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Droughts cause a range of impacts and are often worsened by the effects of climate change on the water cycle: a dry riverbed in France; sandstorm in Somaliland due to drought; droughts negatively impact agriculture in Texas; drought and high temperatures worsened the 2020 bushfires in Australia.

A drought is a period of drier-than-normal conditions.[1]: 1157  A drought can last for days, months or years. Drought often has large impacts on the ecosystems and agriculture of affected regions, and causes harm to the local economy.[2][3] Annual dry seasons in the tropics significantly increase the chances of a drought developing, with subsequent increased wildfire risks.[4] Heat waves can significantly worsen drought conditions by increasing evapotranspiration.[5] This dries out forests and other vegetation, and increases the amount of fuel for wildfires.[4][6]

Drought is a recurring feature of the climate in most parts of the world, becoming more extreme and less predictable due to climate change, which dendrochronological studies date back to 1900. There are three kinds of drought effects, environmental, economic and social. Environmental effects include the drying of wetlands, more and larger wildfires, loss of biodiversity.

Economic impacts of drought result due to negative disruptions to agriculture and livestock farming (causing food insecurity), forestry, public water supplies, river navigation (due to e.g.: lower water levels), electric power supply (by affecting hydropower systems) and impacts on human health.[7]

Social and health costs include the negative effect on the health of people directly exposed to this phenomenon (excessive heat waves), high food costs, stress caused by failed harvests, water scarcity, etc. Drought can also lead to increased air pollution due to increased dust concentrations and wildfires.[8] Prolonged droughts have caused mass migrations and humanitarian crisis.[9][10]

Examples for regions with increased drought risks are the Amazon basin, Australia, the Sahel region and India. For example, in 2005, parts of the Amazon basin experienced the worst drought in 100 years.[11][12] Australia could experience more severe droughts and they could become more frequent in the future, a government-commissioned report said on July 6, 2008.[13] The long Australian Millennial drought broke in 2010. The 2020–2022 Horn of Africa drought surpassed the severe drought in 2010–2011 in both duration and severity.[14][15]

Throughout history, humans have usually viewed droughts as disasters due to the impact on food availability and the rest of society. People have viewed drought as a natural disaster or as something influenced by human activity, or as a result of supernatural forces.

Definition

[edit]
Fields outside Benambra, Australia suffering from drought in 2006.

The IPCC Sixth Assessment Report defines a drought simply as "drier than normal conditions".[1]: 1157  This means that a drought is "a moisture deficit relative to the average water availability at a given location and season".[1]: 1157 

According to National Integrated Drought Information System, a multi-agency partnership, drought is generally defined as "a deficiency of precipitation over an extended period of time (usually a season or more), resulting in a water shortage". The National Weather Service office of the NOAA defines drought as "a deficiency of moisture that results in adverse impacts on people, animals, or vegetation over a sizeable area".[16]

Drought is a complex phenomenon − relating to the absence of water − which is difficult to monitor and define.[17] By the early 1980s, over 150 definitions of "drought" had already been published.[18] The range of definitions reflects differences in regions, needs, and disciplinary approaches.

Categories

[edit]

There are three major categories of drought based on where in the water cycle the moisture deficit occurs: meteorological drought, hydrological drought, and agricultural or ecological drought.[1]: 1157  A meteorological drought occurs due to lack of precipitation. A hydrological drought is related to low runoff, streamflow, and reservoir and groundwater storage.[19] An agricultural or ecological drought is causing plant stress from a combination of evaporation and low soil moisture.[1]: 1157  Some organizations add another category: socioeconomic drought occurs when the demand for an economic good exceeds supply as a result of a weather-related shortfall in water supply.[17][18] The socioeconomic drought is a similar concept to water scarcity.

The different categories of droughts have different causes but similar effects:

  1. Meteorological drought occurs when there is a prolonged time with less than average precipitation.[20] Meteorological drought usually precedes the other kinds of drought.[21] As a drought persists, the conditions surrounding it gradually worsen and its impact on the local population gradually increases.
  2. Hydrological drought happens when water reserves available in sources such as aquifers, lakes and reservoirs fall below average or a locally significant threshold. Hydrological drought tends to present more slowly because it involves stored water that is used but not replenished. Due to the close interaction with water use, this type of drought is can be heavily influenced by water management. Both positive and negative human influences have been discovered and strategic water management strategies seem key to mitigate drought impact.[22][23] Like agricultural droughts, hydrological droughts can be triggered by more than just a loss of rainfall. For instance, around 2007 Kazakhstan was awarded a large amount of money by the World Bank to restore water that had been diverted to other nations from the Aral Sea under Soviet rule.[24] Similar circumstances also place their largest lake, Balkhash, at risk of completely drying out.[25]
  3. Agricultural or ecological droughts affect crop production or ecosystems in general. This condition can also arise independently from any change in precipitation levels when either increased irrigation or soil conditions and erosion triggered by poorly planned agricultural endeavors cause a shortfall in water available to the crops.

Indices and monitoring

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Percent of U.S. experiencing drought intensity of at least level D2 (severe drought), during the weeks of 2000 to 2024.

Several indices have been defined to quantify and monitor drought at different spatial and temporal scales. A key property of drought indices is their spatial comparability, and they must be statistically robust.[26] Drought indices include:[26]

  • Palmer drought index (sometimes called the Palmer drought severity index (PDSI)): a regional drought index commonly used for monitoring drought events and studying areal extent and severity of drought episodes.[27] The index uses precipitation and temperature data to study moisture supply and demand using a simple water balance model.[27][28][29]
  • Keetch-Byram Drought Index: an index that is calculated based on rainfall, air temperature, and other meteorological factors.[30]
  • Standardized precipitation index (SPI): It is computed based on precipitation, which makes it a simple and easy-to-apply indicator for monitoring and prediction of droughts in different parts of the world. The World Meteorological Organization recommends this index for identifying and monitoring meteorological droughts in different climates and time periods.[26]
  • Standardized Precipitation Evapotranspiration Index (SPEI): a multiscalar drought index based on climatic data. The SPEI accounts also for the role of the increased atmospheric evaporative demand on drought severity.[26] Evaporative demand is particularly dominant during periods of precipitation deficit. The SPEI calculation requires long-term and high-quality precipitation and atmospheric evaporative demand datasets. These can be obtained from ground stations or gridded data based on reanalysis as well as satellite and multi-source datasets.[26]
  • Indices related to vegetation: root-zone soil moisture, vegetation condition index (VDI) and vegetation health index (VHI). The VCI and VHI are computed based on vegetation indices such as the normalized difference vegetation index (NDVI) and temperature datasets.[26]
  • Deciles index
  • Standardized runoff index

High-resolution drought information helps to better assess the spatial and temporal changes and variability in drought duration, severity, and magnitude at a much finer scale. This supports the development of site-specific adaptation measures.[26]

The application of multiple indices using different datasets helps to better manage and monitor droughts than using a single dataset, This is particularly the case in regions of the world where not enough data is available such as Africa and South America. Using a single dataset can be limiting, as it may not capture the full spectrum of drought characteristics and impacts.[26]

Careful monitoring of moisture levels can also help predict increased risk for wildfires.

Causes

[edit]
Contraction and desiccation cracks in the dry earth of the Sonoran Desert, northwestern Mexico

General precipitation deficiency

[edit]
See also: Precipitation

Mechanisms of producing precipitation include convective, stratiform,[31] and orographic rainfall.[32] Convective processes involve strong vertical motions that can cause the overturning of the atmosphere in that location within an hour and cause heavy precipitation,[33] while stratiform processes involve weaker upward motions and less intense precipitation over a longer duration.[34]

Precipitation can be divided into three categories, based on whether it falls as liquid water, liquid water that freezes on contact with the surface, or ice.

Droughts occur mainly in areas where normal levels of rainfall are, in themselves, low. If these factors do not support precipitation volumes sufficiently to reach the surface over a sufficient time, the result is a drought. Drought can be triggered by a high level of reflected sunlight and above average prevalence of high pressure systems, winds carrying continental, rather than oceanic air masses, and ridges of high pressure areas aloft can prevent or restrict the developing of thunderstorm activity or rainfall over one certain region. Once a region is within drought, feedback mechanisms such as local arid air,[35] hot conditions which can promote warm core ridging,[36] and minimal evapotranspiration can worsen drought conditions.

