Topic 5 EQ2 Flashcards
General definition of drought from the UN
An extended period of deficient rainfall relative to the statistical multi-year average for a region
What are the four types of droughts?
Meteorological
Hydrological
Agricultural
Socio economic
What is meteorological drought?
- When long term precipitation is much lower than normal
-region specific since the atmospheric conditions that result in deficiencies of precipitation are highly variable between climate.
What is agricultural drought?
- Occurs when there is insufficient soil moisture to meet the needs of a particular crop at a particular time.
- caused by a number of factors such as precipitation shortages
- typically evident after a meteorological drought
What is hydrological drought?
- occurs when there are deficiencies in surface and subsurface water supplies as measured in rivers, reservoirs, lakes and groundwater.
- it originates with precipitation deficiency but occurs after an agricultural drought
What are socio-economic droughts?
- Occurs when the water water demand for social and economic purposes (such as crop irrigation or hydro electric power) exceeds water availability.
- result of a weathered shortfall in water supply or overuse of the available water supplies.
Percentage of land area with severe droughts
10% of land surface which includes 18% of the population
Main physical cause for drought
The Global Atmospheric Circulation system
First process of atmospheric circulation
Intense solar radiation at the equator warms the air which rises and starts convection. The air cools as it rises and condenses to form clouds and rain.
Second process of atmospheric circulation
The Subtropical high pressure is created where air that had risen at the equator has cooled and so sinks to form a belt of high air pressure and hot, dry conditions.
What is the third process of atmospheric circulation?
The air returns to ground level at the equator, creating trade winds
What is the fourth process of atmospheric circulation?
The trade winds meet at the inter tropical convergence zone where the warmed air rises.
The position of the ITCZ?
- Moves with the seasons
- in the Northern hemisphere summer, the ITCZ is North of the equator.
- in December to February, the Southern Hemisphere is tilted towards the sun and therefore the ITCZ is south of the equator.
- this movement creates alternating wet dry seasons in the tropics
Fifth process of atmospheric circulation
Warm air moving from the sub tropics to mid-latitudes meets cold polar air at the polar front where the warm, less dense air rises causing condensation and rainfall.
Sixth process of atmospheric circulation
The warmer air rises into the polar front jet stream and is transferred at high altitude toward the poles, where it cools and sinks. Creates a movement of air at ground level back towards the equator .
What is the intertropical convergence zone?
A belt of low pressure located around the equator. It moves North or south of the equator seasonally.
What is a high pressure block?
When the descending part of the Hadley cell blocks the high humidity, rain bearing air masses associated with the ITCZ.
What is the jet stream?
A very fast moving, meandering belt of air in the upper troposphere
What happens when the loops of the jet stream break down or stabilise?
Allows high pressure (anti cyclones) areas from the sub tropics to move northwards
What do anticyclones bring?
Stable weather conditions with very little precipitation (heat waves) while the rain bearing depressions are forced around them causing drought in mid latitude countries such as the UK.
Consequence of stability of anti cyclones
The sinking air and calm conditions means means that they can persist and block weather systems from the west for up to two weeks
What is an ENSO?
A naturally occurring large mass of very warm seawater in the equatorial Pacific Ocean
Normal conditions in the Pacific
- trade winds blow warm air westwards
- upwelling of nutrient rich cold water lead to optimum fishing conditions
- pressure of the trade winds result in sea levels in Australia being 50cm higher and sea temperatures being 8°c higher
Conditions in El Niño years
- trade winds in the Western Pacific weaken and die
- piled up water in the west moves back east with the convectional uplift. Distorts the path of the jet stream which can lead to teleconnections.
- Eastern Pacific becomes 6-8°c warmer which overrides the cold humboldt current which breaks the food chain
Conditions in La Niña years
- extremely strong trade winds
- push warm water westward, increasing sea level
- low pressure develops with very strong conventional uplift - heavy rain in South East Asia
- very strong upwelling results in strong high pressure and extreme drought on the West coast of South America
How do humans influence the impacts of droughts?
The Sahel contains several of the poorest countries in the world. Human factors act as a positive feedback in enhancing its impacts.
What is desertification?
Land degradation in dry areas resulting from climate variations and human activities.
What are the physical short term causes of desertification?
Less rainfall in the amount and reliability
What are the long term causes of desertification?
