Literature review on Urban Heat Island(UHI)

Are cities’ densification and revegetation the

best options to preserve the

environment?

Introduction

Climate change forces us to rethink the city structure according to new criteria. 

The expansion of very dense built environments and the increase in human activities.

• The first article analyses the environmental benefits of the compact city and the environmental disadvantages of overcrowding, and finds out the most suitable population density in China through simulation.

• The second article simulates some idealised cities’ structures, exploring the links between different variables such as urban density, thermal comfort, air quality and energy consumption.

• Third and Fourth: these articles discusses urban heat island mitigation measures and make a comparative analysis and sensitivity analysis of different climate zones in the United states and Paris.

The first part focuses on the challenges raised by climate change and high urban densities, namely the problem of Urban Heat Islands. The second part discusses the benefits and limitations of compact cities, asking whether the compact city model is a viable answer to climate change or not.

1. Climate change in urban context: the effects of Urban Heat Islands

• Two case studies: Paris and American cities in various climate 

An Urban heat Island (UHI) is a metropolitan area which is significantly warmer than its surroundings.

Different solutions exist to measure or more broadly to quantify UHI, either from observations or from modelling. In literature, the UHI is often defined as the difference in temperature between the city centre and a reference point in the surrounding countryside, i.e. it informs the maximum effect reached at city-scale but does not provide spatialized information on all areas potentially affected. They enable to objectively assess the vulnerability of the city to heat waves, and to compare the various urban sprawl strategies.

 

According to the Environmental Protection Agency, many United State cities have air temperatures up to 5.6°C warmer than the surrounding natural land cover. The main causes are changes in the land surface by urban development along with waste heat generated by energy use. As population centres grow, they tend to change greater areas of land which then undergo a corresponding increase in average temperature.

 

How Do Heat Islands Form?

 

Heat islands form as vegetation is replaced by asphalt and concrete for roads, buildings, and other structures necessary to accommodate growing populations. These surfaces absorb rather than reflect the sun's heat, causing surface temperatures and overall ambient temperatures to rise. Waste heat from vehicles, factories, and air conditioners may add warmth to their surroundings, further exacerbating the heat island effect.

 In the third article, the Paris urban area will evolve by the end of the century, as a result of macro-economic trends and demographic pressure, as well as under the influence of local urban planning policies, evolution of technologies and lifestyles. These combined effects can play a significant role on the structure of the city and, therefore, potentially, on the microclimate.

Urban expansion is one of the main causes of urban heat islands. To construct the urban expansion scenarios that we use here, long term socio-economic scenarios are downscaled at city scale using the method developed by Viguié at al. (2014). To do that, the land-use transport interaction model NEDUM-2D is used. This model is based on a dynamic extension of the classical urban economic theory, and explains the spatial distribution of land and real estate values, dwelling surfaces, population density and buildings heights and density across the city. A validation over the 1900-2010 period shows that the model reproduces available data and captures the main determinants of city shape evolution over this period.

 

In this study, a simple city expansion scenario is used, called the “spread-out city” scenario and referred to as SPR afterwards. This scenario follows broadly current trends in terms of total population increase and population density decrease, leading to urban sprawl. This figure shows a map of projected Paris urban area extension between 2010 and 2100 in this scenario. It is assumed in this case that the extension of the city is entirely guided by the market: we introduce no policy or regulation limiting the extension of the city or preferentially developing certain areas. The idea is that this scenario is used here to represent the “natural” trend of development of the city; this trend does not necessarily match the development that will occur in practice, but it allows understanding and anticipating land pressures, and therefore future local challenges.

 

For example, the urban heat island has been investigated for the Paris region and it indicates that heat waves will become more frequent at the end of the 21st century, but also, they will last longer and will be more intense than today. Some authors showed from the analysis of a large set of regional climate model (RCM) projections that the Paris basin will be frequently affected at the end of the 21st century by heat waves which will be on average longer and more intense than today. Each of these heat waves is characterised by a maximum intensity, that is maximum temperature TX reached during the event, and a duration in number of days. They have been classified according to four classes of intensities, that is temperature TX between 32–36 °C, 36–40 °C, 40–44 °C, and greater than 44 °C.

 

For example, the case study of towns of USA shows that, high UHI intensity levels were found in Phoenix (hot and dry); the average nighttime UHI intensity in summer is 5°C, and can increase up to 11°C. In Chicago, the largest nighttime UHI intensity among eight different neighbourhoods was 2.34°C in the summer of 2010. According to records from Global Historical Climate Network data (2004-2013), the average minimum summer nighttime UHI intensities were 1.84°C, 1.89°C, 2.67°C, 3.11°C, and 3.77°C, in Houston, Chicago, Memphis, Boise, and Phoenix, respectively.

• What solutions to Urban Heat Islands?

1.     Increase shade around your home

Planting trees and other vegetation lowers surface and air temperatures by providing shade and cooling through evapotranspiration.

