City

• 1. Introduction

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  A city covers, on average, a sliver of the planet. Cities are estimated to occupy about 3% of the land surface of the Earth. Yet after two centuries of rapid urbanization, more than half of the world population now lives on that sliver. The Population Division of the United Nations Department of Economic and Social Affairs marked the crossing point in 2014, reporting that for the first time, more than half of humanity lived in cities.

  The word itself carries an argument. "City" means different things around the world, and in some places the settlement it describes can be very small. There is no universally agreed definition of the lower boundary for a city's size. So what actually makes a place a city, rather than a village that happens to be crowded? Why did the first ones appear at all? And why does the international community now treat them as central to questions of climate, health, and survival? The answers run from a Mesopotamian settlement of the mid-fourth millennium BC to gated communities and master-planned towns built from scratch on greenfield sites.

• 2. What Makes A City

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  Common population thresholds for an urban area range between minimum values of 1,500 and 50,000 people. Most U.S. states use a minimum between 1,500 and 5,000 inhabitants, while some jurisdictions set no such criteria at all. National censuses reach for different yardsticks entirely: population density, number of dwellings, economic function, and infrastructure.

  The "functional definition" abandons headcount as the deciding factor. A city, in this view, is distinguished by the role it plays within a larger political context, serving as an administrative, commercial, religious, and cultural hub for its surrounding area. A typical city has professional administrators, regulations, and some form of taxation to support government workers. This vertical arrangement contrasts with the horizontal relationships of a tribe or village, where neighbors accomplish common goals through informal agreements or the leadership of a chief.

  A modern metric tries to settle the matter with numbers again: the degree of urbanization, defined as a population of at least 50,000 inhabitants in contiguous dense grid cells holding more than 1,500 inhabitants per square kilometer. This standard was devised over years by the European Commission, the OECD, the World Bank, and others, then endorsed by the United Nations in March 2021, largely for the purpose of international statistical comparison.

  The definitions fray most at the edges. In the United Kingdom, city status is awarded by the Crown and then remains permanent. The lack of official qualifying criteria has produced some very small cities, notably St Davids, with a population of 1,751. The word's deeper roots run through the Latin civitas, meaning citizenship or community, which eventually came to correspond with urbs, the city as physical streets and buildings.

• 3. The First Cities

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  Jericho and Çatalhöyük, dated to the eighth millennium BC, rank among the earliest proto-cities known to archaeologists. Most archaeologists, however, reserve the title of first true city for Uruk, in ancient Iraq, from the mid-fourth millennium BC. Uruk innovated many of the characteristics that later cities would inherit, and lent its name to the Uruk period.

  Mohenjo-daro, in the Indus Valley civilization of present-day Pakistan, existed from about 2600 BC and held a population of 50,000 or more, served by a sophisticated sanitation system. The conventional view holds that the city followed the development of agriculture, which produced surplus food and thus a social division of labor and trade. Early cities often featured granaries, sometimes within a temple. A minority viewpoint argues cities may have arisen without agriculture, through fishing, through use as communal seasonal shelters, or through their value as bases for military organization.

  In the ancient Americas, the Norte Chico civilization of north-central coastal Peru is the oldest known in the hemisphere, flourishing between the 30th and 18th centuries BC and including as many as 30 major population centers. West Africa offers a striking counterexample to Western assumptions about what a city must be. Jenné-Jeno, in what is today Mali, dates to the third century BCE. According to Roderick and Susan McIntosh, it lacked monumental architecture and a distinctive elite class, arranging its settlement mounds according to a horizontal rather than vertical power hierarchy. Glass beads found there, dating from the third century BCE to the fourth century CE, point to pre-Arabic trade contacts between Jenné-Jeno and North Africa.

• 4. How Cities Grow

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  Access to water has long governed where cities take root, and through the present, most of the world's urban population lives near the coast or on a river. An urban area cannot grow its own food, so it must develop a relationship with a hinterland that sustains it. Only in special cases, such as mining towns vital to long-distance trade, are cities cut off from the countryside that feeds them.

  The shapes cities take fall into recognizable patterns: geomorphic, radial, concentric, rectilinear, and curvilinear. Dutch cities such as Amsterdam and Haarlem grew as a central square surrounded by concentric canals, each ring marking another expansion; in Moscow the same pattern remains clearly visible. The grid plan has been used for millennia across Asia, Europe, and the Americas. The Indus Valley Civilization built Mohenjo-Daro and Harappa on a grid, following ancient principles described by Kautilya and aligned with the compass points.

  Urbanization, the migration from rural to urban areas, broke an old equilibrium. Until the 18th century, a balance held between the rural agricultural population and towns with markets and small-scale manufacturing. In England, the share of the population living in cities jumped from 17% in 1801 to 72% in 1891. In 1900, just 15% of the world's population lived in cities.

