The Pump Handle and Casaubon’s Book are writing posts this week about the global trend of urbanization. More than half of the world’s population now lives in cities, and this shift has implications for the health of the planet.

I grew up in suburban Delaware, and my first experience with urban living came in college when I spent a month on a study abroad trip to London. I fell in love with the Tube, the neighborhood markets and pubs, the profusion of cuisines, and the array of theatrical performances listed in Time Out magazine (it was the nineties, and we relied on the paper version). I vowed to move to a city after graduating, and it didn’t take me long to settle in Washington, DC.

While some of the things I love about living here are specific to DC, others could be true in many other cities. I love that I don’t need to own a car, and can get to most of my destinations by Metro or bus. I can walk to three grocery stores, a farmers’ market, countless restaurants, and several friends’ homes. There are plenty of museums, theaters, and parks nearby, and lots of opportunities to learn about other countries and cultures (it’s not unusual to hear three different languages spoken during a single bus ride).

Of course, my experience of living in DC is shaped by my income. I can afford to live in a relatively safe neighborhood that’s well served by public transit and has plenty of high-quality food for sale. When I get out of class at 9pm and don’t want to face a metro ride involving multiple lines, I can pay for a cab ride home. My budget has room in it for restaurant meals and theater tickets.

If I were a low-income DC resident – as nearly one-fourth of this city’s population is – my experience of living here would be different. And if I were living in a slum in Nairobi or Rio de Janeiro, my life would be so different that it’s hard to imagine.

Urbanization, Growth, and Inequality
Before I get into the issue of slums, though, it’s useful to look at the worldwide urbanization picture. In 2008, the world reached the point of having half its population in urban areas, although the picture differs from region to region. Latin America and the Caribbean has the greatest proportion of people living in urban centers, while Asia and Africa are the least urbanized. UN-HABITAT projects that virtually all of the world’s demographic growth over the next three decades will be concentrated in urban areas. Much of that growth will be in smaller cities.

Migration is one of the key drivers of cities’ growth, as rural residents move to urban areas in search of economic opportunities. Urbanization and economic development go hand in hand, explains UN-HABITAT in State of the World’s Cities 2010/2011:

No country has ever achieved sustained economic growth or rapid social development without urbanizing (countries with the highest per capita income tend to be more urbanized, while low-income countries are the least urbanized). Thanks to superior productivity, urban-based enterprises contribute large shares of gross domestic product (GDP). … High urban densities reduce transaction costs, make public spending on infrastructure and services more economically viable, and facilitate generation and diffusion of knowledge, all of which are important for growth.

As economic engines and recipients of public spending, cities can indeed offer economic opportunity to new residents – but the benefits and opportunities are not equitably distributed. Slum dwellers can’t count on the basic services – water and sanitation, electricity, roads – that other residents of their cities often take for granted. State of the World’s Cities offers this definition of slum households:

A slum household consists of one or a group of individuals living under the same roof in an urban area, lacking one or more of the following five amenities:

(1) durable housing (a permanent structure providing protection from extreme climactic conditions);
(2) sufficient living area (no more than three people sharing a room);
(3) access to improved water (water that is sufficient, affordable and can be obtained without extreme effort);
(4) access to improved sanitation facilities (a private toilet, or a public one shared with a reasonable number of people); and
(5) secure tenure (de facto or de jure secure tenure status and protection against forced eviction)

In 2005, one in three urban dwellers lived in slum conditions. Many slums are located on the outskirts of cities, and physical separation makes it hard for slum dwellers to improve their circumstances. State of the World’s Cities explains:

When slum areas are physically isolated and disconnected from the main urban fabric, residents become cut off from the city, often enduring longer commuting times and higher transportation costs than they would if their neighbourhoods were more integrated into urban networks. On top of low incomes and shelter deprivations, these residents find themselves underprivileged in terms of access to the urban advantage. Combined, the physical and social distance between poor and rich neighbourhoods represents a spatial poverty trap marked by six distinct challenges: (a) severe job restrictions; (b) high rates of gender disparities; (c) deteriorated living conditions; (d) social exclusion and marginalization; (e) lack of social interaction, and (f ) high incidence of crime.

Prospects for Slums
State of the World’s Cities reports that “a number of countries have, to some extent, managed to curb the further expansion of slums and to improve the living conditions prevailing there.” Over the past decade, China and India have achieved the most substantial improvements in slum conditions; Egypt, Morocco, Tunisia were the most successful African countries at reaching this goal, and Argentina, Colombia, and the Dominican Republic reduced their proportions of slum dwellers by one-third.

The share of the developing-world population living in slums has dropped from 39% in 2000 to 33% in 2010, but the absolute number of slum dwellers has grown from 767 million in 2000 to an estimated 828 million. And the problem may get worse, the UN warns:

The recent housing crisis, which contributed to the larger financial and economic downturn, may offset the progress that was made since 1990. Although the crisis did not originate in developing regions, it has hit their populations and cities, where millions continue to live in precarious conditions, often characterized by a lack of basic services and serious health threats. In many cases, public authorities have exacerbated the housing crisis through failures on four major counts: lack of land titles and other forms of secure tenure; cutbacks in funds for subsidized housing for the poor; lack of land reserves earmarked for low-income housing; and an inability to intervene in the market to control land and property speculation. Low incomes in the face of rising land prices virtually rule out the possibility that the working poor can ever own land, contributing to the problem of urban slums.

We all hope that the global economy will stabilize soon, and when it does, cities will likely continue to function as economic engines offering opportunities for residents and migrants to improve their lives. But will those opportunities be equitably distributed?