If you would be known, and not know, vegetate in a village; if you would know, and not be known, live in a city. Charles Caleb Colton
What is a city? Why do cities exist? Why do urban economists call them (with some exaggeration!) our greatest invention? Those are some basic questions we will tackle from time to time in this blog.
The simplest definition of a city: a place with above average density. Think of a world without cities as a world in which our population (and other things, like capital goods) are randomly scattered across the land. That would be a very strange world, indeed. Much better is the world we live in, where most people cluster together in some form of urban settlement; and where most of those who remain in areas we call "rural" actually live in smaller clusters (think villages) that are actually functionally urban, albeit at a small scale.
There are many other definitions of "city" out there. My venerable second edition Merriam Webster's says a city is "from the Latin, civitas, citizenship. Vaguely, any large, important or noted town or unlabeld place, so called by way of distinction. The collective body of citizens, or inhabitants of a city."
More colorfully, Robert Bevan, writing in The Guardian, libels the U.S. Midwest, where he claims "... the word ‘city’ has been appended with abandon to any one-brothel main street that once offered relief to travelers across the prairies.” Waxing a little more poetic, polite, and concise, Richard Sennett offers "a place where strangers meet." Gideon Sjoberg defines a city as “a community of substantial size and population density that shelters a variety of non-agricultural specialists, including a literate elite.”
Getting back to a more analytical focus, in a classic 1938 paper Louis Wirth defines a city as a place with: (1) a ‘large’ population; (2) in a ‘dense’ settlement; (3) more or less permanent; (4) with some social heterogeneity. Of course this brings up a set of hard to answer questions, namely what’s "large?" What’s "dense?" What do we mean by "social heterogeneity?"
In my urban economics class, I suggest a quadrapite model of a city: (1) a city is a land and real estate market; (2) a city is a labor market and a collection of people; (3) a city is a collection of capital, both tangible and financial; and finally, (4) a city is also comprised of a selection of governments and institutions.
"Above average density" and "where strangers meet" and the like get us started, but for some purposes are a little vague and abstract. For many purposes – data collection, legal matters, governance – we need very specific, to some extent arbitrary, definitions of the city.
When we speak of cities, then, we have to be clear whether we are discussing "the" city in a functional sense -- the way an urban economist things of "the city" -- or "a" city in a legal/political sense -- the way governments define cities with specific boundaries and governance structures. Let us now focus on the latter.
Cities in the United States
In the U.S., cities are legally “creatures of states.” The U.S. Constitution discusses the national government, and the states, extensively; cities and other local jurisdictions are never mentioned!
So each state defines its cities, towns, villages, etc.; and every state is different.
In some of its data collection, the Census Bureau works with these definitions; among many other units of observation, Census tracks data on “places,” which can be incorporated, usually cities and villages. Practices vary by state, but incorporated places can also be labeled boroughs or towns or municipalities; these can vary by size, and by the specific kinds of functions states delegate to them. Places can also be unincorporated places; the Census refers to these as Census Designated Places, or CDPs.
A U.S. city or other incorporated place has received a charter from the state government, and will have elected officials. Unincorporated places exist by tradition. Nationally, there are about 19,000 incorporated places, and about 4,000 CDPs.
To recap, different states have different rules for designating cities. These rules often include, but are not limited to, minimum population sizes in most states. In the event, U.S. cities (as defined by the political-legal Census "place") range in size greatly from 8 million (New York) down to as few as single digit population in a few odd cases.
For more details, see Census web sites here and here.
Closely related to the idea of a city is the idea of an urban area, of urbanization. Merriam Webster, 2nd ed., defines urban as "of, or pertaining to cities."
Census defines "urban" a little differently than a "city;" urban areas are densely settled areas that are more fine grained, built up of densely settled census tracts and census blocks. In a city like Madison, where some areas are still rural in character even though they are contained within the incorporated boundary, we can have a Census tract that is within the city but not counted as urban by Census.
Thus, in many respects, Census' definition of "urban areas" comes closer to a functional definition than does the legal definition of many U.S. cities.