Dry season

[edit]
See also: Dry season

Within the tropics, distinct, wet and dry seasons emerge due to the movement of the Intertropical Convergence Zone or Monsoon trough.[37] The dry season greatly increases drought occurrence,[38] and is characterized by its low humidity, with watering holes and rivers drying up. Because of the lack of these watering holes, many grazing animals are forced to migrate due to the lack of water in search of more fertile lands. Examples of such animals are zebras, elephants, and wildebeest. Because of the lack of water in the plants, bushfires are common.[39] Since water vapor becomes more energetic with increasing temperature, more water vapor is required to increase relative humidity values to 100% at higher temperatures (or to get the temperature to fall to the dew point).[40] Periods of warmth quicken the pace of fruit and vegetable production,[41] increase evaporation and transpiration from plants,[42] and worsen drought conditions.[43]

El Niño–Southern Oscillation (ENSO)

[edit]
Further information: El Niño–Southern Oscillation

The El Niño–Southern Oscillation (ENSO) phenomenon can sometimes play a significant role in drought. ENSO comprises two patterns of temperature anomalies in the central Pacific Ocean, known as La Niña and El Niño. La Niña events are generally associated with drier and hotter conditions and further exacerbation of drought in California and the Southwestern United States, and to some extent the U.S. Southeast. Meteorological scientists have observed that La Niñas have become more frequent over time.[44]

Conversely, during El Niño events, drier and hotter weather occurs in parts of the Amazon River Basin, Colombia, and Central America. Winters during the El Niño are warmer and drier than average conditions in the Northwest, northern Midwest, and northern Mideast United States, so those regions experience reduced snowfalls. Conditions are also drier than normal from December to February in south-central Africa, mainly in Zambia, Zimbabwe, Mozambique, and Botswana. Direct effects of El Niño resulting in drier conditions occur in parts of Southeast Asia and Northern Australia, increasing bush fires, worsening haze, and decreasing air quality dramatically. Drier-than-normal conditions are also in general observed in Queensland, inland Victoria, inland New South Wales, and eastern Tasmania from June to August. As warm water spreads from the west Pacific and the Indian Ocean to the east Pacific, it causes extensive drought in the western Pacific. Singapore experienced the driest February in 2014 since records began in 1869, with only 6.3 mm of rain falling in the month and temperatures hitting as high as 35 °C on 26 February. The years 1968 and 2005 had the next driest Februaries, when 8.4 mm of rain fell.[45]

Climate change

[edit]
See also: Effects of climate change on the water cycle and Effects of climate change on agriculture
There will likely be multiplicative increases in the frequency of extreme weather events compared to the pre-industrial era for heat waves, droughts and heavy precipitation events, for various climate change scenarios.[46]

Globally, the occurrence of droughts has increased as a result of the increase in temperature and atmospheric evaporative demand. In addition, increased climate variability has increased the frequency and severity of drought events. Moreover, the occurrence and impact of droughts are aggravated by anthropogenic activities such as land use change and water management and demand.[26]

The IPCC Sixth Assessment Report also pointed out that "Warming over land drives an increase in atmospheric evaporative demand and in the severity of drought events"[47]: 1057  and "Increased atmospheric evaporative demand increases plant water stress, leading to agricultural and ecological drought".[48]: 578 

There is a rise of compound warm-season droughts in Europe that are concurrent with an increase in potential evapotranspiration.[49]

This section is an excerpt from Effects of climate change § Droughts.[edit]

Climate change affects many factors associated with droughts. These include how much rain falls and how fast the rain evaporates again. Warming over land increases the severity and frequency of droughts around much of the world.[50][51]: 1057  In some tropical and subtropical regions of the world, there will probably be less rain due to global warming. This will make them more prone to drought. Droughts are set to worsen in many regions of the world. These include Central America, the Amazon and south-western South America. They also include West and Southern Africa. The Mediterranean and south-western Australia are also some of these regions.[51]: 1157 

Higher temperatures increase evaporation. This dries the soil and increases plant stress. Agriculture suffers as a result. This means even regions where overall rainfall is expected to remain relatively stable will experience these impacts.[51]: 1157  These regions include central and northern Europe. Without climate change mitigation, around one third of land areas are likely to experience moderate or more severe drought by 2100.[51]: 1157  Due to global warming droughts are more frequent and intense than in the past.[52]

Several social factors may worsen the impact of droughts. These are increased water demand, population growth and urban expansion in many areas.[53] Land restoration techniques, such as agroforestry, can help reduce the impact of droughts.[54]

Vegetation changes, erosion and human activities

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See also: Aeolian processes

Human activity can directly trigger exacerbating factors such as over-farming, excessive irrigation,[55] deforestation, and erosion adversely impact the ability of the land to capture and hold water.[56] In arid climates, the main source of erosion is wind.[57] Erosion can be the result of material movement by the wind. The wind can cause small particles to be lifted and therefore moved to another region (deflation). Suspended particles within the wind may impact on solid objects causing erosion by abrasion (ecological succession). Wind erosion generally occurs in areas with little or no vegetation, often in areas where there is insufficient rainfall to support vegetation.[58] Woody plant encroachment can increase soil porosity and therewith the chances of soil drought.[59][60]

Impacts

[edit]
Global drought total economic loss risk
Pair of dead oryx in Namibia during the 2018–19 Southern Africa drought.
After years of drought and dust storms the town of Farina in South Australia was abandoned.

Drought is one of the most complex and major natural hazards, and it has devastating impacts on the environment, economy, water resources, agriculture, and society worldwide.[26]

One can divide the impacts of droughts and water shortages into three groups: environmental, economic and social (including health).

Environmental and economic impacts

[edit]
Western red cedar dying from drought, US, 2018

Environmental effects of droughts include: lower surface and subterranean water-levels, lower flow-levels (with a decrease below the minimum leading to direct danger for amphibian life), increased pollution of surface water, the drying out of wetlands, more and larger wildfires, higher deflation intensity, loss of biodiversity, worse health of trees and the appearance of pests and dendroid diseases.[61][6] Drought-induced mortality of trees lacks in most climate models in their representation of forests as land carbon sink.[62]

Economic losses as a result of droughts include lower agricultural, forests, game and fishing output, higher food-production costs, lower energy-production levels in hydro plants, losses caused by depleted water tourism and transport revenue, problems with water supply for the energy sector and for technological processes in metallurgy, mining, the chemical, paper, wood, foodstuff industries etc., disruption of water supplies for municipal economies.

Further examples of common environmental and economic consequences of drought include:

Agricultural impacts

[edit]
Further information: Effects of climate change on agriculture
Impacts of climate change on soil moisture at 2 °C of global warming. A reduction of one standard deviation means that average soil moisture will approximate the ninth driest year between 1850 and 1900.

Droughts can cause land degradation and loss of soil moisture, resulting in the destruction of cropland productivity.[70] This can result in diminished crop growth or yield productions and carrying capacity for livestock. Drought in combination with high levels of grazing pressure can function as the tipping point for an ecosystem, causing woody encroachment.[71]

Water stress affects plant development and quality in a variety of ways: firstly drought can cause poor germination and impaired seedling development.[72] At the same time plant growth relies on cellular division, cell enlargement, and differentiation. Drought stress impairs mitosis and cell elongation via loss of turgor pressure which results in poor growth.[73] Development of leaves is also dependent upon turgor pressure, concentration of nutrients, and carbon assimilates[clarification needed] all of which are reduced by drought conditions, thus drought stress lead to a decrease in leaf size and number.[73] Plant height, biomass, leaf size and stem girth has been shown to decrease in maize under water limiting conditions.[73] Crop yield is also negatively effected by drought stress, the reduction in crop yield results from a decrease in photosynthetic rate, changes in leaf development, and altered allocation of resources all due to drought stress.[73] Crop plants exposed to drought stress suffer from reductions in leaf water potential and transpiration rate. Water-use efficiency increases in crops such as wheat while decreasing in others, such as potatoes.[74][75][73]

Plants need water for the uptake of nutrients from the soil, and for the transport of nutrients throughout the plant: drought conditions limit these functions leading to stunted growth. Drought stress also causes a decrease in photosynthetic activity in plants due to the reduction of photosynthetic tissues, stomatal closure, and reduced performance of photosynthetic machinery. This reduction in photosynthetic activity contributes to the reduction in plant growth and yields.[73] Another factor influencing reduced plant growth and yields include the allocation of resources; following drought stress plants will allocate more resources to roots to aid in water uptake increasing root growth and reducing the growth of other plant parts while decreasing yields.[73]

Social and health impacts

[edit]

The most negative impacts of drought for humans include crop failure, food crisis, famine, malnutrition, and poverty, which lead to loss of life and mass migration of people.[26]

There are negative effects on the health of people who are directly exposed to this phenomenon (excessive heat waves). Droughts can also cause limitations of water supplies, increased water pollution levels, high food-costs, stress caused by failed harvests, water scarcity, etc. Reduced water quality can occur because lower water-flows reduce dilution of pollutants and increase contamination of remaining water sources.[76][77]

This explains why droughts and water scarcity operate as a factor which increases the gap between developed and developing countries.[78]

Effects vary according to vulnerability. For example, subsistence farmers are more likely to migrate during drought because they do not have alternative food-sources. Areas with populations that depend on water sources as a major food-source are more vulnerable to famine.

