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Global Hazards


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  • 1. World at risk Global hazards
  • 2. Hazards and disasters (1)
      • A natural hazard is a natural event with the potential to harm people and their property.
      • A natural disaster is the realisation and impacts of a natural hazard, I.e. the deaths, injuries, disruption and damage.
  • 3. Hazards and disasters (2)
    • The EM-DAT international database ( ) suggests a hazard becomes a disaster when at least one of the following criteria are met:
      • 10+ people are killed.
      • 100+ people are affected.
      • A state of emergency is declared.
      • International assistance is called for.
    Dregg’s model of a natural disaster
  • 4. Classifying natural hazards
    • Natural hazards can be classified by physical process.
    • Hydro-meteorological hazards result from weather systems.
    • Geophysical hazards result from tectonic processes.
    • Some hazards are termed context hazards because they have the potential to affect the entire planet.
    • Global warming is a context hazard.
  • 5. Depressions (1)
      • Depressions, or mid-latitude cyclones, are cyclonic storms.
      • They form when cool polar maritime (Pm) and warm tropical maritime (Tm) air meet at the polar front.
      • The less dense Tm air rises, forming clouds and precipitation along warm and cold fronts.
      • Depressions rotate around a low pressure centre.
      • As they develop strong winds and heavy rain (or snow) intensify. Gale force winds can cause property damage and floods may occur.
      • In some cases, severe depressions produce significant hazards.
  • 6. Depressions (2)
    • Weather associated with the passage of a depression
  • 7. Hurricanes (1)
      • Hurricanes are intense storms in the subtropics.
      • They begin as tropical depressions and intensify into hurricanes.
      • Hurricanes need sea surface temperatures of 27°C+ to generate intense convection, plus weak upper level winds to allow the storm to develop its characteristic spiral.
      • Hurricanes form 500 km north and south of the equator where the Coriolis effect is strong enough to generate spin.
      • Hurricane strength is measured using the Saffir-Simpson scale ranging from weak Category 1, to intense Category 5.
      • Hurricane force winds, driving rain and storm surges batter coastlines as a hurricane passes.
  • 8. Hurricanes (2) Cross-section of a hurricane
  • 9. Drought
      • Drought is a lack or shortage of water for an unusually long period of time. It can take many months or even years to develop.
      • Droughts hit hardest in areas which rely directly on agriculture, i.e. the developing world.
      • Eventually famine sets in as food supply runs out.
      • This often leads to large-scale migration.
  • 10. Floods
      • Flooding occurs when the capacity of a river channel is exceeded by the water discharge.
      • Persistent rain, over a period of days or weeks, leads to more widespread flooding.
      • Widespread flooding tends to cause property damage as houses and fields remain underwater for days or weeks.
      • Flooding has complex causes, often partly human, e.g. deforestation and poor river management.
      • Flash floods often occur due to intense precipitation over a small area. These sudden floods can have a devastating effect.
  • 11. Tornadoes (1)
      • Tornadoes are small-scale, short-lived storms. They remain in one place for only a few seconds.
      • They begin as large thunderstorms (supercells) where warm and cold air meet.
      • Rapidly convecting warm air produces towering clouds which are twisted by strong upper level jet stream winds.
      • Winds can reach up to 350 km h –1 while the tornado itself moves at an average of about 60 km h –1 .
      • Tornado wind speed is measured using the Fujita scale . An F5 tornado has winds in excess of 323 km h –1 .
      • Tornadoes can be locally devastating, ripping narrow paths of complete destruction.
  • 12. Tornadoes (2)
    • The structure of a tornado
  • 13. Fire
      • Wildfires are a natural phenomenon, often started by lightning.
      • They are common during heatwaves and droughts when vegetation is especially dry.
      • Strong, dry winds such as the Santa Ana winds of California combined with drought conditions can create ‘fire weather’, raising wildfire risk.
      • Wildfires become extreme when the canopy (or crown) of trees catches fire. Such fires can spread at incredible speed.
      • The tendency for people to try to extinguish all forest fires can actually raise risk. Unburned leaves, twigs and branches build up over time to create a vast fuel source making fires more powerful.
  • 14. Volcanoes (1)
      • Volcanoes occur when magma is forced to the surface through cracks and fissures in the Earth’s crust.
      • The degree of volcanic hazard is measured using the VEI (volcanic explosivity index) scale ranging from 0 to 8.
      • Explosivity depends on magma viscosity. The more viscous the magma, the more hazardous the volcano.
      • Viscosity depends on temperature, gas and silica content.
      • Highly explosive volcanoes erupt low temperature, viscous lava with a high silica content.
      • Volcanoes are often places of multiple hazards.
  • 15. Volcanoes (2)
    • Cross-section of an erupting volcano
  • 16. Earthquakes (1)
      • Earthquakes most commonly occur when two tectonic plates move suddenly against each other.
      • Rocks fracture underground at the earthquake focus and the Earth’s crust shakes as energy is released.
      • Waves spread from the epicentre , the point on the surface above the focus.
      • Earthquakes are measured using the Richter magnitude scale , and Mercalli intensity scale .
      • Severe earthquake damage can occur when unconsolidated sediment undergoes a process called liquefaction. This is often responsible for the worst ground shaking and damage.
  • 17. Earthquakes (2)
    • Cross-section across oceanic/continental plate convergence at a destructive plate boundary
  • 18. Tsunamis (1)
      • Tsunami waves are caused by the rapid displacement of water.
      • Submarine earthquakes are the most common cause.
