In a context of global warming and increasing urban populations, the general and localised impacts of Urban Heat Islands are a pertinent concern. Working from the international classification system for "Local Climate Zones", the IAU îdF has established a reference basis for identifying vulnerability of urban areas to temperature increases.
The climatological effect most widely-recognised as having an impact on urban zones is the formation of Urban Heat Islands (UHI), as underlined by researchers whose work is focused on urban climatology. Cities, as geographical areas forged and inhabited by humans, produce a variety of radiative, thermal, hydrological and aerological disruptions to the surrounding climate, due to their particular mineral composition, materials, "roughness length" (see below for a glossary of terms), scarcity of water and vegetation, and the presence of heat-emitting and polluting anthropological activity. Air temperatures therefore tend to be higher in central urban zones that in peripheral rural areas.
Today, cities are viewing the UHI phenomenon in tandem with ever-increasing urban population density in the overall context of global warming, as reaffirming by the GIEC 2013/2014 Climate Report. As city authorities are being asked to take swift action in order to limit the production of greenhouse gases and in favour of sustainable development, they must also remain active on other fronts by pursuing initiatives designed to identify and diminish their own specific vulnerability.
It has become imperative to anticipate the most likely effects of this particular global disturbance; it is also crucial to explain how, in the Île-de-France region, elements such as urban formation, the presence of water and vegetation in a city, and activity and population density in metropolitan areas can affect climate change by aggravating the effects of heat waves. Improved appreciation of the specific territoriality of cities, districts and city blocks, all of which are factors in an area's vulnerability to UHIs, will help authorities to identify the most pertinent courses of action and adaptation.
The UHI effect: a very Parisian problem
While the heat waves experienced in 2003 and 2006 had a serious impact on the population of the Île-de-France region, the UHI phenomenon could affect an even larger portion of the population, and in the future may make life in the inner city very uncomfortable indeed. The sanitation watchdog (InVS), the National Institute for Health and Medical Research (Inserm), the Regional Health Observatory for Île-de-France (ORS) and the Parisian Public Health Workshop have been studying the connection between the mortality rate observed during the 2003 heat wave and the deaths that occurred as a result; the cases in question show that the Île-de-France region had the highest overall increase in mortality (127%-171%, depending on the area) and underlines the significant impact that UHIs can have upon air quality.
Heat Wave action plans implemented by the central government since 2004 have proven to be effective, but the capacity to maintain the robustness of these alert and crisis management systems, at a time when the frequency of such episodes in increasing, raises questions as to the performance of the early warning systems and role of local representatives, such as with the measures introduced at the level of mayoral responsibility concerning the identification of vulnerable populations (as outlined in Law N°2004-626, June 30th 2004).
The City of Paris, for example, updates the Chalex (extreme heat) File every year, in order to provide assistance to elderly and disabled residents who have requested it (24,000 subscribers as of the end of summer 2011); as InVS points out, apart from the likely increase in frequency of heat waves, the ageing population is another important factor to consider in terms of crisis management (by 2050, 15.6% of the region's population will be aged 75 or over, compared to 8.8% in 2003).
Another form of prevention is fighting the effects of UHIs. This is one of the major goals for the region, as outlined in the Île-de-France Regional Climate Plan (PRC) and backed by the Regional Air, Energy and Climate Scheme (SRCAE) and the Regional Île-de-France Management Scheme (Sdrif) in 2013. In effect, in light of the level of vulnerability already observed and the results of climatological, sociodemographic and urban forecasting, these planning documents have been drawn up precisely to respond to this urgent issue.
The vulnerability of the region’s towns and cities to the effects of summer heat waves constitutes a major public health issue, and involves questions for the wider metropolitan ecosystem in terms of water availability and resources, energy sourcing for air conditioning units, air quality reduction (smog), thermal comfort, alteration of natural elements in cities and the risk of degradation to buildings and infrastructures. As such, the challenge of reducing vulnerability to heat waves at a time of increasing population density has become a serious and substantial one.