People displaced by a drought in Somalia arriving at a camp in Dolo Ado, Ethiopia, 2011

Further examples of social and health consequences include:

Loss of fertile soils

[edit]
See also: Aeolian processes

Wind erosion is much more severe in arid areas and during times of drought. For example, in the Great Plains, it is estimated that soil loss due to wind erosion can be as much as 6100 times greater in drought years than in wet years.[81]

Loess is a homogeneous, typically nonstratified, porous, friable, slightly coherent, often calcareous, fine-grained, silty, pale yellow or buff, windblown (Aeolian) sediment.[82] It generally occurs as a widespread blanket deposit that covers areas of hundreds of square kilometers and tens of meters thick. Loess often stands in either steep or vertical faces.[83] Loess tends to develop into highly rich soils. Under appropriate climatic conditions, areas with loess are among the most agriculturally productive in the world.[84] Loess deposits are geologically unstable by nature, and will erode very readily. Therefore, windbreaks (such as big trees and bushes) are often planted by farmers to reduce the wind erosion of loess.[57]

Regions particularly affected

[edit]
Main article: List of droughts

Amazon basin

[edit]
See also: Amazon Rainforest § Impact of early 21st century Amazon droughts

In 2005, parts of the Amazon basin experienced the worst drought in 100 years.[11][12] A 2006 article reported results showing that the forest in its present form could survive only three years of drought.[85][86] Scientists at the Brazilian National Institute of Amazonian Research argue in the article that this drought response, coupled with the effects of deforestation on regional climate, are pushing the rainforest towards a "tipping point" where it would irreversibly start to die. It concludes that the rainforest is on the brink of being turned into savanna or desert, with catastrophic consequences for the world's climate. According to the WWF, the combination of climate change and deforestation increases the drying effect of dead trees that fuels forest fires.[87]

Australia

[edit]
Further information: Drought in Australia

The 1997–2009 Millennium Drought in Australia led to a water supply crisis across much of the country. As a result, many desalination plants were built for the first time (see list).

By far the largest part of Australia is desert or semi-arid lands commonly known as the outback. A 2005 study by Australian and American researchers investigated the desertification of the interior, and suggested that one explanation was related to human settlers who arrived about 50,000 years ago. Regular burning by these settlers could have prevented monsoons from reaching interior Australia.[88] In June 2008 it became known that an expert panel had warned of long term, maybe irreversible, severe ecological damage for the whole Murray-Darling basin if it did not receive sufficient water by October 2008.[89] Australia could experience more severe droughts and they could become more frequent in the future, a government-commissioned report said on July 6, 2008.[13] Australian environmentalist Tim Flannery, predicted that unless it made drastic changes, Perth in Western Australia could become the world's first ghost metropolis, an abandoned city with no more water to sustain its population.[90] The long Australian Millennial drought broke in 2010.

East Africa

[edit]

East Africa, including for example Ethiopia, Eritrea, Kenya, Somalia, South Sudan, Sudan, Tanzania, and Uganda, has a diverse climate, ranging from hot, dry regions to cooler, wetter highland regions. The region has considerable variability in seasonal rainfall and a very complex topography. In the northern parts of the region within the Nile basin (Ethiopia, Sudan), the rainfall is characterized by an unimodal cycle with a wet season from July to September. The rest of the region has a bimodal annual cycle, featuring long rains from March to May and the short rains from October to December. The frequent occurrence of hydrological extremes, like droughts and floods, harms the already vulnerable population suffering from severe poverty and economic turmoil.[91] Droughts prompted food shortages for example in 1984–85, 2006 and 2011.

The Eastern African region experiences the impacts of climate change in different forms. For instance, below-average rainfall occurred for six consecutive rainy seasons in the Horn of Africa during the period 2020–2023 leading to the third longest and most widespread drought on record with dire implications for food security (see Horn of Africa drought (2020–present)). Conversely, other parts experienced extreme floods, e.g., the 2020 East Africa floods in Ethiopia, Rwanda, Kenya, Burundi, and Uganda, and the 2022 floods in South Sudan.[91][14][15]

A key feature in the region is the heterogeneous distribution of hydrologic extremes in space and time. For instance, El Niño can cause droughts in one part of the region and floods in the other. This is also a common situation within a country, e.g., in Ethiopia. The recent years with consecutive droughts followed by floods are a testament to the need to better forecast these kinds of events and their impacts.[91]

Himalayan river basins

[edit]
Drought-affected area in Karnataka, India in 2012.

Approximately 2.4 billion people live in the drainage basin of the Himalayan rivers.[92] India, China, Pakistan, Bangladesh, Nepal and Myanmar could experience floods followed by droughts in coming decades. Drought in India affecting the Ganges is of particular concern, as it provides drinking water and agricultural irrigation for more than 500 million people.[93][94][95] In 2025, the UN warned that retreating glaciers could threaten the food and water supply of 2 billion people worldwide.[96]

North America

[edit]

The west coast of North America, which gets much of its water from glaciers in mountain ranges such as the Rocky Mountains and Sierra Nevada, also would be affected.[97][98]

By country or region

[edit]

Droughts in particular countries:

See also:

Protection, mitigation and relief

[edit]
See also: Water scarcity, Water supply, and Water security
Water distribution on Marshall Islands during El Niño.

Agriculturally, people can effectively mitigate much of the impact of drought through irrigation and crop rotation. Failure to develop adequate drought mitigation strategies carries a grave human cost in the modern era, exacerbated by ever-increasing population densities.

Strategies for drought protection or mitigation include:

  • Dams – many dams and their associated reservoirs supply additional water in times of drought.[99]
  • Cloud seeding – a form of intentional weather modification to induce rainfall.[100] This remains a hotly debated topic, as the United States National Research Council released a report in 2004 stating that to date, there is still no convincing scientific proof of the efficacy of intentional weather modification.[101]
  • Land use – Carefully planned crop rotation can help to minimize erosion and allow farmers to plant less water-dependent crops in drier years.
  • Transvasement – Building canals or redirecting rivers as massive attempts at irrigation in drought-prone areas.

When water is scarce due to droughts, there are a range of options for people to access other sources of water, such as wastewater reuse, rainwater harvesting and stormwater recovery, or seawater desalination.

History

[edit]
See also: List of droughts
A South Dakota farm during the Dust Bowl, 1936

Throughout history, humans have usually viewed droughts as disasters due to the impact on food availability and the rest of society. Drought is among the earliest documented climatic events, present in the Epic of Gilgamesh and tied to the Biblical story of Joseph's arrival in and the later Exodus from ancient Egypt.[102] Hunter-gatherer migrations in 9,500 BC Chile have been linked to the phenomenon,[103] as has the exodus of early humans out of Africa and into the rest of the world around 135,000 years ago.[104]

Droughts can be scientifically explained in terms of physical mechanisms, which underlie natural disasters and are influenced by human impact on the environment.[105] Beliefs about drought are further shaped by cultural factors including local knowledge, perceptions, values, beliefs and religion. In some places and times, droughts have been interpreted as the work of supernatural forces.[106] Globally, people in many societies have been more likely to explain natural events like drought, famine and disease in terms of the supernatural than they are to explain social phenomena like war, murder, and theft.[107][108]

Historically, rituals have been used in an attempt to prevent or avert drought. Rainmaking rituals have ranged from dances to scapegoating to human sacrifices. Many ancient practices are now a matter of folklore while others may still be practiced.[109]

In areas where people have limited understanding of the scientific basis of drought, beliefs about drought continue to reflect indigenous beliefs in the power of spirits and Christian philosophies that see drought as a divine punishment. Such beliefs can influence people's thinking and affect their resilience and ability to adapt to stress and respond to crises.[106] In the case of Creationism, curricula sometimes give religious explanations of natural phenomena rather than scientific ones. Teaching explicitly denies evolution, that human agency is affecting climate, and that climate change is occurring.[110]

Some historical droughts include:

See also

[edit]

References

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Drought

View on Grokipedia
from Grokipedia
A drought is a prolonged deficiency in relative to normal amounts for a given , resulting in shortages that adversely affect ecosystems, , and human activities. Droughts occur as a normal and recurrent feature of across virtually all , driven primarily by natural patterns such as persistent high-pressure systems that suppress rainfall. They are categorized into meteorological drought, defined by shortfalls; agricultural drought, characterized by inadequate for crop growth; hydrological drought, marked by diminished , reservoir levels, and ; and socioeconomic drought, encompassing demand exceeding supply with broader human costs. While natural variability predominates, human factors like overexploitation of and land-use changes can intensify deficits once initiated. Consequences span , including habitat loss and decline; economic damages from crop failures and livestock losses; and societal strains such as reduced food production, heightened fire risks, and public health challenges from .

