Up until the middle of the eighteenth century, western civilization regarded hazards as ‘acts of God’ in the strict biblical sense, as punishment for people sins. As noted by Bryant (2005), ‘natural hazards afflict all corners of the Earth; often unexpected, seemingly unavoidable and frequently catastrophic in their impact’.
Natural hazards, classified based on their origins, are natural processes or phenomena occurring in the biosphere that may constitute a damaging event (ISDR, 2004). A disaster is differentiated from a hazard by being defined as, ‘a serious disruption of the functioning of a community or a society causing widespread human, material, economic or environmental losses which exceed the ability of the affected community or society to cope using its own resources’ (ISDR, 2004). Peters (2007) explains that a natural disaster is the consequence of the combination of a natural hazard (a physical event such as flooding) and human activities, and is associated with great human and economic losses and suffering.
Two main categories of natural disasters are:
(1) Hydrometeorological Hazards which are natural processes or phenomena of atmospheric, hydrological or oceanographic nature. These include flood, debris and mudflows, tropical cyclones, storm surges, wind, rain and other severe storms, blizzards, lightning, drought, desertification, wildland fires, temperature extremes, sand or dust storms permafrost and snow avalanches
(2) Geological Hazards which are natural earth processes or phenomena that include processes of endogenous origin or tectonic or exogenous origin, such as mass movements. These include earthquakes, tsunamis, volcanic activity and emissions, mass movements, landslides, rockslides, liquefaction, sub-marine slides, surface collapse and geological fault activity.
Root causes of these natural disasters
Earthquakes occur, as a result of the release of pressure, wherever the tectonic plates that make up the Earth’s crust sub-duct (one plate slides under another plate), crumple or grind past each other. The result is parts of the surface may experience a jerking movement. The ‘focus’ refers to the point at which the release in pressure occurs (within the crust). Above this, on the surface is the epicentre which usually receives the worst of the shock or seismic waves. Earthquakes can occur at anytime with little or no warning and may only last a few seconds. However, damage can be extensive, such as to buildings, gas and water pipes, roadways and power and communication lines.
Seismic Ocean waves where once incorrectly termed ‘tidal waves’ but are now commonly referred to as tsunamis (Japanese for ‘harbour waves’). Tsunamis originate from undersea or coastal seismic activity caused by earthquakes, landslides or volcanic eruptions, which displaces a large volume of ocean water. The sea water is displaced with a violent motion, swells up and surges over the land mass with great destructive power. In deep oceans, tsunamis can reach speeds of over 500 mph (800 kph) and have wave heights averaging 30 feet (9 metres). Tsunamis lead to coastal flooding.
Volcanoes can be described as vents or chimneys in the earth’s surface through which magma (molten rock), gases, and other materials erupt, deep from within the crust of the earth. Some may erupt explosively or others slowly. Explosive volcanoes present many potential threats, including the release of toxic gases, pyroclastic flows (mixtures of hot gas, ash and volcanic rocks), nuee ardentes (fast moving clouds of extremely hot gases and fine ash) and large volumes of ash.
Landslide is a general term covering a wide variety of landforms and processes involving the movement of earth, rock or debris downslope under the influence of gravity (UNDP and DHA, 1997). Landslides occur as a result of natural or human modifications of the landscape which ultimately results in the disturbance of the equilibrium of the materials in the slope.
A tropical wave is a low-pressure system forming in tropical latitudes as a trade wind easterly with wind speeds of up to 36 kilometres per hour (22mph). Tropical waves may be beneficial after the dry season by providing essential rainfall. However, they become hazardous when they develop into tropical storms. A tropical depression is a low-pressure system forming in tropical latitudes with wind speeds of 62 kilometres per hour (38 mph). A tropical depression becomes a tropical cyclone (hurricane) when winds reach gale force of 117 kilometres per hour (74mph).
A tropical storm is a low-pressure system which has a maximum sustained surface wind speed which ranges from 63 116 kilometres per hour (39-73mph). Tropical storms and waves can develop into hurricanes (tropical cyclone). Tropical cyclones are areas of very low atmospheric pressure over tropical and sub-tropical waters which build up into a huge circulating mass of wind and thunderstorms up to hundreds of kilometres across.
