Natural Hazards

Natural hazard = A sudden severe event which makes the natural environment difficult to manage. 

Types: 

Atmospheric - Droughts, tornadoes, lightning, heatwaves, blizzards, wildfires 

Geological - Earthquakes, volcanic eruptions, landslides, sink holes 

Flooding - Glacial bursts, storm surges 

Factors affecting hazard risk: 

Urbanisation - densely-populated urban areas concentrate those at risk. 

Poverty - expense of housing leads to building on risky ground. 

Farming - the attraction of nutrient-rich floodplains puts people at risk. 

Climate change - global warming raises sea levels and generates more extreme weather.

Tectonic plates

• The Earth's crust is split into 7 major and several minor tectonic plates. 
• There are 2 types of crust - dense, thin, oceanic crust and less dense, thicker continental crust. 
• Plates move, driven by convection currents within the mantle and under gravity. 

Pattern of earthquakes - Most earthquakes occur at the margins of slow-moving tectonic plates. Friction and sticking between plates create enormous pressures and stresses which build to breaking point. 

Distribution of volcanoes - Most occur in belts along plate margins (e.g. the 'Pacific Ring of Fire' and the Mid-Atlantic Ridge). However, some occur at hot spots where the crust is thin and magma breaks through the surface (e.g. Hawaiian Islands). 

Constructive margin:

• 2 plates move apart and magma forces its way to the surface. 
• As it breaks the crust it causes mild earthquakes. 
• The magma is very hot and fluid, allowing the lava to flow a long way before cooling, resulting in typically broad and flat shield volcanoes. 

Destructive margin: 

• 2 plates move towads each other and the dense oceanic plate is subducted beneath the less dense continental plate.
  
• Friction causes strong earthquakes. 
• The sinking oceanic plate creates sticky, gas-rich magma, resulting in steep-sided composite volcanoes which erupt violently. 

Conservative margin: 

• 2 plates move past each other at different rates.
• Friction between the plates build stresses and trigger earthquakes when they slip. 

Why people live at risk from tectonic hazards: 

• Poor people have no choice - money, food and family are seen as more important. 
• Plate margins often coincide with favourable areas for settlement and trade (coastal areas).
• Some people have no experience or knowledge of the risks.
• Volcanoes can bring benefits such as fertile soils, rich mineral deposits and hot water. 
• Earthquakes and volcanic eruptions are rare, so not seen as a great threat.
• Earthquake-resistant building designs reduce risk. 
• Effective monitoring of volcanoes and tsunamis allow evacuation warnings to be given. 

Living on a plate margin: Iceland 

• Naturally occurring hot water and superheated steam provides hot water and central heating for 90% of all buildings e.g. greenhouses and swimming pools. 
• Volcanic rocks are used in road and building construction. 
• Geothermal energy generates 25% of Iceland's electricity. 
• Icelands dramatic landscapes with waterfalls, geysers, lava fields, volcanoes and glaciers support a huge tourism industry. 

Monitoring

• Remote sensing - satellites detect heat increases. 
• Seismicity - seismographs record microquakes.
• Ground deformation - laser beams measure changes in the shape of the ground. 

Prediction

Monitoring is now allowing accurate prediction and effective evacuation. 

Protection 

Little can be done to protect property, but earth embankments and explosives have been used to successfully divert lava flows. 

Planning

Risk assessment and hazard mapping to identify areas to practise evacuation or restrict building.

What is global atmospheric circulation?

• Sinking air creates high pressure, and rising air creates low pressure. 
• Surface winds move from high to low pressure, transferring heat and moisture from one area to another. 
• These winds curve due to the Earth's rotation and change seasonally as the tilt and rotation of the Earth causes relative changes in the position of the overhead sun. 

How global circulation affects the world's weather: 

• Cloudy & wet in the UK - 60 degrees north is close to where cold polar air from the north meets warm, subtropical air from the south. Surface winds from the south-west usually bring warm and wet weather (rising air cools and condenses forming clouds and rain). 
• Hot & dry in the desert - Most deserts are found about 30 degrees north and south where sinking air means high pressure, little rain, hot daytime temperatures and very cold nights. 
• Hot & humid at the equator - Low pressure marks where the sun is directly overhead. Hot, humid air rises, cools and condenses, causing heavy rain - hence the tropical rainforests. 

Tropical storms = huge storms known as hurricanes, cyclones and typhoons in different parts of the world. 

