Urban Sustainability and Megacities

🏗️ What is a Megacity?

A megacity is a city with a population of more than 10 million people. The term was first widely used in the 1970s, when planners began to recognise that cities of this scale created challenges that smaller urban areas simply did not face. A city of 500,000 needs a bus network and a water supply. A city of 22 million needs all of that, multiplied many times over — and it needs it to work simultaneously, every day, for people spread across hundreds of square kilometres.

The geography of megacities has changed dramatically in the last 70 years. In 1950, megacities were a feature of the wealthiest nations: New York and Tokyo. By 2024, the list looks very different:

The critical geographical pattern: approximately 90% of megacity growth is now happening in LICs (Low Income Countries) and NEEs (Newly Emerging Economies) in Africa and Asia. Cities like Lagos, Kinshasa, and Karachi are projected to enter the megacity category within the next decade. This matters because growth in a wealthy country like Japan looks very different from growth in Bangladesh — the infrastructure, governance, and resources available to manage rapid urban expansion differ enormously.

The Speed of Change

Numbers alone do not convey how fast this transformation is happening. Delhi adds roughly one million people per year. Lagos has grown from 290,000 people in 1950 to over 15 million today — a fiftyfold increase in 70 years. No city in human history ever grew this fast, for this long, across this many different countries simultaneously. The infrastructure, planning, and governance systems that megacities need take decades to build. The populations arrive far faster than those systems can be created.

🔍 Why Do Megacities Grow? The Push-Pull Model

Megacities do not grow simply because people are born in them. They grow primarily because people migrate to them from rural areas — a process called rural-urban migration. To understand why, geographers use the push-pull model: a framework that distinguishes between factors that drive people away from the countryside (push factors) and factors that attract them towards the city (pull factors).

Push and pull factors always operate together. A farmer whose harvest has failed (a push factor) will not necessarily move to the city unless she believes the city offers something better (a pull factor). The decision to migrate is rational — people weigh the costs and benefits, gather information from family members who have already moved, and make a calculated choice about where their livelihood is most secure.

🇮🇳 Case Study 1: Mumbai, India (NEE)

Mumbai is India's financial capital and its most economically productive city. With a metropolitan population of approximately 21.4 million people in 2024, it contributes around 6.6% of India's entire GDP — more than the whole of many Indian states. It is home to the Bombay Stock Exchange, the headquarters of most of India's major banks and corporations, and Bollywood, the world's most prolific film industry by output, producing over 1,000 films per year.

Understanding Mumbai requires holding two realities simultaneously. Mumbai is a place of extraordinary wealth, ambition, and cultural vitality. It is also a place where one of the densest human settlements on Earth has grown in conditions of severe poverty, inadequate sanitation, and chronic flood risk — and where those two realities coexist within a few hundred metres of each other.

Dharavi: The Slum That Has a £1 Billion Economy

The name most associated with Mumbai's inequality is Dharavi. Covering approximately 2.4 km² — roughly the size of 340 football pitches — Dharavi houses an estimated one million people. By population density, it is one of the most crowded places on the planet. Western media has frequently portrayed it simply as a place of suffering, made famous internationally by the 2008 film Slumdog Millionaire. The reality is considerably more complex.

• Dharavi has an estimated annual economy worth over £1 billion, generated by approximately 15,000 single-room factories producing leather goods, pottery, textiles, and recycled plastic
• The recycling industry alone processes 80% of Mumbai's plastic waste, running an operation that formal businesses cannot match for efficiency or cost
• Approximately 85% of Dharavi's residents have lived there for more than 10 years — Dharavi is not a transit camp for recent migrants but an established community with deep social roots, schools, temples, mosques, and local governance structures
• Average household income in Dharavi is significantly higher than the Indian rural average — residents have chosen the city because it offers more, not because they had no alternative
• However: housing typically consists of single rooms of 10–15 m² shared by entire families; access to clean water is limited to a few hours per day; toilet facilities are shared between many households; alleyways between structures are too narrow for emergency vehicles

Dharavi demonstrates a central tension in development geography: informal settlements are not simply problems to be solved. They are communities with economies, social networks, and cultural identities. Approaches that simply demolish them to build formal housing often destroy the social capital that made those communities function — and frequently price out the very residents they were meant to help.

