Although the immediate cause of of any mass movement of people may appear to be political upheaval and civil or military violence, the underlying causes often include the deterioration of the natural resource base and a significant reduction in its capacity to support the population.
One estimate is that environmental or ecological refugees will number in the hundreds of millions by the end of the first quarter of the 21st century. In the case of Ethiopia it has been reported that the primary cause of recent famines was not drought but rather a combination of long-continued bad land use and steadily increasing human and livestock populations. Expanding population, land degradation and drought in Senegal and Mauritania helped spur a violent conflict over irrigable land in the Senegal River basin, resulting in tens of thousands of refugees. In Haiti, the irreversible clear-cutting of forests and loss of soil worsen the economic crisis and violent social strife, which in turn have caused an exodus of boat people.
Refugees are forced to make unrestricted assaults on the natural environment for their survival. Refugee numbers reached an all-time high of 27.4 million in 1995 (UNHCR 1998).