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Droughts and wildfires are slow-onset hazards that can persist for months or years, affecting agriculture, water supply, ecosystems and human health. They are often interconnected: prolonged drought dries vegetation, creating the conditions for catastrophic wildfires. As climate change intensifies, these hazards are becoming more frequent and severe in many parts of the world. Understanding their causes, impacts and management is essential for the AQA Hazards specification.
Key Definition: A drought is a prolonged period of abnormally low precipitation relative to the statistical average for a region, leading to a shortage of water for people, agriculture and ecosystems. Unlike other hazards, drought has a gradual onset and no precise beginning or end.
| Type | Definition | Indicator |
|---|---|---|
| Meteorological drought | A sustained period in which precipitation is significantly below the long-term average for the region | Measured by precipitation deficit (e.g., < 75% of average for 3+ months) |
| Agricultural drought | Insufficient soil moisture to meet the needs of crops at a particular growth stage | Measured by soil moisture levels and crop yield data |
| Hydrological drought | Reduced streamflow, reservoir levels and groundwater levels, usually lagging behind meteorological drought by weeks or months | Measured by river discharge, reservoir levels, groundwater monitoring |
| Socio-economic drought | The point at which drought begins to affect the supply and demand of economic goods — food, water, energy | Measured by food prices, water rationing, economic losses |
graph LR
A["Meteorological<br/>Drought<br/>(weeks)"] --> B["Agricultural<br/>Drought<br/>(weeks-months)"]
B --> C["Hydrological<br/>Drought<br/>(months)"]
C --> D["Socio-economic<br/>Drought<br/>(months-years)"]
| Cause | Mechanism |
|---|---|
| Changes in atmospheric circulation | Shifts in the jet stream or subtropical high-pressure belts can redirect moisture-bearing weather systems away from a region. Persistent blocking anticyclones prevent rainfall |
| El Nino-Southern Oscillation (ENSO) | El Nino events suppress rainfall in Australia, Indonesia, southern Africa and northeast Brazil while increasing rainfall in western South America. La Nina events can cause drought in southwestern USA and the Horn of Africa |
| Indian Ocean Dipole (IOD) | A positive IOD (warmer western Indian Ocean, cooler east) reduces rainfall in Australia and Southeast Asia |
| North Atlantic Oscillation (NAO) | Influences winter rainfall patterns across Europe; a negative NAO phase can reduce rainfall in southern Europe |
| Monsoon variability | Weakened or delayed monsoon seasons cause drought across South and Southeast Asia — affecting billions of people |
| Factor | How It Worsens Drought |
|---|---|
| Deforestation | Reduces evapotranspiration, decreasing atmospheric moisture and rainfall (a positive feedback). The Amazon generates ~50% of its own rainfall through transpiration — deforestation risks pushing the system past a tipping point (Lovejoy and Nobre, 2018) |
| Over-abstraction of water | Groundwater depletion lowers water tables; rivers are over-extracted for irrigation. The Aral Sea has lost 90% of its area since 1960 due to Soviet-era irrigation projects diverting its feeder rivers |
| Soil degradation | Compacted or degraded soils have reduced infiltration capacity, increasing runoff and reducing soil moisture storage |
| Climate change | Higher temperatures increase evapotranspiration, exacerbating drought even without reduced rainfall. Some regions are experiencing reduced precipitation as well |
| Urbanisation | Impermeable surfaces prevent infiltration; increased water demand from growing populations |
Key Definition: Desertification is the degradation of land in arid, semi-arid and dry sub-humid areas, resulting from climatic variations and human activities (UNCCD, 1994). It does not mean the expansion of existing deserts but rather the degradation of productive land.
The Sahel — the semi-arid transition zone south of the Sahara Desert stretching 5,000 km from Senegal to Eritrea — is the most cited example of desertification:
| Factor | Detail |
|---|---|
| Climate | Annual rainfall 200–600 mm; highly variable from year to year; devastating droughts in 1968–1974 and 1983–1985 killed an estimated 250,000 people and millions of livestock |
| Population growth | Population has increased from ~30 million (1950) to ~140 million (2020), placing enormous pressure on land and water resources |
| Overgrazing | Livestock numbers have increased; overgrazing removes vegetation, exposing soil to wind erosion |
| Overcultivation | Shortened fallow periods deplete soil nutrients; marginal land is brought into cultivation |
| Deforestation | Firewood is the primary fuel source; trees are cleared for agriculture; roots that bind soil are lost |
| Wind erosion | Exposed, degraded soil is blown away by the Harmattan wind, creating dust storms that can reach Europe |
Key Definition: A wildfire is an uncontrolled fire in an area of combustible vegetation. Wildfires are a natural part of many ecosystems but become hazards when they threaten lives, property and livelihoods.
Three elements must be present for fire:
graph TD
A["FUEL<br/>Vegetation, organic matter"] --- B["HEAT<br/>Ignition source<br/>(lightning, human)"]
B --- C["OXYGEN<br/>Wind supplies O2"]
C --- A
A --> D["FIRE"]
B --> D
C --> D
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