The growing problem of urban heat
The intensive rate of urbanization in the 20th and 21st century means that addressing climate-related elements such as urban heat, that endanger urban population health and well-being, needs to be a top political priority, and a central component of mitigation and adaptation plans. Currently, we are witnessing a trend towards urbanization, with a significant increase in urban populations (Seto et al. 2012). Urban areas are composed of numerous climatically active artificial surfaces and, on the other hand, limited green and blue areas. These surfaces consist of a range of fabrics that have different climatic properties, including radiative, thermal, moisture, or aerodynamic. Hence, regardless of the type of building and road materials, urban surfaces have a positive correlation in terms of absorbing solar energy and increasing the surface temperature. In addition, various in-situ anthropogenic factors also contribute to urban heat originating from the energy use in buildings, the combustion of fossil fuels for transportation purposes, and the energy consumption needed for industries located in urban areas (Depietri et al. 2012).
Contrary to this, areas covered by plants or with surrounding water may effectively reduce the heat effect in cities. Evapotranspiration from soil-vegetation systems is a proven moderator of near-surface climates in warm and dry local climate conditions. As such, it may create cooler “oases” in urban heat surroundings (Oke et al. 2017).
Apart from influencing urban areas, the high temperatures also provide suitable conditions for forest fires. In North America, for instance, data from the National Interagency Fires Centre (NIFC) shows that between January and July 2021, 36.467 fire spots have been detected, which have claimed a total area of 2.770.454 acres (National Interagency Fire Center 2021). In Turkey, there have been reported more than 130 wildfires across 30 Turkish provinces, affecting more than 136,000 hectares (525 square miles) (NASA Earth Observatory 2021). According to Trenberth (2007), there is a direct local contribution to drying and high temperatures in the absence of evaporative cooling. This was observed in Russia in 2010, when the record high temperatures led to wildfires that affecting human lives (Lau and Kim 2012).
A recent study (Leal Filho et al. 2021) has shown that, due to their intensity, excessive urban heat causes not only thermal discomfort, but also reductions in the levels of life quality; and this still remains as an invisible risk in many cities (Brimicombe et al. 2021)
Based on the need to shed some light on the various factors that characterize urban heat, the aim of this communication is to address the impacts of urban heat on human health, presents an international study identifying current trends, and suggesting some measures to address the problem.
Impact of urban heat on human health
In many European cities, high-resolution urban monitoring networks (UMNs) are deployed to empower the urban atmosphere monitoring and improve urban planning, emergency preparation, infrastructure needs, decision making, etc. (Muller et al. 2013). In terms of finer-scale urban monitoring, Stewart and Oke (2012) defined a "local climate zone" (LCZ) classification system that provides a research framework for urban heat islands and standardizes the worldwide exchange of urban temperature observations. LCZs represent a uniform surface cover, structure, material, and human activity that span hundreds of meters to several kilometers in horizontal scale, and each LCZ has a characteristic temperature regime that is most apparent over dry surfaces, on calm, clear nights, and in areas of simple relief in cities and their surroundings. Based on the existing LCZs classification system and UMNs, the intra-urban assessments of cities is important for ensuring better public health and well-being and urban environment by obtaining more detailed atmospheric conditions between different urbanization types (Kousis et al. 2021).
From the preindustrial period, the world has already warmed by more than 1.2 °C with the expectation that this increasing heat will be intensified in cities (Kousis et al. 2021). It may worsen public health effects, i.e., increasing morbidity and mortality due to heat stress, heat stroke, or cardiovascular/respiratory diseases. New assessments show that the population's vulnerability to extreme heat events increases in almost every region globally and urban areas. In 2019, there were 475 million additional exposures to heatwaves affecting vulnerable populations. It was represented by 2,9 billion additional days of heatwaves (Kousis et al. 2021). As a result of these heat effects, the heat-related mortality in people older than 65 years increased almost 54% from 2000 to 2018, mostly in central Europe, Japan, eastern China, and northern India. Only in 2018 was recorded 296,000 excess deaths (Watts et al. 2021).
The year 2003 has gained significant international notoriety since Europe's heatwave in July and August caused more than 73,000 deaths. The 2010 heatwave also had a significant impact on the population of Russia, where the death toll was more than 56,000 people (Center for Research on the Epidemiology of Disasters 2020). The EM-DAT database has recorded 217 heatwaves since 1936. These heat waves have caused 171,296 deaths in 56 countries (Center for Research on the Epidemiology of Disasters 2020). Although heatwaves have affected countries on five continents, the impact on the European population has been more significant, according to EM-DAT records (Table 1); a previous study supports this (Watts et al. 2021). Table 1 shows the impacts on heat-related mortality have affect many countries.
Table 1. Top 10 heatwaves and affected countries.
Given that trends show that temperatures under different scenarios may increase, it is necessary to improve the capacity to record these impacts worldwide to also allow the identification of solutions suitable to developing countries.
Unrelented heat: international trends
Based on the need for research which may enable a comparison of trends related to urban heat between different urban areas around the world, the increase in temperatures for 15 selected cities (listed in Table 2) was estimated from the long-term temperature records of the Climatic Research Unit gridded Time Series - Version 4 dataset (CRU TS v4) (Royal Netherlands Meteorological Institute 2021). This tool was used, since it provides a high-resolution monthly grid of ground observations from standardized individual station series dating back to 1901. Temperature trends were estimated based on data of controlled quality from meteorological stations located in urbanized areas, and the corresponding gridded data that were assumed to represent surrounding non-urbanized or rural areas.
Table 2. Cities selected to take part in the study
The mean temperature differences between urban and rural areas in the analyzed cities were approximately 5°C. The highest values were recorded in Mexico City, New Delhi, Cairo, Brisbane, and Sydney (Fig. 1a). The cities that registered the greatest variations in these differences throughout the analyzed period were Sydney, Mexico City, Jakarta, Sao Paulo, and Istanbul (Fig. 1a). These variations can at least in part be attributed to the rapid population growth that these cities registered from the middle of the 20th century, which generated an accelerated process of urban expansion and the occupation of peripheral areas in the surroundings of the main urban centers (Rushayati et al. 2016; Lima and Magaña Rueda 2018; Ünal et al. 2020).
The data series for the period 1901 - 2020 show an average increase of 1.5°C in the temperatures of the analyzed cities, which has occurred mainly since the 1980s (Fig. 1b. The cities that recorded the largest increases in average temperatures were Moscow, Sao Paulo, and New York City. The smallest increases were registered in Lagos, New Delhi, and Kinshasa. However, the trends for these cities may be affected by the absence of consistent networks of meteorological stations in their territories, decreasing the precision of the interpolations of monthly climatic anomalies from the CRU TS v4 climatic dataset (Royal Netherlands Meteorological Institute 2021).
The simulations carried out from Phase 5 of the Coupled Model Intercomparison Project (CMIP5) (Royal Netherlands Meteorological Institute 2021) under two greenhouse gas emission scenarios (RCP2.6 - low emissions and RCP8.5 - high emissions) show mean and maximum temperature trends for 2050 and 2100 in the 15 selected cities (Figs. 1c and 1d). Increases of more than 1.5 °C for 2050 and 2.5 °C for 2100 are observed in the RCP8.5 scenario, with the highest values generally registered in cities located in humid temperate regions.
Data from the models provided in Figure 1 suggest that increases in urban temperatures may be expected and that appropriate responses to this trend are needed.