• acute onset movements with the possibility of return. This is one of the most visible forms of flight, caused by natural disasters such as cyclones, earthquakes, floods and, volcanic eruptions. Man-made tragedies such as industrial accidents such as the Bhopal gas disaster in India are also behind such temporary displacements.
• acute onset movements, without the possibility of return. This is applicable to groups displaced by nuclear contamination or dumping of other hazardous wastes or destruction by especially severe natural disasters.
• slow onset movements with predicability without possibility of return. In this case, displacement is caused, for example, by dam construction and other large-scale development projects.
• slow onset movements, with the possibility of return. Chronic water shortages, deforestation, agricultural failure and land-related problems such as unclear land tenure and even extensive pollution of resources (land, water and air) are major causes of flight in this category.
• low onset movements, without the possibility of return because of natural conditions in the area of departure. This is a reference to irreversible natural phenomena, such as desertification or rising sea water levels.

This ordering completely leaves out the other major migrant who comes in the shape of a refugee, fleeing not destitution or an environmental crisis but seeking succour from a threat to his life and liberty. A political refugee is not a migrant. The former is regarded as a temporary traveller, while the latter is viewed as a settler and thereby a competitor for space in every form: land, water, services and jobs.

The political refugee has a chance of returning to his homeland after the threat to his personal life or that of his community has abated. This has happened time and again, not just in Rwanda-Burundi, witness to the Hutu-Tutsi civil strife. In 1970-71, nine million refugees fled a Pakistani military massacre of Bengalis in East Pakistan. They returned from India only when their land gained independence as Bangladesh, and the Pakistani killing machine had been crushed by the Indian Army and the insurgent Mukti Bahini.

Among the cases of refugees not returning to their country of origin are the Ugandan Asians who were driven out of Uganda by the dictator Idi Amin; they settled in Britain. Then there are the two million Afghan refugees who continue to live in Pakistan’s North West Frontier Province, more than a decade after the exit of the Soviet Army, which intervened in 1979. Bitter ethnic fighting and civil war continues to ravage Afghanistan.