Relatively sudden and widespread disturbance of the social system and life of a community or region may be caused by one or more of the destructive forces of nature. Natural disasters are usually the result of geophysical or meteorological disturbances, the causes and mechanisms of which are now relatively well understood even though their occurrence and the detailed consequences cannot be predicted. The phenomena which mainly cause disasters are earthquakes and cyclonic storms, usually of tropical origin, but seismic activity under the sea can cause floods far from the centre of disturbance. Besides direct damage due to flooding, wind forces and earth movement, landslips and outbreaks of fire may occur to cause further damage and loss of life. Volcanic activity, besides being the cause of some earthquakes, can also cause damage from lava and ash.
The UNEP data book of 1991 notes that the frequency of natural disasters has increased in recent decade, adding that one reason may be a heightened awareness of disasters through reports in the media. It also suggests that natural disasters are becoming more significant in terms of magnitude and numbers. The most widely accepted reason is that the "hit rate" has increased considerably due to the continuing growth of world population. More than one third of the world's largest and fasting growing cities are now in regions of high seismic risk. Many of these are in developing countries where poor construction methods exacerbate the dangers. Many huge populations of non-urban people are concentrated in regions of environmental risk, such as variable rainfall; thousands of people have migrated to these regions in the past few decades as a result of population pressure and social conflict.
The world's societies and environment are increasingly suffering from the effects of natural disasters. A disaster usually results from the combination of a vulnerable environment and a hazard such as floods, tropical storms, earthquakes, landslides, wild fires, volcanic eruptions, and tsunamis. They can cause the loss of life, property and livelihoods.
[Developing countries] A natural event may be an Act of God but the disaster which follows is caused or magnified by human and environmental mismanagement. In the rural Third World, growing populations are forced by poverty to overcultivate, overgraze and deforest their land. This makes drought and floods more destructive. In mushrooming cities of the least developed countries, the poor live on the most dangerous ground: in shantytowns on river flood plains or coastal mudflats, an in heavy mud-brick shacks on steep hillsides and ravines. This multiplies the casualties from floods and earthquakes. In cases like prolonged drought, major floods or cyclones, the affected countries' ability to renew their developmental efforts regresses by several years. Developing countries in 1991 are spending between 2 and 7% of their GNP on natural disasters.
Over the past twenty years, natural disasters are estimated to have caused the deaths of 3 million people and to have affected the lives of 800 million others. For example, in 1988 there were serious floods in Nile, Ganges, and Brahmaputra, violent hurricanes in the Caribbean, fatal typhoons in the Philippines and the China Sea, earthquakes in Nepal, Yunnan and USSR Armenia. For one sole year, the death toll rose to more than 50,000; millions of people were left homeless.
The incidence of natural disasters such as storms and floods is increasing in frequency and magnitude (Munich Re 1997), and some of these natural phenomena – particularly floods – are being exacerbated or triggered by human degradation of the environment and disturbance of formerly stable ecosystems. Trends in disaster events are disturbing. Statistics show that during the period 1900-91, there have been more than 3 500 disasters – roughly 40 a year – and that they have killed more than 27 million people. There is also evidence that the frequency of disasters is increasing. For the top 10 most disaster-prone countries in the Asia-Pacific region—Australia, Bangladesh, China, India, Indonesia, the Islamic Republic of Iran, Japan, New Zealand, Philippines and Viet Nam – there were a total of 1,312 disasters during the 25 years 1966-90, which killed 1.7 million people and affected more than 2,000 million. The frequency of disasters in this period was 52.5 a year, compared to only 24.8 a year over the period 1900-91.
Already, 96 per cent of all deaths from natural disasters occur in developing countries. One billion people are living in the world's unplanned shanty towns and 40 of the 50 fastest growing cities are located in earthquake zones. Another 10 million people live under constant threat of floods (International Federation of Red Cross and Red Crescent Societies, World Disasters Report 1999).