In 2008, the world reached an important milestone: For the first time in history more than half of its human population - 3.3 billion people - were living in urban areas. By 2030, their number is expected to swell to almost five billion. Many of the new urbanites will be poor and their future will depend, to a large extent, on decisions made now.

Rapid urbanization is related in part to population growth and also to migration--both domestic and external--that many countries are experiencing. Frequently, the causes are rural poverty, the search for better social and employment opportunities, or flight from political persecution and violence.

An example of the last situation is the urbanization process in Colombia. Unlike that characteristic of most other Latin American countries the process in Colombia was stimulated, and to some extent defined, by episodes of violence, which occurred principally in rural areas. Since the 1930s, violence has been an inescapable fact of Colombian civilian life.

As families were uprooted and displaced by successive waves of violence, they fled en masse to the country’s main cities, where the majority among them now resides in poverty-stricken marginal areas. As a result of the violence either witnessed or experienced first-hand, many of Colombia’s young generation have internalized the culture of aggression into which they were born.

Colombia's case is certainly not unique. More recently, the rural poor in many other countries throughout the world have been uprooted by violence and forced to flee en masse toward the large urban centers. The recent tragedy in Haiti exemplifies a massive population movement of people from rural areas to the capital city of Port au Prince, where they ended up living in precarious tenements that were destroyed by the earthquakes that cost the lives of over 200,000 people.

Large migrations will intensify as changing climate conditions will lead to abandonment of flooded or arid and inhospitable environments. This will lead to serious health problems both from the various stresses of the migration process and from the civil strife that could be caused by the chaotic movement of people. Every year, climate change causes the death of approximately 300,000 people, and seriously affects 325 million, according to the Global Humanitarian Forum.

A climate refugee is a person who is forced to relocate, either to a new country or to a new location within their country, due to the consequences of global warming. Sometimes, climate refugees are classified as environmental refugees. The number of environmental refugees will reach 150 million over the next 50 years, according to Professor Norman Myers of Oxford University.

In Africa, desertification and its consequences in agricultural production is displacing increasingly large amounts of people. Approximately 10 million people in Africa have been forced to migrate over the last two decades as a consequence of desertification and environmental degradation.

In addition, most people in Africa move into mostly marginal urban areas because of poverty, environmental degradation, political persecution, and religious strife. In addition, food insecurity and lack of basic services in the rural areas encourage people’s migration into the cities, where they all too often end up living in marginal areas.

These marginal areas, known as bidonvilles in French-speaking West Africa, ishish in some Arab countries, kampungs in Indonesia, villas miseria in Argentina, favelas in Brazil, pueblos jóvenes in Peru, and ranchitos in Venezuela, may contain from 30% to 60% of the population of many Third World cities, according to Worldwatch Institute.

Many governments attempt to discourage migration from rural areas to the cities, but these measures are by and large unsuccessful. Since large cities enjoy preferential treatment in terms of infrastructure and industrial development, they serve as magnets for the "have-nots."

Regardless of the big city's allure, many observers now feel that conditions for the ever-growing numbers of urban poor are most likely worse than for their rural counterparts. The true dimensions of this phenomenon remain elusive, according to World Health Organization expert Dr. I. Tabibzadeh, because the poor are either omitted from official statistics or are not considered separately.

Migrations between countries also continue unabated, usually stimulated by similar factors responsible for internal migration. The Latin American country that has produced the greatest number of migrants is Mexico. Among Mexicans living abroad, 99% can be found in the United States, where income opportunities are greater. In the Southern Cone, Argentina is the main destination for migrants from Paraguay, Uruguay, and Bolivia. In Central America and the Caribbean, the U.S. is the most frequent destination, although there are also significant migratory flows from the Dominican Republic to Venezuela and Puerto Rico and from Haiti to the Dominican Republic.

Several European countries have attracted a large number of Africans and many Africans form Sub Saharan countries have migrated to north-African countries. In addition, the traditional pattern of migration within and from Africa is changing. A male-dominated process is becoming increasingly feminized.

Women’s migration is increasingly being affected by the host countries’ family reunification policies. But women are also traveling alone in search of better job and educational opportunities. In many cases, they end up working in low status, low wage jobs and are particularly prone to exploitation and abuse.

Migration within and from Asian countries is not a new phenomenon. The current trends and characteristics of migration in the region have been shaped by the political and economic changes in recent decades. It is estimated that more than six million migrants are working in East and Southeast Asia, one third of whom are in irregular situation. Until the recent economic crisis oil-rich Arab countries have attracted large numbers of Asian workers.

The economic, social and political trends influencing migration will continue for the next few decades. The challenge for governments is to design migration policies that take into account the needs of the migrants as well as those of the host population. Industrialized countries’ economic investments in developing countries as well as more fair trade policies can foster long-term cooperation and ease migration pressures.

- In this blog series, Dr. Cesar Chelala explores the many challenges presented by urbanization, the impact of urban migration, challenges to health, and challenges of providing clean water. - Ed.

Cesar Chelala, MD, PhD, is an international public health consultant and a writer on human rights issues and foreign affairs.