Drought

• 1. Introduction

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  Drought is a period of drier-than-normal conditions, and it has been shaping human history for as long as records exist. It appears in the Epic of Gilgamesh. It is tied to the Biblical story of Joseph and the Exodus from ancient Egypt. Hunter-gatherer migrations in 9,500 BC Chile have been linked to it. So has the exodus of early humans out of Africa and into the rest of the world around 135,000 years ago.

  By the early 1980s, over 150 definitions of the word had already been published. That number alone speaks to how difficult drought is to pin down. It can last for days, months, or years. It strikes wetlands and forests, crop fields and city reservoirs. It pushes animals like zebras, elephants, and wildebeest across continents in search of water.

  The Food and Agriculture Organization of the United Nations has calculated that droughts caused US$278 billion of damage between 1991 and 2023. That figure does not count the mass migrations, the famines, or the political collapses that drought has contributed to across centuries. What turns a dry spell into a catastrophe, and who bears the heaviest cost when water runs out, is what this documentary sets out to understand.

• 2. What Counts as Drought

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  The IPCC Sixth Assessment Report defines drought simply as drier than normal conditions, meaning a moisture deficit relative to the average water availability at a given location and season. That deceptively simple definition conceals a significant problem: normal is different everywhere.

  The National Weather Service office of NOAA defines drought as a deficiency of moisture that results in adverse impacts on people, animals, or vegetation over a sizeable area. The National Integrated Drought Information System, a multi-agency partnership, specifies that the deficiency must persist for a season or more before qualifying as a drought.

  Scientists have organized drought into at least three major categories based on where in the water cycle the deficit occurs. Meteorological drought is the simplest: less rain than average for a prolonged period. Hydrological drought follows more slowly, as aquifers, lakes, and reservoirs fall below normal thresholds. Agricultural or ecological drought strikes when plant stress builds from a combination of low soil moisture and high evaporation, sometimes even when rainfall has not dropped dramatically. A fourth category, socioeconomic drought, occurs when demand for water as an economic resource outpaces what the weather-related shortfall in supply can meet.

  Meteorological drought tends to arrive first and triggers the others in sequence, worsening gradually as the conditions surrounding it compound.

• 3. How Droughts Form

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  High levels of reflected sunlight and above-average dominance of high-pressure systems are among the triggers that can block rainfall over a region. Winds carrying continental rather than oceanic air masses reduce the moisture available to form clouds. Ridges of high pressure aloft can shut down thunderstorm activity entirely.

  Once drought takes hold, feedback mechanisms make it self-reinforcing. Arid local air, hot conditions that promote warm-core ridging, and minimal evapotranspiration all conspire to deepen the deficit.

  Within the tropics, the movement of the Intertropical Convergence Zone creates distinct wet and dry seasons. In dry season, watering holes and rivers dry up. Grazing animals including zebras, elephants, and wildebeest migrate in search of water. Bushfires become common as plant moisture drops. Periods of warmth quicken the pace of evaporation and transpiration, compounding the stress on vegetation.

  The El Nino-Southern Oscillation, known as ENSO, adds another layer of complexity. La Nina events are associated with drier, hotter conditions in California, the southwestern United States, and parts of the US Southeast. El Nino, by contrast, drives drier conditions in parts of the Amazon River Basin, Colombia, and Central America. In December to February, south-central Africa, particularly Zambia, Zimbabwe, Mozambique, and Botswana, experiences drier-than-normal conditions during El Nino. Singapore recorded its driest February since records began in 1869 during 2014, with only 6.3 mm of rain in the month and temperatures reaching 35 degrees Celsius on the 26th of February.

  Human activity accelerates formation as well. Over-farming, excessive irrigation, deforestation, and erosion all reduce the land's ability to capture and hold water. In arid climates, wind erosion lifts small particles and moves them to other regions, a process called deflation. In the Great Plains, soil loss due to wind erosion in drought years has been estimated at up to 6,100 times greater than in wet years.

• 4. Measuring the Invisible

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  Palmer drought index, sometimes called the Palmer Drought Severity Index or PDSI, uses precipitation and temperature data to model moisture supply and demand through a simple water balance calculation. It is one of the most widely used tools for monitoring drought events and studying how far a drought has spread and how severe it has grown.

  The World Meteorological Organization recommends the Standardized Precipitation Index, known as SPI, for identifying and monitoring meteorological droughts across different climates and time periods. It is computed based on precipitation alone, making it straightforward to apply.

  The Standardized Precipitation Evapotranspiration Index, or SPEI, goes further by accounting for the role of increased atmospheric evaporative demand, which becomes especially dominant during periods of precipitation deficit. Computing the SPEI requires long-term, high-quality precipitation and evaporative demand datasets from ground stations, satellite records, or gridded reanalysis data.

  Vegetation indices offer another window. The vegetation condition index and vegetation health index are computed from the normalized difference vegetation index, known as NDVI, combined with temperature datasets, tracking plant stress from space.

  Researchers have found that using multiple indices drawn from different datasets gives a more accurate picture of drought than relying on any single measure. This matters most in regions like Africa and South America, where data coverage is thinnest. High-resolution drought monitoring helps identify exactly where conditions are worsening and supports the design of site-specific responses, including early warning of elevated wildfire risk.

• 5. The Weight of Impact

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  Drought is one of the most complex and major natural hazards on the planet, and its consequences spread across three interlocking domains: environment, economy, and human health.

  Environmental damage includes lower surface and groundwater levels, reduced river flow that threatens amphibian life, increased surface-water pollution, the drying of wetlands, more and larger wildfires, and loss of biodiversity. Drought-induced tree mortality is largely absent from most climate models, a gap that affects how scientists calculate forests as land carbon sinks.

