The relationship between climate change and the city

Since the Fourth Assessment Report of the Intergovernmental Panel on Climate Change—IPCC (2007), issues related to climate change have passed the stage of debate and have become unequivocal truths. The global warming caused by anthropic actions, especially those associated with GHG emissions, has shown itself to be unquestionable. This global change caused by humans is affecting the whole of the environment and the biosphere. Over the decades, GHG emissions have altered climate standards and have resulted in worldwide warming. Oceans, forests, glaciers, deserts, and all the other natural environments are changing, so that life must adapt in order to survive. Climate change is already provoking humans to change how they live and consequently to consider how buildings and cities should be designed.

Studies related to this theme have been developed since the middle of the last century, but the discussions have become more intense, and with the involvement of society in general since the 1980s, easily noticeable, due to the governmental and non-governmental actions aimed, in particular, at the search for a reduction of environmental loads. In this respect, the various agreements established by international meetings and conferences with the participation of leaders from the most important nations stand out. However, although such events and the commitments made by participating nations have been widely celebrated and publicised by the media, the desired effects have not been obtained, especially in countries whose economies are based on industrial activity.

The main agreement currently in force is the so-called Paris Agreement, the international treaty on climate change adopted by 196 countries at COP 21 (Conference of the Parties) in Paris on December 12, 2015, which came into force on November 4, 2016 (UN-Climate Change 2019). It aims to limit global warming to less than 2 °C, but preferably to 1.5 °C, compared to the temperatures recorded before the Industrial Revolution in the 18th century, when the population growth in cities started.

However, the planet is currently far from meeting the 1.5 °C or 2 °C targets of the Paris Agreement. In recent years, heat records have been broken; 2019 was the second warmest year on record and the decade from 2010 to 2019 was the warmest on record. Since the 1980s, every decade has been warmer than any previous decade since 1850 (UN-News 2020). Additionally, new researchers are revisiting methods and datasets and updating the warming estimate in increments of at least 1 °C (IPCC 2021).

As a result of the global pandemic (COVID-19), due to travel bans and the economic slowdown, it is estimated that there may have been a drop of approximately 6% in GHG emissions, however, there is no doubt that this improvement was only temporary. There has been no halt to ongoing climate change, and, as the economy starts to recover, emissions are also returning to previous high levels (UN 2020).

Most people currently live in cities. In 2018, about 55% (4.22 billion) of the world population lived in urban areas, and according to projections, this number will be 68% (6.88 billion) by 2050. This represents 2.6 billion more people living in cities, an increase of 63% in just 30 years, mainly located in developing and less developed countries. This massive urbanisation will raise the impacts of climate change to a new level and make the preservation of the quality of cities and their resilience increasingly important.

More and more data and findings show that climate changes, especially warming, are inducers of extreme events, as shown by the increase in their frequency and magnitude of occurrence. Such warming intensifies the continuity of, and accelerates the increase in, the average global temperature. It is estimated that among the total disasters that have occurred in the last 20 years, 91% have been climate-related disasters.

With the increase of urbanisation and climate change, in addition to the interference with air quality, aspects related to water availability and quality, land use, and the production and management of waste, it should be noted that extreme events tend to have more significant consequences for the most vulnerable populations, increasing further social inequities.

The health of the urban population can be strongly impacted, while extreme heat can cause reactions in the human body ranging from simple cramps to even exhaustion and heat stroke. These reactions, which often affect the body's ability to maintain its normal temperature, occur more frequently in vulnerable groups, such as children, the elderly, and people with pre-existing illnesses. Moreover, most of the impact factors are out of man's control, such as their geographic location, altitude, proximity to the ocean, water accessibility, etc. Nonetheless, these impacts cause profound effects on human life, city planning, infrastructure, and public services, which require immediate attention and action from governments and society. For example, in 2019, high temperatures caused more than 100 deaths in Japan and 1462 deaths in France (UN-News 2020).

New challenges arise every day. Globalisation has brought development to areas otherwise forgotten, rapidly connecting people and places never seen before. Likewise, it presents unthinkable challenges that a world in global change must be prepared to face.

Nevertheless, only in the first decade of this century have issues related to the need for cities to adapt become part of the agenda for discussion in international climate-related conferences. As a result, cities have become the main contributors to the intensification of climate change problems and also the recipients of its effects (Alvarez and Bragança 2018).

Urban resilience, which is the ability of a city to resist, absorb, adapt, and recover from exposure to threats, is becoming a concept that is integrated into public policies, whether local or global. In parallel, it is necessary to adopt measures that effectively reduce GHG emissions, slowing the growth rate of climate change, in particular, global warming.

It is essential to consider studies related to the expected direct and indirect impacts, given the +1.5 °C scenario established by the Paris Agreement (and also the IPCC +2.2 °C scenario), and how these environmental impacts will affect cities' infrastructures, housing, transport, health, ecosystems, water, energy, and solid waste management. The questions that should be asked are how cities can respond in order to adapt and to mitigate effects, and how they can do it by being more inclusive, resilient, and efficient.