Definition and Classification

Meteorological and Hydrological Definitions

Meteorological drought refers to a period of below-average precipitation that persists long enough to cause a serious hydrological imbalance, typically measured relative to long-term statistical norms for a specific region. This definition emphasizes the degree of rainfall deficit and the duration of the dry spell, often spanning weeks to months or longer, without immediate consideration of impacts on vegetation, soil, or water storage. Unlike other drought types, meteorological drought focuses solely on atmospheric conditions, such as reduced convective activity or persistent high-pressure systems that inhibit rainfall, and it serves as the primary trigger for subsequent drought categories. Hydrological drought, in contrast, arises from deficiencies in surface and subsurface water supplies, manifesting as abnormally low streamflows, levels, lake volumes, and tables, even after resumes. It typically lags meteorological drought by several months due to the time required for deficits to propagate through the hydrological cycle, including infiltration, runoff, and storage dynamics. Thresholds for hydrological drought are often defined using of historical flow data—for instance, below the 20th of long-term records—or deviations from mean annual volumes in aquifers and impoundments. This type reflects integrated effects of prior shortfalls, evaporation rates, and human withdrawals, making it distinct from purely atmospheric measures. The distinction between these definitions underscores a causal sequence: meteorological conditions initiate water shortages, but hydrological drought quantifies their persistence in the physical water system, independent of economic or ecological consequences. Empirical monitoring relies on networks for meteorological assessment and gauging stations or altimetry for hydrological evaluation, with no single universal threshold due to regional climatic variability. For example, in arid basins, even modest declines can trigger rapid hydrological responses, whereas humid regions may buffer deficits through larger storage capacities.

Types of Drought: Meteorological, Agricultural, Hydrological, and Socioeconomic

Meteorological drought is defined as a prolonged period of deficient relative to the climatological norm for a specific , measured by the degree of rainfall deficit and the duration of dry conditions. This type focuses solely on atmospheric and patterns, such as reduced rainfall or snowfall, without immediate consideration of downstream effects. Definitions vary regionally due to differences in baseline regimes and atmospheric drivers, with thresholds often set as deviations from long-term averages, for instance, less than 60% of normal rainfall over in some monitoring systems. It serves as the initial trigger for other drought types but can occur independently if impacts remain confined to anomalies. Agricultural drought arises when moisture deficits in the and reduced water availability impair growth, evapotranspiration, and overall plant health, often preceding visible yield losses. Unlike meteorological drought, it emphasizes biophysical responses in , including , stunted development, and heightened vulnerability to pests, driven by factors like , stage, and access. For example, in rain-fed systems, agricultural drought may manifest when falls below critical levels for root zones, as quantified by indices tracking potential versus actual deficits. This type highlights the causal link between shortfalls and food production constraints, with empirical showing failures correlating to cumulative rainfall anomalies exceeding 20-30% below norms during growing seasons. Hydrological drought refers to prolonged shortfalls in and supplies, evident in diminished streamflows, reservoir levels, lake volumes, and aquifer recharge rates following extended deficits. It lags behind meteorological drought due to the time required for water storage systems to deplete, often persisting for months or years after rains resume if abstractions exceed inflows. Measurements typically involve percentiles of historical flows, such as stream discharge below the 20th percentile for consecutive months, impacting , generation, and ecosystem baseflows. In regions like the U.S. Midwest, hydrological droughts have been documented with table declines of 1-2 meters during multi-year events, exacerbating issues from concentrated pollutants. Socioeconomic drought emerges when water shortages disrupt the balance between for economic dependent on water, such as food, energy, and industrial outputs. It integrates elements of prior drought types with human factors like , policy decisions, and market dynamics, where unmet leads to , price spikes, or shortages. For instance, during the 2011-2017 drought, socioeconomic effects included agricultural water allocations cut by up to 80% in some districts, resulting in $2.7 billion in direct economic losses from fallowed land. This type underscores that vulnerability stems not only from natural deficits but also from inadequate or over-allocation, with often exceeding sustainable yields by 10-20% in stressed basins under prolonged dry spells.

Monitoring and Assessment

Key Drought Indices and Metrics

The Standardized Precipitation Index (SPI) quantifies meteorological drought by standardizing precipitation anomalies against long-term climatological averages, computed for timescales from 1 to 48 months to capture short-term deficits or multi-year events. Values below -1 indicate drought, with severity increasing as SPI drops (e.g., -2 for extreme); it relies solely on data, enabling global comparability without needing or inputs. The SPI's flexibility across timescales supports early warning for agricultural and hydrological impacts, though it overlooks temperature-driven evaporative demand. The Palmer Drought Severity Index (PDSI) estimates overall drought through a water balance model incorporating , temperature-derived potential evapotranspiration, and runoff, yielding values from -10 (extreme dry) to +10 (extreme wet), with thresholds like -3 to -4 for severe drought. Developed in 1965, it simulates antecedent conditions via a two-layer model but assumes fixed parameters for soil capacity and available water-holding, leading to regional biases and a inherent 9-month timescale lag that delays detection of rapid-onset droughts. Recent self-calibrating variants address some shortcomings by adjusting for local , improving projections under warming scenarios where original PDSI overestimates drought intensification. The Standardized Precipitation Evapotranspiration Index (SPEI) builds on SPI by balancing precipitation against potential evapotranspiration (PET), typically calculated via the Thornthwaite method, to reflect atmospheric water demand influenced by temperature rises. Like SPI, it is multiscalar and standardized to a , with negative values signaling deficits; SPEI better detects hydrological and agricultural droughts in warming climates, as PET increases amplify even without precipitation shortfalls. Global datasets from 1982 onward enable monitoring of trends, such as intensified droughts in semi-arid regions. Composite tools like the U.S. Drought Monitor (USDM) aggregate indices such as SPI, PDSI, streamflow percentiles, soil moisture models, and vegetation health from satellites (e.g., VegDRI) into categorical classifications: D0 (abnormally dry) to D4 (exceptional drought), validated weekly by expert consensus incorporating local impacts. This multi-indicator approach mitigates single-index limitations, weighting meteorological (40%), hydrological (40%), and agricultural/socioeconomic (20%) factors, though it remains U.S.-centric and subjective in boundary delineation. Agricultural metrics often supplement these with soil moisture deficits, measured as anomalies or percentiles from models like in NLDAS-2, correlating strongly with crop yield reductions (e.g., maize losses exceeding 10% under prolonged deficits). Yield impact indices, such as standardized anomalies in harvested production, link drought to economic losses but require post-event validation due to confounding factors like practices. These metrics prioritize empirical thresholds, such as deficits below 50%, for region-specific assessments.

