The number of people who are on the move involuntarily worldwide may be as high as 184 million-roughly equivalent to the entire population of Brazil, or one out of every 36 persons on Earth.¹ Among them are 16 million refugees (including 4.6 million Palestinians) and 26 million internally dis­placed people (IDPs-those who, unlike refugees, did not cross an international border).² (See Figures 1 and 2.) Another 12 million people are stateless-they are vulnerable because they lack the protection of citizenship, although they are not necessarily displaced.³ Some 25 million people have been uprooted by natural disasters.⁴ And Christian Aid, a London-based advocacy group, estimates that as many as 105 million people are made homeless by a variety of so-called development projects, including dams, mines, roads, factories, plantations, and wild­life reserves.⁵

Because these estimates come from different sources, the total of 184 million needs to be regarded with some caution. This is especially so because the Christian Aid figure is a rough estimate and may partially overlap with the other categories.

Environmental and resource pressures are increasingly a driver of displacement. They also have an impact on the number of long-term migrants-people who leave voluntarily and live outside their home country for a year or longer-whose numbers rose from 75 million in 1965 to some 200 million in 2005.⁶ In relative terms, however, the number of long-term migrants has remained at roughly 2-3 percent of global population.⁷

The U.N. High Commissioner for Refugees (UNHCR) has traditionally been tasked with assisting those who seek refuge from war and repression. The largest groups of refugees under UNHCR's care in 2007 were 3 million Afghanis (mostly residing in Pakistan and Iran) and 2 million Iraqis (mainly in Syria and Jordan).⁸ UNHCR also helped Colombians (552,000), Sudanese (523,000), and Somalis (457,000).⁹ (A separate agency, the United Nations Relief and Works Agency for Palestine Refugees in the Near East, or UNRWA, is responsible for Palestinian refugees, who number 4.6 million.)¹⁰ In 2007, Sudan was the country with the highest number of IDPs (5.8 million), followed by Colombia (up to 4 million), Iraq (2.5 million), the Democratic Republic of the Congo (1.4 million), and Uganda (1.3 million).¹¹

According to U.N. High Commissioner for Refugees Antonio Guterres, it will be increas­ingly difficult to easily categorize the displaced by separate causes.¹² Environmental degrada­tion, for example, is now often seen to be a factor contributing to both involuntary and voluntary population movements. But environmental problems are often closely intertwined with socioeconomic conditions (poverty, inequality of land ownership, etc.), resource disputes, and poor governance.¹³

The concept of "environmental refugees" has been discussed since the mid-1980s, when Essam El-Hinnawi offered the following definition: "People who have been forced to leave their traditional habitat, temporarily or permanently, because of a marked environmental disruption (natural and/or triggered by people) that jeopardized their existence and/or seriously affected the quality of their life."¹⁴ El-Hinnawi has estimated that there are now perhaps 30 million environmental refugees.¹⁵ A projection by environmental researcher Norman Myers that there could be as many as 250 million such refugees by mid-century has been widely cited but also has been criticized for some of its assumptions.¹⁶

No one is systematically collecting data on environmentally driven displacement-in part because there is no generally accepted definition and methodology. Some analysts argue that the category of refugees-legally defined as people fleeing persecution without access to protection by their own country-should not be muddied by other factors such as environmental degradation.¹⁷ Others point to the fact that not every­one uprooted by environmental change crosses a border-and thus does not technically become a refugee, but rather an "environmentally dis­placed person."¹⁸

And there are now also increasing references to "climate refugees." Climate change will have serious human repercussions-in the form of sea level rise, more frequent and more devastating weather events, freshwater short­ages, disruption of agricultural systems, impaired ecosystem services, and health epidemics-that are bound to force people to relocate.¹⁹

Some people may be more aptly described as environmental migrants-moving, sometimes seasonally or temporarily, before the "push" of environmental degradation forces them to leave and with expectations of the "pull" of a better life elsewhere (or the prospect of being able to send money back home). As climate change takes center stage, however, it is likely that "push" will outweigh "pull."²⁰

More than 600 million people live in low-lying coastal zones worldwide.²¹ By some pro­jec­tions, at least 160 million people living in such areas may be at risk of flooding from storm surges by 2010.²² Bangladesh, for instance, is already experiencing growing storm surges and rising salinity in coastal areas.²³ One third of the country could be flooded if the sea rises by one meter, affecting 20 million of its 140 million people.²⁴ In 2005, Hurricane Katrina caused 1.5 million people to be displaced temporarily; some 300,000 may never return to their former homes.²⁵ Meanwhile, small island states like the Maldives in the Indian Ocean and Tuvalu in the Pacific face the danger of being entirely devoured by sea level rise.²⁶ Low-lying, heavily populated deltas similarly face the specter of inundation.

Arid and semiarid areas cover about 40 percent of Earth's land surface and are home to more than 2 billion people.²⁷ Desertification processes put an estimated 135 million people worldwide at risk of being driven from their lands.²⁸ Where people-typically women-already have to walk many kilometers each day to fetch water, such as in the Sahel, longer journeys are simply not an option.²⁹ Water short­ages could affect anywhere from 75 million to 250 million people in Africa by 2020 and more than 1 billion people in Asia by 2050.³⁰

The precise nature of environmental change can make a big difference in terms of displacements. Fast-onset impacts like floods and storms will affect people in different ways than a gradual process like drought and desertification or sea level rise. The severity and frequency of disasters, too, has important impacts on the habitability and economic viability of affected areas.

Resilience is a key factor determining whether vulnerability translates into flight. The poor are typically most exposed to environmental hazards. Population pressures and social marginalization often compel them to live in risky places-steep hillsides likely to be hit by landslides, low-lying areas susceptible to flooding, or coastal strips whose natural buffers (wetlands, mangroves, and coral reefs) have been stripped away. And they often have limited capacity to deal with these challenges, some­times even lacking the necessary monetary resources, family networks, or other connec­tions needed to migrate.³¹

Adaptation measures can help reduce vul­nerability: disaster and famine early warning systems, livelihood diversification, drought-resistant crops, restoration of ecosystems, flood-defense infrastructure, crop insurance, and others. But even in the wake of floods or storms, well-calibrated emergency and recovery aid can make the difference between people staying and leaving. Resilience is also a function of overall economic capacity, demographic pressures, governance structures, and good leadership, as well as social and political cohesiveness.³²

So far, international funding for adaptation in poorer and more vulnerable countries is woefully limited.³³ Yet timely adaptation-along with mitigation measures to prevent the worst of climate change-will be much less costly in economic and human terms than dealing with disasters and displacements. UNHCR already struggles to provide adequate support for refugees and internally displaced people, and the same is true for agencies providing humanitarian aid. They will be overwhelmed if the large-scale climate-related displacements now predicted indeed come to pass.