2.Install green roofs

A green roof, or rooftop garden, is a vegetative layer grown on a rooftop. Green roofs provide shade and remove heat from the air through evapotranspiration, reducing temperatures of the roof surface and the surrounding air.

3. Install cool roofs or reflective roofs

Cool or reflective roofs help to reflect sunlight and heat away from your home, reducing roof temperatures. This allows for your home to stay cooler, reducing the amount of air conditioning needed during hot days.

4. Awareness and implementation of heat reduction policies and regulations.

The EU directives regarding environmental policies, such as low carbon fuel standards and the uses of renewable energy, can significantly regulate and mitigate the problem of urban heat island effect. With fewer emissions, the level of greenhouse gases in the atmosphere can be reduced. This, in turn, decreases the effects of climate change and global warming. Education and community outreach can also help to ensure that communities are made aware of the economic and social benefits of sustainable practices such as planting trees,and eco-roofing.

2. Compact cities: a viable answer to climate change?

1. A case study: Chinese cities + benefits and limitations of compact cities

compact city: relative high-density, mixed-use city, based on an efficient public transport system and dimensions that encourage walking and cycling.From the definition, it seems that compact city have many positive effects on the environment. But in fact, is the compact city model really effective for protecting the environment ?

 

There are environmental benefits but also environmental problems for a compact city. First of all, the dominant environmental benefits are preservation of the countryside, we protect arable land and green fields in the urban fringe by restricting the city expansion， while also helping to maintain biodiversity. But higher urban density implies heavy exploitation of urban green or open space for development.

 

In terms of transportation, urbain compaction limits travel distance to reduce emission and greenhouse gases. At the same time, public transport in compact cities will be more convenient and popular, there is less car dependence, less fuel consumption for traffic.However, high population density will lead to traffic congestion. Heavy traffic volume will increase travel time, then more fuel consumption, and bad air quality.

 

In terms of resource consumption, concentration of buildings and populations in compact cities are associated with less road length, shorter service run and more efficient use of urban resources. This enable energy and resource savings for infrastructure provision, thus leading to cheaper infrastructure costs. But higher standard and energy intensive materials are required for large or tall buildings in forming compact built form.

Moreover, as urban density increases, economies of scale are exploited because minimum densities of population are required for facilities. Economies of scale , which means increasing the number of people in facilities like hospitals, schools and libraries to encourage social service provision. However, overcrowding in compact neighbourhoods   results in bad neighbourhood effects such as noise, ill health, poverty and crime.

So urban compaction has been found to be beneficial for some environmental aspects but harmful to others.

 We can see that the environment has a limit or capacity up to which it can absorb activities without irreparable harm. So is there an urban density that maximises environmental advantages and minimises disadvantages? We used data from 45 Chinese cities to do this research. In the Chinese context, 16 variables are employed for representing agglomerated environmental performance. Net population density, namely non-agricultural population density in built-up areas of the city is employed for measuring urban compactness. Through Best-to-fit analysis, we found that environmental efficiency generated by urban compaction may be positive only up to168 person per hectare, after which the relationship becomes negative.

 

Of the 45 cities, only two, Shanghai and Wuhan, have a population density above the set level, while the rest are below it. This may suggest that Chinese cities still have the potential to absorb more people in existing cities.

2. Simulations of city structures: is there a climatically optimal urban organisation?

We will now see some cities simulations to discuss the model of compact cities. 

It is based on an article, in which the main question is the following: 

 Is there a climatically optimal city structure?

To answer that, they made 44 simulations of idealised cities (22 for summer and 22 for winter).

The cities are characterised by the population density and the fraction of vegetated

areas.

They chose three variables:

• Thermal comfort

• Air quality

• Energy consumption

Those are idealised cities, so they are not meant to account for the diversity of city structures. It’s more of a tool created to study general trends. 

We will not detail the mode of calculation, but here are the results :

• Compact cities, with buildings with low surface-to-volume ratios, minimise the building energy consumption for space heating and cooling, but maximise the outdoor heat stress in summer

• Compact cities have the lowest energy consumption and consequently emissions, but also the highest emission density amount of pollutant emitted per unit surface.

• For air quality, it’s a bit more complex : it seems that the optimum is for cities with intermediate population densities.

• The inclusion of vegetation is most of the time positive, and never detrimental, in this climate.

To conclude:

• it shows the complex interactions between the urban structure (population density and vegetation fraction), energy consumption, thermal comfort and air quality

• but other variables could be used, such as the building materials, the albedos, the variability of building heights, etc.

Conclusion

This document discusses  to what extent compact cities could deal with the issues raised by urban growth. It accounts for the complexity of cities' dynamics and interactions. With all the solutions aforementioned, it is very vital to implement them so as to mitigate Urban Heat Island effects in order to have a serene healthy environment to live in.

The answer might be to find the better compromise between on the one hand high density and on the other hand vegetation, air quality and temperature.

Marie Amélie

Marie Merveille

Fang Yé

Sanusi Ahmed.