GHG emissions due to human activities have been increasing since pre-industrial times, driven by population growth and economic development. These carbon dioxide, methane, and nitrous oxide emissions concentrate in the atmosphere and have reached an unprecedented level. Studies show that they have been the main cause of warming since the mid-20th century (IPCC 2014).

Also, according to the IPCC reports, the rates of global carbon dioxide (CO₂) emission have increased by almost 50% since 1990. In addition, the GHG emissions have grown more rapidly in the period between 2000 and 2010 than in each of the previous three decades (UN 2020).

Anthropogenic effects are responsible for approximately 1.0 °C of global warming above pre-industrial levels. Studies show a wide range of possible future global mean temperature increases, from 1.5 °C to 4.8 °C from the pre-industrial era to the end of the 21st century, based on different scenarios for GHG emission, atmospheric concentration, air-pollutant emission, and land use. The IPCC (2014) defends a rigorous mitigation procedure, limiting emissions to achieve the 1.5 °C scenario.

Five emission scenarios called SSPs—Shared Socioeconomic Pathways—were simulated by the 6th Assessment Report of the IPCC. First, the scenarios consider very low and low GHG emissions, with CO₂ emissions declining to net zero by around 2050, followed by varying levels of net negative CO₂ emissions (SSP 1–1.9 and SSP 1–2.6). Next followed an intermediate scenario (SSP 2–4.5), with CO₂ emissions remaining at around current levels until 2050. Finally, the report described the most critical scenarios with high and very high GHG emissions (SSP 3–7.0 and SSP 5–8.5), and CO₂ emissions doubling from current levels by 2100 and 2050, respectively. In the most adverse scenario, in some regions of the planet, especially in the far North, a temperature increase of up to 11 °C would be observed, in addition to extremes of heat noticed mainly in Latin America for more than 160 days a year.

The IPCC report further states that it is still possible, through technological measures, changes in behaviour, and institutional actions to limit the increase in the average global temperature to 2 °C above pre-industrial levels. However, the measures used to reach the increasingly unlikely scenario of a maximum temperature increase of up to 1.5 °C need to be urgent. In this sense, the establishment of public policies for effective action is of fundamental importance in changing the scenery and responsibility for this necessary change cannot be transferred to the individual or to an alleged collective consciousness.

Observations and measurements throughout the last decades have shown changes in climates and weather. Observed temperatures show that in the last three decades, Earth's surface has become successively warmer. In the Northern hemisphere, the period from 1983 to 2012 was probably the warmest of the last 1400 years. The ocean's accumulated energy has increased drastically, corresponding to more than 90% of the energy stored in the oceans compared to only 1% in the atmosphere between 1971 and 2010. In addition, the excessive CO₂ uptake by oceans has resulted in acidification and the pH of surface water has decreased by 0.1; precipitation measured in the mid-latitude areas of the Northern hemisphere has increased. It should also be remembered that high temperatures and intense solar radiation enhance the photochemical reactions responsible for emitting gases and polluting particles into the atmosphere.

Climate change impacts can already be observed in some natural systems. For example, hydrological systems are being altered by the change of precipitation and snow/ice melt; faunal observations show species that have changed their interactions, geographic distribution, seasonal activities, migration patterns, and population due to climate change; and the Greenland and Antarctic ice sheets have been losing more and more mass over the period from 1992 to 2011 (figure 1.1).

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According to studies in this field, the effects related to climate change will be especially felt in terms of temperatures, rainfall, droughts, floods, and runoff from rivers, oceans, cyclones, and extreme events.

According to the IPCC (2018), continental temperature changes are already widely known and are mainly detectable in North America, Europe, and Australia, with a global increase in the frequency and intensity of hot days and nights and a decrease in the intensity and frequency of cold days and nights.

Projections of future temperatures are linked to the mitigation scenarios predicted by the IPCC (2018). Considering the 2046–2065 timeframe, a strict mitigation scenario could lead to a global mean temperature increase of 1.0 °C, with a likely range of 0.4 to 1.6 °C; intermediate scenarios could lead to a mean increase of 1.4 °C, with a likely range of 0.9 °C to 2 °C, and a high GHG emission scenario could lead to a 2.0 °C mean increase, with a likely range of 1.4 °C to 2.6 °C (IPCC 2014).

There is an expected increase in the average temperature variability for tropical regions, which are more susceptible to heatwaves and thermal extremes, with a progressive number of warmer days in the pessimistic future scenarios. This thermal tendency in tropical cities aggravates the imbalance between social, economic, and environmental aspects in developing countries.

