By Matt Burdett, 5 November 2019
On this page, we look at urban microclimate modification and management, including the urban heat island effect.
- Green roofs are one way to reduce the urban heat island intensity, but retrofitting them to existing buildings must be done very carefully. This green roof at City University collapsed due to the extra weight of soil and plants. Source: EJI Insight, 2016.
Impacts of urban microclimates
An urban microclimate has many impacts on the people who live in the urban area.
In colder climates, the increased temperature of an urban heat island reduces weather-related hazards such as ice on the roads. In hotter periods of the year, the increased temperature can lead to exacerbated heat waves.
Modification and management of urban microclimates
City authorities can manage the impact of urban microclimates in two main ways. First, they can modify the urban area to reduce the intensity of the microclimate. Secondly, they can manage the impacts of the microclimate intensity by changing the way people respond to the microclimate.
Modification of the urban environment
Since urban microclimates are always a human-caused change to the natural climate of an area, it follows that humans can take action to reverse the impact of these changes. The following ways have been shown to reduce the urban heat island intensity (UHII) (partly sourced from EPA, 2019):
- Ensure buildings are designed to reduce their impact on urban heat island intensity, such as by being well insulated to keep in cool air conditioned air, or keep in warm centrally heated air.
- Plant vegetation wherever possible throughout the urban area, including in traffic islands and other ‘wasted’ urban space.
- Add trees to provide shade over road spaces, which if unshaded result in fast evaporation of moisture.
- Build green roofs on the tops of buildings. These are rooftops that are covered with gardens. This must be done carefully – the green roof in the photograph at the top of this page would have killed hundreds of people had the building been occupied when it collapsed.
- Painting buildings white can reduce the absorption of solar energy, which reduces the resulting increase in urban air temperatures.
Management of an urban heat island: Paris in 2003, 2006 and 2019
In summer 2003, an anticyclone (area of high pressure which brings low wind speed and clear skies) over Europe caused a heatwave which was blamed for the deaths of over 30,000 people. Most of these were in cities including Paris where 4867 deaths were caused (Doucet et al., 2010).
The city of Paris sits in a lowland area surrounded by hills which reduced the free movement of air. Daytime temperatures exceeded 38˚C. The highest number of deaths was among elderly women who lived alone. Part of the contributing factors was that this was the high summer in Paris and many families were on holiday and left elderly relatives behind. Bodies were left in the mortuary into September without being claimed – 57 remained on 3 September 2019.
The response was to create a heatwave plan for future events. The Plan Canicule aimed to better coordinate the various agencies involved in dealing with problems including transport, health care, humanitarian organisations, local government and national government. The plan was tested – and succeeded – in 2006 when a similar heatwave claimed after fewer lives across France (Fouillet et al., 2008) and only 46 lives in Paris.
In 2019, a new protocol was put in place to deal with future heat waves (O’Sullivan, 2019):
- People are encouraged to place themselves on a list of vulnerable people by registering with a free telephone call
- Temporary air conditioners are provided to at-risk locations such as kindergartens
- Public swimming pools stay open into the late evening
- ‘Misting’ machines which add water to the air result in heat energy in the air being ‘used up’ by evaporating the water moisture into water vapour, which reduces the air temperature perceived by people
- More drinking water fountains were placed around the city
The plan was tested immediately when Paris experienced its highest ever recorded temperature of 42.6°C in August 2019 (Leister, 2019). Across the country, France experienced an increase in the death rate with around 1500 extra deaths. This was significantly lower than the 15000 deaths in 2003, due to the preventative measures that the authorities introduced (Agence France-Presse, 2019).
Doucet et al., 2010. Climate change and heat waves in Paris metropolitan area. https://www.nasa.gov/pdf/505253main_dousset.pdf Accessed 5 November 2019.
EJI Insight, 2016. Fears grow over green roofs after City University collapse. http://www.ejinsight.com/20160523-fears-grow-over-green-roofs-after-city-university-collapse/ Accessed 5 November 2019.
EPA [United States Environmental Protection Agency], 2019. Reduce Urban Heat Island Effect. https://www.epa.gov/green-infrastructure/reduce-urban-heat-island-effect Accessed 5 November 2019.
Fouillet et al., 2008. Has the impact of heat waves on mortality changed in France since the European heat wave of summer 2003? A study of the 2006 heat wave. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2652641/ Accessed 5 November 2019.
Leister, 2019. Paris breaks all-time high temperature as deadly heat wave shatters records across Europe. https://www.accuweather.com/en/weather-news/scorching-heat-produces-all-time-record-highs-in-belgium-netherlands-as-western-europe-swelters-under-heat-wave/70008886 Accessed 5 November 2019.
O’Sullivan, 2019. As Record Heat Roasts Europe, Paris Prepares for the Worst. https://www.citylab.com/environment/2019/06/europe-heatwave-paris-climate-resilience-weather-hot/592729/ Accessed 5 November 2019.
Urban microclimates: impacts and responses: Learning activities
- Suggest how some cities benefit from urban heat islands and/or urban microclimates. 
- Distinguish between modification and management in relation to the response to urban heat islands. 
- Identify four ways to modify urban infrastructure to reduce the UHII. 
- Describe the response of Paris in 2019 to the heatwave. 
- Explain the difference in the number of deaths in the European heatwaves between 2003 and 2019. 
Investigate your local environment. What microclimates exist, and what could be done about them? Is it possible to modify the local urban infrastructure? Is a plan needed to manage the impacts of the microclimate? Is it desirable to minimise the impact of humans on the immediate climate, or does it have a positive impact?
This video outlines several impacts of heatwaves. Search for similar videos that describe the impacts of urban microclimates in other locations.
© Matthew Burdett, 2019. All rights reserved.
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