Kyra Marty

Of the 7.2 billion people on earth, more than 50% are living in urban areas.¹ Where would be a better place to witness the impacts human actions have on the environment? In fact “urban climate-change” exists, and we can take it as a threat or as a chance.

What are Urban Heat Islands?

Urban heat islands (UHI) are one of the easiest ways to see how human impact can change our climate, and at the same time, cities might be, where we experience the first fatal outcomes of global warming. Even today in Switzerland, city temperatures are on average 1-3 degrees higher than in rural areas, a discrepancy that is expected to increase. While that does not sound like much, on a clear summer night this difference can amount to more than 10° C, resulting in significantly less heat days per year in rural areas.² And when adressing human health we should consider peaks not averages: Because our metabolism produces heat, we depend on a environment that can take up excess heat, to keep us at core temperature of 37° C.⁷ However, temperature is not everything when it comes to human heat management, because if  humidity exceeds critical values, sweating becomes ineffective.⁷ So if our environmental temperature is too close to 37° or the air is too humid, the environment can not take up heat fast enough and our body temperature will get too high. Heat illness is easily underestimated, because exeedance of the optimal body temperature results in dysfunction of multiple organs, making it prone to misdiagnoses.⁷ The symptoms of exposure to heat reach from an amplification of excisting conditions to hospitalisation and death due to stroke or respiratory ilness.⁴ The most vulnerable are the elderly and/or people with preexisting conditions. Using two different datasets from 2000, Mora et al.⁷ found that between 13 to 16% of our planet’s land area, where 30 to 37% (values depending on the data) of the worlds population lives, were exposed to 20 or more days when temperature and humidity contidions became deadly. The heat wave in Europe in 2003 is a great example of how dangerous this can get. It lead to more than 70000 additional deaths³ and big european cities were especially affected⁴. However, UHI’s can occure in any urbanised area and are not limited to metropoles like london or paris. According to climate modelling for Zurich, the annual number of heat days (days when temperature exceeds 30° C) in city centers are expected to rise to an average of more than 35 within the next twenty years.²  UHI does directly impact human health and this is a risk that will increase in combination with rising temperatures, raising serious concerns given the global climate change.^4,7

Why do UHI exist?

The factors leading to UHI are well known, however their relations and separate impacts probably differ for each city. One of the most prominent reasons for UHI is the use of low albedo materials in house and street construction: Objects that are low in albedo gather heat when they are irradiated by sunlight. Often, those materials do have a high heat capacity, meaning that they release long-wave radiation slowly, and therefore emit the captured heat until far into the next morning. This leads to so called “tropical nights”, meaning nights in which the temperature does not drop below 20° C. But not only do our pavements capture heat, they also make water drain faster, preventing evaporation, with its natural cooling effect. What rural areas have and most cities lack are green areas. Natural soils allow water retention and vegetation is high in albedo and have a low heat capacity. However, nearly 30-45% of urban areas is covered by pavements that are generally made of materials that reach maximum temperatures of 50-70°C on summerdays⁶. Cities produce massive amounts of CO₂, a gas that is very well known for its heating effect, as well as other air pollutants that can trap solar radiation. If there is not enough airflow passing through the streets, these emissions will stay concentrated in the surrounding area of the city. CO₂ is emitted in our daily traffic. High density of individual motorised traffic still is the daily routine of most city. Another important emitter of CO₂, especially in this context, are cooling systems. But also the layout of a city can influence it’s heat management. For example, dense standing, tall buildings can even intensify the heat themselves, by reflecting it from one wall to the other and poorly placed buildings can prohibit windformation, trapping the hot air inside the city. ^2,5,6

What can be done?