They form 5-15 degrees north and south of the Equator, in summer and autumn where:

• ocean temperatures are highest (above 27 degrees). 
• the spinning (Coriolis) effect of the Earth's rotation is very high. 
• intense heat and humidity makes the air unstable. 

How do tropical storms form? 

• Rising air draws evaporated water vapour up from the ocean surface which cools and condenses to form towering thunderstorm clouds. 
• The condensing releses heat which powers the storm and draws up more water vapour.
• Multiple thunderstorms join to form a giant rotating storm. 
• Coriolis forces spin the storm at over 70mph creating a vast cloud spiral with a central, calm eye of rapidly descending air. 
• Prevailing winds drift the storm over the ocean surface like a spinning top, gathering strength as it picks up more and more heat energy. On reaching land, the energy supply is cut off and the storm will weaken. 

Structure of a tropical storm 

• The central eye is a small area where relatively cold air sinks towards the ground and warms up. There are no clouds here and conditions are calm. 
• On either side of the eye is a tall bank of cloud called the eye wall. Here there are strong winds in excess of 120km/h, heavy rain, thunder and lightning. 
• Beyond the eye wall there are further banks of clouds with thunderstorms and occasionally tornadoes. There will also be strong winds and heavy rain. 

Will climate change affect tropical storms? 

• Over the last few decades, sea surface temperatures in the Tropics have increased by 0.25-0.5 degrees. 
• Tropical storms may extend into the South Atlantic and parts of the sub-tropics. 
• Tropical storms may become more powerful. 
• In the North Atlantic, hurricane intensity has risen in the last 20 years. 
• Currently there is no clear evidence that the numbers or intensities of storms are increasing. 

Monitoring and prediction: 

Developments in technology, including satellite tracking, allow prediction maps to be prepared and warnings issued. The government of the Philippines send out Tropical Cyclone Warning Signals graded on the severity of winds and time frame expected. 

Protection: 

Methods of protection usually involve anticipation in design - reinforced walls, roofs and window shutters; storm drains; sea walls; houses built on stilts and on raised ground. 

Planning: 

Planning involves education and media campaigns to raise individual and community awareness  so that people understand the dangers and are able to respond. 

Orbital changes - the Milankovitch cycles

Eccentricity - every 100,000 years or so the orbit changes from almost circular, to mildly elliptical (oval) and back again. 

Axial tilt - every 41,000 years the tilt of the Earth's axis moves back and forth between 21.5 degrees and 24.5 degrees. 

Precession - over a period of about 26,000 years the axis wobbles from one extreme to the other.

Solar & volcanic activity 

The surface of the sun has sunspots which mark short-term regions of reduced surface temperature. They are usually accompanied by explosive, high-energy solar flares increasing heat output. 

Volcanic ash can block out the sun, and releases sulphur dioxide which converts to suphuric acid and reflects solar radiation back into space. These reduce Earth's temperature. 

Alternative enegry sources - hydroelectric power, nuclear power, solar, wind, and tides. 

Carbon capture - Carbon capture and storage uses technology to capture CO2. Once captured, it is compressed, piped and injected underground for long-term storage in suitable geological reservoirs such as depleted oil and gas reservoirs. 

Planting trees - Trees act as carbon sinks, removing CO2 from the atmosphere by the process of photosynthesis. They also release moisture, producing more cloud and so reducing incoming solar radiation. 

International agreements - Governments are negotiating towards a more sustainable future. The Paris Agreement was the first legally binding global climate deal that aimed to limit global temperature increases to 1.5 degrees above pre-industrial levels. 

Global impacts of climate change - Reduced crop yields and water supplies, more heat related illness and disease, low-lying coastal areas threatened by flooding, changing ecosystems and animal habitats, more extreme weather events. 

Agricultural adaptation - Farmers will need to: 

• manage water supply by storing water 
• use efficient irrigation systems 
• grow drought-resistant strains of crops 
• plant trees to shade seedlings from strong sunshine 
• introduce new cropping patterns e.g changing planting/sowing dates 

Reducing risk from rising sea levels 

The Maldives have already began tackling this problem by:

• constructing sea walls - a 3m sea wall is being constructed around the capital Male with sandbags.
• building houses that are raised off the ground on stilts 
• restoring coastal mangrove forests - their tangled roots trap sediment and offer protection from storm waves 
• constructing artificial islands up to 3m high so that people most at risk could be relocated.