Study of urban morphology and its effects on climate
Since the first observation of specifically urban climate zones by Luke Howard, a British pharmacist whose work The Climate of London was published between 1818 and 1820, understanding of the phenomenon has increased greatly(1). Howard reported a difference in nocturnal temperature of 3.7°C between the centre of London and the surrounding countryside, as a result of what we now term an Urban Heat Island. The causes of the UHI effect are today well-documented by the global scientific community, all representing a difference in radiative and energetic base levels in urban surfaces (cf. text box "Causes of the UHI effect by Timothy R. Oke (1982)").
In the Île-de-France region, the constitution of a UHI began to be properly studied during the 1990s by climatological researchers interested in the effects of urbanisation on regional climates (Gisèle Escourrou).
The creation of warm breezes and light winds and their effect on atmospheric pollution, as well as the risk of storms during the warmer months and the connection with the instability of masses of hot and humid air, which pass over the city and become "rougher", were underlined as key factors(2).
As a result, the shape of the UHI fluctuates according to the strength and reaction of dominant winds and warm breezes, as well as the time of day. Its effects are more intense during the summer, and at night. In effect, during the nocturnal period, air cools less quickly because of the heat released by materials in the form of infrared rays, which are themselves trapped by the dense, compact surfaces of vertical buildings. Night time is a crucial period for organisms to recover from the effects of heat. A territorial indicator showing tropical nocturnal temperatures (above 20°C) may help scientists to visualise the contours of the UHI.
Beyond the metropolitan UHI, whose behaviour, intensity and delimitation merit further observation and study, notably via
Classification of Local Climate Zones by Iain D. Stewart and Timothy R. Oke according to age
The system of LCZ classification for rural and urban areas according to their thermal differentiation was established by Iain D. Stewart and Timothy R. Oke (Department of Geography, University of British Columbia, Vancouver, Canada). LCZs are defined as uniform spatial entities by distribution of combined air and surface temperatures between 100 and 1000m on a horizontal plane. Each LCZ expresses a characteristic geometry and type of land use that generates a specific climate - a unique surface temperature - during times of calm weather and clear skies. Zones are differentiated according to surface properties, which have a direct influence on references temperatures (1.5 - 2 m above ground level), such as the proportion of vegetation, height and distribution of buildings and trees (roughness length/compactness), ground humidity and anthropogenic heat fluctuations. According to these differentiations, the urban-rural continuum gives a hierarchy of 17 climate zones: 10 "Building" LCZs; 7 "Land cover" LCZs.
modelling and forecasting programmes such as Épicéa(3) and Muscade(4), thermal contrasts also exist in the wider urban area, forming hot blocks and cold blocks(5).
It is therefore essential to be aware of the morphological distinctions of individual cities, districts and urban blocks. Improving this knowledge base has become a major strategic objective, as evidenced by the regional energy-climate plans that have recently been adopted.
As part of its application of the PRC, the IAU îdF has endeavoured to determine more precisely which areas of the Île-de-France region are subject to the effects of UHIs, in order to identify their level of vulnerability to the dangers posed by heat waves. The discussion of vulnerability is an attempt to draw connections between the climatological risks exacerbated by the effects of UHIs and the level of exposure of the areas affected, as well as their ability to manage the dangers. Here we encounter a methodological hurdle: identifying and measuring the vulnerability of particular areas to the effects of UHIs (so as to increase the effectiveness of localised adaptation processes) requires the mobilisation of large amounts of urban geographical data drawn from a variety of disparate resources.
