Health impacts of urban heat island mitigation in Europe
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This assessment aims to quantify health impacts of heat exposure in large European cities and the effectiveness of different urban heat island mitigation policies in reducing these effects. It is a part of the Intarese Common Case Study.
Scope
Purpose
What are the current and future annual health impacts of heat exposure in large European cities? What is the potential and relative effectiveness of different urban heat island mitigation measures in reducing these impacts?
Boundaries
- Year: 2010, 2020, 2030, 2050
- Geographical area: EU-27, excluding Bulgaria, Cyprus, Latvia, Romania
- North-continental region: Austria, Belgium, Czech Republic, Denmark, Estonia, Finland, France, Germany, Hungary, Ireland, Lithuania, Luxemburg, Netherlands, Poland, Slovakia, Sweden, United Kingdom
- Mediterranean region: Greece, Italy, Malta, Portugal, Slovenia, Spain
- Health impacts:
- Natural mortality
- Cardiovascular mortality
- Respiratory mortality
Scenarios
Future climate scenarios:
- IPCC A1B
- IPCC B1
Intended users
- Intarese/Heimtsa Common Case Study
- Anyone interested
Participants
- The National Institute for Health and Welfare (THL), Finland
- University of Stuttgart
Definition
Decision variables
Evaluated policies for mitigating urban heat island effect
1) Reference (Business-as-usual (BAU))
- No new UHI mitigation measures
2) Vegetation
- Urban forestry is increased by planting trees on open grassy areas and street curbsides
- Trees are assumed to be deciduous and mature
- 10.8% of the total urban area is redeveloped from open grass to trees (open area planting)
- 6.7% of the total urban area is redeveloped from street to trees (curbside planting)
3) Albedo
- Albedo of the city is increased by converting impervious roof and street surfaces (sidewalks and roadways) to light colored surfaces
- For light roof surface, albedo is assumed to increase form 0.15 to 0.5
- For light street level surface, albedo is assumed to increase from 0.15 to 0.2
- 13.6% of the total urban area is redeveloped from impervious roof surface to light roof surface
- 34.4% of the total urban area is redeveloped from impervious street surface to light street surface
4) Veg+alb
- Both urban forestry and albedo is increased
- 10.8% of the total urban area is redeveloped from open grass to trees (open area planting)
- 6.7% of the total urban area is redeveloped from street to trees (curbside planting)
- 13.6% of the total urban area is redeveloped from impervious roof surface to light roof surface
- 34.4% of the total urban area is redeveloped from impervious street surface to light street surface
- Trees are assumed to be deciduous and mature
- For light roof surface, albedo is assumed to increase form 0.15 to 0.5
- For light street level surface, albedo is assumed to increase from 0.15 to 0.2
The fraction of the total urban area in large European cities available to be redeveloped from one land use or surface cover type to another is based on an urban heat island mitigation study conducted in New York City [1]
Indicators
Other variables
- Ambient air temperature in Europe
- Ambient air dew point temperature in Europe
- Effect of urban land use change on ambient air temperature
- Heat exposure in Europe
- Population of Europe
- Mortality in Europe
- ERF of ambient temperature on mortality
- Following WP3.7-work and WP4.3-screening, DRF will be based on the PHEWE project, which investigated the acute health effects of weather in 15 European cities and provided both pooled estimates (Mediterranean region and North-Continental region) of the impact of heat on mortality (Michelozzi et al. 2007; Baccini et al. 2008). DRFs are based on a linear threshold model. Heat exposure is defined as follows: °C Daily maximum Apparent Temperature over threshold temperature, warm season (April-September).
- The PHEWE ERFs describe the association between background ambient temperature level and mortality risk in a city. However, because of the urban heat island effect, heat exposure in the city is likely to be higher than the background temperature indicates. Therefore, the impact of the urban heat island effect is embedded in and inseparable from these ERF estimates, and the health impacts of heat exposure and the changes in these impacts due to different UHI mitigation policies have to be evaluated against ambient background temperature data.
- Threshold and the DRF differ for the Mediterranean region and the North-continental region; therefore, the countries included in the assessment need to be divided into two larger sub-regions: Mediterranean and North-continental.