The year 2021 may have been one of the hottest years, with forest fires, droughts, storms, and intensified melting of glaciers. Recently, torrential rains in Belgium and Germany have flooded several rivers, destroying thousands of buildings and killing many people. In the Chinese city of Zhengzhou, the equivalent of a whole year's rain fell in just three days. In Canada, in June, the maximum temperature record was reached (49.6 °C) and in July, Finland experienced its hottest night ever. Wildfires in Australia, California, and the Mediterranean basin are increasingly longer and more intense; they devastate larger areas, causing health problems due to the smoke, deaths, and massive damage. All of this happened with an Earth warmed by just 1.1 °C–1.3 °C. The climatic collapse signs are becoming more visible each year. With the emissions estimates for the year 2030, the planet is on its way to a 3.2 °C rise in temperature this century, even if the promises of governments under the Paris Agreement (known as Nationally Determined Contributions (NDCs)) are fully implemented.

In the models and simulations carried out so far, regardless of the database used, the results invariably indicate that the distribution of the global warming will be highly unequal. The Arctic region will warm more and faster than other regions. High latitudes in the Northern hemisphere will concentrate great temperature changes. Globally, land areas will capture more heat than oceans. Tropical regions are likely to have a smaller increase in mean temperature, but as they approach threshold temperatures, such an increase can lead to great ecosystem impacts (IPCC 2018).

As already mentioned, this temperature rise can lead to adverse reactions in humans, such as cramps, exhaustion, and even fatal medical emergencies, if not quickly treated. In addition, pre-existing cardiorespiratory pathologies can suffer sudden worsening.

The aspects related to precipitation also deserve to be highlighted, since changes in precipitation will not be uniform. The projections for precipitation are not as accurate as those for temperatures, but it is expected that more areas will experience increases in frequency and intensity than the areas with a rain reduction.

Projections show that North America, North Europe, parts of Asia, and Africa will have equal or high precipitation levels. Meanwhile, North and South Africa, South and Central America, Southern Europe, and East Asia may have less precipitation (IPCC 2018).

Research in Africa found that the average precipitation may change significantly due to fewer rainy days that are compensated for by heavier rains and less rainfall in early summer and more in late summer. This scenario can be extremely dangerous, especially for the most socially vulnerable communities.

In Europe, changes in precipitation may also occur. Studies show that while the average precipitation is expected to increase in Central and Northern Europe in the winter, this happens only over Northern Europe in the summer, while it decreases in Central and Southern Europe.

Although confidence in the projected precipitation changes is not as high as for the temperature changes, the responses in the model simulation are robust and predictably linear. Significant differences in rain distribution were not found between the 1.5 °C and the 2 °C global warming scenarios. However, in the 2 °C scenario, there will be more heavy precipitation (IPCC 2018). It should also be noted that many platforms that offer simulation programs obtain quite different results, even when the input data are similar. Thus, special attention must be paid to the reliability of these sites.

In scenarios for temperature changes between 3.2 °C and 5.4 °C, an increase up to 50% in the average rainfall is expected in the extreme north of the planet at the end of the century. Regions such as the Northeast of South America, Central America and Southern Africa will, in turn, suffer from a reduction in rainfall of up to 30%.

There is low confidence in the drought and dryness trends on a global scale, but more confidence in regional observations. The Mediterranean and Western African regions may experience increases in dryness, and Central North America and Northwest Australia will see drought decrease. Modelling studies also project an increase in drought years in the Mediterranean region. An increase in drought and dryness, or a reduction in precipitation is foreseen in some regions as a result of considering the 1.5 °C and 2 °C global warming levels. However, there are many variability issues that affect the precision, except for the Mediterranean region, which has more data. Analysis that considered evapotranspiration has identified hotspots of drying in the Mediterranean region, Northeastern Brazil, and Southern Africa.

The probability of dryness increases with global warming, given that the 2 °C or more global warming scenarios tend to further reduce water availability in regions such as the Mediterranean and Southern Africa. However, according to the IPCC (2018) studies, constraining global warming to 1.5 °C will avoid the most extreme risks of change.

Additionally, the ingestion of, or contact with, polluted water can result in many types of disease. Pathogens that cause illness may reproduce faster due to climate change effects. Floods also potentiate the transport of too many pollutants and sediments into water and food supplies. The scarcity in quantity and the reduced quality of food and water, due to droughts, can impact human nutrition and health.

The likely changes in precipitation, associated with temperature changes that affect the water cycle, will also have consequences for cities located close to rivers, deltas, and islands. Studies show that in some regions there will be an increase in flood frequency, extreme streamflow , mudslides, and flood hazards (IPCC 2018). Extreme hydrological events will occur, and the annual global exposure to floods is expected to increase by between four and fourteen times over the 21st century compared to the 20th century.

The global projected changes in average annual streamflow are smaller for the 1.5 °C scenario than for 2 °C of global warming. A study of 21 of the most important river basins showed that most of them will experience an increase in runoff; the exceptions are the Guadiana, the Danube, the Amazon, and the Orange basin, for which the projections forecast a decreased flow.

There is a lot of confidence that global warming will lead to an expansion of areas of flooding and increases in runoff, thus incrementing hazards. Once again, the most socially vulnerable riverside populations will be the most affected, given the impossibility of relocating to safer places.

