Half of the world’s population lives in cities
Since time immemorial, the human population of every country and geographical region in the world has been divided into city and town people on the one hand, and country people on the other hand. Historically, cities have offered better conditions for crafts and industry – the first factories were in cities – and they have traditionally been safer places to live, being protected by walls. People living outside towns and cities have been focused on agriculture: growing crops and raising cattle.
Since the end of the 19th century there has been a major population influx to towns and cities. This process is called urbanization.
Figure 2.9.1 The old town of Berne in 1820
URBANIZATION
is the process by which towns and cities be- come dominant in a society. It is caused by the growth of industry in cities, development of their cultural and political functions and a deepening of the territorial division of labour.
By 2008, because of urbanization, the share of the world’s population living in cities rose above 50% for the first time and continued to grow thereafter (Fig 2.9.2). By mid-2023, approximately 4.6 billion of the more than eight billion people worldwide lived in towns or cities. This represents 57% of the global population. According to the United Nations, the urban population is set to increase by almost 600 million by the year 2030, reaching a total of 5.2 billion (about 60% of the global population).
At present there are 34 cities worldwide with more than 10 million inhabitants. Most of these megacities are situated in Asia (21), Latin America (6) and Africa (3). The largest city is the urban agglomeration of Tokyo with a population of over 38 million (Table 2.9.1). This is followed by Delhi (26.5 million) and Shanghai (24.5 million). By United Nations calculations, the number of megacities is expected to increase to 43 by 2030. Delhi will then be the largest city in the world, with a population of almost 39 million.
So, the study of the climate in cities is important for at least half the population of our planet.
Figure 2.9.2 Share of urban population in total population and the largest cities of the world in 2018
Table 2.9.1 The 15 largest cities in the world
| № | City | Country | Population (million people) |
| 1 | Tokyo |  Japan | 38.1 |
| 2 | Delhi |  India | 26.5 |
| 3 | Shanghai |  China | 24.5 |
| 4 | Mumbai | India | 21.4 |
| 5 | São Paulo |  Brazil | 21.3 |
| 6 | Beijing | China | 21.2 |
| 7 | Mexico City |  Mexico | 21.2 |
| 8 | Osaka | Japan | 20.3 |
| 9 | Cairo |  Egypt | 19.1 |
| 10 | New York |  USA | 18.6 |
| 11 | Dhaka |  Bangladesh | 18.2 |
| 12 | Karachi |  Pakistan | 17.1 |
| 13 | Buenos Aires |  Argentina | 15.3 |
| 14 | Kolkata | India | 15.0 |
| 15 | Istanbul |  Türkiye | 14.4 |
Figure 2.9.3 Tokyo with more than 38 million inhabitants is the largest city by population in the world
Why are cities called heat islands?
Cities are unique environmental hot spots on our planet, taking the word ‘hot’ quite literally: emissions of various substances from factories and motor vehicles ‘stagnate’ in the surface layer of the atmosphere above the city, creating a greenhouse effect, which raises its air temperature by several degrees compared with the surrounding territory. Scientists therefore call cities heat islands.
HEAT ISLAND
is an area in the centre of a big city, where the air temperature is higher than in outlying areas. The urban heat island effect is most noticeable in the evening and at night, especially in the spring and autumn, when the temperature difference between the centre of the city and outlying areas can be as much as 10–15°C.
The heat island effect in large metropolitan areas is being intensified by the process of climate warming.
We all know about the urban heat-island effect from personal experience: if you step out of a city building in the evening of a hot summer’s day, the temperature on the street is warm enough for a stroll in light clothing, but outside the city you would find it quite chilly outdoors in the evening without a jacket, even during the hottest summer month. This is because in an urban environment, surface air cools more slowly: it is kept warm by the walls and roofs of buildings that have soaked up heat during the day.
Figure 2.9.4 Air temperature distribution over a city (urban heat island)
The first studies of city climate
The first studies of city climate were carried out by Luke Howard (1772– 1864), an amateur meteorologist in London with a lifelong fascination with clouds and the weather.