I don't have a chart handy for Madison, but a demographic services firm called ProximityOne has posted a map for Lawrence, Kansas, using 2000 Census data and definitions:
The Lawrence city boundary in green; the urban urban area in yellow. Obviously there is a lot of overlap, but clearly some parts of the incorporated city are outside the urban area; and some of the urban area is outside the city.
To sum up, and extend a bit, here is a summary of how Census defines "urban." An urban area comprises (1) a densely settled core of census tracts and/or census blocks that meet minimum population density requirements (at least 500 people per square mile), along with:
(2) adjacent territory containing non-residential urban land uses, and (3) territory with low population density that links outlying densely settled territory with the densely settled core.
Furthermore, an urban area must encompass at least 2,500 people, at least 1,500 of which reside outside institutional group quarters.
Just to keep us on our toes, the Census Bureau identifies two types of urban areas: (1) Urbanized Areas (UAs) of 50,000 or more people; and (2) Urban Clusters (UCs) of at least 2,500 and less than 50,000 people.
“Rural” encompasses all population, housing, and territory not included within an urban area.
The figure shows the evolution of U.S. urban and rural population over time, since the first Census in 1790. Two caveats attach to the figure. First, the definition of "urban" and "rural" have changed over time, but the data are not revised to fit a single definition. That accounts for some of the apparent blips in the data.
Second, the stacked area chart above gives us a look at the evolution of the levels of urban and rural population over time, but is very misleading with regard to growth rates. To the untutored eye, it looks like U.S. population growth (especially urban growth) is accelerating, because the line is moving up at an increasing rate. But growth rates are changes over levels, and as we move forward in time, both the change and the level are going up. Eyeball the chart. If you think it's showing faster growth in recent years, then you are a prime candidate for a future post on data transformations that will convince you that U.S. population growth is slowing down, in both the city and the countryside.
In the 2010 Census, about 250 million, or 81 percent, of the U.S. population resided in urban areas. About 60 million, or 19 percent, of the U.S. population resided in rural areas.
Because urban areas are denser, the fraction of U.S. land that's urbanized will be less than the fraction of population. But it surprises many how little U.S. land is urbanized. Separately, the U.S. Department of Agriculture estimates how much land area is urbanized. In 2007, the last detailed accounting, about 25 million hectares (61 million acres), or about 3 percent of our 0.9 billion hectares (2.3 billion acres), was urbanized. About another 2 percent comprised rural built up areas (e.g. farm buildings, rural housing and other real estate, and highways).
World Urbanization: A First Look
The figure above shows the urban population of the world, by decade. Data come from the United Nations World Urbanization Prospects.
Today there are roughly 7.5 billion people spread over about 13 billion hectares (32 billion acres) of land. Of course much of this land is arid or otherwise inhospitable to settlement, but more on that another day. Of those 7.5 billion people, about 4 billion live in cities/urban settlements, and 3.5 billion live in urban areas.
In 1950, just before I was added to the red bars, the world's urban population was only about 3/4 billion; that was only 30 percent of our total 2.5 billion population.
It's an auspicious time to study urbanization! Human history, at least that of homo sapiens, is currently thought to extend about 150,000 years, and some of us have lived in urban settlements for about ten millennia. According to the best estimates – and these are rough numbers – in 2007 the red bar became bigger than the blue bar, and we very recently crossed over, for the first time, to a world where the majority of human population lives in cities.
Obviously we are unable to actually date the transition to majority urban as precisely as a given year, much less the date “May 23, 2007” announced, I presume tongue-in-cheek, by several U.S. sociologists a few years back.
The actual data up through 2010 are themselves subject to error, as they are built up from country estimates. In addition to sampling errors, the UN's demographers use each country's own definition of "urban." Roughly, for the U.S, as noted above, our threshold for urban is a place of 2,500 or more. As it happens, 2500 happens to be a common but not ubiquitous threshold for urban place in other countries. Country thresholds range from 1,000 to 10,000 and higher, although 2500 is the most common. See the "Data Sources" page of World Urbanization Prospects for list of definitions by country as well as additional detail.
