Drought is a pervasive global hazard that severely impacts water resources, agricultural productivity, and natural ecosystems across all continents. It develops gradually, often going unnoticed until the resulting water shortage begins to affect human and environmental systems. Understanding where droughts occur requires recognizing that this hazard is measured and experienced in several distinct ways.
Understanding the Different Types of Drought
Drought conditions are categorized based on the specific part of the water cycle that is affected. The first type is meteorological drought, defined by a prolonged period of below-average precipitation compared to the historical norm. This deficit is the initial trigger for all subsequent drought impacts.
Agricultural drought quickly follows, characterized by a deficit of soil moisture. When water in the soil is insufficient to support crop growth and meet vegetation needs, agricultural yields decline. This type of drought is particularly relevant for rain-fed farming regions, impacting food security quickly.
The third category is hydrological drought, a slower-onset condition reflecting reduced water levels in surface and subsurface reservoirs. This includes low streamflow, decreased water levels in lakes and reservoirs, and the depletion of groundwater aquifers. Hydrological drought tends to last longer than the other types, often persisting for months or years after rainfall has returned to normal.
Major Global Drought Hotspots
The Sahel region of Africa, stretching from Senegal to Sudan, is a persistent hotspot characterized by severe agricultural drought due to high reliance on seasonal rains. This region, along with the Horn of Africa (Ethiopia, Somalia, and Kenya), frequently experiences multi-year rainfall failures driven by shifts in the monsoon system. The resulting food insecurity and famine risk demonstrate the link between meteorological and agricultural drought in this vulnerable area.
Eastern Australia is regularly subjected to drought, often linked to oceanic cycles that suppress rainfall. These dry periods lead to substantial hydrological impacts, including low river flows and reduced water storage for major cities and agricultural centers. The southeastern portion of the continent has seen extended periods of severe drought affecting rangelands and grain production.
The Mediterranean Basin, encompassing areas like Spain, Morocco, and Türkiye, faces persistent drought conditions exacerbated by high summer temperatures and changing winter rainfall patterns. Spain has seen significant impacts on its agricultural sector, such as a sharp reduction in olive oil production. Morocco has struggled with livestock losses due to lack of forage, as high evaporative demand creates intense water stress.
The Southwestern US and Northern Mexico have been gripped by one of the most prolonged mega-droughts in over a millennium. This region relies heavily on winter snowpack in mountain ranges like the Sierra Nevada and the Rockies. Reduced snowfall translates directly into hydrological drought, severely depleting major water sources like the Colorado River system.
Central Asia, including Afghanistan and Iran, also experiences frequent and intense drought. In South America, the Amazon Basin recently experienced record-low river levels that disrupted transport and aquatic ecosystems. Southeast Asia and India face drought that can severely disrupt the production of key global commodities like rice, coffee, and sugar due to rainfall variability.
Geographic and Climatic Drivers of Vulnerability
Atmospheric Blocking
Hotspots are often influenced by persistent, large-scale atmospheric patterns that suppress precipitation. One mechanism is atmospheric blocking, where a stationary, high-pressure system remains over a region for weeks or months. This pattern diverts storm tracks and causes air to sink, warming and drying the atmosphere, which prevents cloud formation and rainfall.
Oceanic Oscillations
Major oceanic oscillations drive drought vulnerability by shifting global heat and moisture distribution. The El Niño-Southern Oscillation (ENSO) is a primary influence, often leading to drought in Eastern Australia and parts of Southeast Asia during its warm phase. Another element is the Indian Ocean Dipole (IOD), an irregular oscillation of sea surface temperatures that creates a strong east-west gradient. A positive IOD phase typically cools waters in the eastern Indian Ocean, leading to severe drought in Indonesia and Australia.
Orographic Effect and Monsoons
The inherent geography of a region can pre-dispose it to dry conditions through the orographic effect. When moist air is forced over a mountain range, it cools and releases moisture on the windward side, creating a rain shadow on the leeward side. The descending air warms and dries out, resulting in naturally arid or semi-arid climates, such as the Great Basin area. High variability of major monsoon systems, like the West African and South Asian monsoons, also contributes to drought. Shifts in the timing of these seasonal wind patterns result in a sharp reduction in expected annual rainfall for regions that rely heavily on them.