Mumbai's Urban Challenges

• Traffic congestion: Mumbai's average vehicle speed is approximately 10 km/h — slower than a brisk walking pace. The city has approximately 3 million registered vehicles on a road network designed for a fraction of that number. Annual productivity loss from congestion is estimated at $1 billion.
• Solid waste: Mumbai generates approximately 5,000 tonnes of solid waste per day. The Deonar landfill, one of Asia's largest, regularly catches fire, generating toxic smoke that blankets nearby communities. Only a fraction of waste is formally recycled — the rest goes to landfill or is burned informally.
• Air pollution: PM2.5 particulate levels in Mumbai regularly exceed 5× the WHO safe limit. Air pollution causes an estimated 16,000 premature deaths in Mumbai each year.
• Flooding: Mumbai's location on a narrow peninsula, combined with inadequate drainage infrastructure and annual monsoon rains, makes flooding a perennial crisis. In July 2005, 944 mm of rain fell in 24 hours — the highest single-day rainfall ever recorded in India. Over 1,000 people were killed, hundreds of thousands of homes were flooded, and the city was paralysed for days. Dharavi's ground-floor homes were among the worst affected.
• Housing shortage: Approximately 55% of Mumbai's population lives in informal settlements, despite the city's economic importance. The gap between Mumbai's GDP contribution and its ability to house its own workforce is one of the starkest expressions of urban inequality in the world.

Mumbai's Responses

• Mumbai Metro: Metro Line 3 — a fully underground, air-conditioned rapid transit line running 33.5 km through the densest parts of the city — opened in stages from 2023 to 2024. The project cost approximately ₹33,000 crore (around £3 billion). It is projected to carry 1.7 million passengers per day, removing hundreds of thousands of car and bus journeys from the city's surface roads.
• Slum Rehabilitation Authority (SRA): A government body that negotiates with developers to redevelop slum land. Developers receive permission to build commercial towers; in exchange, they must provide free replacement housing for slum residents on the same site. The scheme has rehoused tens of thousands of families, but has also been criticised for demolishing communities before replacement housing is complete, and for building tower blocks that replicate the social isolation of formal housing without the community networks of informal settlements.
• Storm drainage investment: Following the 2005 flood disaster, Mumbai invested heavily in upgrading the Mithi river channel and constructing new storm water drains capable of handling 50mm of rain per hour (previously only 25mm per hour). However, climate change means extreme rainfall events are becoming more frequent, and the upgraded infrastructure may already be insufficient for the storms of the 2030s and 2040s.

🇧🇩 Case Study 2: Dhaka, Bangladesh (LIC)

Dhaka is one of the most densely populated cities on Earth and one of the fastest growing. With a population of approximately 22.5 million in 2024 and a growth rate of around 300,000 new residents per year, Dhaka faces every urban challenge in its most extreme form. It sits in a nation that is simultaneously one of the world's most climatically vulnerable countries — 70% of Bangladesh is less than 6 metres above sea level, and the country sits on the confluence of three major rivers: the Ganges, the Brahmaputra, and the Meghna.

Bangladesh is classified as a Low Income Country (LIC), with a GNI per capita of approximately $2,800. This means the resources available to manage Dhaka's explosive growth are a fraction of what Mumbai can draw on — and Mumbai's resources are already severely strained.