Global warming: higher temperatures - increased evaporation, less condensation
Human cause of desertification in the Sahel
- increase of livestock on fragile grassland. Livestock numbers increased by 40% preceding the mid 1960’s. Overgrazing leads to nutrient poor soil.
Sahel population growth
- High birth rates and lower death rates - population double every 20-30 years
- farmers use more land. Over cultivation leads to reduced soil fertility, soil left exposed to wind and rain.
- immigrants including refugees from civil wars and roughs . More demand for wood for fuel, cooking
Consequences of desertification
Vegetation dies - decrease in protective layer
Vegetation cannot re establish itself - increase in evaporation
Vegetation removed - increased risk of soil erosion
Reasons for drought in Australia
- sub tropics high pressure - low and variable rainfall
- El Niño led to drought in Southern Queensland
What was the ‘Big Dry’
- a 1 in 1000 year drought event associated with longer term climate change, leading to a trend of warmer, drier climate
Impacts of the Big Dry
- 30% of country affected
- affected more than 50% of farmlands which provides 50% of the nations agricultural outputs.
- reservoirs fell to 40% of capacity
- growing affluence water consuming population
What is a wetland?
- An area of marsh, fen, peatland or water whether natural or artificial, permanent or temporary with water that is static or flowing fresh, brackish or salt.
- cover 10% of the Earths land surface
What are the key function of wetlands?
- act as temporary water stores within the hydrological cycle which mitigates river flows downstream.
- chemically, wetlands trap and recycle nutrients and pollutants which maintains water quality
- biologically, supports a diverse food web: nursery areas for birds and refuges for mitigating birds
Why are wetlands valued by humans ?
- provides resources (fish, fuelwood)
- hydrology within the water cycle and carbon store within the carbon cycle
What leads to a less functional wetland?
- increase in evaporation
- less protected surface
- decrease in transpiration
What is desiccation ?
Removal of moisture from an area. Accelerates destruction by wildfire
How many square kilometres of wetlands have been lost in the USA and Europe?
2.5 million square kilometres have been destroyed for agricultural and urban purposes and exploiting fuel resources such as peat.
Schemes that have led wetland drainage
- Jonglei canal project which diverted the white Nile discharge away from the Sudd Swamp to the dry land areas of South Sudan.
Scheme developed to protect wetland
The 1991 Ramsar convention on wetlands has listed over 1800 wetlands of international importance covering 1.7 million square kilometres.
Environments more at risk of high flows of water
- low lying parts of floodplains which are subject to river and groundwater flooding.
- low lying partially urbanised areas
- small basins, short lag time
Climatological causes of flooding
- rain
- snowmelt
- ice melt
- combined rain and melt
Part climatological causes of flooding
- coastal storm surges
Other causes of flooding
- earthquakes
- landslides
- failure of dams and other control works
Usual causes of flooding in the UK
- prolonged heavy rainfall
- traditional time for this to occur is Autumn or Early winter but unusual positions of the jet stream can change the time of year. Eg summer floods of 2007 and 2008
Usual causes of flooding in Southern and Eastern Asia
- intense seasonal and monsoonal rainfall
- Pakistan suffered floods in 2010, primary cause was the 9000 mm of water received in one week, 10x the yearly average
Floods caused by snow and ice melt
- responsible for many flood events usually in higher latitudes or mountainous areas
- melting snow in late spring causes extensive flooding
- in the Himalayas, glacial outburst floods occur as ice dams melt
Human causes of flooding
- grazing animals trample soil
- ploughing compacts soil
- dams
- channelisation
- impermeable areas
- sewers feed water into channel
- bridge support
- deforestation
Cause of floodplains to be built on
Economic and population growth caused many floodplains to be built on for agricultural, industrial and urban
Why Researchers suggest that urbanisation is the key factor for flooding
- impermeable surfaces
- artificial conduits speeding up drainage
- impede channel flow by building alongside the river
- straightening channels of increase the flow result in flooding downstream
- changing land use - increased run off and increased sediment
Blame for the 2015 Cumbria floods
- extreme weather induced by climate change
- budget cuts in the amount of money being spent on flood defences
- the EU water framework put environmental concern before regular maintenance
- poor land management, for example blocking ditches or improving pasture then over grazing it
What is the flood return period?
