Drought is a slow disaster. Unlike floods, hurricanes, or earthquakes that strike rapidly, drought builds over months or years. Rivers shrink, reservoirs dry, crops fail, ecosystems collapse, and entire civilizations can be uprooted. The Sahel, the American Dust Bowl, the Australian Millennium Drought — all show how drought reshapes societies. Yet droughts often start quietly with just slightly less rain than usual.
The Short Answer
Droughts are caused by prolonged periods of below-average precipitation, often combined with increased evaporation from high temperatures. The underlying cause is typically a shift in atmospheric circulation patterns that diverts rain-producing weather systems away from a region. Other factors — ocean temperature patterns, jet stream shifts, vegetation feedbacks — can intensify and prolong droughts.
Types of Drought
Scientists distinguish several types of drought:
- Meteorological drought: Defined by reduced precipitation over a specific period. The starting point of all droughts.
- Hydrological drought: Reduced river flows, lake levels, groundwater. Lags meteorological drought by months.
- Agricultural drought: Insufficient soil moisture for crops, even if precipitation has recovered.
- Ecological drought: Ecosystem-level impacts on vegetation and wildlife.
- Socioeconomic drought: When supply can't meet human demand — drinking water, irrigation, hydropower.
What Drives Drought
Several atmospheric and oceanic factors create drought:
- High-pressure ridges: Persistent high pressure suppresses rain-producing weather systems.
- Jet stream shifts: Storm tracks moving away from a region.
- El Niño / La Niña: Pacific Ocean temperature patterns shift rainfall worldwide.
- North Atlantic Oscillation: Affects European and Mediterranean drought.
- Indian Ocean Dipole: Drives drought in Australia and East Africa.
- Atlantic Multidecadal Oscillation: Multi-decade rainfall patterns in the Americas and Africa.
The Role of Heat
Drought isn't just about reduced rainfall — heat amplifies it. Higher temperatures cause:
- More evaporation from soil, lakes, reservoirs
- Increased water loss from plants ("evapotranspiration")
- Faster snowmelt, reducing summer water supply
- Lower humidity, drying soils faster
Climate change is increasing global temperatures, which means even when precipitation is normal, drought conditions can intensify due to evaporation. "Hot droughts" are becoming more common.
Famous Historical Droughts
- American Dust Bowl (1930s): Combined drought and poor farming practices destroyed Great Plains agriculture; 2.5 million people displaced.
- Sahel Drought (1968-85): Devastated West African savanna; contributed to ~100,000 deaths.
- Ethiopian Famine (1983-85): Drought triggered famine; over 1 million deaths.
- Australia's Millennium Drought (1997-2009): Worst since European settlement; reshaped water policy.
- California Drought (2012-2017): Most severe in 1,200 years; followed by intense flooding in 2017.
- Cape Town "Day Zero" (2017-18): Almost ran out of water entirely.
- Horn of Africa drought (2020-23): Worst in 40 years; affected 23+ million people.
Effects on Agriculture
Droughts devastate farming:
- Crop failures and reduced yields
- Livestock losses from lack of pasture and water
- Farmer bankruptcies and rural depopulation
- Food price increases globally
- Forced migration of agricultural communities
- Long-term soil degradation
Ecological Impacts
Droughts reshape ecosystems:
- Forest die-offs (especially as droughts combine with insect outbreaks)
- Wildfires (drought-stressed vegetation burns more easily)
- Fish kills in shrinking water bodies
- Wildlife migration and starvation
- Shifts in species ranges
- Long-term changes in vegetation composition
Drought and Civilization Collapse
Many historians link drought to past civilizational collapses:
- Maya: Multi-century drought may have contributed to the abandonment of major Classic-period cities.
- Akkadian Empire: Drought around 4,200 years ago.
- Khmer Empire: Droughts at the end of Angkor's greatness.
- Anasazi/Ancestral Puebloans: Drought drove abandonment of cliff dwellings.
- Late Bronze Age collapse: Drought may have triggered mass migrations.
The Sahara's Drying
The Sahara wasn't always a desert. About 11,000-5,000 years ago, the "Green Sahara" period brought monsoon rains that supported lakes, grasslands, and human populations. The transition to desert happened gradually as Earth's orbital geometry changed, shifting monsoon patterns. Today's Sahara is the largest hot desert on Earth, but it represents a regional drought lasting thousands of years.
Megadroughts
Tree-ring records reveal "megadroughts" lasting decades:
- Western North America has experienced megadroughts lasting 20-40+ years multiple times in the past 1,200 years
- The current 21st-century drought in the southwestern US (2000-present) is now classified as a megadrought
- Drought paleoclimate (using tree rings, lake sediments) shows climate variability far exceeding 20th-century weather records
Climate Change and Drought
Climate change is intensifying drought in several ways:
- Higher temperatures increase evaporation regardless of precipitation
- Some regions are getting drier (Mediterranean, southwestern US, parts of southern Africa)
- Earlier snowmelt reduces summer water supply
- Reduced soil moisture amplifies heat waves
- Vegetation stress from heat creates positive feedback
Some regions face very different rainfall futures than the past, requiring different water infrastructure and agriculture.