Modern Detection Methods and Data Sources

Modern drought detection integrates satellite remote sensing, ground-based sensor networks, and atmospheric reanalysis data to quantify deficits in , , and water storage with high spatial and temporal resolution. These methods enable early identification of meteorological, agricultural, and hydrological droughts by tracking anomalies relative to climatological norms. Satellite platforms provide broad-scale observations critical for global and regional monitoring. NASA's GRACE and GRACE-FO missions, launched in 2002 and 2018 respectively, measure terrestrial water storage variations through gravity field changes detected by twin satellites orbiting at 485 km altitude, facilitating hydrological drought assessment via groundwater and soil moisture anomalies. The Atmospheric Infrared Sounder (AIRS) instrument on NASA's Aqua satellite derives vapor pressure deficit and relative humidity data, offering early drought signals up to three months in advance by capturing atmospheric aridity. Vegetation-based indicators, such as the Vegetation Health Index (VHI) from NOAA's Advanced Very High Resolution Radiometer (AVHRR) and Moderate Resolution Imaging Spectroradiometer (MODIS), detect agricultural drought through anomalies in vegetation greenness and temperature, with VHI operational since the 1980s and updated weekly. The Evaporative Demand Drought Index (EDDI), developed by NOAA, uses reanalysis of atmospheric evaporative demand from sources like North American Mesoscale Forecast System data to identify "flash" droughts driven by high temperatures and low humidity. Ground-based networks complement satellite data with direct measurements for validation and local precision. The National Coordinated Soil Moisture Monitoring Network (NCSMMN), coordinated by NOAA's National Integrated Drought Information System (NIDIS) with USDA partners, standardizes in-situ sensors across federal, state, and private sites to produce comparable datasets, enhancing early warnings for flash droughts and improving model inputs for and risk. USGS gauges track and levels to signal hydrological deficits, while community networks like CoCoRaHS collect volunteer reports for impact verification. Integrated platforms aggregate these sources for operational use. The U.S. Drought Monitor (USDM), updated weekly by the National Drought Mitigation Center, incorporates GRACE-FO, AIRS, , and expert analysis to map drought intensity from D0 (abnormally dry) to D4 (exceptional). Drought.gov portals from NOAA and partners provide access to multi-agency datasets, including evapotranspiration products and USDA vegetation indices, supporting real-time decision-making without reliance on single metrics. Globally, NASA's GRACE data informs systems like the USGS Famine Early Warning Systems Network (FEWS NET) for monitoring in vulnerable regions.

Causal Mechanisms

Natural Variability and Climatic Cycles

Natural variability in systems, driven by internal oscillations in ocean-atmosphere interactions, has long produced episodic droughts through alterations in global patterns, independent of anthropogenic forcing. These cycles operate on interannual to multidecadal timescales, modulating sea surface temperatures (SSTs), , and moisture transport, often resulting in prolonged dry spells in vulnerable regions. Empirical reconstructions from paleoclimate proxies, such as rings and sediment cores, indicate that severe droughts, including North American "megadroughts" lasting 20-40 years during the medieval period (circa 900-1400 CE), were primarily attributable to such natural fluctuations rather than external forcings. The El Niño-Southern Oscillation (ENSO), the dominant interannual mode, exemplifies this variability: during El Niño phases, anomalous warming of equatorial Pacific SSTs suppresses rainfall in regions like the , , and northeastern Brazil, while La Niña phases—characterized by cooler SSTs—exacerbate droughts in the and southern . For instance, the 2020-2023 triple-dip La Niña contributed to consecutive years of severe drought in , , and , affecting millions through crop failures and famine risks. Globally, ENSO extremes have triggered twice as many drought-related disasters in their second year compared to non-ENSO periods, as documented in analyses of historical weather events. On decadal scales, the (PDO) and (AMO) further shape drought persistence by amplifying or dampening ENSO teleconnections. The PDO, resembling an El Niño-like pattern over 20-30 years, in its positive (warm) phase enhances drought risk across the U.S. and southwestern states, as seen in the 1930s era when combined with favorable ENSO conditions; conversely, its negative phase correlates with wetter conditions in the but drier ones in the southwestern U.S. Similarly, the AMO's positive phase, marked by North Atlantic warming, has been linked to elevated U.S. drought frequencies since the mid-1990s, modulating ENSO impacts such that El Niño events produce broader dry anomalies during warm AMO periods. These oscillations explain much of the multidecadal clustering of droughts observed in instrumental records, such as the U.S. 1950s event tied to cool Pacific SSTs. Interactions among these cycles underscore their causal role: for example, a positive PDO strengthens El Niño-driven droughts over land, while AMO phases can intensify in the U.S. Midwest through altered positions and deficits. Tree-ring data spanning the last millennium confirm that such natural drivers accounted for the most severe pre-industrial droughts in the , with no evidence of systematic intensification until recent decades potentially influenced by other factors. This variability highlights the inherent unpredictability of drought onset, necessitating adaptive strategies attuned to cycle phases rather than assuming unidirectional trends.

Human Activities and Resource Management

Human activities significantly alter the hydrological cycle, often exacerbating drought conditions through overexploitation of and modifications to land and water management practices. Excessive pumping for and urban use depletes aquifers faster than natural recharge, creating "anthropogenic droughts" where persists even during periods of adequate precipitation. For instance, in the United States, groundwater withdrawals have outpaced recharge in many aquifers, leading to declines of up to several meters per decade in regions like the High Plains . Globally, rapid depletion has been observed in overexploited basins, with human extraction contributing to non-recovery in 30% of monitored aquifers between 2003 and 2014. Deforestation and land-use changes disrupt local patterns and , reducing retention and intensifying drought vulnerability. In tropical regions, forest loss has been linked to declines of up to 1.25% in annual totals, as canopy removal diminishes atmospheric moisture recycling. Peer-reviewed analyses indicate that biophysical effects from , including decreased and , contribute to net warming and drier conditions, independent of carbon emissions. For example, simulated large-scale scenarios show increased drought frequency across multiple climate zones by altering moisture feedbacks. Inefficient practices in , which accounts for 70% of global freshwater use, amplify drought impacts by accelerating during dry spells. Flood and furrow methods result in losses exceeding 50% in arid areas, leaving less available for subsequent needs and hastening hydrological deficits. In water-scarce basins, such mismanagement shifts regions toward unsustainable extraction, where demands outstrip supply, prolonging agricultural droughts. Urbanization exacerbates drought by expanding impervious surfaces like and asphalt, which reduce by up to 50% in affected catchments through increased runoff and diminished infiltration. Studies in cities such as demonstrate that lowers potential recharge rates, intensifying water shortages during low-precipitation periods. This effect compounds with population-driven demand, as seen in rapid urban growth areas where hydraulic heads decline due to combined impervious cover and extraction. Large-scale dam construction alters river , often worsening downstream drought propagation by regulating flows unevenly and reducing natural flood-recession cycles essential for replenishment. The in , for example, has steepened the propagation of meteorological to hydrological droughts, increasing severity slopes by factors observable in post-2003 data. Cascade dams in drought-prone basins further disrupt dynamics, amplifying shortages when storage releases are curtailed.

Influence of Anthropogenic Climate Change

Anthropogenic climate change, driven by elevated atmospheric concentrations of greenhouse gases, primarily affects drought through two main mechanisms: increased atmospheric evaporative demand (AED) from higher temperatures and altered patterns resulting from shifts in , such as expansion. AED rises nonlinearly with temperature, amplifying soil moisture deficits even in regions without precipitation declines, as warmer air holds more moisture and accelerates from land surfaces. Observations from 1901 to 2020 show global land has increased by about 1-3% per decade in some datasets, partially offsetting AED effects, but regional disparities persist, with drying in subtropical zones like the Mediterranean and . Empirical attribution studies using detection and attribution methods, comparing observed data with climate model simulations under natural versus anthropogenic forcings, indicate human influence has increased the frequency, duration, and intensity of meteorological droughts globally since the mid-20th century. For instance, standardized precipitation indices (SPI) reveal anthropogenic forcing accounts for roughly 20-50% of observed changes in drought characteristics in drylands covering 12.6% of global land area, exacerbating desertification risks for over 200 million people. In western North America, warming has shifted droughts toward "hot droughts," where temperature deficits contribute more than precipitation shortfalls since 2000, with anthropogenic signals detectable in event attribution analyses. Similarly, in California, model-based assessments attribute a 15-20% increase in drought risk to anthropogenic warming during events like 2012-2016. Despite these findings, global observational records, including Palmer Drought Severity Index (PDSI) and soil moisture datasets from 1950-2020, exhibit no robust upward trend in drought frequency or area, with some metrics showing stability or declines due to CO2 fertilization enhancing plant water-use efficiency and global greening. IPCC AR6 assesses low confidence in widespread observed increases for meteorological droughts but medium confidence for human contributions to agricultural and ecological drought intensification in specific regions, such as the Mediterranean (high confidence for hydrological drought rise) and parts of . Critiques of attribution studies highlight potential over-reliance on models that underestimate natural variability or overestimate AED sensitivity, leading to uncertain global-scale claims. Projections under high-emissions scenarios (SSP5-8.5) anticipate 20-50% increases in drought frequency and severity by 2100 in mid-latitude and subtropical , driven by AED outpacing projected gains of 1-2% per degree warming. However, these rely on model ensembles that have historically diverged from observations in trends, underscoring the need for integrated assessments incorporating land-use and feedbacks.