From 1806 to 1831, Howard carried out daily measurements of atmospheric pressure, air temperature and humidity, rainfall, and evaporation in the suburbs of London. For his observations he used newspaper reports on specific weather events. Howard did not study the specific climate in London, but carried out general climate studies, using London as the base for his observations. But what gives him a claim to be the founder of urban climatology was his attempt to compare the data from his own meteorological measurements with those of the Royal Society at a site in central London. The comparison revealed what modern researchers refer to as the ‘urban heat-island’ effect.
How does climate change affect the health of city dwellers?
Climate change has a substantial impact on human life and health. We already know that our health depends on lifestyle, food security, heredity, occupation, environment, and access to health care, but it is now becoming clear that it also depends on climate change.
Climate change is particularly noticeable in cities, and most of all in large cities. For example, the increase in air temperature in Moscow over the last century has been more than 2°C, while the increase in global average temperatures during the same period has been only 1.2°C. Air temperatures are also rising quickly in other major cities around the world (Fig. 2.9.5).
Figure 2.9.5 Change in average annual daily maximum temperature in major Australian cities in 1980–2012. Linear trend increase is per decade
Experts at the World Health Organization expect that global warming will cause extremely hot spells in cities to become more frequent, intense, and long-lasting. It is well-known that fluctuations in pressure, temperature and humidity can make living conditions in cities uncomfortable; there are more and more instances of excessive city heat taking a tragic toll among the elderly, young children, and people in poor health. Intense heat is accompanied by higher concentrations of pollen and other particles that cause allergies and asthma.
People who live and work in the city centre and those whose jobs require spending much time out of doors (road workers, construction workers, delivery men) are particularly at risk on hot days.
If a heat wave lasts for more than a week, it can lead to heart problems and even death among the elderly, and people with poor health.
According to a study published in the journal, Nature Medicine, nearly 62,000 people died of heat-related causes in Europe during the summer of 2022. The study analysed temperature and mortality data between 2015 and 2022 for 35 European countries, representing a total population of 543 million, and used it to create epidemiological models to calculate heat-related deaths. Researchers found that Italy was the hardest-hit country, with around 18,000 deaths, followed by Spain with over 11,000, and Germany with around 8,000. The extreme heat disproportionately harmed the elderly and women. Of the nearly 62,000 deaths analysed, the heat-related mortality rate was 63% higher among women than men. Age was also an important factor, with the death toll rising significantly among people aged 65 and over.
Timely forecasting of a coming heat wave is of great importance, since it gives medical personnel a chance to prepare for it. The World Meteorological Organization (WMO) recommends that such a warning forecast be given at least two days before the period of intense heat begins.
The USA, Canada, France, and some other countries have already taken steps to address the challenges posed by the heat island effect in the context of global warming. For example, the US city of Philadelphia has advocated a system of ‘good offices’ during heat waves: the media regularly report on the changing weather conditions and offer advice on how to avoid heat-related illnesses. The number of a telephone hot line is published in the newspapers and displayed prominently on a large screen in the city centre. Emergency medical services and fire departments take on extra staff. Special air-conditioned premises are provided for elderly people, who are brought there by a special, free-of-charge transport service to take a break from the heat.
Temperatures that people from a hotter climate consider normal can be termed a heat wave in cooler areas if they are outside the normal climate pattern. In the USA, for example, the definition of a heat wave depends on the region. In the northeast, it is typically defined as three consecutive days of temperatures reaching or exceeding 90°F (+32.2°C). In California, where the climate is hotter, a heat wave has a higher threshold of 100°F (+37.8°C) for three or more consecutive days. The National Weather Service issues heat advisories and excessive heat warnings when unusual periods of hot weather are expected.
Precautions to take in hot weather
- Wear clothes made of natural fabrics: they help to prevent overheating by allowing the skin to breathe.
