We are probably experiencing the last “cool” summer of our time and the generations to come. The aim of this short article is to examine the phenomenon of Urban Heat Island. What is it (the causes and impacts)? Which are the most vulnerable regions? What is its relation with energy demand and greenhouse gases? Are there any mitigation mechanisms?
The Urban Heat Island phenomenon constitutes a phenomenon that exacerbates the risks of extreme summer heat and one may observe it in large cities. It is two hundred years old and first, it was first studied by The temperature of city centers is higher than that of the suburban area. As extreme summer temperatures are expected to occur more frequently in the future – due to climate change-, limiting the consequences of the urban heat island effect will become increasingly necessary. To put it another way, UHI constitutes a substantial difference in the temperature of urban and rural areas during both day and night time affecting the lives of citizens.
Growth in population and urbanization made necessary the expansion of cities, replacing the natural surroundings with buildings, roads, and pavements that absorb solar radiation and heat and emit it at night. Among the most common causes of this burning issue are anthropogenic heat release, surface cover, air pollutants, and unsuitable planning of cities. According to Santamouris et al. (2007); Akbari et al. (2001) and Oke (1987) the following are the causes of UHI: -Low amount of evapotranspiration because of less vegetation -Absorption of solar radiation due to low albedo -Hindrance to the flow of air because of higher rugosity -High amount of anthropogenic heat release.
In particular, the creation of the phenomenon is primarily related to the changes in the characteristics and geomorphology of an area, which arise due to the replacement of the natural environment by construction materials (eg, asphalt and cement), as previously stated. Artificial surfaces, whose characteristics are among others higher heat storage potential, the complex geometry of roads and buildings, reduced percentage of vegetation, and anthropogenic heat emissions (e.g. air conditioning) affect several natural processes such as the energy balance of the soil, ultimately leading to a warmer environment. Heatwaves enhance the Urban Heat Island effect, because of higher ambient temperature, especially during nighttime. Urban canopy, wind blockage, destruction of vegetation, and human activity are the roots of the problem.
According to US Environmental Protection Agency, the damaging impacts of the Urban Heat Island are enumerated: a) the increased energy consumption as more electricity is used for cooling in the summer months, b) the increased emissions of air pollutants and GHGs related to the above and c) the higher mortality rates from heat associated deaths in the summer. Tropical and arid regions are the most vulnerable.
Overexposure to extreme heat causes stress, cardiovascular and respiratory diseases, and even death. Extreme heat is the most popular weather-related cause of mortality. Moreover, UHIs have poor air and water quality compared to rural areas. Pollutants, such as waste products from cars and public transportation, industrial waste, and everyday materials are pumped into the atmosphere and are blocked there due to buildings, roads, and pavements. Moreover, water spaces, it’s getting warmer provoking stress on the aquatic species that usually live in a cooler natural habitat.
UHI and GHG
One should elaborate further on the interrelation between UHIs, energy demand, and greenhouse gases. As people need the energy to cool down their homes comfortably, they emit more and more greenhouse gases. According to Akbari (2001), for every 1o C temperature increase, the energy demand may go up by 2-4% in the summer months. More than twenty years later the percentages are higher. To cover the demand and in a time when electricity prices soar, the exploitation of fossil fuels leads to higher than expected emissions taking into account NDCs and the Paris Agreement. Increased usage of air conditioning leads also to higher public expenditures and bills for the citizens. Air conditioners may relieve us in the short term, but in the long run, they jeopardize the environment, contributing to global warming and worsening the UHI effect.
Taking everything into consideration, the Urban Heat Island effect is most certainly degrading the environment and people’s quality of life. However, multiple mitigation techniques have been studied, developed, and applied. Cool pavements and green spaces are among the most popular. According to Qin (2015a), cool pavements are paving materials that keep cooler the pavements than conventional pavements.”
Increasing the albedo to reduce the thermal absorption is a quite simple solution. White or as light as possible does not absorb heat or radiation but rather reflects it back to the atmosphere (reflective pavements). The creation of cool pavements is an asset in the fight against Urban Heat Island. Evaporative and water-retentive pavements are another potential mitigation mechanism. Last but not least, permeable pavements that use a different technique are another option. However, due to the author’s lack of expertise, we will not discuss the specifics of those technologies.
Another effective strategy, is the expansion of vegetation, as trees aid the reduction of the UFI effect by their evapotranspiration. The shade of the trees also manages to cool down the urban areas and contributes to a more comfortable living. Furthermore, green roofs absorb heat and greenhouse gases as well, therefore they have a double role, filtering the air and cooling the ambient atmosphere. Not to mention that they moderate the energy demand of the urban population. Urban planning is, also, of great importance because when done properly allows the city to breathe. It goes without saying that each city has its own characteristics and needs tailored solutions (or a combination of them).
P.S.: The phenomenon of the Urban Heat Island effect is not treated on a global scale so far, but rather on a regional level. Municipals around the world have taken great initiatives. A great example is the city of Athens (Greece). It is the first European capital to appoint a Chief Heat Officer, Eleni (Leino) Myrivili, emphasizing the importance of tackling this issue. Extreme heat should and shall be at the forefront when discussing climate change.