Dhaka, an urban area with a population of some 12 million.

1While humid tropical forests and ocean areas play an important role in climate regulation, urban zones generate the major proportion of greenhouse gas emissions. Large cities are centres for industry and also for the consumption of fossil resources for transport, heating and air-conditioning. The countries in the North are historically the main contributors to these urban emissions. However, the economic slumps and mitigation policies set up in Europe in parallel with strong population and economic growth of the megalopolises in the South are gradually reversing the trend.

2The cities in the tropics are particularly exposed to climate impacts, mainly because vulnerability is high and town planning policies and measures to combat natural risks are less developed there. The IPCC Fifth Assessment Report stresses the urgency of addressing urban areas with regard to both mitigation policies and capacity to adapt as the impacts on society—poorly assessed as yet—are worrying. The immediate consequences for the population are the effects of pollution as regards public health. The rapid rise in temperatures, related in particular to the urban heat island phenomenon, also has marked consequences. In the medium term, the increase in extreme events and the rise in sea level could have catastrophic results for the stability of societies in the South, whereas with the extension of precarious districts and coastal towns, present urban dynamics increase the risks for populations.

3The research sector must therefore gain better understanding of vulnerability and the impacts of climate change in cities in order to attempt to propose solutions adapted to increasing urban concentration. Such urban research is still little developed in countries in the tropics. IRD has long addressed similar themes such as population-environment relations, natural risks and public policies in cities and the trend today is that of refocusing these themes in a context of climate crisis.

An increasingly urban and coastal world

4Nearly 4 billion people, that is to say 58 % of the population of the world, now live in urban environments. This massive urbanisation is relatively recent dynamics as the urban population has increased nearly 5-fold since 1950. Urban growth has displayed two new features since the 2000s. First, it is accompanied by concentration in increasing large towns in the South (Fig. 27). In 1975, there were 18 megalopolises with populations of over 5 million, the whole totalling 170 million people. In 2014, there were 73 megalopolises with a total of 800 million people. The main megalopolises are on the American and Asian continents, mainly in emerging countries (Brazil, India and China). But many other cities in the tropics have become megalopolises at the regional level, such as Lima with 30 % of the population of Peru and Lagos with three-quarters of the population of Nigeria.

Figure 27. Evolution of large cities. Exposure to 10 μm particles in the atmosphere in urban zones (2008-2013).

Lima, Peru.

5Another major feature of this urban growth is the greater development of coastal cities. According to IPCC, nearly 145 million people live at an elevation of 1 metre above sea level and 397 million at less than 10 metres. The trend will become more marked in the coming decades because of the intensification of world sea trade.

Increased vulnerability in the face of natural risks

6In spite of the lack of observation systems focused on the urban climate in countries in the South, impacts of warming, increased climate variability and extreme events and the rise in sea levels are observed. The IPCC Fifth Assessment Report lays particular stress on two phenomena: landslides and the rise in sea level.

Marine submersion

7More than half of the cities in the tropics are on the coast—among other things a heritage of the trading centres of the old colonies. And current urban dynamics continue to encourage the growth of these coastal cities. Increasingly large urban populations are thus exposed to the rise in sea level and, more precisely, to erosion and marine submersion. Among others, this is the case of Nouakchott, Lagos, Lomé, Dhaka, Ho Chi Minh City and Rio de Janeiro. The rise in sea level of several tens of centimetres in the coming decades will accentuate these phenomena, with the destruction of housing and infrastructure and movements of populations. Marine submersion will also cause a loss of coastal resources that are important for local economies.

Landslide in February 2011 in the city of La Paz (Bolivia).

Floods and landslides

8Through more violent precipitation or serious drought, climate variability will also affect cities further inland. The increase in extreme rainfall events heightens the flood threat that is already accentuated by the imperviousness resulting from town development. Strong precipitations also increase landslide risks. The consequences of these phenomena are often amplified by the vulnerability of urban environments. Human activities over which there is little control, urban concentration in sometimes dangerous zones and precarious dwellings are all factors that can turn climate risks into urban catastrophes. The danger is particularly strong in highland cities. For example, most cities in the Andes are exposed to these phenomena that have been increasing steadily in recent decades.

