According to the UN's 2018 Revision of World Urbanization Prospects, what share of the world's population is projected to live in urban areas by 2050?
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Environmental Sustainability · Foundations
Sustainable Cities
On this page 9 sections
In 30 seconds
Most people now live in cities, and the share keeps climbing. A sustainable city meets residents' needs while shrinking its environmental footprint. The key lever is Urban form The physical layout of a city: its density, street pattern, block size, and how land uses are arranged. Full entry →: compact, mixed-use, walkable, transit-served development that uses less land and energy than Sprawl Low-density, single-use development spread across large areas, typically dependent on cars for most trips. Full entry →. Cities add Green infrastructure Vegetated and permeable systems (rain gardens, permeable pavement, green roofs, bioswales, tree canopy) that manage rainwater where it falls. Full entry → to manage stormwater and cool streets, fight the Urban heat island effect The tendency of built-up areas to be warmer than surrounding rural areas because they replace vegetation with heat-absorbing surfaces. Full entry →, and follow smart-growth ideas. UN Sustainable Development Goal 11 The UN goal to make cities and human settlements inclusive, safe, resilient, and sustainable, part of the 2030 Agenda adopted in 2015. Full entry → sets the global target.
Why this matters
By 2050 roughly two of every three people will live in cities, so how cities are built largely decides whether humanity's footprint grows or shrinks. Urban form shapes transportation, energy use, flooding, heat exposure, and land consumption for decades, because streets and buildings last a long time. Students in planning, environmental science, public health, engineering, and policy use these ideas to read development proposals, weigh density against sprawl, and evaluate climate plans. Understanding sustainable cities also clarifies a common misconception: dense urban living, done well, can lower per-capita emissions rather than raise them, which reframes how we think about growth.
The college version
An urban planet
Humanity crossed a threshold in the last two decades: more than half of us now live in cities. The UN Department of Economic and Social Affairs, in its 2018 Revision of World Urbanization The rising share of a population that lives in cities and towns rather than rural areas. Full entry → Prospects, put the urban share at about 55% in 2018 and projected it to reach 68% by 2050. Urbanization combined with overall population growth could add roughly 2.5 billion people to urban areas by 2050, with close to 90% of that increase occurring in Asia and Africa; India, China, and Nigeria alone are expected to account for about 35% of the growth. This matters for sustainability because cities concentrate energy use, construction, waste, and emissions in a small footprint. The scale of building still to come means the choices made now about where and how cities grow will lock in environmental outcomes for generations. A sustainable city is one that meets residents' needs for housing, mobility, and services while keeping its demands on land, water, energy, and the atmosphere within limits that can last.
Why urban form is the main lever
The single most important sustainability variable a city controls is its physical form. Compact development places homes, jobs, shops, and services close together at moderate to high density. Mixed-use zoning lets those functions sit side by side rather than in separate zones connected only by car. Transit-oriented development Concentrating housing and jobs at higher density around public-transit stations so frequent transit is viable. Full entry → concentrates that density around stations so frequent transit becomes practical, and walkability means everyday trips can be made on foot. The opposite pattern, sprawl, spreads low-density single-use development across the landscape, forcing long car trips, paving more land, and raising the cost of every pipe and road per household. The IPCC's Sixth Assessment Report (AR6, Working Group III, 2022) found that compact, walkable urban form is strongly correlated with low greenhouse-gas emissions, because co-locating people and destinations cuts transport demand and denser buildings lose less heat. This is why dense cities can have lower per-capita footprints than suburbs, even though their absolute impact is large: the average resident drives less and lives in more energy-efficient housing. AR6 describes avoiding sprawl as a necessary condition for decarbonizing settlements.