To mitigate the risk of dangerous heat waves, measurements in city planning and reorganization are key, and if executed well, they can contribute to a better city atmosphere in more than one way. In planted areas temperature is significantly lower: Green parks offer water retention capacity and therefore enable latent heat flux through evaporation. While open green spaces provide maximum heat relief at night, they tend to heat up rather quickly during the day. On the other hand, areas with trees offer shade during the day, but tend to capture more thermal energy at nighttime². Trees are essential when it comes to a sustainable city management, as they are a natural air cleaning facility: they absorb aerosol particles and reduce CO₂ concentration by photosynthesis. But greenery does not only have a positive effect on thermal comfort and pollution. Urban vegetation does have a positive psychological effect as well: People benefit from parks as a recreation locality and during heat stress periods, plants can give citizens the perception of well-being6. In order to reduce heat stress on cities, vegetation can be implementet by creating new parks or adding trees along transport corridors. But it can also be placed on facades or roofs, creating an entirely new perspective of what a city can look like. Urban farming, for example, would not only function as a green space. It could also reduce the amount of traffic needed for food supply. In general, there is a lot of room for improvement in city-transport. Reducing the number of vehicles in a city would be the most straight foreward approach to reducing reat production due to transport. This can be achieved by better public transport infrastructure and the promotion of walking or cycling. But also the replacement of internal combustion engines (ICE) with electric vehicles would lead to a significant decrease of heat prodution in transport, as the operation temperature of a ICE is 90-100°C and batteries of electric vehicles is 30-40° C.⁷ Water contributes to a better city climate by enhancing latent heat flow. Water bodies, like lakes or rivers, offer retreat for citizens on a hot summer day. Spray mist and fan combinatin can instantly decrease the skin temperature, providing heat relief in areas where shadowing is not an option. When considered individually, vegetation is of greater benefit to heat mitigation than water. Together however, they can provide ecosystem services, that exceed the benefit of heat mitigation alone. At last, it is also highly important, how those cooling elements are placed. It is important, that tree shading is used effectively and that buildings are placed in a way that they can provide shadow to one another, but do not stand to close, so they wont form street canyons. However the most important factor that needs to be considered in placement is wind formation: Due to pressure differences, there is air exchange from places with lower temperatures towards places with higher temperatures². Therefore, when cooling elements are thoughtfully distributed, it is possible to create wind, which can then distribute the heat more evenly. Building arrangement and even form can intensify this effect. Building forms with low site coverage or gaps in their structure are especially well suited. But also variation in between building height and wider streets are elements that allow to establish wind. Most of those measurements have various positive outcomes, often including attractiveness of the city. A well planned city can therefore provide the ideal combination of citizens recreation and heat mitigation.^2,6

But why bother curing symptoms?

Measurements like the mentioned above are not themselves going to prevent global warming. And as the main reason why UHI are so threatening to us, is climate change, one could argue we should rather target the underlying problem. However, UHI’s differ from most other climate related problems in three ways: On the global scale, cities are relatively small and tangible therefore consequences are much easier to controll. Solutions do not need to be international, making the decision for measurements easier and consequences can be felt by many people, directly where they live. The most important difference might be, that unlike the needed changes in travel behavior or food supply chains, the mitigation of UHI does not come with the feeling of restriction for citizens. In fact, they will even benefit from a better urban atmosphere. In other words, heat mitigation in cities is not only technically but also politically comparatively unproblematic. But that does not mean that there is not a lot to be won: Successfully creating cool cities could promote necessity and feasibility of actions against global warming worldwide, leading not only to more sustainable cities but hopefully also to more sensitive minds.

References

1. United Nations (2018), Department of Economic and Social Affairs
   https://www.un.org/development/desa/en/news/population/2018-revision-of-world-urbanization-prospects.html, Last access: 04.05.2020
2. AWEL (2018), Klimakarten. https://awel.zh.ch/internet/baudirektion/awel/de/luft_klima_elektrosmog/klima/klimakarten.html, Last access: 04.05.2020
3. Robine, J.-M. et al. (2008), Death toll exceeded 70,000 in Europe during the summer of 2003. Comptes Rendus Biol.331, 171–178.
4. Heaviside, C., Macintyre, H., & Vardoulakis, S. (2017), The Urban Heat Island: Implications for Health in a Changing Environment. Current Environmental Health Reports, 4(3), 296–305.doi:10.1007/s40572-017-0150-3
5. Md. Nuruzzaman (2015), Urban Heat Island: Causes, Effects and Mitigation Measures – A Review. International Journal of Environmental Monitoring and Analysis. Vol. 3, No. 2, pp. 67-73. doi: 10.11648/j.ijema.20150302.15
6. Ruefenacht, L., Acero, J. A. (2017), Strategies for cooling Singapore. https://static1.squarespace.com/static/586dfed8b3db2bba412a8919/t/5d33a948b5c4a100011cb82d/1563666897377/CS_Catalogue_of_Strategies_online.pdf, last access: 04.05.2020
7. Mora, C., Dousset, B., Caldwell, I. et al. (2017). Global risk of deadly heat. Nature Clim Change7, 501–506. https://doi.org/10.1038/nclimate3322

Picture: Meteo Zurich. (2015). https://www.meteozurich.ch/?tag=urban-heat-island