Urban Climate Mitigation Techniques

Mat Santamouris, Denia Kolokotsa

• Publisher: Taylor & Francis
• Year: 2016
• Print ISBN: 9780367669980
• eBook ISBN: 9781317658627
• Edition: 1
• Topic: Architecture
• Subtopic: Urban Planning & Landscaping

The urban climate is continuously deteriorating. Urban heat lowers the quality of urban life, increases energy needs, and affects the urban socio-economy. Urban Climate Mitigation Techniques presents steps that can be taken to mitigate this situation through a series of innovative technologies and examples of best practices for the improvement of the urban climate.

Including tools for evaluation and a comparative analysis, this book addresses anthropogenic heat, green areas, cool materials and pavements, outdoor shading structures, evaporative cooling and earth cooling. Case studies demonstrate the success and applicability of these measures in various cities throughout the world.

Useful for urban designers, architects and planners, Urban Climate Mitigation Techniques is a step by step tour of the innovative technologies improving our urban climate, providing a holistic approach supported by well-established quantitative examples.

1

URBAN WARMING AND MITIGATION

Actual status, impacts and challenges

Mat Santamouris

University of Athens – Athens, Greece

Introduction

Almost four billion people actually live in cities and official forecasts predict a very large increase in the next few years (United Nations, 2014). Urban population is increasing rapidly because of childbirth rates and the important migration of the rural population into cities caused by expectations for a better life, local conflicts and lack of resources in the country areas. As mentioned by PWC (2014), the increase of the world’s urban population is close to 1.5 million people per week. High density and increased consumption patterns make cities the higher consumer of global resources as they are responsible for almost 75 per cent of the world’s assets (PWC, 2014). In parallel, the urban environment experiences a very significant territorial expansion, known as urban sprawl, combined with a significant change of land use. Losses in green spaces and sealing of urban land because of the extensive use of opaque surfaces of paving, in combination with a very high increase of released anthropogenic heat, have affected the urban climate, resulted in a serious environmental degradation and have increased significantly the urban ecological footprint (Oke, 1997).

Global climatic change caused by the increased concentration of greenhouse gases in the atmosphere contributes to increased urban temperatures and the frequency and length of extreme climatic phenomena like heat waves. The magnitude of the ambient temperature increase caused by the global climate change is forecasted by the IPCC Committee. For the period 1990–2005, predictions indicated an increase between 0.15 K to 0.3 K, which is already confirmed by measurements (IPCC, 2014). At the same time, predictions for the period 1990–2100 indicate that the possible ambient temperature increase will range between 1.8 K to 4 K.

Additionally with global climate change, the thermal balance of cities is highly affected by the increased absorption of solar radiation, the corresponding increase of sensible heat released by urban structures, higher anthropogenic heat, reduced latent heat, higher emission of infrared radiation and other specific sources (Landsberg, 1981). Additional heat accumulated and released in the urban environment results in a positive thermal balance and increased urban ambient temperatures compared to the surrounding urban environment. Such a phenomenon is known as the ‘urban heat island’ and it is the most documented phenomenon of climate change. The difference between the urban temperature and the corresponding rural or suburban one is referred as the ‘urban heat island intensity’ and its magnitude is a function of the physical, structural and morphological characteristics of the cities, the urban layout, local climatic parameters, the synoptic weather conditions and also the total anthropogenic heat generated and released in the city (Oke et al.,1991). Studies on the heat island characteristics are available for most of the medium and large cities in the world and the reported urban heat island intensities reach values up to 10 K.

Global climatic change and urban heat islands affect the urban climate, but their specific impact may not always act in a synergistic way. The specific mechanisms of the interaction between global climatic change, in particular heat waves, and urban heat islands are analysed by Li and Bou-Zeid (2013). Three mechanisms of interrelationship are identified:

1. Given that heat waves strengthen secondary circulations, the warm air over the city moves upwards, and fresh and cool air enters the city from the neighbouring zones. This mechanism has a negative feedback on urban heat islands.

2. Heat waves are mostly associated with low wind speeds. Under the same conditions the urban heat island is always stronger. Thus, under the specific conditions the two phenomena may have a synergistic effect.

3. During heat waves, the surface temperatures are increasing and thus more evapotranspiration and storage to the ground occurs. Given that in the rural areas there is a higher availability of surface moisture, the temperature difference between the urban and rural areas tends to increase.

Simulations and future projections on the possible impact of global climatic change on the magnitude of urban heat islands are quite contradictory. According to Oke (1997), the magnitude of the urban heat island may not be modified even for more intensive conditions of climate change, while Brázdil and Budíková (1999) predicted a lower magnitude of the urban heat island under intensified global climatic change conditions because of the possible increase of the vertical instability and the corresponding dissipation of the heat in the urban environment. Another study performed by McCarthy et al. (2010) concluded that stronger climatic change phenomena may cause a reduction of the urban heat island magnitude of 6 per cent; however, in urban zones with an intensive urbanization, urban heat island may increase up to 30 per cent.

Increased urban temperatures have a serious impact on the global environmental quality of cities. Urban warming increases the energy consumption for cooling purposes, and the peak electricity demand during the summer period raises the concentration of harmful pollutants like the tropospheric ozone and VOCs, increases the emissions of CO₂ to the atmosphere, deteriorates indoor and outdoor thermal comfort during the warm periods, seriously affects health conditions, and increases mortality, while it has an important environmental and economic impact. A quite detailed analysis of the specific impact of urban warming on energy, environment, health an...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• List of figures and tables
• About the editors
• About the contributors
• 1 Urban warming and mitigation: Actual status, impacts and challenges
• 2 Understanding and reducing the anthropogenic heat emission
• 3 Valuing green spaces as a heat mitigation technique
• 4 Mitigating the urban heat with cool materials for the buildings’ fabric
• 5 Cool pavements to mitigate urban heat islands
• 6 The effect of evaporative cooling techniques on reducing urban heat
• 7 Exploiting earth cooling to mitigate heat on cities’ scale
• 8 Urban climate mitigation techniques: The role of spatial planning
• 9 Urban climate models
• 10: Urban heat island mitigation technologies: Case studies
• Index