  Economic losses cascade through agriculture, fishing, forestry, hydropower, river transport, and municipal water systems. Drought has been shown to harm metallurgy, mining, the chemical industry, paper, wood, and food production. In 2016, high levels of microcystin appeared in San Francisco Bay Area saltwater shellfish and freshwater supplies across California, an example of how drought concentrates cyanotoxins in remaining water sources.

  The social toll is borne most heavily by the most vulnerable. Subsistence farmers are more likely to migrate during drought because they lack alternative food sources. Communities that depend on water bodies as a primary food source face elevated famine risk. Reduced water flow shrinks dilution of pollutants, degrading what water remains. Prolonged droughts have caused mass migrations and humanitarian crises. In the 1921-22 drought in the Soviet Union, over five million people perished from starvation, driven by the combined effects of severe drought and the pressures of war.

  Drought can also provoke social unrest and, in some cases, has preceded periods of political upheaval. The gap between developed and developing countries widens under drought pressure, as wealthier nations can invest in infrastructure and alternatives that poorer ones cannot.

• 6. Regions on the Edge

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  In 2005, parts of the Amazon basin experienced the worst drought in 100 years. A 2006 article reported findings suggesting the forest in its current form could survive only three years of drought before irreversible damage set in. Scientists at the Brazilian National Institute of Amazonian Research argued that drought responses, combined with the effects of deforestation on regional climate, were pushing the rainforest toward a tipping point where it would begin to die back irreversibly, potentially turning into savanna or desert.

  Australia's 1997-2009 Millennium Drought drove a water supply crisis across much of the country and led to the construction of desalination plants on a scale the country had never seen before. Australian environmentalist Tim Flannery predicted that unless drastic changes were made, Perth in Western Australia could become the world's first ghost metropolis. The Millennial Drought broke in 2010, but a government-commissioned report dated the 6th of July 2008 warned that more severe and frequent droughts could follow.

  In the Horn of Africa, below-average rainfall occurred for six consecutive rainy seasons during the period 2020-2023, producing the third longest and most widespread drought on record. It surpassed the severe 2010-2011 drought in both duration and severity, with dire consequences for food security across Ethiopia, Eritrea, Kenya, Somalia, South Sudan, Sudan, Tanzania, and Uganda.

  Approximately 2.4 billion people live in the drainage basin of the Himalayan rivers. In 2025, the UN warned that retreating glaciers could threaten the food and water supply of 2 billion people worldwide. Drought affecting the Ganges alone is of particular concern, as that river provides drinking water and agricultural irrigation for more than 500 million people.

  The longest drought in recorded history started 400 years ago in the Atacama Desert in Chile and continues to this day.

• 7. History and Human Response

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  The 4.2-kiloyear event, a megadrought that struck Africa and Asia between 5,000 and 4,000 years ago, has been linked with the collapse of the Old Kingdom in Egypt, the Akkadian Empire in Mesopotamia, the Liangzhu culture in the lower Yangtze River area, and the Indus Valley Civilization. A drought in 1540 across Central Europe is described as the worst drought of the millennium, with eleven months without rain and temperatures running 5-7 degrees Celsius above 20th-century averages.

  Historically, societies have tried to prevent drought through ritual. Rainmaking practices have ranged from dances to scapegoating to human sacrifice. Many of those practices now survive only as folklore, though some may still be observed. In communities with limited access to scientific explanations, drought has been interpreted as divine punishment or the work of supernatural forces, and such beliefs can shape a population's resilience and capacity to adapt.

  Mitigation strategies have grown more practical over time. Dams and reservoirs supply buffer water. Cloud seeding attempts to induce rainfall, though a United States National Research Council report in 2004 found no convincing scientific proof of its efficacy. Carefully planned crop rotation minimizes erosion and allows farmers to plant less water-dependent varieties in dry years. Canals and river redirection, a practice called transvasement, have been attempted as large-scale irrigation solutions.

  Around 2007, Kazakhstan was awarded a substantial sum by the World Bank to restore water that had been diverted from the Aral Sea under Soviet rule. That same diversion placed Balkhash, Kazakhstan's largest lake, at risk of drying out entirely, illustrating how water management decisions made generations earlier can determine which regions become drought-prone decades later.

• 8. Climate Change and What Comes Next

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  Dendrochronological studies, which use tree-ring data to reconstruct past climates, date the increased extremity and unpredictability of drought back to 1900. The IPCC Sixth Assessment Report states directly that warming over land drives an increase in atmospheric evaporative demand and in the severity of drought events, and that increased atmospheric evaporative demand leads to greater plant water stress, producing agricultural and ecological drought even without a corresponding drop in rainfall.

  In Europe, compound warm-season droughts, those that coincide with elevated potential evapotranspiration, have been rising in frequency. Globally, the combination of higher temperatures, greater climate variability, land use change, and intensified water demand has made droughts both more common and harder to predict.

  Heat waves worsen drought conditions by accelerating evapotranspiration, drying out forests and other vegetation and increasing the fuel load available for wildfires. Wildfire monitoring and drought monitoring have become inseparable disciplines, with careful tracking of soil moisture used as an early warning system for fire risk.

  The Amazon basin, where deforestation compounds the drying effect of drought on dead trees that feed forest fires, stands as one of the starkest cases of interacting pressures. According to the WWF, the combination of climate change and deforestation amplifies that drying cycle. With the forest already having survived what scientists described in 2006 as potentially only three years of severe drought before hitting a tipping point, the trajectory of global temperatures will determine whether that threshold is ever crossed.