Green infrastructure and the heat island
Conventional cities are largely impervious: roofs, roads, and parking lots that shed rainwater into pipes and drains, known as Gray infrastructure Conventional engineered drainage of pipes, gutters, and storm drains that channels stormwater away from where it lands. Full entry →. This produces polluted runoff, downstream flooding, and few cooling surfaces. Green infrastructure takes the opposite approach, managing rainwater where it falls with vegetated and permeable features. The EPA's examples include rain gardens and bioretention basins, permeable pavement, bioswales, green roofs, and expanded urban tree canopy. These slow, absorb, and filter stormwater while adding shade and greenery. Green infrastructure also helps with a distinct problem: the urban heat island effect. Because cities replace vegetation with heat-absorbing materials and trap heat in 'urban canyons' between buildings, EPA reports that U.S. urban areas run roughly 1-7 degrees F warmer than surrounding areas during the day and about 2-5 degrees F warmer at night; dark conventional roofs can reach as much as 66 degrees F above the surrounding air. Trees, green and cool roofs, and cool, permeable pavements are the standard mitigations. Heat islands are not evenly distributed; neighborhoods with less tree cover, often lower-income, bear more heat, which links urban design to equity.
Smart growth and the global goal
To turn these ideas into practice, the EPA promotes Smart growth An EPA-promoted approach of development and conservation strategies emphasizing compact, mixed-use, walkable, well-served communities. Full entry →, defined as a set of development and conservation strategies that protect health and the environment while making communities more attractive and economically resilient. Its ten principles, developed with the Smart Growth Network, include mixing land uses, taking advantage of compact building design, creating a range of housing choices, building walkable neighborhoods, fostering a strong sense of place, preserving open space and farmland, directing development toward existing communities, providing varied transportation choices, making development decisions predictable and fair, and encouraging community collaboration. At the global scale, the same agenda is captured in UN Sustainable Development Goal 11, adopted in 2015 as part of the 2030 Agenda: 'make cities and human settlements inclusive, safe, resilient and sustainable.' Its targets call for adequate and affordable housing (11.1), sustainable transport systems (11.2), reduced per-capita environmental impact including air quality and waste (11.6), and universal access to safe, green public spaces (11.7). Together, smart growth and SDG 11 show that sustainability at the city scale is less about any single technology than about the pattern in which people, buildings, and streets are arranged.
Eli explains
The same idea, in plain words
Explain it like I’m 10
Cities are where most people live now, and that keeps growing. A city uses a lot of energy, water, and land, so how we build it really matters. If homes, shops, schools, and jobs are close together, people can walk, bike, or take a bus instead of driving everywhere, which saves energy and land. Cities also get hot because streets and rooftops soak up sunshine, so planners add trees, parks, and special surfaces that soak up rain and keep things cooler. Building cities this smart way is the goal, and the UN even made it one of its official world goals.
Picture it like this
Think of a city like a backpack for school. If you pack it smart, with the heavy books flat against your back and everything in the right pocket, it is easy to carry and nothing gets crushed. If you just toss everything in loosely, it is bulky, heavy, and hard to walk with. A compact, well-organized city is the smartly packed backpack: things are close and easy to reach, so you waste less energy getting around.
Where the picture stops working
The backpack analogy is about arranging fixed items, but a real city keeps growing and changing, and it has living systems, like trees, rivers, and millions of people making their own choices, that a backpack does not. Packing a backpack is a one-time act; keeping a city sustainable is continuous.
Worked example
A mid-sized city must house 40,000 new residents. Plan A zones farmland at the edge for single-family homes on large lots, reachable only by car. Plan B rezones underused land near an existing rail line for mid-rise apartments with ground-floor shops. Compare them against what this lesson taught. Plan A is sprawl: it paves open space, adds impervious surface that worsens runoff and the heat island, and locks in long car commutes, raising per-capita emissions. Plan B is transit-oriented, mixed-use, compact development: residents can walk to shops and ride rail, buildings share walls and lose less heat, and the city can add green roofs and street trees to manage stormwater and cool the blocks. Plan B aligns with smart growth principles and SDG 11 targets on housing, transport, and environmental impact, and its residents are likely to have lower per-capita footprints even though the neighborhood is denser.
Key takeaway
A sustainable city is defined less by any single technology than by its form: compact, mixed-use, walkable, transit-served development, supported by green infrastructure and heat-island mitigation, lets a growing urban population live with a smaller per-capita footprint, the goal expressed globally in UN SDG 11.
Quick check
3 questions here, of 5 in this lesson’s practice set. Answers stay hidden until you check.
Which feature best distinguishes green infrastructure from gray infrastructure for managing stormwater?
Choose an answer, then check it.
Per the EPA, which is the primary driver of the urban heat island effect?
Choose an answer, then check it.