  That growth has not slowed. Batam in Indonesia, Mogadishu in Somalia, Xiamen in China, and Niamey in Niger rank among the world's fastest-growing cities, with annual growth rates of 5 to 8%. Asia alone is home to over two billion city-dwellers. The UN predicts an additional 2.5 billion city dwellers worldwide by 2050, with 90% of that expansion occurring in Asia and Africa.

• 5. Cities And War

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  War may have served as the social rationale and economic basis for the very earliest cities. Their economic, demographic, symbolic, and political centrality makes them crucial in warfare, and for the same reasons makes them targets in asymmetric warfare. Many cities were founded under military auspices, a great many incorporated fortifications, and military principles continue to shape urban design.

  Garrison towns, America's Strategic Hamlet Program during the Vietnam War, and Israeli settlements in Palestine all show fortified settlements built into military strategy. While occupying the Philippines, the U.S. Army ordered local people to concentrate in cities and towns, isolating insurgents so it could battle them freely in the countryside. During World War II, national governments sometimes declared cities open, surrendering them to an advancing enemy to avoid damage and bloodshed.

  The Battle of Stalingrad proved urban warfare could be decisive, as Soviet forces repulsed German occupiers amid extreme casualties and destruction. Capture is the usual aim, but warfare has occasionally meant a city's complete destruction, a fate captured in the Latin motto Carthago delenda est. Since the atomic bombings of Hiroshima and Nagasaki, nuclear strategists have weighed "counter-value" targeting: crippling an enemy by annihilating its valuable cities rather than aiming at its military forces.

• 6. The Urban Climate

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  The temperature in New York City exceeds nearby rural temperatures by an average of 2 to 3 degrees Celsius, with differences of 5 to 10 degrees recorded at times. Concrete, asphalt, and other artificial surfaces heat up in sunlight and channel rainwater into underground ducts, giving cities their own microclimates. Aerial particulates increase rainfall by 5 to 10%, and urban areas see earlier flowering and later leaf dropping than the surrounding country.

  Urban soil carries higher concentrations of heavy metals, especially lead, copper, and nickel, and a lower pH than comparable wilderness. Air pollution arises from fireplaces, wood or coal-burning stoves, heating systems, and internal combustion engines. Industrialized cities and today's third-world megacities are notorious for veils of smog that pose a chronic threat to millions of inhabitants.

  The burden falls unevenly. Poor and working-class people face disproportionate exposure to environmental risks, a pattern called environmental racism where it intersects with racial segregation. Within the urban microclimate, less-vegetated poor neighborhoods bear more of the heat while having fewer means to cope with it. Because dense cities consume less land and move people efficiently, they hold the potential for a smaller ecological footprint per inhabitant, which is why compact cities are often called crucial to fighting climate change.

  Green space is one remedy. A study published in Scientific Reports in 2019, drawing on data from almost 20,000 people in the UK, found that those who spent at least two hours per week in nature were 23 percent more likely to be satisfied with their life and 59 percent more likely to be in good health than those with zero exposure. Benefits increased for up to 300 minutes of exposure, and people who fell short of two hours gained nothing. The study excluded time spent in one's own yard, yet most nature visits took place within two miles of home.

• 7. Cities Beyond Borders

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  Saskia Sassen coined the term "global city" in her 1991 work, The Global City: New York, London, Tokyo, naming a city's power, status, and cosmopolitanism rather than its size. Global cities form the capstone of a global hierarchy, exerting command and control through economic and political influence. Multinational corporations and banks plant their headquarters there, and American firms dominate the international markets for law and engineering while maintaining branches in the biggest foreign global cities.

  Not everyone accepts the ranking. Paul James argues the term is "reductive and skewed" in its focus on financial systems, missing places significant for other reasons. The phenomenon of cities acting across borders is itself old, traceable to the Silk Road, Phoenicia, and the Greek city-states, through the Hanseatic League and other alliances of cities.

  Cities now conduct world politics independently of their nation-states. Hamburg, Prague, Amsterdam, The Hague, and the City of London maintain their own embassies to the European Union at Brussels. United Cities and Local Governments serves as an umbrella organization at the global level, while environmental networks such as the C40 Cities Climate Leadership Group and ICLEI carry climate norms between member cities.

  The United Nations has tracked this rise through a sequence of gatherings. The Habitat I conference in 1976 adopted the Vancouver Declaration on Human Settlements; the 1992 Earth Summit in Rio de Janeiro produced Agenda 21; and the Habitat III conference of 2016 set out a "New Urban Agenda." Because of these influences on global issues, the international community has prioritized investment through Sustainable Development Goal 11, the goal that now ties the fate of cities to the fate of the planet.