Impacts and Consequences

Environmental and Biodiversity Effects

Drought induces widespread vegetation die-off, particularly in and , where hotter and more intense events exceed physiological tolerances of trees and plants. Global observations indicate that tree mortality surges when water deficits combine with elevated temperatures, occurring outside 98% of historical drought-stress ranges, as documented in die-off events across multiple biomes. In semi-arid , prolonged droughts have led to extensive canopy loss, with examples including woodlands in experiencing record die-off following the 2022 hot drought. productivity declines by an average of 36% under experimental drought conditions, with extreme events amplifying aboveground biomass loss by 60% beyond prior estimates. These vegetation changes cascade to soil degradation, accelerating and contributing to in arid and semi-arid regions. Reduced plant cover exposes soil to wind and water , depleting nutrients and , which impairs recovery and fosters long-term affecting over 40% of global terrestrial land in . In , multi-year droughts from 2012-2016 caused significant dieback, correlating with drops and persistent shifts in composition. Freshwater ecosystems suffer as rivers and wetlands shrink, concentrating pollutants and altering aquatic habitats, which degrades and exacerbates in riparian zones. Biodiversity declines as drought favors drought-tolerant or while eliminating sensitive ones, reducing overall and altering community structures. In grasslands, droughts have caused disproportionate losses of annual species and certain perennials, with recovery dependent on dominant species traits rather than diversity alone. populations face heightened risks from and resource scarcity; for instance, severe droughts reduce juvenile and adult survival by 10-12% in modeled populations, while concentrating animals increases predation and disease vulnerability. In the continental , projected increases in year-long droughts—nearly fivefold by mid-century—threaten diverse taxa through amplified physiological stress and behavioral shifts. Aquatic species in drying wetlands experience mass die-offs, as seen in the Delta's 2022 vegetation collapse from during extreme drought.

Agricultural and Food Production Losses


Drought impairs agricultural production by reducing soil moisture essential for seed germination, root development, and nutrient uptake, leading to stunted growth and diminished crop yields. In rain-fed systems, which constitute about 80% of global cropland, water deficits directly limit photosynthesis and transpiration, causing widespread harvest shortfalls. Livestock sectors face forage scarcity, compelling early culling, reduced milk yields, and higher mortality rates due to inadequate water and feed. Globally, droughts account for over 34% of crop and livestock production losses in low- and lower-middle-income countries, resulting in an estimated $37 billion in damages. Empirical analyses indicate that a single drought event reduces global agricultural by an average of 0.8%, with impacts varying by type and region; for instance, and yields can decline by 4-13% under combined and drought stress, exacerbated by vapor pressure deficits that hinder plant water efficiency. Flash droughts, characterized by rapid onset, amplify losses in rain-fed by 5-20% compared to irrigated ones, as they outpace adaptive measures like supplemental watering. In the United States, the 2012 Midwest drought slashed corn production by 13% and soybean yields by 7%, contributing to $30 billion in losses. More recently, in 2023, droughts combined with excessive caused over $16.6 billion in U.S. damages, predominantly affecting grains and forages. Regional case studies underscore disproportionate vulnerabilities: in , the 2011 Horn of Africa drought halved maize harvests in affected areas, precipitating food price spikes and dependency on imports. Australia's Millennium Drought (1997-2009) reduced output by up to 50% in peak years, with numbers dropping 30% due to failure. In , the 2022 drought cut grain production by 8-10% across the continent, straining feed supplies and elevating import needs. These events highlight how prolonged dry spells compound losses through degradation and pest proliferation, though mitigates effects in water-abundant basins. Projections from models suggest escalating yield risks under future scenarios, with drought-driven losses for major staples like and rising 9-19% in key producers.

Economic Costs and Sectoral Disruptions

Droughts inflict substantial economic costs globally, with annual losses exceeding $307 billion, driven primarily by direct damages in vulnerable sectors and indirect ripple effects across supply chains. These costs have risen, as evidenced by a 29% increase in recorded droughts over recent decades, affecting 1.5 billion people and costing $125 billion in the decade to 2017. The agricultural sector suffers the most acute disruptions, accounting for over 50% of drought-related economic losses through reductions, degradation, and declines, which elevate food prices and strain rural economies. In the United States, the 2012 Midwest drought triggered crop losses estimated at $30–40 billion and record federal payouts of $17.3 billion, with 80% attributed to drought and . Australia's Drought (2001–2009) halved agricultural output in key periods, reducing growth by 1.8 percentage points per year. Industrial and manufacturing sectors face heightened production costs and operational halts due to water shortages, affecting processes in water-intensive industries like semiconductors and . A severe drought in could cost the $2.7 billion annually through water rationing and facility slowdowns. Low river levels further disrupt fluvial transport, impeding and supply chains. In the energy sector, droughts curtail output, forcing reliance on costlier fuels or imports, with Europe's 2022 event reducing generation by 11,233 GWh in , , and alone during the first half of the year. The 2021 drought exemplified broader impacts, yielding $1.7 billion in total economic losses and 14,634 job cuts across sectors. Projections from the indicate that by 2035, an average drought will cost 35% more than today, underscoring the need for adaptive measures in water management and sector resilience.

Human Health, Social, and Migration Pressures

Drought exacerbates human health risks through reduced water availability, leading to , , and heightened susceptibility to infectious diseases. In regions with prolonged dry spells, diminished surface and supplies compromise quality, fostering outbreaks of waterborne pathogens such as and typhoid; for instance, the 2022 Somalia epidemic was intensified by ongoing drought conditions that strained and infrastructure. Empirical studies indicate that severe droughts correlate with elevated all-cause mortality, particularly among vulnerable populations, due to compounded effects of food scarcity and heat stress. Mental health burdens also rise, with increased reports of anxiety, depression, and linked to livelihood losses and resource uncertainty during extended dry periods. Social pressures from drought manifest in heightened interpersonal and communal tensions, often escalating into conflicts over scarce water and pasture resources. In pastoralist areas of northern and the , recurrent droughts have triggered inter-clan violence, as seen in clashes between herding groups competing for dwindling lands, exacerbating humanitarian crises. Historical precedents, such as drought-induced factional strife among the ancient Maya in the Yucatán Peninsula around 800-900 CE, demonstrate how prolonged aridity can destabilize societies by amplifying pre-existing rivalries. These dynamics strain social cohesion, increase , and overburden local governance, with evidence from the Sudanese conflicts where drought-fueled contributed to regional insurgencies. Drought drives significant , both internal and cross-border, as populations flee unlivable conditions of and agricultural failure. In 2022, over 1 million Somalis were internally displaced by severe drought, marking one of the largest climate-related movements in recent years. Projections estimate that climate-induced factors, including droughts, could displace 44 to 216 million people internally by 2050, predominantly in , , and , with accelerating rural-to-urban shifts. Studies forecast at least a 200% rise in drought-triggered migration through the , underscoring the role of hydrological stress in reshaping demographics, though outcomes vary by and policy responses.