Coastal and low-lying areas have great vulnerability to storms and heavy precipitation. The impact of these depends heavily on their specific characteristics, but they can directly cause deaths and injuries and indirectly cause effects such as the contamination of soil and water, along with damage to respiratory and mental health. Mental health impacts are particularly common and strong, as they frequently come after the stress of evacuation, property damage, economic loss, and death traumas.

It is essential to realise that climate change can provoke an imbalance in ecosystems and directly influence the reproduction of species. For example, malaria and dengue are mosquito-borne diseases that strike many developing countries' populations and directly correlate with warming trends. The incidence of dengue virus increased in 2019 due to higher temperatures, which facilitated mosquitoes' transmission of the disease (UN-News 2020). Likewise, a tick, a vector for Lyme disease, has expanded from the US to Canada due to an increase in temperature.

Regarding the oceans , although it is estimated that they will continue to warm up, this increase in temperature will occur at rates lower than the rate of increase in the global temperature. As a result, the frequency of occurrence of marine heatwaves will grow. Sea ice levels between 1979 and 2012 showed a decrease of 3.5% to 4.1%; projections show that this process will continue in all scenarios, and at least once a century, the Arctic summer will be ice-free (IPCC 2018).

Ocean warming is expected to be greater in tropical and northern subtropical regions and more significant in the southern ocean. As Atlantic Meridional Overturning Circulation (AMOC) becomes weaker, global warming will increase (IPCC 2014).

The global mean sea level will continue to rise in all scenarios, at rates greater than 2 mm yr⁻¹. The sea-level change will not be uniform, although it will rise over more than 95% of the ocean area, and 70% of coastlines worldwide. The sea-level rise is related to CO₂ emissions, and the sooner its reduction occurs, the better (IPCC 2014).

In the absence of anthropogenic climate change, sea levels on the planet would have risen by less than half of the amount seen during the 20th century and could even have fallen. However, the rate of sea-level rise in the past two decades is almost double that of the last century, and each year it is accelerating slightly. From 1901 to 2010, the average sea level on the planet increased by 19 cm because the oceans expanded due to the warming and melting of ice. In the Arctic, sea ice has shrunk every decade since 1979, by 1.07 million km² per decade (UN 2020).

Ocean chemistry is also changing. The increase in the water surface temperature impacts water oxygenation, which has reduced since 1960, affecting ocean life. As a result of increasing the CO₂ in the atmosphere, the ocean is also increasing in CO₂ content and becoming more acid. Since the beginning of the Industrial Revolution, the acidity of the surface ocean waters has increased by about 30%, as its pH has fallen by 0.1 units. Ocean salinity is also changing with hotter temperatures, becoming less salty in the Arctic (due to ice melting) and saltier in other areas (due to increased evaporation), potentially affecting large-scale changes in water movement.

The fact that the oceans are becoming more acidic affects many species, such as oysters—due to their shells—and corals, which may even start to dissolve if the pH gets too low. Researchers have even discovered severe levels of dissolution of the shells of some species in the Southern Ocean, which surrounds Antarctica. In addition, the ability of some fish to detect predators is reduced in more acidic waters. However, algae and seagrass can benefit from the higher CO₂ conditions in the ocean, as they require it for photosynthesis. Some studies have examined whether cultivating seaweed could help to slow the ocean's acidification, as seaweed is mainly responsible for the oxygenation of the planet.

With the modification of temperatures, precipitation, and water cycles, there is a natural tendency for changes to also happen in the wind system. However, there is low confidence in the changes in cyclones observed since the pre-industrial age. Projections show an increase in heavy precipitation associated with tropical cyclones (IPCC 2018).

Although the simulation results point to a global decrease in the number of cyclones due to the increase in temperatures, there is a tendency for the frequency of occurrence of the highest-intensity cyclones to increase (IPCC 2018).

In 2019, extreme weather events occurred, some on an unprecedented scale, in different parts of the world. The monsoon season featured above-average long-term rains in India, Nepal, Bangladesh, and Myanmar, and flooding in the region killed 2200 people. In 2019, more tropical cyclones occurred than the average: 72 in the northern hemisphere and 27 in the southern hemisphere. Cities on the east coast of Africa, Japan, and the Bahamas suffered from destruction and/or flooding. Part of South America was hit by floods in January, while Australia had its driest year on record. South Africa, Central America, and parts of South America had abnormally low rainfall rates. In the United States, floods generated losses of US $ 20 billion (UN-News 2020).

Cities are central areas for human settlement and economic activities; projections show that by 2050, two-thirds of the world's population will live in cities. Most of this urbanisation will occur in Asia and Africa. Regarding GHG emissions, cities are part of both the problem and the solution. Approximately 70% of global anthropogenic GHG emissions are emitted into the urban environment, but the per-capita energy consumption of urban residents in developed countries is lower than that of rural residents (UN-Habitat 2016).

Climate change impacts will have very extensive consequences for urban areas on every continent. Cities have unique climate risks, such as urban heat islands, impervious surfaces that exacerbate flooding, and coastal developments which are threatened by sea-level rise. In addition, urban areas' characteristics, such as their heat-absorbing materials, reduced evaporative cooling, lack of vegetation, and produced waste can lead to excessive heat compared to other areas.