- Keep a bottle of water with you, preferably water that is not too cold. A person should drink at least three litres of water a day in hot
- Keep out of direct sunlight so far as The sun is at its strongest from midday until about 1600 hours, so try to stay indoors during this time.
- Always wear a hat or
- Do not buy perishable products: bacteria multiply very quickly in high temperatures, so there is a risk of severe food poisoning.
- Eat plenty of fruits, vegetables, various salads, and cold
- Avoid oily and salty
- Do not overdo sport and physical
- Stay relaxed: any nervous stress increases the risk of heat stroke, sun stroke and cardiovascular disorders.
- Do not sit directly under air conditioning: the temperature difference between the hot streets and an air-conditioned room is very large, and such temperature swings can induce colds and pneumonia.
Climate change has negative impacts on human health (Fig. 2.9.6). Dangerous infectious diseases, such as encephalitis and malaria, spread to areas where they were not previously present, and the period of the year when there is danger of infection becomes longer.
Climate change intensifies infection risk
Tick-borne encephalitis is a viral infection. The virus enters the human body through a bite from an infected tick. Encephalitic ticks, the main virus carriers, live in taiga and forest areas of Siberia, the Urals, and the Russian Far East. But there have recently been an increasing number of cases of infection in the central part of European Russia, the North-West and the Volga region. Cases of tick-borne encephalitis are being recorded for the first time in parts of European Russia, and scientists attribute this to global warming.
Warmer weather in the winter and spring favour the spread of ticks: they are more likely to survive the winter and can multiply rapidly in the spring. Typically, only a small fraction of all ticks is infected with encephalitis. But an increase in the total number of ticks leads to an increase in the number of infected individuals.
Malaria (from the Italian ‘mala aria’, meaning ‘bad air’), also known as swamp fever, is an infectious disease transmitted to humans by bites of malarial mosquitoes, causing a high fever. Malaria transmission depends on the presence of malarial mosquitoes in a given area and ambient temperatures, at which the viral agent that causes the disease can develop in mosquitoes.
Malaria usually occurs in tropical countries, but the disease is much less common in tropical highlands, where colder temperatures slow down the mosquito and the development of the parasite within it.
As the climate warms, the boundary of the area where malaria occurs moves into milder climate zones further and uphill from the equator, and the ‘malaria season’ (the time of year when outbreaks are most likely to occur) grows longer. Some studies have shown that malaria has spread into higher altitude areas in Kenya, Colombia, and Ethiopia, where it was previously too cold for the disease to thrive. This puts millions of people at risk of the disease, necessitating extra measures to prevent outbreaks of malaria.
Figure 2.9.6 Impacts of climate change on human health
A recent study on the health impacts of climate change published in the Lancet Medical Journal provides a set of indicators that help to understand the impacts of climate change on human health, such as heat mortality, food insecurity and air pollution exposure.
On extreme heat, for example, the study finds that in the Small Island Developing States, 103 days of health-threatening temperatures every year are attributable to climate change over 2018-2022. Across Europe, North America and Oceania, this number is less than 30.
The study estimates that under a 2°C warming scenario, for example, 525 million additional people will experience food insecurity by 2041-2060, compared with the 1995-2014 average.
A combination of climate change, urbanization and human movement are driving up the incidence of diseases such as dengue fever. The study finds that ‘cases of dengue have doubled every decade since 1990, and almost half of the world population is now at risk of this life-threatening disease.’
Floods also pose a threat to human health, since flooding disrupts water supply and sewerage systems, which increases the risk of intestinal diseases. In some parts of the world, flooding may bring poisonous snakes and crocodiles with it, as happened in Australia in 2011.
Changes in the environment and lifestyles can induce psychological stress and depression. You have probably noticed that sometimes, when the weather is bad, you don’t want to go anywhere or do anything. So how will people feel if they experience bad weather more often?
How does climate change affect the urban economy?