9The change in the rainfall regime also results in more floods, exposing towns near rivers to risk. The historic floods of the Amazon and its tributaries in 2009, 2012 and 2014, linked with exceptional precipitation and deforestation, affected several hundred thousand people. A state of emergency was declared in several regions of Peru, Brazil and Bolivia and numerous towns were flooded.

Avenue Patria Quito, Ecuador

Makeshift accommodation in an outlying district of Cairo (Egypt).

Greater Bangkok has a population of more than 14 million, forming over 20 % of the population of Thailand.

The direct sanitary effects of emissions

10The effects of climate warming must not hide the direct effects of urban emissions on air pollution and the warming of urban climates. These local effects are very marked today in cities in the South (Fig. 28). Mexico City, Sao Paulo, Cairo, Karachi, Dakar, Bamako, etc. seriously exceed the atmospheric pollutions thresholds accepted by the World Health Organization (WHO) in particular because of ageing public transport and vehicles and the absence of regulation of domestic use of wood or gasoil. The particle count in New Delhi is 153 μg/m³, that is to say 15 times more than the WHO threshold and 10 times more than in Paris.

11Mastering urban heat islands (UHI) is also a challenge in the face of climate change. Increased temperature operates differently in town and in the country. In town, the albedo effects on the surfaces of buildings and tarmac roads combined with heat emissions from transport, building heating or air-conditioning together with high pollution levels can cause local temperature increases of 4 to 6°C. The effects are felt especially at night when the energy stored in concrete walls is released. The phenomenon is particularly marked in Mediterranean and tropical environments where air-conditioning is used continuously.

12The combined impacts of thermal stress, heat islands and air pollution form a new health risk, with increased respiratory disease and vulnerability in children and old people (dehydration and cardio-vascular diseases). These risks are particularly marked in zones where there is already thermal stress, such as the Sahel, and in the Mediterranean zone.

Figure 28. Map of air pollution in large cities.

Access to resources endangered

13Another impact of climate change in urban zones is the availability of resources to supply an entire city. Access to water is a particularly crucial issue with the threat of extreme climate events. This covers shortage of water during severe droughts, the pollution of groundwater following heavy rainfall (because of contamination resulting from poor treatment of sewage) or the destruction of infrastructure during natural catastrophes. Broken pipes can soon affect an entire city. After the La Paz landslide in 2011, the potable water supply for about 300,000 people was halted for several weeks. This example highlights the fragility that is a feature of centralised urban facilities (Box 44).

14Another phenomenon related to climate change and that forms a threat to water supply is the retreat of glaciers. The melting of ice during dry periods is an important source of water. In arid regions, such as those in Peru and Bolivia, the contribution of glaciers to irrigation, hydroelectricity and water supply for the local population can be very significant. Although the melting of glaciers caused by warming is currently increasing the discharge of watercourses, the disappearance of the glaciers is a threat to this resource in the medium term.

Distribution of food in a school used temporarily as a refuge for local victims in La Paz (Bolivia).

A favela in Rio de Janeiro, Brazil.

15More generally, the availability of resources raises the question of the population-environment relation in megalopolises. The context of demographic change (rural exodus) and change in agriculture (the replacement of family farming by commercial farming) is causing the reorganisation of the supplying of food but this affects the food security of city-dwellers when the distribution networks are down.

Modelling cities

16Most of the effects of climate change in urban areas remain to be discovered. Several operations in which IRD is participating are beginning, aimed at observing and understanding changes in the urban environment under the effect of pollution and climate. For example, the operation for monitoring urban environments in Marseilles and Algiers set up by IRD is installing experimental set-ups for the monitoring of fauna and flora in urban environments. This is aimed at evaluating warming (especially in urban heat islands) according to urban structure (height, density, layout of urban components) and explaining their impacts on urban biodiversity.

17One of the future challenges is that of modelling urban microclimates and their impacts according to urban structure and the ways of life of the population.