Existing risks will be amplified, and new threats will be created, especially for people in disadvantaged and low-income areas. Climate extreme events, such as heatwaves, droughts, heavy precipitation, and floods will be exacerbated in frequency and severity, leading to casualties and damage, leading to calls for measures to increase resilience to mitigate such impacts (figure 1.2).

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The effects of the global change in each city vary not only according to the kind of impact it will suffer but also according to the level of development of the city and how prepared it will be for the impacts of global change. Therefore, climate change risks need a transdisciplinary approach to the climate system, natural and managed ecosystems, human systems, and responses (IPCC 2018).

This is because climate change can result in a ripple effect across urban infrastructure systems, which are interdependent on each other, such as the water, sanitation, energy, and transportation sectors. The vulnerability of these systems to climate changes depends on their degree of development, resilience, and ability to adapt. Climatic variations can aggravate the problems, especially in developing countries, where a significant part of the population still lacks basic sanitation services, there are serious traffic problems, and energy security is still an issue for discussion (PBMC 2016).

Among the various parts that make up an urban community, the housing sector can be considered the heart of the city, accounting for a large part of the land area, the energy consumption, and the population's wellbeing. Most of the global energy demand for buildings is generated by developed countries. Building renovations are opportunities to apply long-term energy-saving solutions using better house designs and to retrofit to improve energy consumption and reduce GHG emissions. Renovated spaces can also improve quality of life by providing better public areas and local job creation (Jean-Baptiste et al 2018).

Low-income and informal settlements are especially vulnerable to climate change threats due to their geophysical locations, land availability, poor infrastructure, low house quality, and limited support in disasters. Table 1.1 shows the possible impacts of global warming changes on urban areas and how they can impact poor areas and informal settlements. Among those impacts, many are related to health, like the increased rate of disease, access to water, and life expectancy; others are related to housing, such as durability, and still more are related to the economy, like employment and ownership.

Some impacts listed in table 1.1 have already been noticed due to climate change. In 2019, about 6.7 million people on the planet had been displaced from their homes due to natural disasters—such as storms, floods, and devastating cyclones (UN-News 2020).

Low- and middle-income countries still face a high demand for housing; an estimated 500 million people will be in need of a home by 2050 (Jean-Baptiste et al 2018). This represents a big challenge and an opportunity to build cost-efficient, low-carbon, and resilient dwellings while improving access and reducing climate-related risks. In addition, it should also be considered that, in the future, the COVID-19 pandemic may change both population growth and the relationship between the user and the environment, probably making people more dependent on air conditioners and more energy for household and office activities.

Associated with the issue of housing, aspects related to urban mobility should be discussed. A great proportion of GHG emissions are from urban transportation systems. Cities are responsible for 70% of CO₂ emissions, and the transport sector accounts for 30% of the total. Urban transportation accounts for 40% of total transport emissions, with a growth of about 2% to 3% annually. The majority of the GHG stock belongs to developed countries, but 90% of the growth corresponds to lower-income countries (Mehrotra and Zusman 2018).

Adequate planning for public transport systems can provide several mobility options, increase public safety, reduce vehicle travel distances, reduce air pollution, reduce energy consumption, conserve natural resources and open spaces, reduce infrastructure costs, and contribute to more affordable housing. Areas of relatively high density are necessary to generate greater efficiency in the use of public transport. In general, density is one of the factors that affects energy expenditure and GHG emissions. Dealing with these issues requires an ongoing analysis of urban processes (UN-Habitat 2011).

Regarding air pollution, it should be noted that one in seven deaths around the world is linked to air pollution. Furthermore, climate variations can potentially change emissions, transport, dilution, chemical patterns, and the deposition of air pollutants. For example, the increase in wildfires leads to particle emissions and respiratory disease.

Urban transportation is vital to city living and there are many interdependencies between transport and other areas. Climate-related events can cause damage to the transportation system, resulting in cascade failures, such as problems with access to hospitals, energy supply, water availability, and rescue arrival. Emergency systems also rely on a functional transport system.

The increase of extreme events, such as heatwaves, intense rainfall, and sudden snowstorms, will directly impact transportation availability. Table 1.2 shows climate hazards that will likely impact the urban transportation system.

Low-carbon transport strategies involve a high-capacity mass transport system, more efficient multimodal networks, better vehicle design and cleaner propulsion technologies. This can also have other benefits, such as improving public health, better air quality, and redundancy of routes. Risk reduction solutions are also necessary, based on local risk assessment, supportive policy, and public–private participation.

As already mentioned, climate change directly impacts the so-called ' urban health ', including its buildings. Extreme heat, storms, floods, and landslides are among the originators of sequential impacts that can reach cities.

Besides infrastructure and geographic location, other factors that aggravate urban health vulnerabilities are the proportions of children, elderly, sick, and poor populations, which can vary drastically from city to city. Existing health conditions and diseases can also be exacerbated by climate change.