Causes of the UHI effect by Timothy R. Oke (1982)
• Greater absorption of solar rays due to multiple reflections as well as trapping in vertical building surfaces and walls of apartment blocks in cities (multiplication of surfaces, lower albedo of urban materials (cf. "Glossary"). • Greater retention of infrared rays in "canyon" roads (due to high, compact surrounding buildings) at night, because of reduced albedo "sky view factor" (cf. "Glossary", ). • Increased trapping and slower release of heat by buildings and artificially covered surfaces in cities (thermal properties of various materials, trapping of solar and infrared rays, reduced loss of heat by convection at canopy level where air flow is slowed). • Greater proportion of solar rays absorbed by surfaces are converted into sensible heat (changing the temperature of a surface) rather than latent heat (where a material's physical state changes rather than its temperature). This effect is due to the replacement of humid ground surfaces and vegetation with artificially-covered, impermeable surfaces leading to lower levels of surface evaporation. • Greater emission of sensible and latent heat due to the combustion of petrol by urban transport systems, industrial procedures and the heating/cooling of dwellings and residences.
Zones affected by UHIs using the LCZ approach
Without meteorological or climatological observation data, the chosen approach has been to use the new Local Climate Zones (LCZ) classification from Iain D. Stewart and Timothy R. Oke to classify urban units according to typomorphology, thermal and radiative behaviour, and potential for air cooling, in order to evaluate their individual climatological influence. This reference system will provide a research foundation for IHU studies and will standardise the exchange of information regarding urban temperature patterns at a global level(6).
One initial methodological challenge was to agree upon a clearly understandable territorial reference entity: the îlot morphologique urbain (IMU), an urban unit roughly equivalent to a city block. This approach allows researchers to incorporate the descriptive reality of the terrain in question, as opposed to methods that assume a homogenous urban fabric (currently used by many data modelling methods). Defining the contours and primary characteristics of each UHI using urban reference databases has enabled the foundation of a new layer of digital information within the IAU îdF's Geographical Information System (SIG)(7).
Among the forty or so typomorphological indicators calculated for each UHI, a number are retained to help identify the corresponding type of LCZ from the 17 climate zones described in the frame of reference: the predominant use of land, compactness and average height of buildings and effect on urban roughness length, surface permeability rates, water presence, vegetation, etc.
Working from this initial step in the classification of urban units within LCZs, a "theoretical" calculation is undertaken of the 10 determining properties in the creation and intensity of a UHI. The seven properties relating to geometrical characteristics and the nature of surfaces, such as the Sky View Factor (see "Glossary"), are directly or indirectly specified using the attributes of a particular urban unit (IMU). For radiative and thermal properties (such as average albedo and surface capacity to trap and release heat), the fixed values established for each type of LCZ by Steward and Oke are used.
Finally, the anthropogenic heat fluctuation is measured using a composite indicator that takes into account the presence of industrial activity and linear CO2 emissions from road traffic (Source : Airparif, 2010) and electricity consumption of tertiary buildings in terms of air conditioning, cooking and electricity (Visiau Énergie CENTER, 2005).
The makeup, typology and properties of an LCZ can therefore be used to specify the potential interaction of certain urban formations and land use with their climate, summarising the presence of determining factors – whether favourable or unfavourable - in the formation and reinforcement of UHI effects at the level of each urban unit.
From LCZ classification to evaluation of potential health risks
It is only by taking into account the ensemble of diagnostic criteria - UHI effect zone, sensitivity, ability to deal with effects - that efficient development actions may be implemented. The étude des impacts socio-économiques de l’adaptation au changement climatique (study of the socio-economic impacts of climate change, carried out by Artelia & RCT for the Île-de-France region and Ademe, October 2012) and the Indicateurs de vulnérabilité d’un territoire au changement climatique. Recueil de littérature internationale (Territorial Indicators of Vulnerability to Climate Change, Ademe, February 2013) provide necessary information regarding the most pertinent indicators for vulnerability and resilience to the effects of heat waves to be considered.
Taking into account the number of challenges associated with the risk of UHIs (thermal comfort and health of populations, availability of water resources, energy provision, weakening of buildings and infrastructures, etc.), several types of vulnerability scale will be examined for use. We have chosen to begin with Health and Sanitary Vulnerability.