So far our brief discussion has focused on the levels of global urban and rural population. The chart also shows annual growth rates for each decade. In the 1950s, urban population was growing a bit over 3 percent per year, while urban population was growing about 1 percent; overall population growth was a little under 2 percent. Today, urban population is growing about 2 percent, while rural population is flat, for a total global growth rate of about 1 percent. In several decades, the UN's forecast is that urban population growth will slow down to about 1 percent while rural population will start to decline.
Until recently, the UN and a number of other global projections suggested that population growth would level off and begin to decline sometime during this century. More recent projections (see chart above, from Kunzig) suggest that population growth may continue, albeit at a declining rate, through the rest of this century.
The forecast growth is concentrated in a relatively small group of the world's 200+ countries, mainly those that are large, with especially high fertility rates. The UN World Population Prospects projects that half the world’s population growth will come from just nine countries; in order: India, Nigeria,Pakistan, Democratic Republic of the Congo, Ethiopia, United Republic of Tanzania, the U.S., Indonesia and Uganda.
Note that the U.S. is the only "developed" country in the list; the U.S. has a much higher growth rate than most other developed countries, partly because of immigration, and partly because of higher fertility rates. (Interestingly, the higher U.S. fertility rate is in turn due in large part to immigration, since it's recent immigrants and their children who have the highest fertility rates within our borders). U.S. population grows about 1 percent per year, close to the current global average; many other rich countries like France, Germany or Japan, are flat or declining. Note also the absence of China in the short list; China is large, but has a modest rate of population growth, around 0.5 percent.
More to Come: Metropolitan Definitions
Another basic set of city-related definitions revolve around metropolitan areas. These are collections of counties, that are economically connected to cities. Metropolitan areas -- and related concepts, like micropolitan areas, "combined statistical areas," and the like -- are a big topic, so we will undertake a separate post to examine them.
If you can't wait -- and there's no reason why you should wait! -- see Frey et al. (2004) for a detailed discussion.
Reading for Life
Bevan, Robert. "What Makes a City a City - and Does It Really Matter Anyway?" The Guardian, May 8, 2014 2014.
Colton, Charles Caleb. Lacon: Or, Many Things in Few Words: Addressed to Those Who Think: Longman, Orme, Brown, Green, & Longmans, 1837.
Frey, William H, Jill H Wilson, Alan Berube, and Audrey Singer. "Tracking Metropolitan America into the 21st Century: A Field Guide to the New Metropolitan and Micropolitan Definitions." The Brookings Institution, 2004.
Frug, Gerald E. "The City as a Legal Concept." Harvard Law Review 93, (1980): 1057-1154.
Glaeser, Edward L. Triumph of the City: How Our Greatest Invention Makes Us Richer, Smarter, Greener, Healthier, and Happier: Penguin, 2011.
Kunzig, Robert. "A World with 11 Billion People? New Population Projections Shatter Earlier Estimates." National Geographic, (2014).
Malpezzi, Stephen. "Urban Growth and Development at Six Scales: An Economist's View." In Global Urbanization, edited by Eugenie L. Birch and Susan M. Wachter. Philadelphia: University of Pennsylvania Press, 2011.
________. "Population Density: Some Facts and Some Predictions." Cityscape 15, no. 3 (2013): 183-201.
Nickerson, Cynthia, Robert Ebel, Allison Borchers, and Fernando Carriazo. Major Uses of Land in the United States, 2007: United States Department of Agriculture, Economic Research Service, 2011.
Sjoberg, Gideon. "The Origin and Evolution of Cities." Scientific American 213, no. 3 (1965): 55-63.
United Nations. World Urbanization Prospects: The 2014 Revision: UN Department of Economic Social Affairs. Population Division, 2014.
________. World Population Prospects: The 2015 Revision: UN Department of Economic Social Affairs. Population Division, 2015.
Wirth, Louis. "Urbanism as a Way of Life." American Journal of Sociology, (1938): 1-24.