The growth of the urban population puts pressure on urban biodiversity and the ecosystem. Urban ecosystems and biodiversity are extremely sensitive to climate change effects, and some changes are already visible. Urbanisation and climate change are already increasing the vulnerability of urban species, habitats, and critical ecosystem services.

Urban ecosystems have a critical role in climate change adaptation and mitigation. They provide a natural environment between built spaces, with cost-effective nature-based solutions that can improve quality of life, human health, and social wellbeing. Urban ecosystems also promote sustainable urban development, a green economy, and social equity.

Temperatures rise can affect species' physiological processes, such as photosynthesis, respiration, growth, flowering, and plant development by altering growth and reproduction rates, either positively or negatively. Droughts can increase evapotranspiration, which can lead to reduced water availability and groundwater resources, with impacts on salinisation and water stress.

The shift of species in response to climate change has already been documented. For example, species can move north in search of cooler temperatures. Species with sensitive characteristics can suffer from local extinction. An urban health issue will be caused by species with unique attributes that benefit from climate change, such as the mosquito Aedes aegypti, a vector of various diseases.

Flood hazards can have impacts such as sediment movement, soil processes, and the distribution of pathogens that were once concentrated. In coastal cities, sea-level rise and floods can increase salinisation and reduce groundwater recharge, which affect habitat quality and biodiversity (IPCC 2014).

Modifications of species for any reason can also have cascading effects on the ecosystem, on other species, on soil fauna, or any other component of biodiversity; these can also have impacts on human activities and infrastructure.

The suppression of ecosystems is considered to be one of the most significant factors in reducing the resilience of cities. This is because they are more vulnerable to future problems that will be accentuated by global warming. Among the most important ecosystem services that can mitigate global warming are the provision of drinking water, the regulation of extreme events, the local climate, air and water quality, erosion, and carbon sequestration. Wrong decisions about ecosystems can involve huge losses of natural capital, for which the costs of restoration are much greater than the costs of prevention (PBMC 2016).

Water is an essential resource for life, humanity, and the economy. Good-quality water is fundamental for the increasing number of people living in cities and economic activities. Historically, many cities were located near water bodies; this applies to 13 of the 20 most populous cities and more than 70% of world trade.

Climate change imposes additional pressure, because temperature rises result in trends of increasing water consumption and hazards related to water. The excess or lack of precipitation, floods, pollution, droughts, and sea-level rise can not only damage essential infrastructure, but also reduce adequate water availability (Forman 2018).

Water security is a central issue for cities. UN-Water (UNU-INWEH 2013) defines it as the ability of the population to have sustainable access to adequate water, to ensure protection against water-borne hazards, and to preserve ecosystems. This means conflict mediation, investments in infrastructure, and management between various stakeholders. For cities in low-income countries, this is particularly important, as many of them struggle to deliver basic services with a minimum quality and quantity of water. Moreover, as their populations grow, demand and competition will increase with climate change, leading to social stress and possible conflicts.

The impacts of climate change on water security can be categorised in two ways: water as a resource and water as a hazard. Increasing temperatures, changes in weather and precipitation, sea-level rise, storm surges, and changes in surface and groundwater availability are all threats that affect urban water security. Table 1.3 shows the risks that arise from climate change in terms of water as a resource and water as a hazard.

The adaptation to climate change must be built on a solid basis of good planning and governance, sources of investment, social equity in water access, and a resilient infrastructure made to support fast-changing conditions. Specifically, in low-income countries, the growth of the urban population imposes another challenge that requires improvements in the efficiency of the system and even the use of alternative sources of water.

Some alternatives for adapting water resources to urban infrastructure involve efficient water use, water storage, conservation, reuse, and rainwater use along with reviews and modifications of the surface and underground sources of water collection and transfer. It is also necessary to increase the water storage capacity and promote the recovery of hydrographic basins (PBMC 2016).

The continuing trend of urbanisation, fast population growth, and climate change will impact the global energy infrastructure even more. The challenges for the energy sector involve three main issues: environmental impacts, energy access, and vulnerabilities to climate-related events.

The main resources of energy are still fossil fuels. Oil, gas, and coal support 84.9% of global energy use and are the biggest source of GHG emissions. The current trends of demographics and urbanisation will further increase the total amount of emissions. Although clean energy use is growing rapidly, it will still take a long time to change the game (Marcotullio et al 2018).

On the demand side, the warmer climate, the growth of urban populations, and global development put pressure on expanding energy use. On the supply side, extreme events and technological changes bring risks and opportunities for future development.

The problem is that energy emissions grow with the increase in the population and GDP of countries. The promotion of energy efficiency involves the complete decarbonisation of an economy's activities, reaching an almost 100% clean and renewable energy matrix. To achieve the goals of the Paris Agreement, countries such as Brazil should have net emissions equal to zero by 2050 and negative net emissions after that date.