      • Across the open ocean tsunami waves travel at speeds up to 700 km h –1 . The wavelengths are hundreds of kilometres, but their height is only about 1 m.
      • Tsunami waves cannot be seen out at sea. Only as they approach the shore, slow down and increase in height does their potential for destruction become clear.
  • 19. Tsunamis (2)
    • How a tsunami is generated
  • 20. Tsunamis (3)
    • How a tsunami is generated
  • 21. Tsunamis (4)
    • How a tsunami is generated
  • 22. Landslides
      • Landslides are downslope movements of rock and soil under the influence of gravity. They are part of a wider group of processes termed mass movement .
      • The term ‘landslide’ implies a relatively rapid movement, ranging from seconds to a few days.
      • Most hazardous landslides involve water, and heavy rain is often one of the key causes. Earthquakes often trigger landslides too.
      • Rapid liquid flows (e.g. mud and debris flows, volcanic ash lahars) are the most devastating.
      • Like floods, landslides often have complex causes and humans frequently play a part, e.g. increasing risk due to deforestation.
  • 23. Avalanches
      • Avalanches are a type of mass movement which involves snow, ice and other debris.
      • They occur on mountains, typically on slopes of about 30  –45  .
      • Avalanches occur within snowpacks which contain different layers of snow, some weak and some strong.
      • Changing wind conditions, temperatures and further snow falls can cause weak snow layers to fail. Human actions (e.g.skiing) and other triggers (e.g.Earth tremors) also contribute.
      • Failure is often in the form of slab failure, when a large area of snow begins to move. Speeds of 300 km h –1 can be reached as the slab breaks up and rushes downslope.
      • Most avalanche fatalities result from burial.
  • 24. Risk (1)
      • Risk, as shown in the disaster risk equation , increases as hazardous events become more common, people become more vulnerable and their capacity to cope decreases.
      • Risk can be reduced by reducing vulnerability, increasing capacity or reducing hazard frequency and/or magnitude.
      • R = risk, H = frequency or magnitude of hazard,
      • V = vulnerability level, C = capacity to cope
  • 25. Risk (2)
    • Risk The probability of harmful consequences, or expected losses, resulting from interactions between natural or human-induced hazards and vulnerable conditions.
    • Hazard A potentially damaging physical event, phenomenon or human activity that may cause the loss of life or injury, property damage, social and economic disruption or environmental degradation.
    • Vulnerability The conditions determined by physical, social, economic and environmental factors or processes, which increase the susceptibility of a community to the impact of hazards.
    • Capacity to cope A combination of all the strengths and resources available within a community, society or organisation that can reduce the level of risk, or the effects of a disaster.
  • 26. Vulnerability (1)
      • Vulnerability is important because it helps to determine the exact impact a hazard will have on a population.
      • In general, developing countries and regions are more vulnerable than developed ones.
      • Capacity to cope is generally lower in the developing world, so hazard impact lasts longer than in the developed world.
      • Vulnerability has social, economic, environmental and physical aspects.
      • Poverty, poor social conditions, environmental degradation and unfavourable physical geography all increase vulnerability.
  • 27. Vulnerability (2)
    • Increasing vulnerability
      • Population growth
      • Urbanisation and urban sprawl
      • Environmental degradation
      • Loss of community memory about hazards
      • Ageing population
      • Ageing infrastructure
      • Greater reliance on power, water, communication systems
      • Over-reliance on technological fix
    • Decreasing vulnerability
      • Warning and emergency-response systems
      • Economic wealth
      • Government disaster-assistance programmes
      • Insurance
      • Community initiatives
      • Scientific understanding
      • Hazard engineering
  • 28. Vulnerability compared $73 million in damage, including 1,200 buildings damaged Economic losses about $10 billion; up to 90,000 people displaced; 18,000 buildings destroyed Property damage 0 deaths/several hundred minor injuries At least 25,000 deaths/around 30,000 injured Deaths/ injuries Hawaii, USA October 2006 6.7 Richter scale earthquake Bam, Iran December 2003 6.6 Richter scale earthquake
    • In Bam 90% of buildings were mud (adobe) brick built, with no structural frame. In Hawaii, most buildings could resist the ground shaking with only minor damage.
    • In Bam, many emergency service buildings and vehicles were damaged by the earthquake.
    • Average incomes in Hawaii are $30,000 per year compared to $3,900 in Iran. Bam is in one of Iran’s poorer, more isolated regions.
  • 29. Global warming: a context hazard (1)
      • Global warming is described as a chronic hazard because it is continually present.
      • Other context hazards, eg. an asteroid strike or super-volcanic eruption, are described as rare because they are ‘one offs’.
      • Global warming is potentially a global hazard because its impacts could be very widespread, e.g causing whole climate zones to shift .
      • Global warming could cause increases in the frequency and magnitude of hydro-meteorological hazards (see next slide).
      • It could also increase vulnerability to tectonic hazards by reducing food supply and water availability.
      • It might be argued that global warming is unjust, as approximately 80% of pollution is caused by the developed world but it is likely to be the more vulnerable developing world which is most at risk as climates change.
      • As a global problem, it requires a global solution which is by its very nature, complex.
  • 30. Global warming: a context hazard (2) Possible impacts of global warming on hydro-meteorological hazards These could become more frequent, and more intense over areas such as the UK. Depressions These may become more intense, and possibly more frequent. New areas could become affected. Hurricanes Mountain areas could experience more variable weather patterns, reducing predictive ability. Avalanches Already vulnerable areas such as the Sahel could experience increased drought. Drought Changing rainfall patterns could increase risk in some areas; floods may become more common. Floods