Extreme weather events can disrupt transport, electricity, and water supply in cities. Flooding may inundate buildings, roads, railways, seaports, and airports. Higher temperatures lead to faster deterioration of road surfaces, which need more frequent repairs. Sudden temperature drops in the winter cause the formation of ice that damages power lines, leaving homes, schools, hospitals, and businesses without electricity.
Residents in northern countries with colder climates may be able to reduce the cost of heating their homes as air temperature in the cold season rises. But cities in southern countries with hot climate conditions will face higher costs as there will be greater need for air conditioning in the summer.
In-depth studies of climate change impacts on cities over the last decade can help improve our understanding of the possible consequences of global warming and offset some of the costs. For example, the cost of clearing damage caused by flooding can be partly compensated by savings on heating in the winter.
How can we adapt and live in healthier and climate-resilient cities and settlements?
IPCC scientists noted in the AR6 that rapidly growing urban populations and unmet needs for healthy, decent, and sustainable housing and infrastructure pose a persistent challenge. But urbanization also offers many opportunities to build adaptation measures into the planning and development of cities and their infrastructure. This is a critical priority, given that an additional 2.5 billion people are projected to be living in urban areas by 2050, with up to 90% of this increase concentrated in Asia and Africa.
Many cities and urban settlements are already taking actions to adapt to current and near-term climate impacts and risks as part of their social and economic planning and policies. These actions include hard engineering interventions and planning, such as further development of urban public transportation, engineering flood defence, redesigning and fortifying buildings, setting up cyclone shelters, introducing hurricane-resistant building codes, developing standards, regulations, and guidelines for construction, improving stormwater management, drainage and flood protection systems, and, where relevant, strengthening infrastructure to withstand permafrost conditions. There is also growing interest in adaptive urban design and nature-based solutions aimed at green cooling, such as green roofs, walls, and parks. Expanding green and natural spaces simultaneously enhances biodiversity, improves air quality, and moderates the hydrological cycle; it also helps reduce health risks associated with heat stress and respiratory illnesses, and mitigates mental health challenges arising from congested urban living. Green roofs in the USA (Fig. 2.9.7), are an example of how cities are working to mitigate the effects of stormwater runoff, which is a major polluter of waterways, while at the same time keeping buildings cooler.
Health-related measures include redesigning and retrofitting homes, schools, and health care facilities to reduce the impacts of extreme heat, managing increases in infectious diseases such as malaria, and improved access to water and sanitation services. Actions taken in different sectors to address the risks of climate change can generate benefits for human health and well-being. Transitioning away from internal-combustion vehicles and fossil fuel-powered generating stations to renewable energy mitigates greenhouse gas emissions, improves air quality, and lowers the risks of respiratory illnesses. Policies and designs that facilitate active urban transport (walking and bicycling) increase efficiency in that sector, reduce emissions, and improve air quality and the physical and mental health of residents. Energy-efficient buildings and urban design improve indoor air quality and reduce risks of heat stress and respiratory illness. Food systems that emphasize healthy, plant-centred diets reduce emissions in the agricultural sector while helping in the fight against malnutrition.
Figure 2.9.7 Green roofs in the USA help with stormwater runoff and to cool buildings
QUESTIONS
1
Does more of the world’s population live in cities or outside cities?
2
Where is it warmer – in the city or in the city suburbs?
3
Why are heat islands bad for human health?
4
Can you list three measures for adaptation to climate change in cities?
5
What are the negative impacts of global warming on human health? What are vector-borne diseases?
6
What precautions should be taken in hot weather?
TASKS
1
If you take your summer holiday in the countryside, try placing a thermometer outdoors in the shade at the level of a person’s height above the ground and write down the temperature it shows in the early morning (before the sun starts to raise the
air temperature). Compare it with the forecast for night air temperatures in the nearest large city on the same day.
Are the figures different? Why?
2
Using textbooks, reference books and the Internet, find out and write down how you can help someone suffering from heat stroke, sunburn, frostbite, and severe allergic reaction to pollen, or someone who has been bitten by a tick.
What preventive measures can you take to protect your health during a period of intense heat?