Also known as the flood recurrence interval. Estimate of the likelihood of a flood of a certain size occurinh
Impacts of flooding
- 900 million people live in flood prone areas
- 90% of all flood deaths occur in Asia
- 50% of flood damages occur in Asia
- cost of £1.3 billion quoted for 2015 UK floods
Socio economic impacts of flooding
- post flood morbidity is extremely likely in low-income countries
- Psychological stress post-flood
- livelihoods affected
- damage to property
- concerns on flood insurance post flood
- fall in property prices
Positive environmental impacts of flooding
- in natural ecosystems floods play an important role in maintaining key ecosystem functions and biodiversity by linking the river with land.
- recharge groundwater systems
- fill wetlands
- more sediment and nutrients into marine environment
- flood events trigger breeding , migration and dispersal
- resilient to moderate flooding leading to increased productivity and maintenance of recreational environments
Negative environmental impacts of flooding
- amplify effects of human activities
- excessive overland flow can lead to oversupply of sediment and nutrients with possible eutrophication and the destruction of aquatic plants
- pollution from nitrates chemicals and heavy metals
- biodiversity loss
What is eutrophication?
Excess richness of nutrients causes dense growth of plant life and death of animal life from a lack of oxygen.
Impacts of the Summer 2007 floods in England and Wales
- rainfall along the river Severn
- a powerful storm sent the river into floods
- a dozen people were killed
- £6 million
- upper Severn at Shrewsbury was 6 times the normal level
- mid Severn around Worcester, river levels rose 4.5 m above normal
What do most scientists agree about on the impacts of climate change on the hydrological cycle?
Climate change will result in an intensification, acceleration or enhancement of the global hydrological cycle.
How does climate dynamics make modelling climate change trend more complex?
The way the atmosphere, ocean, terrestrial, cryosphere and biosphere systems all interact with each other is only partially understood by science
How does teleconnections make climate change trends complex to model?
Teleconnections make it difficult to distinguish between the impacts of oscillations such as ENSO and climate warming
Why do global records make it difficult to model climate change trends?
Global records are incomplete; in many parts of the world there is insufficient depth or detail of evidence to establish reliable trends for the impact of climate change.
Trend of the precipitation input
- widespread increases in intense rainfall events have occurred although overall amounts remained steady or decreased
Which areas have seen a change of precipitation?
- High latitude areas and the tropics have seen an increase of precipitation
- 10-30° North and South of the equator have seen decreases
- length, frequency and intensity of heat waves has increased widely especially in southern Europe and Southern Africa
- more precipitation now fall as rain instead of snow in Northern regions.
Evaporation and evapotranspiration trends
- some research suggested that in large areas of Asia and North America actual evaporation is increasing but increased cloud cover may work against this
- transpiration is linked to any changes in vegetation and soil moisture
Soil moisture key findings
- results are ambiguous as the amount of soil moisture is related to many factors.
- where precipitation increases , soil moisture will increase
Run off and stream flow key findings
- a 1° rise in temperature could increase global run off by 40%
- reduce infiltration
Groundwater flow key findings
Evidence is again limited with no definitive link between groundwater amounts and climate change as human abstraction is the dominant influence
Reservoir, lake and wetland storage key findings
- regional variations have been linked to regional changes in climate eg Lake Chad
- wetlands are affected where there are decreasing water volumes and higher temperatures.
Permafrost key findings
- changes in climate at high latitudes, primarily increasing air and ocean temperatures are leading to permafrost degradation in Northern areas
- deepening active layer
- impacts groundwater supplies
- releases methane from thaw lakes
- leads to positive feedback and accelerating change
Snow trends
- most studies suggest that the length of snow cover season has decreased especially in the Northern hemisphere
- spring melt has occurred earlier in the last 50-100 years
Ice trends
- strong evidence that glaciers have retreated globally since the end of the little ice age
- downwasting (thinning of a glacier) accelerating as a result of rapid temperature increase and changes in precipitation
Ocean trends
In areas of ocean warming, increased evaporation will occur
How is diminishing water supply increasing uncertainty?
- increase temperatures lead to greater evaporation of surface waters and reservoirs
- greater evapotranspiration leads to desication
- impact of oscillations is leading to unreliable patterns of rainfall
- more frequent cyclone and monsoon events threaten water supplies intermittently