Once again, the methodological challenge lies in managing to mobilise the available data at IMU level in order to proceed with appropriate geographical cross-references. In terms of vulnerability indicators, we have considered the presence of young children (0-5 years), elderly residents (65 or older), and the human density of the area (resident population + employment). Regarding indicators demonstrating an area's ability to cope with UHI effects, the criteria considered were access to hospital facilities, access to nearby green spaces and the financial resources of inhabitants (proportion of households with average or above income levels).
With the vulnerability characterisation phase complete, the final outcome of the study is the production of an interactive cartographic tool providing geographical data for each IMU: characteristics, types and levels of vulnerability to the effects of UHIs. As a result, an initial series of suitable adaptation solutions may now be proposed, including physical measures and improvements to urban organisation:
Increased use of surfaces covered with vegetation, surfaces providing shade and water surfaces.
Use of building materials with specially-adapted thermal and optical properties.
Reduction of anthropogenic heat sources.
Morphology and configuration of various development zones, blue and green tramways.
Health services, district cooling, water management, etc.
Improved understanding of the current vulnerability of zones subject to the effects in UHIs (at each individual local level in the Île-de-France region) as well as of the principles of adaptation, may aid in regional and local decision-making (hierarchies), in particular with regard to diagnostics and the formulation of activities within the scope of the Territorial Climate/Energy Plan (PCET).
GLOSSARY • Albedo: this physical value represents the proportion of energy reflected by a surface compared to the amount of solar energy it has absorbed. The more solar rays that are reflected back towards space (closer to albedo level 1), the less the planet is warmed. • Sky View Factor, SVF: sky view factor indicates the portion of the sky that can be observed from the surface in question. It ranges from 0 to 1 (in the case of a flat surface with no surrounding obstacles). • Urban roughness length: a parameter that is implicitly linked to the effects of built-up areas on air flow: the greater the roughness length, the more the air drags the ground surface, slowing down the air mass.
1. Cordeau Erwan, Valette Emmanuelle, Urban Heat Blocks L’adaptation de la ville aux chaleurs urbaines, Book 1, Répertoire de fiches connaissance, Book 2, IAU îdF, November 2010.
2. Cordeau Erwan, Escourrou Gisèle, "Climat et microclimat urbain, pollutions atmosphériques et nuisances météorologiques localisées ", Note rapide no. 6, Iaurif, 1995.
3. Projet Épicéa: multidisciplinary study of the impacts of climate change on the Paris urban area, carried out in partnership with Météo France (the French national meteorological service), the CSTB (Scientific and technical centre for buildings) and the office of the Mayor of Paris.
4. Muscade: Urban data modelling and adaption strategies for climate change, in order to anticipate energy demand and production requirements. Project initiated by the ANR (National Research Agency), 2009-2013, coordinated by the Meteorological Atmospheric Study Group (Game), also involving the International Research Centre for Development and the Environment (Cired), the CSTB, the Coastal, Environmental the Business union (LIENSs), the Architectural Research laboratory (LRA), the IAU îdF, and the Parisian Urbanism Workshop (Apur).
5. Cordeau Erwan, "Caractériser les zones sujettes aux îlots de chaleur urbains", published in Les Cahiers, no. 170-171, IAU îdF, September 2014, pp. 80-81. Note rapide no. 661 La vulnérabilité de la ville à la chaleur par l’approche Zones climatiques locales (Vulnerability of towns and cities to rising temperatures, assessed using the Local Climate Zones approach), September 2014
6. Stewart Iain D., Oke Thimoty R., "Local climate zones for urban temperatures studies", Bulletin of the American Meteorology Society, December 2012.
7. Cordeau Erwan, "Caractériser l’environnement urbain à l’échelle de l’îlot", (Identifying Environments at City Unit level), Les Cahiers, no. 168, IAU îdF, 2013, pp. 61-63.