Analyses
Qualitative and quantitative uncertainty analyses
Indices
Result
Results
Annual deaths (mean and 95% confidence limits) attributable to heat exposure in large cities.
| Year | UHI mitigation scenario | Climate scenario A1B | Climate scenario B1 |
| 2010 | Current | 14639 (8142-22847) | 14639 (8142-22847) |
| 2020 | Reference | 25172 (13682-40539) | 25761 (13721-41806) |
| Vegetation | 24123 (13112-38929) | 24568 (13063-39917) | |
| Albedo | 22376 (12156-36211) | 22665 (11905-37098) | |
| Veg+alb | 21402 (11627-34718) | 21605 (11345-35529) | |
| 2030 | Reference | 41253 (22284-66631) | 38446 (20884-61501) |
| Vegetation | 39459 (21266-63937) | 36808 (19969-58953) | |
| Albedo | 36574 (19543-59504) | 34303 (18569-55122) | |
| Veg+alb | 34732 (18391-56714) | 32878 (17759-52977) | |
| 2050 | Reference | 78357 (42046-123275) | 46341 (24366-77494) |
| Vegetation | 75248 (40184-118849) | 44436 (23406-74422) | |
| Albedo | 70462 (37534-111824) | 41426 (21876-69871) | |
| Veg+alb | 67882 (36154-108226) | 39692 (20897-67081) |
Uncertainties
- The temperature estimates modelled for the future are uncertain due to both assumptions and simplifications in the model as well as assumptions made in the IPCC scenarios on world development.
- The temperature data data may lead to underestimation of heat exposure in coastline cities. This is especially the case for those cities, or parts of the cities, which are located in grid cells where greater part of the cell area consists of sea.
- In the UHI mitigation policy analysis, uncertainties relate to both estimates on the amount of surface area that could be redeveloped from one surface cover type to another, as well as the actual cooling effects of these changes. It is unknown how well the estimates from the mitigation study conducted in New York City translate to the average European urban setting.
- Heat exposure assessment is complicated because urban areas contain numerous microclimates. The actual heat exposure of the city inhabitants, therefore, depends on the microclimatic conditions in which people actually spend their time. It is unclear how changes in the urban surface energy budget and average urban heat island intensity relate to changes in actual heat exposure.
- It is assumed in the assessment, that the absolute air humidity stays the same regardless of the implementation of UHI mitigation measures. This may very well not be true. For example, increasing urban vegetation would likely increase air humidity through higher evapotranspiration. This would subsequently be reflected in the level of discomfort related to a given level of air temperature.
- The interpretation of the results is complicated because the health impact of heat island intensity is actually embedded in the exposure-response function. Thus, in reality the cooling effect of the mitigation measures would be reflected in the association between background ambient temperature and mortality risk in a city, not in the used exposure indicator. In addition, the urban infrastructure is also likely to go through other changes in the future, which is ignored in the assessment. For example, the use of air conditioning could increase in European cities because of the increasing summer temperatures, which would lead to less severe heat exposure even if ambient temperatures rise. These types of factors would also be reflected in the exposure-response function.
Conclusions
Currently, almost 15000 deaths are attributable to summertime heat exposure annually in the densely populated European metropolitan areas within the countries included in the assessment. Over 70% of these deaths occur in those aged 75 or older. The number of heat related deaths is likely to increase drastically during the next 40 years as climate becomes warmer and the population ages.
Thousands of heat exposure related deaths could be averted annually by implementing the evaluated urban heat island mitigation policies. The benefits from the policies increase substantially in the future and are higher in the IPCC climate scenario A1B, which represents stronger climate change and warming and seems, at the moment, to be the more realistic one of the evaluated climate scenarios.
When conducting an assessment on the scale of the whole Europe, it is necessary to make many simplifying assumptions. It is also evident, that there is a lack of simple modelling tools for this type and scale of assessment. The majority of the studies that are conducted on urban heat island mitigation are more local in their nature and make use of complex regional climate models. This kind of approach is not feasible in the time and budget frame of this particular assessment. Given these constraints, the validation of the used methodology is also difficult, as it would require comparisons with more sophisticated modelling approaches. Nevertheless, despite the various sources of uncertainty, the simple methodology used in this assessment can provide valuable insight into the magnitude of health impacts caused by heat exposure in Europe, as well as the potential and relative effectiveness of different policy options in mitigating urban heat island effect and heat exposure in cities.