Study tools & related lessonsYou’ll learn to · Common mistakes · Easily confused · Key vocabulary · Related
You’ll learn to
- Describe the global urbanization trend using dated UN figures.
- Explain why compact, mixed-use, transit-oriented urban form is more sustainable than sprawl.
- Distinguish green infrastructure from gray infrastructure for stormwater management.
- Explain the urban heat island effect and identify mitigation strategies.
- Summarize EPA smart growth principles and the aim of UN Sustainable Development Goal 11.
- Evaluate the claim that dense cities can have lower per-capita footprints.
Common mistakes
Assuming denser cities are automatically worse for the environment.
Absolute impact is large because many people are concentrated, but per-capita emissions are typically lower in compact cities, since residents drive less and live in more efficient buildings (IPCC AR6 WGIII, 2022).
Treating 'green infrastructure' as just adding parks for looks.
Green infrastructure is functional stormwater management: rain gardens, permeable pavement, bioswales, and green roofs absorb and filter rainwater where it falls, doing work that gray pipes do differently.
Thinking the urban heat island comes mainly from waste heat like air conditioners.
The larger drivers are loss of vegetation and heat-absorbing impervious surfaces and urban geometry; waste heat contributes but is not the primary cause per EPA.
Confusing smart growth with simply stopping all development.
Smart growth directs and shapes development toward compact, mixed-use, walkable patterns in existing communities; it is a way to grow, not a ban on growth.
Believing sustainable cities are only about new technology or gadgets.
The main lever is urban form, the arrangement of density, land uses, and streets, which determines transport and energy demand more than any single device.
Easily confused
Compact, mixed-use city vs. Sprawl
Compact form co-locates homes, jobs, and services so trips are short and often walkable or transit-served; sprawl separates uses at low density and forces long car trips, paving more land.
Green infrastructure vs. Gray infrastructure
Green infrastructure absorbs and filters stormwater where it falls using vegetation and permeable surfaces; gray infrastructure uses pipes and drains to move runoff away.
Absolute urban footprint vs. Per-capita urban footprint
A big city's total emissions are large because of its population, yet each resident's emissions can be lower than a suburban resident's because of density and transit.
Key vocabulary
- Urbanization
- The rising share of a population that lives in cities and towns rather than rural areas.
- Urban form
- The physical layout of a city: its density, street pattern, block size, and how land uses are arranged.
- Sprawl
- Low-density, single-use development spread across large areas, typically dependent on cars for most trips.
- Transit-oriented development
- Concentrating housing and jobs at higher density around public-transit stations so frequent transit is viable.
- Mixed-use development
- Placing residential, commercial, and other uses close together or in the same buildings rather than in separate zones.
- Green infrastructure
- Vegetated and permeable systems (rain gardens, permeable pavement, green roofs, bioswales, tree canopy) that manage rainwater where it falls.
- Gray infrastructure
- Conventional engineered drainage of pipes, gutters, and storm drains that channels stormwater away from where it lands.
- Urban heat island effect
- The tendency of built-up areas to be warmer than surrounding rural areas because they replace vegetation with heat-absorbing surfaces.
- Smart growth
- An EPA-promoted approach of development and conservation strategies emphasizing compact, mixed-use, walkable, well-served communities.
- Sustainable Development Goal 11
- The UN goal to make cities and human settlements inclusive, safe, resilient, and sustainable, part of the 2030 Agenda adopted in 2015.
Sources & references
- 68% of the world population projected to live in urban areas by 2050 (2018 Revision of World Urbanization Prospects) — United Nations Department of Economic and Social Affairs (UN DESA), Population Division
- Goal 11: Make cities and human settlements inclusive, safe, resilient and sustainable — United Nations, Department of Economic and Social Affairs (Sustainable Development Goals)
- Learn About Heat Islands — U.S. Environmental Protection Agency (EPA)
- About Smart Growth — U.S. Environmental Protection Agency (EPA)
- Types of Green Infrastructure — U.S. Environmental Protection Agency (EPA)
- IPCC AR6 Working Group III, Chapter 8: Urban Systems and Other Settlements — Intergovernmental Panel on Climate Change (IPCC)
EliExplains lessons are original prose written from the open, credible references above. See Copyright & Licensing.
Researched 2026-08-19
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