Drought Management
Strategies to cope with drought:
- Water conservation: Reduced lawn watering, low-flow fixtures, efficient irrigation.
- Drought-resistant crops: Breeding and selecting plants that need less water.
- Water markets: Allowing water to flow to highest-value uses.
- Reservoir management: Saving water during wet years for dry ones.
- Groundwater management: Preventing over-pumping during droughts.
- Drought monitoring: Tracking conditions to provide early warning.
- Cloud seeding: Attempting to enhance rainfall (limited evidence of effectiveness).
- Desalination: Removing salt from seawater (expensive but increasingly viable).
Drought Indicators
Scientists track multiple indices to characterize drought:
- Palmer Drought Severity Index: Considers precipitation and temperature; traditional standard.
- Standardized Precipitation Index: Based on rainfall departures.
- Standardized Precipitation Evapotranspiration Index: Includes evaporation effects.
- US Drought Monitor: Composite weekly map combining multiple inputs.
- Satellite measurements: Soil moisture, vegetation greenness, groundwater (GRACE satellites).
Drought in a Changing World
Some regions facing significant drought trends:
- Southwestern North America: Multi-decade drought now considered the worst in 1,200 years.
- Mediterranean basin: Drying significantly; major implications for southern Europe, North Africa.
- Southern Africa: Particularly hard hit recently.
- Southern Australia: Long-term drying trend.
- Amazon: Increasing drought frequency threatens forest viability.
- Central Asia: Aral Sea catastrophe linked to irrigation removing river water.
Key Facts
- Droughts are prolonged periods of below-average precipitation.
- Heat amplifies drought by increasing evaporation.
- Drought types include meteorological, hydrological, agricultural, ecological.
- Megadroughts can last decades.
- Climate change is increasing drought intensity globally.
Fun Facts
- The Sahara was green 6,000 years ago with monsoon rains and lakes.
- California's 2012-17 drought was the most severe in 1,200 years.
- Tree rings record droughts going back thousands of years.
- The Sahel drought may have killed 100,000+ people in the 1980s.
- "Day Zero" almost happened in Cape Town in 2018.
Forecasting Droughts
Seasonal drought forecasting has improved significantly. Models combine ocean temperatures, atmospheric patterns, soil moisture data, and historical analogues. The U.S. Drought Monitor releases weekly maps. The European Drought Observatory provides continent-wide assessments. Climate centers in major countries publish seasonal outlooks. Skill in forecasts varies — some 3-6 month predictions are quite reliable, others remain difficult. Improving drought forecasting helps farmers plan, water managers prepare, and emergency responders mobilize. As climate change makes droughts more frequent and severe, forecasting becomes even more important.
Drought-Resistant Agriculture
Agriculture is adapting to increasing drought risk through breeding, technology, and practices. Drought-resistant crop varieties have been developed for wheat, corn, sorghum, and rice. Drip irrigation uses 30-70% less water than flood irrigation. Soil moisture sensors guide precise watering. Agroforestry adds trees to fields to reduce evaporation. Cover cropping maintains soil moisture between harvests. Israel pioneered many water-saving techniques and exports the technology globally. Genetically engineered drought-resistant crops are entering mainstream agriculture. While these innovations help, they don't fully replace the need for adequate water — fundamental adaptation to climate change-driven aridification will be required.
The 2012 US Drought
The 2012 US drought is a recent reminder of drought's economic impact. Affecting more than 60% of the US, it caused $30+ billion in agricultural losses and pushed up global grain prices. Iowa, the nation's largest corn producer, was hit particularly hard. The Mississippi River dropped to record-low levels, disrupting barge transportation. Cattle herds shrank dramatically as ranchers culled animals they couldn't afford to feed. While not as severe as some past droughts, the 2012 event demonstrated that even modern agriculture remains vulnerable to drought, with effects rippling through global food supply chains and pricing.
Drought and Conflict
Research has linked drought to social unrest and conflict. The Syrian civil war was preceded by an unprecedented multi-year drought (2007-2011) that destroyed rural livelihoods and drove urban migration. The Darfur conflict in Sudan involved competition between drought-stressed pastoralists and farmers. Numerous historical revolutions and famines followed drought periods. Climate change projections suggest drought-related migration could displace millions in coming decades. The connection isn't simple — drought is a stress that exacerbates existing tensions rather than directly causing conflict — but the correlation is real and concerning for the future.
The Bottom Line
Droughts are caused by prolonged periods of reduced precipitation, intensified by heat that increases evaporation. They emerge from shifts in atmospheric circulation, ocean temperature patterns, and increasingly from climate change. Despite developing slowly, droughts can be among the most devastating natural disasters — destroying agriculture, ecosystems, and even entire civilizations. Understanding their causes and predicting their patterns is increasingly important as climate change makes many regions drier.