The 2022-2023 Horn of Africa drought displaced over 2.3 million people and pushed 23 million into acute food insecurity, according to the United Nations. Drought ranks among the costliest and most widespread natural hazards globally, yet its slow onset means it often receives less attention than sudden disasters like floods or storms.
What is drought? Drought is a prolonged period of below-normal precipitation that reduces water availability for ecosystems, agriculture, and human use. As one of the 12 physical risk hazards in climate risk assessment, drought risk is measured by the number of severe drought months a location experiences per year. Understanding what drought is and how it develops is essential for managing water-dependent operations, agricultural systems, and supply chains.
What Is Drought?
Drought is a sustained deficit in precipitation relative to what is normal for a region. Unlike aridity, which is a permanent climate characteristic of dry regions, drought is a temporary departure from average conditions that can occur anywhere, including in regions that typically receive abundant rainfall.
What makes drought distinct from other climate hazards is its slow onset and compounding nature. A drought does not begin on a specific day. Precipitation deficits accumulate over weeks and months, gradually depleting soil moisture, reducing streamflows, and lowering groundwater levels. By the time visible impacts appear, the drought may have been building for months.
Drought affects virtually every economic sector. Agriculture loses crop yields and livestock productivity. Energy production drops when hydroelectric reservoirs fall and cooling water becomes scarce. Manufacturing faces water supply constraints. Urban populations face rationing and water quality deterioration. The cumulative economic impact of drought often exceeds that of sudden-onset disasters.
Types of Drought
Four distinct types of drought capture different dimensions of the phenomenon. Each type follows the previous in a cascading sequence, meaning what starts as a meteorological drought can progress through all four stages if precipitation deficits persist.
Meteorological drought. The starting point: precipitation falls below normal for a sustained period. Defined purely by rainfall deficit relative to historical averages, meteorological drought is the trigger for all other drought types. A region experiencing 60% of its normal rainfall for three consecutive months is in meteorological drought.
Hydrological drought. Reduced precipitation eventually depletes surface water and groundwater. Rivers, lakes, and reservoirs drop below normal levels. Hydrological drought typically lags meteorological drought by weeks to months because it takes time for rainfall deficits to propagate through the water cycle.
Agricultural drought. Soil moisture drops below levels needed to sustain crops and pastures. Agricultural drought can begin before hydrological drought because plants respond rapidly to reduced soil moisture. Crop failure, reduced yields, and livestock losses are the primary impacts.
Socioeconomic drought. Water supply fails to meet the demands of people and economic activity. Socioeconomic drought occurs when the physical water deficit translates into tangible human impacts: water rationing, food price spikes, industrial shutdowns, and population displacement.
What Causes Drought?
Drought is caused by persistent disruptions to normal precipitation patterns. Several mechanisms drive these disruptions:
Atmospheric circulation changes. Shifts in large-scale weather patterns, including jet stream position, high-pressure blocking systems, and monsoon timing, can redirect moisture-bearing air masses away from a region. The result is weeks or months of below-normal rainfall.
El Nino and La Nina cycles. The El Nino-Southern Oscillation (ENSO) redistributes heat and moisture across the Pacific, triggering drought in some regions while causing floods in others. El Nino events are strongly associated with drought in Australia, Southeast Asia, and parts of Africa.
Climate change. Rising temperatures accelerate evapotranspiration, meaning more water is lost from soils and plants even when rainfall stays constant. Climate projections show subtropical dry zones expanding poleward, extending drought-prone areas into regions that currently receive adequate rainfall. The IPCC Sixth Assessment Report projects increasing drought frequency and severity across Mediterranean, southern Africa, and Central American regions.
Human water management. Over-extraction of groundwater, deforestation that reduces moisture recycling, and land use changes that alter surface runoff all contribute to drought conditions. These human factors can amplify natural drought or create water scarcity independent of precipitation trends.
How Is Drought Risk Assessed?
Drought risk assessment uses NASA NEX-GDDP-CMIP6 precipitation data to estimate the number of severe drought months per year at each location. The methodology uses a Standardized Precipitation Index (SPI) proxy based on precipitation change ratios:
The baseline assumes 0.8 drought months per year at any location. When future precipitation falls below the baseline (ratio less than 1.0), drought months increase proportionally to the deficit. A region projected to receive 20% less rainfall than baseline would see approximately 2.8 drought months per year.
| Risk Rating | Drought Months/Year | Impact Level |
|---|---|---|
| Low | Less than 1 | Rare, short-duration drought |
| Moderate | 1-2 | Seasonal dry spells, manageable |
| High | 2-4 | Extended drought periods, crop stress |
| Severe | 4-6 | Prolonged drought, water restrictions |
| Extreme | Greater than 6 | Chronic drought, systemic water failure |
Drought risk is assessed under SSP2-4.5 and SSP5-8.5 scenarios across baseline, 2030, 2040, and 2050 time horizons. Regions where annual precipitation is projected to decline (precipitation ratio below 1.0) face increasing drought risk over time. Platforms like Continuuiti assess drought alongside 11 other climate hazards for a complete physical risk profile.
Drought Effects on Agriculture and Economy
Drought effects cascade through interconnected economic systems. Agriculture bears the most direct impact: the Food and Agriculture Organization estimates that drought accounts for over 80% of all agricultural disaster damage globally.
Crop loss. Soil moisture deficit during critical growth stages reduces yields or causes total crop failure. Irrigated agriculture survives longer but depletes groundwater reserves, creating long-term vulnerability.
Energy disruption. Hydroelectric generation drops when reservoir levels fall. Thermal power plants face cooling water shortages. Energy prices spike during prolonged drought, affecting industrial competitiveness.
Supply chain stress. Drought effects propagate through supply chains when agricultural inputs, water-dependent manufacturing, or river transport are disrupted. Companies with operations or suppliers in drought-prone regions face production delays and cost increases.
Frequently Asked Questions
What is drought?
Drought is a prolonged period of below-normal precipitation that reduces water availability. Unlike aridity, drought is a temporary departure from normal conditions that can occur in any climate zone. Precipitation deficits accumulate over weeks and months before visible impacts appear.
What are the four types of drought?
The four types are meteorological (precipitation deficit), hydrological (reduced surface water and groundwater), agricultural (insufficient soil moisture for crops), and socioeconomic (water supply fails to meet human demand). These cascade in sequence from meteorological drought onward.
What causes drought?
Drought is caused by disruptions to normal precipitation patterns, including atmospheric circulation shifts, El Nino/La Nina cycles, and climate change. Human factors like groundwater over-extraction and deforestation can amplify natural drought.
How is drought risk measured in climate assessment?
Drought risk uses NASA NEX-GDDP-CMIP6 precipitation data to estimate severe drought months per year. Risk is rated from Low (less than 1 month) to Extreme (more than 6 months per year) under multiple emissions scenarios.
How does climate change affect drought?
Climate change increases drought risk by raising temperatures that accelerate evapotranspiration and by expanding subtropical dry zones. IPCC projections show increasing drought frequency across Mediterranean, southern Africa, and Central American regions under both moderate and high emissions scenarios.
Understanding what drought is and how it develops is the first step in managing water-dependent risk. Drought effects compound over time and propagate through supply chains, making early identification of drought-prone locations essential. Integrating drought risk into physical risk screening captures where precipitation deficits are projected to worsen and which operations face growing water vulnerability.
