Further research is needed in the heat mitigation techniques to improve the quality of life in our cities

The predictions of population growth, published by the UN in 2014, indicate that in 2050 about 66 % of the world population will live in urban areas. The massive migration of rural population to the cities recorded in recent decades is caused mainly by expectations of a better life and greater opportunities due to economic development and industrialization.

However, the increase in urban population often carries serious problems of territorial extension and dispersion, and modifies consumption patterns of its inhabitants. Although cities occupy only 0.5 % of the earth’s surface, they consume about 75 % of global resources. In most cases, urban sprawl (often uncontrolled) and the change in land use involve serious climatic and environmental degradation, such as air pollution, noise, deterioration of water quality, etc. Consequently, a reduction of green areas, soil sealing through intensive paving and construction, and increase of outdoor air temperature due to anthropogenic activities occurs.

Urban Heat Island (UHI) Effect

The temperature rise that is registered in urban areas compared to rural areas is called Urban Heat Island (UHI) and is one of the most documented phenomena during the analysis of climate change effects. Numerous studies, among which are the ones developed by Professor Hashem Akbari and Professor Mattheos Santamouris in cities around the world show that this temperature difference can reach 10 ºC and is a phenomenon that is repeated in most of the urban areas. Moreover, in many cases the effects that are attributed to the UHI are similar or greater in magnitude than those caused by the effect of global warming.

The UHI increases the concentration of pollutants and affect air quality in cities, increases the energy consumption of air conditioning (as the peak electricity demand), increases CO2 emissions and ecological footprint and influences negatively on human health, thermal comfort and economy.

It is well documented by the World Health Organization that exposure to high temperatures cause serious respiratory and cardiovascular problems, causing an increment of hospitalization rates and even deaths. These effects are much greater in periods of heat wave (that are increasing in length and number). At this point it is important to emphasize that the upper reference temperature limit related to health problems in the Mediterranean cities is 29.4 °C while in the Nordic and continental European countries it reduces to 23.3 ºC. Therefore, it is not just a problem of the cities on hot climates but the UHI effects are also extrapolated to most of European cities.

Technologies and techniques for cooling

Counteract the impact of urban warming is one of the most serious challenges that the scientific community has to face. Therefore, it is necessary to work on the development and implementation of technologies and passive cooling techniques to mitigate these adverse effects.
Among the most effective strategies are those aimed to expand green areas in cities, the proliferation of green roofs, the use of highly reflective materials on pavements, roofs and building envelopes, and the decrease in anthropogenic heat emissions and the use of the territory as a heat sink.

Green areas, in which parks, street trees and open spaces are included, help to reduce the air temperature and mitigate the urban heat. They provide sun protection and cooling of the atmosphere through evapotranspiration as well as favoring air movement. Moreover, they help mask the noise, filter air pollution, reduce stress of visitors, increase the level of satisfaction and improve the attractiveness of cities.

Green roofs provide solar and thermal protection for buildings as well as improving air quality and reducing the emission of sensible heat to the atmosphere.

Reflective materials or “cool materials” have a very high reflectivity of solar radiation and high emissivity coefficient. Lower absorption of solar radiation and higher infrared emission maintain the surface of the materials at a lower temperature causing a reduction of sensible heat which is dissipated into the atmosphere.

Pavements greatly affect the urban microclimate. Paved surfaces of cities are usually made of asphalt or concrete (both with a high absorption coefficient). Also they extend through most of the territory so that its importance is even more relevant. High temperatures that pavements reach increase outdoor temperature, so it is important to take note of it and begin to replace actual material by the so-called “cool materials”, as is already been done successfully in many cities.

To reduce the contribution of anthropogenic heat to the atmosphere we should focus on the transportation modes and the location of power generation (individual or collective) and try to find alternatives.

Finally, taking advantage of the ground temperature being relatively constant and lower than the ambient temperature in summer and higher in winter (deeper than 2 m.), nowadays techniques called “Earth to Air Heat Exchangers” are been developed and implemented. These technologies are based on the concept of circulating ambient air through underground horizontal pipes.

Further research and increasing awareness on heat mitigation techniques are required. Specifically, the application of passive systems on our cities can continue to provide a good quality of life for its inhabitants.

To read more:
-Santamouris, M. (2007) “Advances in Passive Cooling”. Earthscan, London, UK
-Akbari, H. et al (1992) “Cooling our communities: A Guidebook on Tree Planting and Light-Colored Surfacing”. US Environmental Protection Agency, Office of Policy Analysis, Climate Change Division, Washington, DC.