegetation cover and green spaces as solution concept. Every person who has visited a forest on a hot summer day, appreciates its cooling properties. Yet the very same cooling properties have also been observed in parks of different sizes [16,12], urban forests [17] as well as street trees [18], resulting in the term “Park Cooling Island” (PCI) within urban settings (Visible in Figure 1). The effect has been demonstrated using a variety of approaches and experimental set-ups, including field measurements, simulations, and more recently satellite-based evaluation [19,20]. Typically ranging from around 1°C [21], to up to 4.8 °C [22], the strength substantially depends on the experimental setup and the type of temperature measured (air vs. surface temperature). Besides the size of the park itself [23], different plant species [24] will equally influence both the intensity and the range of the cooling effect. Zooming onto the individual tree and shrub level, the origin of the cooling effect lies in the comparatively high heat absorption of the vegetation, the provision of shade and its so-called evapotranspiration (simplified: a plant self-cooling mechanism through water emission) [21]. Still not convinced? Have a look at Figure 2 showcasing a street in Australia in summer!
Figure 2. Vegetation cooling captured
with a thermal camera in Australia
Source: Based on Hiemstra et al. [11]
Local data collection with thermal cameras as depicted in Figure 2, however, is expensive and limited to the observation area. Satellite-based temperature assessment has recently risen the ranks as a viable alternative. While it does not capture the air temperature itself, it can appropriately assess land surface temperature over a large area without incurring high costs. The observed land surface temperature is in turn significantly positively correlated to air temperature, thus a viable approximation. [25,26]
urban trees as temperature regulator.
Let's take a trip to the biggest city in the world - Tokyo, Japan. Tokyo is located in a humid subtropical climate zone (Cfa, Köppen classification) and is characterized by hot, humid summers and mild winters. The city offers large-sized parks and green spaces in the city center, notably Yoyogi Park, the Meiji Jingu shrine grounds and the Imperial Gardens of the Japanese Emperor. Building upon publicly available satellite imagery by Landsat 8, an American Earth observation satellite launched on 11. February 2013, we can observe that Tokyo exhibits the cooling effect of urban vegetation (Figure 3). The cross-sectional image through Yoyogi Park and Meiji Jingu Shrine portrays the temperature differences vividly. Juxtaposing the land surface temperature data recorded by the satellite on a cloud-free day (for example in April 2020) with vegetation data, we can observe how cooler locations (purple tones, Figure 4) are accompanied by higher tree cover (red tones in blue graphic).
Figure 3. Land Surface Temperature in Tokyo, April 2020 (left)
Source: Global land surface temperature (LST) by Landsat 8, [27]
Figure 4. Temperature Profile across Meiji Jingu and Yoyogi park in Shibuya City, Tokyo (right)
Source: Own research based on Terra MODIS Vegetation Continuous Fields (VCF), published by NASA, [28]
A group of Swiss researchers [29] extended this comparison to 293 cities of Europe while also taking into account climate zone and green space structure data. Using statistical models and over 120,000 satellite scenes, the authors were able to discover exciting results - not only the structure of urban parks, with or without trees, but also the climate zone plays an important role. The authors calculated the (Land Surface) temperature differences between parks and urban fabric (e.g., streets and concrete buildings) and obtained negative median values across all of Europe. In other words, green spaces provide cooling - the results are presented in Figure 5. By comparing different park types, the authors furthermore identified that treeless parks provide less median cooling than their tree hosting counterparts, exemplified by the green bars. Another finding is the different strength of the effect depending on the climate zone. Warmer regions of southern Europe show less pronounced cooling properties, largely attributable to lower leaf coverage of native trees and thus less shade, according to the authors. While urban trees might not serve as silver bullet across every region, they work wonders in cooler climatic zones with broadleaf vegetation.
Figure 5. Median cooling, calculated as temperature difference between green spaces and urban fabric in Europe.
Source: Based on Schwaab et al. [29], simplified version using only median values
more trees, please!
So far, we learned that urban green spaces actually reduce temperature substantially, and how the cooling strength depends both on the park structure and climatic zone. But what does this mean for the average city dweller?
Back to Asia - in Singapore, researchers found that green vegetation reduces local air temperatures by approximately 1 °C. At the same time, they discovered energy savings of up to 4.5% in adjacent buildings, relishing the local temperature reduction. Chen and Wong [30] carried out a similar analysis in China and equally found a 5% reduction in energy consumption in nearby properties, caused by a 1-degree Celsius temperature difference. Without considering any additional biodiversity or recreational benefits, we can conclude – money indeed (indirectly) grows on trees as the summer cooling effect of trees can reduce energy consumption and ultimately electricity costs.
So, what happens when we create new green spaces and enhance existing ones? Research from Jakarta, Indonesia, predicts local temperature reductions of 1.5°C in exchange for 17% more green cover [31]. Huang et al. [32] ultimately translate temperature into electricity demand. According to their findings, an expansion of urban tree canopy cover by just 10% could yield energy savings of approximately 18% of the annual electricity demand across various US cities. Enhancing canopy cover by 25% would even enable annual electricity savings of up to 42%!
Let's say you reside in Tokyo, you're a single household, and you pay exactly the average cost of electricity per month, as calculated by the Statistics Bureau. On an annual basis, your electricity expenditure would amount to about 500 USD per year. How much money would you save if you lived next to a (growing) green space?
Our back-of-the-envelope-calculation promises savings of at least $25 USD (3000 Yen) for a single household, per year. Assuming your neighboring green space gains 25% tree cover, you could even save up to 40% of your annual electricity bill, around 200 USD per year! Now consider the collective savings of not only you, but all of your neighbors. Quite a lot of money, isn’t it?
Figure 6. Average electricity expenditure as Single Household in Tokyo
Source: Own representation, based on Statistics Bureau, Japanese Ministry of Internal Affairs and Communications, Annual Report of Household Survey 2020, Exchange rate of JPY 134.27 per USD [33]
conclusion.
Throughout this article, we learned about the man-made problem of Urban Heat Islands and how parks can counteract its warming effect. Ultimately, we linked the cooling properties of parks to your energy costs. We discovered that trees exert a strong cooling effect, yielding indirect monetary value. Does this mean we should now plant trees everywhere?
Sadly, no. Planting the wrong tree in the wrong place may trigger allergies for residents [34] and interrupt existing wind currents, potentially resulting in pollutant pockets due to stagnant air [35]. We need more parks, more trees, but in the right place. Fortunately, trees and parks offer several additional benefits (e.g., aiding your mental health), as we will discover in our next article of the asset.city series. We are curious about your experiences! Have you ever noticed the cooling effect of your local parks or urban forests?
our two cents
Although our cities suffer from the urban heat island effect, we are not powerless. Green spaces and their vegetation can help us mitigate heightened temperatures while providing us with additional benefits.-
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