Historical Context

Ancient and Pre-Industrial Droughts

The 4.2-kiloyear BP aridification event, centered around 2200 BCE and lasting approximately a century, marked one of the most severe prehistoric droughts, with proxy evidence from lake sediments, speleothems, and pollen records indicating sharply reduced monsoon intensity and precipitation across the Middle East, North Africa, and parts of South Asia. In Mesopotamia, this megadrought coincided with the abrupt collapse of the Akkadian Empire, as archaeological surveys reveal widespread abandonment of irrigated farmlands and urban centers, likely due to salinization and crop failures from diminished Euphrates and Tigris flows. Similarly, in Egypt, the event aligned with the end of the Old Kingdom (circa 2181–2055 BCE), where historical annals and Nile flood records document consecutive low inundations, famine, and social unrest, though pharaonic mismanagement amplified vulnerabilities. Recent geochemical analyses, however, indicate the event's severity varied regionally, with some areas experiencing only moderate drying rather than uniform catastrophe, suggesting overemphasis on climate as the sole driver overlooks adaptive capacities or local factors. In , the same aridification phase contributed to the decline of the Indus Valley Civilization around 1900 BCE, as evidenced by sediment cores from the showing weakened summer monsoons, leading to reduced river flows and the desertion of major cities like and without signs of conquest or disease. Proxy data from tree rings and isotopic records in the American Southwest further document megadroughts during the late , such as those circa 2000–1500 BCE, which stressed early agricultural communities reliant on cultivation, prompting shifts in settlement patterns. Multi-decadal droughts recurred in during the Classic Maya period (250–900 CE), reconstructed via oxygen isotope ratios in speleothems revealing precipitation deficits of up to 70% below normal for periods exceeding 50 years around 800–1000 CE. These events strained reservoir systems and rain-fed agriculture in a landscape with limited , correlating with the abandonment of southern lowland centers like and , though analyses emphasize that deforestation-induced changes and likely intensified local aridity beyond baseline climatic forcing. In , a severe drought around 1200 BCE, inferred from Anatolian tree-ring chronologies showing 50–100 years of growth suppression, overlapped with the Hittite Empire's disintegration, marked by capital abandonment and elite tomb disruptions amid failing grain yields. Pre-industrial European records, including the Old World Drought Atlas derived from 2,000-year tree-ring networks, identify prolonged arid spells during the Spörer Minimum (circa 1400–1480 CE), with enduring droughts twice as severe and four times longer than 20th-century analogs, evidenced by narrowed rings and historical chronicles of river navigability failures and harvest shortfalls. In East Asia, documentary annals from the record extreme droughts like 1585–1590 CE, spanning multiple years with anomalies of -50% or more, linking to famines and disease outbreaks in the , though institutional responses mitigated total societal breakdown. These events underscore droughts' role in amplifying pre-existing pressures like overexploitation of aquifers or inequitable resource distribution, rather than acting as isolated triggers for collapse.

Major 19th and 20th Century Events

In , the Federation Drought of 1895–1903 stands out as one of the most severe prolonged dry periods in the continent's recorded history, affecting vast pastoral regions and causing the loss of an estimated 40 million sheep and 2 million cattle due to starvation and thirst. Rainfall deficits exceeded 50% below average in eastern states like and , exacerbating dust storms, heatwaves, and bushfires that reshaped land use and contributed to the decline of large-scale squatting operations. This event, linked to persistent El Niño conditions, preceded national federation and prompted early investments in water infrastructure, though stock losses crippled rural economies for years. In , mid- to late-19th-century droughts repeatedly struck the and western regions, with notable episodes in the 1850s, 1860s, and 1870s disrupting settlement and agriculture. The 1860s drought in , for instance, ended the dominant ranchero system by killing off over 80% of cattle herds through lack of forage and water, forcing a shift toward smaller-scale farming and urban water imports that laid groundwork for modern ' aqueducts. These events, driven by natural aridity amplified by and poor land practices, coincided with rapid westward expansion, leading to crop failures, Native American displacements, and ecological shifts like the decline of populations in the 1870s Plains drought. The Dust Bowl of the 1930s in the United States, peaking from 1934 to 1936, encompassed a decade-long drought across the , where annual fell 30–50% below normal, turning overcultivated soils into dust storms that blackened skies and buried farms under feet of silt. Affecting states like , , , and , the event displaced over 300,000 people in "" migrations to and caused loss equivalent to 100 million acres of farmland, with economic damages exceeding $1 billion in today's terms amid the . Poor farming techniques, including deep plowing of native grasslands during wetter prior decades, intensified wind erosion, though the core driver was persistent atmospheric high-pressure systems; federal responses like the Soil Conservation Service marked a turn toward sustainable practices. In the of , spanning the and , a major drought reduced annual rainfall by over 30% compared to mid-20th-century norms, leading to widespread crop failures, affecting millions, and livestock deaths estimated in the tens of millions across countries like , , and . The event, characterized by prolonged dry spells from 1968–1974 and 1982–1985, stemmed primarily from shifts in tropical circulation patterns rather than solely local or , as evidenced by later rainfall recoveries without proportional land-use changes. Human toll included over 100,000 deaths from and related causes in the early phase, prompting international aid but highlighting vulnerabilities in rain-fed .

Patterns in Long-Term Records

Long-term drought records, derived from instrumental measurements since the late and extended via paleoclimate proxies such as tree rings, lake sediments, and , reveal cyclical patterns of variability rather than a uniform escalation in frequency or severity. , the Palmer Drought Severity Index (PDSI) from instrumental data spanning 1895 to present indicates multidecadal oscillations tied to ocean-atmosphere modes like the (PDO) and (AMO), with no statistically significant global trend toward increased aridity over the 20th century. Tree-ring reconstructions extending back over 2,000 years in regions like the North American Southwest document megadroughts—prolonged dry periods lasting decades or longer—that exceed the intensity and duration of 20th-century events such as the 1930s , including severe episodes from 900–1100 CE and 1100–1350 CE. Regionally, exhibits patterns of extended dry spells in paleoclimate data, with megadroughts during the often surpassing modern droughts in length and deficit, though contemporary events incorporate higher temperatures. In the , instrumental records from 1916–2013 show drought variability without a clear intensification trend, characterized by clusters of severe events interspersed with wet periods, as evidenced by standardized indices. Paleoclimate syntheses globally highlight that megadroughts, such as those around 4.2–3.9 ka BP, demonstrate inherent dynamics capable of producing extreme independent of anthropogenic influences, with spatial patterns varying by hemisphere and . These records underscore the dominance of internal variability in driving long-term drought patterns, with proxy data indicating that pre-industrial megadroughts in areas like the n and Southwest were comparable or more severe than recent ones when adjusted for . Historical archives complement instrumental data, revealing early 19th-century droughts in northeastern (1790–1830) of comparable magnitude to later events, driven by volcanic and solar forcings rather than monotonic warming. Overall, the absence of a pervasive increase in drought across millennia-long reconstructions challenges narratives of unprecedented modern , emphasizing instead recurrent natural cycles.

Recent and Regional Examples

Global Hotspots in the 21st Century

In the , global drought hotspots have included the , , , the , and , where prolonged dry periods have combined low precipitation with elevated temperatures to exacerbate . These events, analyzed in multimodel ensembles, indicate increased drought severity in regions such as the Mediterranean, , and parts of , with projections of further intensification under warming scenarios. The , particularly , endured the 2012-2016 drought, marked by the lowest five-year runoff on record at 221 million acre-feet and statewide emergency declarations in 2014, leading to $2.7 billion in agricultural losses and over 100 million dead trees from 2012 to 2016. Australia's Drought, spanning 2001-2009 in southeast regions, registered as the worst on instrumental record since 1900, with rainfall deficits up to 30% below average and Murray-Darling Basin inflows reduced by 60% from long-term means, prompting urban rationing and investments. In the , the 2011 drought—deemed the region's worst in 60 years—affected 12.4 million people across , , , and , with crop failures exceeding 40% in pastoral areas and declared in southern , resulting in an estimated 258,000 deaths. Southern Africa's 2015-2018 drought, a one-in-400-year event, depleted reservoirs to 10-20% capacity by early 2018, forcing a 50% reduction in municipal use from pre-drought levels through tariffs and restrictions, averting "Day Zero" shutdowns. Central Europe's 2018-2020 drought established a new benchmark for intensity over at least 250 years, covering 35.6% of the continent for over two years with a +2.8°C , causing €9 billion in agricultural damages in 2018 alone, and navigation halts, and outbreaks killing millions of trees in . South American hotspots, such as the , saw severe events in 2005 and 2010 with rainfall 30% below normal, amplifying wildfires and river level drops, while recurring droughts in southern India, like the 2012 Karnataka crisis, affected 10 million people with depletion and crop losses over 50% in rainfed areas. These hotspots highlight vulnerabilities tied to both climatic patterns and land-use pressures, with ensemble models forecasting heightened risks in southern and eastern Europe, , and through 2100.