A hurricane, as defined by the WMO, is a rotating, intense low-pressure system, which forms over tropical oceans where there are warm waters (usually 26oC or warmer), humid air and converging winds. It derives its energy from the latent heat of water vapour condensation over warm tropical seas and can have wind speeds of 117km/hour and over (74mph and more). A hurricane tends to lose strength when it moves over land or cool water. The ‘eye’ of the hurricane refers to the relatively clear and calm area inside the circular wall of convective clouds which spiral in an anticlockwise direction. A hurricane can have a diameter of 650 kilometres (404 miles) with significant destructive potential. As noted by the WMO, low surface air pressure at the centre of a tropical cyclone allows the sea level to rise as a dome 2-5m high and up to 80km across. Strong onshore winds push water ahead of them which generates large waves that grows higher when they reach shallow water. The combination of water-driven waves and the low-pressure dome produces a storm surge which is a huge volume of water driven ashore at high speed and of immense force. A storm surge varies from one shoreline to the other and those generated by hurricanes have the potential to cause the greatest amount of damage washing away everything in its path.
Floods occur when surface water covers land that is normally dry or when water overflows normal confinements (UNDP & UNDRO, 1992). A flood is defined as ‘an abnormal progressive rise in water level of streams or rivers which may result in overflowing’. Torrential rainfall can be considered a natural hazard since it can result in flooding of low-lying areas.
The deficiency of rainfall is the primary cause of any drought. It occurs when the reduction of water or moisture availability is temporary and significant in relation to the norm (UNDP and UNDRO, 1992). Meteorological drought refers to the reduction in rainfall and hydrological drought is the reduction in water resources.
Climate change, global warming, the El Nio phenomena and sea-level rise in many respects are not natural hazards themselves. However they can be considered triggers for natural hazards. Climate change refers to the variability in weather conditions, both averages and ranges, over extended periods of time. Scientists today are concerned that if climate change occurs too quickly, several dangerous situations would occur such as, melting of the polar ice which would lead to sea level rise and consequently coastal flooding; altering of atmospheric circulation due to changes in temperatures resulting in modification of weather patterns; increase in number and intensity of storms and negative impacts on weather dependant sectors such as agriculture (affecting food production). There has been an increase in global temperatures of 0.3 to 0.6 degrees Celsius over the past century as a result of human additions to atmospheric concentrations of carbon dioxide and methane (for example through burning of fossil fuels) and large-scale changes in land use.
Over the past century global warming has been associated with a consequential observed rise in sea level of 1.0 to 2.5mm per year. Sea-level rise refers to the elevation of ocean or sea levels which may threaten flooding of surrounding land masses. Sea-level rise is one of the anticipated impacts of global climate change, as warmer mean temperatures trigger thermal expansion of ocean waters of 10-25cm over the past century and with a continuation of current trends are anticipated to rise an additional 15-95cm by 2100.
The Earth’s oceans and atmosphere are closely connected and so a change in one can create an instantaneous or postponed change in the other. El Nio involves warming of the sea surface temperatures in the equatorial Pacific Ocean which the temperature change causing the usual positions of the jet streams and pressure cells to shift which ultimately change global weather patterns. This can lead to floods, droughts and other hazards. A La Nia event usually occurs the year after the end of an El Nio event but involves abnormal cooling of the same ocean waters which also alters weather patterns.
This article served to present to you a synopsis of natural disasters and their root cause in a concise manner.
1. Bryant, E. (2005). ‘Natural Hazards Second Edition’, Cambridge University Press, Excerpt Cited on 20/11/06, Available online at www.cambridge.org
EW56Bhttp://www.cambridge.org/WH6PDZ6EFP2. ISDR (2004), ‘Living with Risk -A global review of disaster reduction initiatives’ United Nations/International Strategy for Disaster Reduction Secretariat
3. Mirovitskaya, N., and Ascher, W., (2001) ‘Guide to Sustainable Development and Environmental Policy’ Duke University Press Durham and London
4. UNDP and DHA (1997) ‘Introduction to Hazards’ 3rd Edition, Disaster Management Training Programme
5. UNDP and UNDRO (1992) ‘An Overview of Disaster Management’ 2nd Edition, Disaster Management Training Programme Websites