Regarding vulnerabilities to climate-related events, three different approaches can be presented (Marcotullio et al 2018). The first is primary energy feedstocks related to risks such as reducing biomass crops to produce energy, which primarily affects developing countries, and oil and gas drilling operations, which can suffer from exposure to extreme events such as flooding and high winds.

The second vulnerability occurs in energy generation. It can be related to power plants located near coastal zones that can be damaged by sea-level rise and flooding. Another risk to the energy generation system is the increased amount of high peak demand with rising temperatures, which increases the risks of blackouts and brownouts. Also, droughts can cut off the supply of water, affect the cooling needs of plants, and provoke wildfires and high temperatures that damage infrastructure (Marcotullio et al 2018).

Renewable resource power generation can also be affected. Droughts and the decline of snow levels can affect hydroelectric facilities, an increase in cloudy days can weaken solar radiation by up to 20%, and lower wind speeds may diminish wind energy production.

The third vulnerability occurs in energy transmission and distribution networks. The hotter the temperature, the more conductivity declines, and power lines became more susceptible to failures. Extreme events, such as heavy rains, flooding, storms, high winds, and hurricanes also affect distribution systems.

Essential services depend on energy supply to work correctly. Therefore, reducing vulnerabilities is necessary to avoid cascading failures that threaten hospitals, transport systems, industrial production, and other critical human activities.

Urban solid waste is intimately related to development, population growth, an increase in urbanisation, and climate change (Oteng-Ababio et al 2018). About 3% to 5% of global GHG emissions belong to improper waste management, most of which is methane produced in landfills. In developed countries' cities, for example, in North America and Europe, the waste sector is responsible for 2%–4% of total urban emissions. On the other hand, in less developed cities, like those in Africa and South America, waste sector emission corresponds to 4%–9% of total urban emission. Most of this difference belongs to the methane reduction infrastructure.

However, there are variations in the emission rates associated with waste in different cities, probably due to varying consumption patterns and generation of waste, differences in waste management, and differences in accounting mechanisms. The lack of a standardised urban structure that would allow inventories of GHG emissions can be conducted leads to variations that make it difficult to assess them (UN-Habitat 2011).

Climate-change-related impacts to waste sector infrastructure involve the three main processes of the system: collection, processing, and disposal. Table 1.4 shows climate change risks and their effects on each part of waste management.

Regarding the waste sector, there are many challenges in cities, but they all point in the same direction, which is to increase recycling, reuse, and reduce waste generation. Better landfills with energy recovery systems can also help to reduce local pollution and GHG emissions. For low- and mid-income countries, population growth, an increase in per-capita generation, and poor infrastructure poses great challenges. Improving solid waste management can also create opportunities for collection, recycling, and landfill building and maintenance.

The environmental standards of cities must shift as global change impacts take place. The increase in the intensity and frequency of extreme events will provoke disasters and climate emergencies that cities will need to respond to in the coming decades. Many aspects of these are related to vulnerabilities, such as culture, demographics, economy, governments, technology, the built environment, ecosystems, resource exploitation, and degradation. Cities that do not adequately plan their urbanisation face serious threats, because environmental change can remove natural barriers against storms, increase air and water pollution and the heat-island effect, and exacerbate the overall impacts of climate change.

The transition to low-carbon cities is critical to the sustainability movement. The relation between urbanisation and GHG emissions is complex. Cities are responsible for a great part of GHG emissions, most of the world's population, and its growth. However, they present opportunities to efficiently use resources in large populations, focus on sustainable development goals, reduce carbon emissions, and prepare for upcoming events.

It is fundamental that cities must adapt to global change. Climate change, migrations, and the growth of urbanisation are strong trends; their impacts are already materialising and demanding city action. Making cities and human settlements inclusive, safe, resilient, and sustainable is essential in order to respond to the global change and its impacts and to achieve the SDG 11 'Sustainable Cities and Communities'. Transformative climate change measures create the possibility of generating more extensive moves toward sustainable cities. In this way, three fundamental characteristics emerge for adaptation and mitigation improvements: resilience, efficiency, and inclusivity.

The concept of efficiency is linked to economic theory and comprises the management of resources. It is a way of expressing the level at which a nation's known production capabilities are taken advantage of. The relationship between the effect, the effort, and the results achieved, in terms of a specific period or other best practice define the economic efficiency (Borza 2014).

With the advent of the COVID-19 pandemic, the ideas for the economic crisis recovery have evolved into concepts more appropriate to fight against climate change and its consequences. In this sense, the idea of a Green Recovery Plan has arisen, which consists of a transition strategy for a green economy which is compatible with the fight against inequality, with the generation of employment and income, and with sustainable economic growth. Thus, the concept has established itself by considering the need to reduce or eliminate the economic activities that strongly drive carbon emissions.

The concept of efficiency is also directly correlated to sustainability. To maintain and improve people's wellbeing, it is imperative to do more with fewer resources. The term 'eco-efficiency' complements the efficiency concept by adding the environmental resources and impacts to the equation. Eco-efficiency is defined as the efficiency with which ecological resources are used to attend to human necessities. Eco-efficiency can be expressed as the ratio between the amount of social service provided and the environmental burden generated. For urban areas, eco-efficiency is described as the ratio between citizen wellbeing and the ecological burden caused by the city, so efficient urbanism is achieved when the social services increase more than the environmental or ecological burden.