The Garment Industry: Bangladesh's Economic Engine

Dhaka's growth is inseparable from Bangladesh's garment industry. Bangladesh is the world's second largest clothing exporter (after China), generating approximately $47 billion in annual export revenue — over 80% of the country's total export earnings. The industry employs roughly 4 million workers, of whom approximately 80% are women. For millions of young women from rural Bangladesh, factory work in Dhaka represents economic independence, a wage, and an escape from early marriage — life choices that farming could not offer.

The dark side of that dependency was exposed catastrophically on 24 April 2013. The Rana Plaza building in Savar, on the outskirts of Dhaka, was an eight-storey commercial building containing five garment factories producing clothes for international brands. Cracks had been spotted in the building the day before, and the banks and shops on the ground floor had closed. The garment workers on the upper floors were told to return or lose their pay. The building collapsed during the morning shift, killing 1,134 people — the deadliest garment industry accident in history. It triggered an international reckoning with the working conditions behind fast fashion, and led to the Accord on Fire and Building Safety, a legally binding agreement between brands and unions that has since improved conditions in thousands of Bangladeshi factories.

Dhaka's Urban Challenges

• Informal settlements (bastis): Approximately 40% of Dhaka's population lives in informal settlements known locally as bastis. These areas typically lack piped water, sewage systems, formal waste collection, and legal land tenure. The largest basti, Korail, houses over 100,000 people in an area less than 1 km².
• Extreme flood risk: Dhaka sits at the meeting point of three major rivers and the Bengal delta. Approximately 70% of the city is less than 6 metres above sea level. Each monsoon season brings flooding that inundates low-lying bastis for weeks; streets become rivers; waterborne disease increases sharply. Climate change is making these floods more severe and less predictable.
• Traffic congestion: Dhaka's average vehicle speed is approximately 7 km/h — widely cited as the worst congestion of any megacity in the world. The city has a road network covering just 7% of its area (the global standard for major cities is 25%). Congestion costs Bangladesh an estimated 3% of GDP annually in lost productivity.
• Water and sanitation: Safe drinking water is unavailable to large parts of the population, particularly basti residents. Arsenic contamination of groundwater is a significant health risk in Bangladesh — the WHO has described it as the largest mass poisoning in history, affecting an estimated 30–35 million people across the country.
• Solid waste: Dhaka generates approximately 35,000 tonnes of solid waste per day. There is no formal waste collection for most basti residents. Waste is dumped in rivers and waterways, contributing to flooding by blocking drainage channels and to waterborne disease by contaminating water sources.
• Unemployment and the informal economy: Many migrants from rural Bangladesh cannot find formal employment. They enter the informal economy as rickshaw drivers, street vendors, domestic workers, or day labourers. The informal economy provides income but without legal protections, minimum wages, or sick pay — making workers entirely vulnerable to economic shocks.

Dhaka's Responses

• ADB-funded flood embankments: The Asian Development Bank (ADB) has funded the construction of embankments around flood-vulnerable parts of Dhaka, along with drainage pump stations capable of removing floodwater from the city during monsoon season. These are top-down, infrastructure-heavy solutions that protect formal areas of the city but often do not extend to informal settlements.
• BRAC (Building Resources Across Communities): BRAC is the world's largest NGO, founded in Bangladesh. It operates an extraordinary range of bottom-up programmes: microfinance loans to allow basti women to start small businesses; schools in informal settlements; health clinics with trained community health workers; and sanitation facilities. BRAC has reached over 100 million people across Bangladesh, demonstrating that community-level intervention can achieve at scale what government infrastructure programmes cannot.
• Bus Rapid Transit (BRT): Bangladesh is planning a BRT network for Dhaka — dedicated lanes for express buses that can move large numbers of passengers more efficiently than private cars. The first corridor is under construction, though delays due to land acquisition and funding challenges have pushed back the timeline repeatedly.
• Dutch water management collaboration: The Netherlands — a country that has managed flood risk for centuries — has worked with Bangladesh on advanced flood management techniques, including floating homes, permeable paving, and water retention basins designed to absorb monsoon rainfall before it overwhelms drainage systems.