Since then, their numbers have expanded almost twice as fast as total population growth, to more than 2.5 billion. That is the equivalent to almost five new cities the size of Beijing, every 12 months. By 2025, more than half the developing world's population - 3.5 billion people - will be urban.
While urbanization in Europe and North America took centuries, spurred on by industrialization and steady increases in per capita income, in the developing world it will occur in the space of two or three generations. In many developing countries, urban growth is being driven not by economic opportunity but by high birth rates and a mass influx of rural people seeking to escape hunger, poverty and insecurity.
Most of the world's fastest growing cities are found in low-income countries of Asia and Africa with young populations. Over the next 10 years, the current number of urban dwellers in sub-Saharan Africa is expected to grow by almost 45 percent, from 320 million to 460 million. Kinshasa, capital of one of the world's poorest countries, is now the world's fastest growing future megacity. By 2025, the urban population of least-developed countries in Asia will have grown from 90 million to a projected 150 million, and Dhaka is expected to be the world's fifth largest city, with 21 million inhabitants.
Urbanization in low-income countries is accompanied by high levels of poverty, unemployment and food insecurity. Worldwide, an estimated one billion people live in crowded slums, without access to basic health, water and sanitation services. Around 30 percent of the developing world's urban population - 770 million people - are unemployed or "working poor", with incomes below official poverty lines.
Those urban poor spend most of their income just to feed themselves. Yet their children suffer levels of malnutrition that are often as high as those found in rural areas. To survive, millions of slum dwellers have resorted to growing their own food on every piece of available land: in backyards, along rivers, roads and railways, and under power lines.
The growth of urban slums outpaces urban growth by a wide margin. By 2020, the proportion of the urban population living in poverty could reach 45 percent, or 1.4 billion people. By then, 85 percent of poor people in Latin America, and almost half of those in Africa and Asia, will be concentrated in towns and cities.
That prospect has been described as "the new population bomb" and a nightmare for governance: sprawling, degraded and impoverished cities with large, vulnerable populations that are socially excluded, young and unemployed.
The promise of greener cities
A brighter future for the world's developing cities is both imperative and possible. Historically, cities have been places not of misery and despair but of opportunity - for economies of scale, employment and improved living standards, especially for rural people seeking a better life. They have served as engines of social progress and national economic development.
Creating the conditions to realize that potential - in Kinshasa, Dhaka and other growing towns and cities across the developing world - is crucial now and will be more so in the decades ahead. The challenge is to steer urbanization from its current, unsustainable path, towards sustainable, greener cities that offer their inhabitants choice, opportunity and hope.
The concept of "green cities" - designed for resilience, self-reliance, and social, economic and environmental sustainability - is usually associated with urban planning in more developed countries. It suggests high-tech eco-architecture, bicycle greenways and zero-waste, "closed loop" industries.
However, it has a special application, and significantly different social and economic dimensions, in low-income developing countries. There, the core principles of greener cities can guide urban development that ensures food security, decent work and income, a clean environment and good governance for all citizens.
A starting point for growing greener cities is to recognize and integrate into urban policy and planning many of the creative solutions that the urban poor themselves have developed to strengthen their communities and improve their lives. One of those solutions - and an essential feature of green city planning in developed, and a growing number of developing, countries - is urban and peri-urban horticulture.
How horticulture helps grow greener cities
Urban and peri-urban horticulture (or UPH) is the cultivation of a wide range of crops - including fruit, vegetables, roots, tubers and ornamental plants - within cities and towns and in their surrounding areas. It is estimated that 130 million urban residents in Africa and 230 million in Latin America engage in agriculture, mainly horticulture, to provide food for their families or to earn income from sales.
While the urban poor, particularly those arriving from rural areas, have long practised horticulture as a livelihood and survival strategy, in many countries the sector is still largely informal, usually precarious and sometimes illegal.
But that is changing rapidly.
Over the past decade, governments in 20 countries have sought FAO's assistance in removing barriers and providing incentives, inputs and training to low-income "city farmers", from the burgeoning metropolises of West and Central Africa to the low-income barrios of Managua, Caracas and Bogotá.
Through multidisciplinary projects, FAO has helped governments and city administrations to optimize policies, institutional frameworks and support services for UPH, and to improve horticultural production systems. It has promoted irrigated commercial market gardening on urban peripheries, simple hydroponic micro-gardens in slum areas, and green rooftops in densely populated city centres.
The FAO programme, and similar initiatives by partner organizations, have demonstrated how horticulture helps empower the urban poor, and contributes to their food security and nutrition. But it can also help grow greener cities that are better able to cope with social and environmental challenges, from slum improvement and management of urban wastes to job creation and community development.