The city's answer to the increase in the frequency and severity of extreme events must include a new decision framework and disaster risk reduction measures that build resilience against these impacts. Resilience is a concept first related to ecosystems and how they are applied to urban space.

Resilience is a crucial aspect for mitigating the impacts on the infrastructure due to extreme events. Therefore, every infrastructure system must assess its vulnerabilities and their interactions to improve the city's resilience. Resilience can be measured as a combination of three factors: the quantity of disruption a system can absorb and still remain in the same state, the capacity of the system for self-organisation, and the ability to build and increase capacity for learning and adaptation (UN-Habitat 2017). Other critical aspects for cities are the ability for the municipality to develop long-term planning strategies, management processes, and urban system efficiency.

For low- and mid-income countries, inclusivity plays a special role. The rapid urbanisation process has led to enormous migration from rural areas to cities, which added to the liberalisation and privatisation of public spaces and infrastructure, creating pressure on public services and resulting in poor and informal settlements that decreased the overall quality of life. Additionally, bad infrastructure, poor-quality housing, and the lack of education, jobs, and public services contribute to increased socio-spatial and financial inequality, further expanding the gap between rich and poor people. This creates the poverty trap that puts pressure on infrastructure systems, stressing earning potential, political participation, and population wellbeing.

In poor cities and cities with great inequality, global change can have a magnified impact in comparison with the impact to rich and organised cities, making it difficult to take some types of mitigating measure. For example, the lack of ownership in informal settlements can limit any action of evacuation; since people do not know if they will ever return home, they can hesitate to take this decision.

Impacts on low-income populations can have additional cascading effects. For example, the low quality of dwellings can increase disease and mortality due to extreme weather events or vector-borne disease. Otherwise, the same can be said for the upside effects of mitigation measures. For example, an improvement in public transportation can reduce activity costs and the time spent in traffic and improve population wellbeing.

Mitigation and adaptation efforts must prioritise measures that encompass multiple key aspects. Urban planning decisions have long-term consequences and a high potential to reduce GHG emissions and respond to climate hazards. Sustainable urban planning, in conjunction with regional climate change assessment, can improve urban environments and address resilience, efficiency, and inclusiveness.

According to the IPCC (2014), there are some necessary mitigation efforts. In the area of energy supply, the use of low-carbon technologies including renewables, nuclear sources, and carbon capture and storage can reduce GHG emissions. Energy demand can be reduced by up to 20% in the short term and 50% by 2050 by increasing the efficiency of buildings and achieving changes in residents' awareness and behaviour. For transport, emissions can be reduced by moving to low-carbon transport systems, increasing the efficiency of vehicles and engines, and reducing the carbon intensity of fuels by the use of engines powered by electricity or hydrogen. As for buildings, the renovation of existing constructions can reduce heating and cooling energy needs. New buildings in rapidly growing regions represent an opportunity to mitigate the effects of global warming; in such buildings, GHG emissions can be virtually eliminated. In cities undergoing rapid development, the trajectories of urban development and infrastructure must be shaped. For mature cities, the options are in urban regeneration and rehabilitation.

This chapter has presented how global change can take place and will affect the urban environment, infrastructure, and services. Even if limited to the Paris Agreement level, 1.5 °C above pre-industrial levels, global warming can cause enough damage to change ecosystems and harm populations profoundly. Although it is still somewhat uncertain whether that level will be achieved, studies of a possible 2.0 °C global warming show that most of the projected impacts would significantly increase. Therefore, immediate efforts must be made to avoid an increase in temperature above the proposed levels.

Global warming effects are not uniform and not precisely known. Although the global mean temperature will rise, this increase will be heterogeneous, causing different impacts. Moreover, the combination of effects has the potential to multiply risks. For example, the rise in ocean temperature can raise sea levels, melting glaciers and ice sheets, which leads to even more sea-level rise. In addition, the increase in temperatures and the lack of precipitation can lead to droughts, and, at the same time, increase water consumption in cities, which can aggravate the situation.

The increase in the frequency and severity of extreme events will be a challenge for humanity, especially in low-income cities. Cities must develop strategic plans and prepare for the impacts by improving resilience, efficiency, and inclusivity. Resilience must be built by taking account of the whole city, avoiding cascade failures and resource wastage. Urban planning that fails to adjust zoning, building codes, and standards for the climate-change future will limit the possibilities for adapting the city's infrastructure and put the lives of its citizens at risk.

Low- and mid-income cities face additional challenges due to increasing urbanisation, social and economic inequality, migration, and informal settlements. In addition, many of these cities are in areas that are more vulnerable to climate change impacts, making adaptation and mitigation efforts even more crucial. Climate action in such places must also consider inclusiveness and efficiency as essential aspects for development, improving people's lives, and offering a path towards sustainability.