City shapes and climate change in Africa

CITY SHAPES & CLIMATE CHANGE IN AFRICA

Africa’s cities are the most rapidly growing in the world. In 2020, 685 million people lived in urban areas. By 2050, urban dwellers are expected to reach 1.5 billion. More people will need more buildings, homes, schools, hospitals, markets and other services. Where and how these buildings and amenities are constructed matters, since these decisions will last for decades. Yet, African cities develop spontaneously with limited urban planning, resulting in a proliferation of precarious neighbourhoods, inadequate transport systems, congestion, rising outdoor air pollution and high living costs.

Africa's cities are especially vulnerable to the effects of climate change. Flooding, heatwaves, landslides and extreme climate events mean that building resilient and sustainable urban spaces, and preserving ecosystems and biodiversity is critical.

INDICATORS MEASURING URBAN COMPACTNESS

SWAC/OECD developed two indicators to compare the shape of Africa’s cities, by combining the Google AI Africa Open Buildings dataset and the urban agglomeration data from Africapolis. Using the co-ordinates and surfaces of 183 million buildings constructed in nearly 6 000 African cities, the indicators measure:

DIVERSITY OF AFRICAN URBAN FORMS

The built environment across cities varies according to location within a country as well as city size. Two trends have emerged. First, very large cities (above 4 million inhabitants) are more compact than large and intermediate ones. Second, sprawl and elongation increase with the city size up to 4 million people. This is because very large cities fill in the space between buildings and start to become more circular, reducing distances.

ELONGATED AND SPRAWLED CITIES

FOR CITIES TO TAKE FULL ADVANTAGE OF THE OPPORTUNITIES OFFERED BY THEIR EXPANSION, THEY REQUIRE:

Better, more accessible data on urban forms

Analyses of the constraints and levers to sustainable urban development, a low-carbon future and stronger environmental and ecosystem services.

The most compact city has a round shape with short distances between buildings. The least compact has greater distances between buildings and more elongation.

IMPLICATIONS FOR LESS COMPACT CITIES

While many cities across the world grow upwards, becoming denser over time, African cities tend to grow outwards. This results in sprawl, which means that people travel longer distances to get to their destinations. Longer distances discourage active mobility such as walking and cycling, increase the cost of public transport, lead to higher energy demands and emissions of private vehicles as well as congestion. Longer distances also mean reduced access to services and opportunities. This presents the risk of locking in unsustainable transport modes such as private vehicles instead of low-carbon modes (walking, cycling or public transport).

As a city’s population doubles, the energy demand for transport is expected to triple. This is because the average distance between buildings is predicted to increase by 45% and more people will have to travel longer distances.

Club SAHEL AND WEST AFRICA Secretariat

THE SPACE BETWEEN BUILDINGS OR THE LEVEL OF “EMPTY” SPACE

THE EXTENT TO WHICH THE SHAPE OF THE CITY DEPARTS FROM BEING A CIRCLE

COMPACT CITIES AND TRADE-OFFS

Compact cities aim to bring all the elements for living and working into local neighbourhood communities. They can improve accessibility to services through greater connectivity, more walkable urban landscapes, lower car dependency and reduced emissions and pollution.

While compact cities can become more sustainable and liveable, they also come with their own challenges, such as a loss of green space. As cities become larger and more compact, green spaces are depleted in favour of residential, commercial or industrial land areas. This exacerbates cities’ vulnerability to climate change, diminishes resilience to floods, heatwaves and landslides, and reduces other ecosystem services provided by these spaces, such as carbon absorption and biodiversity conservation.

At the same time, urban agglomerations are at risk of the urban heat island (UHI) effect. The UHI effect describes an urban area where the temperature is considerably warmer than the surrounding suburban or rural areas. Cities have greater concentrations of paved roads, sidewalks, building façades and other surfaces that absorb and retain heat. This leads to higher daytime temperatures but also reduces nighttime cooling. This increased heat takes a toll on urban residents and may result in heat-related illnesses and mortality. Compactness can exacerbate UHI effects.

GREEN SPACES

Patches of trees can help limit the UHI effects and create a cooling effect of up to 2°C. However, the availability of green space is not enough; proximity matters. People need to live close to these green spaces to benefit from their cooling effects.

The availability of green spaces is similar in Accra and Abuja (20%), but accessibility stands at 16% and 55% respectively. Green spaces with trees are much more dispersed in Abuja than in Accra.

DATA AND POLICY TOOLS

SWAC/OECD provides data and tools to help policy makers better plan and guide more sustainable and resilient urban development. These data and analyses need to be complemented by further research into the urban informalities that make up the African urban landscape, their challenges, potential and opportunities. Integrating urban informalities, often precarious constructions, as innovative, resilient and dynamic social and economic spaces into discussions is necessary for the sustainable and inclusive development of African cities.