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Greenery as a mitigation and adaptation strategy to urban heat

Abstract

The absence of vegetation in urban areas contributes to the establishment of the urban heat island, markedly increasing thermal stress for residents, driving morbidity and mortality. Mitigation strategies are, therefore, needed to reduce urban heat, particularly against a background of urbanization, anthropogenic warming and increasing frequency and intensity of heatwaves. In this Review, we evaluate the potential of green infrastructure as a mitigation strategy, focusing on greenery on the ground (parks) and greenery on buildings (green roofs and green walls). Green infrastructure acts to cool the urban environment through shade provision and evapotranspiration. Typically, greenery on the ground reduces peak surface temperature by 2–9 °C, while green roofs and green walls reduce surface temperature by ~17 °C, also providing added thermal insulation for the building envelope. However, the cooling potential varies markedly, depending on the scale of interest (city or building level), greenery extent (park shape and size), plant selection and plant placement. Urban planners must, therefore, optimize design to maximize mitigation benefits, for example, by interspersing parks throughout a city, allocating more trees than lawn space and using multiple strategies in areas where most cooling is required. To do so, improved translation of scientific understanding to practical design guidelines is needed.

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Fig. 1: The urban heat island effect.
Fig. 2: Greenery-related cooling mechanisms in the urban environment.
Fig. 3: Average greenery-related peak temperature reductions.
Fig. 4: Factors contributing to temperature reduction for ground-level greenery.
Fig. 5: Types of greenery on buildings.
Fig. 6: Factors influencing the cooling potential of vertical and rooftop greenery.
Fig. 7: Translation of greenery research into design.

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C.L.T. led the research, discussion, writing and editing of the article. N.H.W., D.D.K. and H.T. contributed to the writing.

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Correspondence to Chun Liang Tan.

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Nature Reviews Earth & Environment thanks Amirhosein Ghaffarianhoseini, Tijana Blanusa, Mohammad Asrafur Rahman, Paul Osmond and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Supplementary information

Glossary

Sensible heat

Heat transfer that results in a change in temperature between objects, without changing the volume or pressure.

Evapotranspiration

The combined processes of evaporation of water from the soil, as well as plant transpiration, where water is transported from the soil through the roots and exits via the leaf stomata and into the atmosphere as water vapour.

UHI intensity

The temperature difference between urban and rural areas; either surface or air temperature can be used.

Albedo

The ratio of reflected radiation over total incident radiation on a surface, indicating its overall reflecting potential. Albedo values can range from 0 to 1, with 1 meaning all radiation is reflected and 0 indicating that all radiation is being absorbed.

Latent heat

Heat transfer that results in a change in state (such as liquid into vapour), without changing the temperature.

Bowen ratio

The ratio of sensible heat flux to latent heat flux above a surface that contains moisture. Commonly used in meteorological and hydrological studies, it is an indication of the abundance of water over surfaces, as the presence of moisture will directly influence latent heat flux density.

Threshold value of efficiency

(TVoE). The value to which an increase in green space ceases to provide substantial cooling.

Leaf area index

(LAI). Total one-sided leaf area per unit horizontal ground surface.

Vapour pressure deficit

The difference between moisture content in in situ air compared with the total moisture the air can hold when it is saturated.

Physiological equivalent temperature

Air temperature at which, in a typical indoor setting, the heat balance of the human body is maintained with core and skin temperatures equal to those under the conditions being assessed. It provides an indication of thermal comfort, applicable for both indoors and outdoors.

Computational fluid dynamics

(CFD). Quantitative modelling of fluid flow based on the laws of mass, momentum and energy conservation that govern fluid motion.

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Wong, N.H., Tan, C.L., Kolokotsa, D.D. et al. Greenery as a mitigation and adaptation strategy to urban heat. Nat Rev Earth Environ 2, 166–181 (2021). https://doi.org/10.1038/s43017-020-00129-5

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