Case Studies: North America, Australia, Africa, and Amazon Basin

North America
The experienced a severe drought from 2020 to 2022, characterized by prolonged low compounded by record-high temperatures that intensified and reduced across the region. This event affected over a dozen states, including , , and , leading to critically low reservoir levels—such as reaching 35% capacity in 2022—and widespread agricultural losses estimated at billions of dollars. activity surged, with over 7 million acres burned in 2020 alone, exacerbating air quality issues and forest die-offs. The U.S. Drought Monitor classified much of the Southwest as D4 (exceptional drought) for extended periods, highlighting the event's rarity compared to 20th-century records. In the Southwest, this drought formed part of a "" persisting since 2000, with paleoclimate data indicating it as the driest 22-year period in over 1,200 years based on tree-ring reconstructions. Hydrological impacts included flows dropping to historic lows, prompting emergency water cuts for and urban use in 2022. Economic disruptions hit farming hardest, with California's Central Valley—producing 25% of U.S. food—facing crop failures in almonds, , and tomatoes, contributing to a 2021 agricultural GDP loss of $1.1 billion in the state alone. Australia
Australia's Tinderbox Drought from 2017 to 2019 was the most severe three-year meteorological drought in the Murray-Darling Basin since records began in 1900, with rainfall deficits exceeding 50% below average and soil moisture at record lows. This event parched southeastern , including and Victoria, fueling the 2019-2020 bushfires that scorched 18 million hectares and killed or displaced billions of animals. River flows in the Murray-Darling system fell to 20-30% of normal, triggering mass fish kills of over 1 million and other species due to hypoxia and thermal stress. Agricultural impacts were profound, with numbers reduced by 15-20% through and destocking, and production dropping 30% nationally in 2019. Water allocations for were cut to near zero in parts of the basin, costing the an estimated AUD 5.5 billion in lost farm output. The drought's intensity, driven by persistent high-pressure systems and Indian Ocean warming, contrasted with preceding wetter decades, underscoring variability in Australia's El Niño-influenced climate.
The Horn of Africa endured a multi-year drought from 2020 to 2023, the longest in four decades, affecting 20 million people across Ethiopia, Kenya, and Somalia with four consecutive failed rainy seasons. Rainfall was 50-80% below average, leading to crop failures that halved maize yields in pastoral areas and triggered acute food insecurity for 23 million individuals by mid-2023. Livestock mortality reached 2-3 million head, devastating nomadic herders' livelihoods in Somalia, where 40% of the population relies on pastoralism. In the Sahel region, overlapping droughts from 2020 onward compounded vulnerabilities, with , , and seeing vegetation deficits and river levels 20-30% below norms, exacerbating conflicts over scarce resources. The 2023 West African drought intensified heatwaves, pushing temperatures above 45°C and contributing to over 1,000 heat-related deaths in urban areas. Empirical records show these events align with historical dry spells since the 1970s, marked by 20-30% rainfall declines from 1950s wetter periods, though recent failures were worsened by poor and . Amazon Basin
The Basin faced its most extreme drought on record in 2023, with the mainstem at dropping to -3.35 meters—18 cm below the prior 2010 low—and tributaries like the halting navigation for months. This affected 36 million residents across , , and , causing water shortages for 1.5 million people and mass fish die-offs exceeding 100 tons in isolated lakes due to low oxygen. activity tripled, burning 2.4 million hectares and releasing 200 million tons of CO2, while output fell 20%, prompting blackouts in southern . Forest canopy drying led to widespread tree mortality, with data showing a 15% increase in defoliation across central Amazonia, threatening the region's function. Indigenous communities, such as the , reported heightened and displacement as rivers became impassable, forcing reliance on air-dropped . The event's severity stemmed from below-average rainfall (20-40% deficits) and temperatures 2-3°C above norms, though long-term experiments indicate some adaptation through deeper rooting in diverse stands.

Controversies and Debates

Overattribution to Climate Change vs. Natural Cycles

Many analyses attribute recent droughts primarily to anthropogenic , emphasizing increased temperatures and evaporative demand as amplifying factors. However, empirical assessments of global drought metrics, such as the Palmer Drought Severity Index (PDSI) and standardized indices, indicate no significant long-term increase in drought frequency, duration, or severity over the past century when accounting for natural variability. For instance, a comprehensive of data from 1950 to 2008 found that only 0.6%–4.1% of global land areas experienced increasing drought trends, with 1.8%–6.8% showing decreases, and the majority exhibiting no detectable change dominated by interannual fluctuations. Similarly, revisions to earlier PDSI-based estimates revealed that reported global drought increases were overestimated due to flaws in potential evaporation modeling, resulting in little net change over the past 60 years. Natural climate oscillations, including the El Niño-Southern Oscillation (ENSO), (PDO), and (AMO), explain a substantial portion of observed drought variability on decadal scales, often overshadowing any anthropogenic signal. These cycles modulate patterns through teleconnections; for example, negative PDO phases and La Niña events correlate with enhanced aridity in regions like the and , as seen in the 2012–2016 drought and the 1997–2009 Millennium Drought in southeastern , where atmospheric anomalies and ocean temperatures were primary drivers rather than alone. Paleoclimate reconstructions further demonstrate that severe megadroughts, such as those in the medieval period (circa 900–1300 CE) across the North American Southwest, exceeded the intensity of 21st-century events without elevated CO2 levels, underscoring the role of internal variability in producing extremes. Critiques of attribution studies highlight methodological issues, including reliance on climate models that underrepresent natural variability and overemphasize thermodynamic effects like warming-induced drying while downplaying dynamic precipitation shifts. The Intergovernmental Panel on Climate Change's Sixth Assessment Report (AR6) assigns low confidence to a global anthropogenic signal in observed meteorological drought trends, noting that regional increases in ecological or agricultural droughts are detectable in some areas (e.g., parts of the Mediterranean and ) but are not ubiquitous, with offsetting wetter conditions elsewhere. This contrasts with frequent media and advocacy claims of unprecedented drought escalation, which often extrapolate from model projections or short-term regional data without contextualizing against historical baselines or cycles, potentially inflating perceived climate impacts. Peer-reviewed syntheses emphasize that while human-induced warming may modestly intensify drought persistence via higher —estimated at 10–20% in heat-dominated events—natural forcings remain the dominant causal factor for occurrence and spatial extent in most cases. In regions like the U.S. West, where recent droughts (e.g., 2000–2021) have been labeled "megadroughts," tree-ring and proxy data indicate that such events fall within the envelope of preindustrial variability, with cool-phase PDO and AMO contributing over 50% of the signal in some reconstructions. Overattribution risks misdirecting resources toward mitigation of uncertain long-term forcings rather than addressing proximate drivers like land-use changes or water mismanagement, as evidenced by the 1930s , which rivaled or surpassed modern U.S. droughts in severity under far lower global temperatures. Sustained monitoring of drought indices, integrated with paleo-records and cycle indices, is essential to distinguish transient variability from emerging trends, avoiding conflation that could undermine adaptive strategies grounded in observed patterns.

Policy Failures in Water Management and Regulation

In many regions prone to drought, policy frameworks have exacerbated through inefficient allocation mechanisms, regulatory barriers to infrastructure development, and subsidies that incentivize overuse. For instance, fragmented water rights systems, often based on historical "prior appropriation" doctrines, prioritize senior rights holders during shortages, leading to inequitable distributions and underutilization of available supplies in junior areas. Institutional failures, including and lack of enforcement, have compounded these issues, as seen in various global cases where political priorities delayed adaptive measures like expansions or pricing reforms. In , regulatory decisions to release water from reservoirs for environmental purposes, such as Delta smelt protection under the Endangered Species Act, have diverted billions of gallons annually to the during droughts, reducing agricultural and urban supplies despite adequate precipitation in wet years. The state's fragmented governance, with over 400 water agencies and outdated riparian rights, hinders coordinated responses, as evidenced by the 2012-2016 drought when policy rigidities prevented flexible reallocations, forcing emergency curtailments. Farmers have reported that man-made restrictions, rather than drought alone, account for up to 80% of water shortages in the Central Valley, where federal and state rules limit pumping from the Sacramento-San Joaquin Delta. The Basin illustrates failures in interstate compacts, where the 1922 agreement allocated 16.5 million acre-feet annually—exceeding the river's average flow by about 1.2 million acre-feet—leading to chronic overuse and reservoir declines, with dropping to 35% capacity by 2022. Senior water rights, favoring early 20th-century claims in Lower Basin states like and , perpetuate tensions and discourage conservation investments, as junior users in the Upper Basin face disproportionate cuts without compensatory mechanisms. Negotiations for post-2026 guidelines have stalled due to these entrenched allocations, ignoring updated hydrological showing overestimated flows. Australia's Murray-Darling Basin Plan, enacted in 2012 with A$13 billion invested over three decades, failed to meet 74% of environmental success indicators by 2024, primarily due to inadequate enforcement of extraction limits, upstream illegal diversions, and overreliance on voluntary buybacks that reduced productive capacity without restoring flows. Mismanagement during the 2017-2019 Drought, including delayed flood releases and poor market design for tradeable entitlements, resulted in class-action lawsuits alleging billions in losses to irrigators from unaccounted "ghost water" extractions. These shortcomings highlight how centralized planning, sidelining local incentives and infrastructure upgrades, amplifies drought impacts over natural variability.

Mitigation, Adaptation, and Response

Engineering and Technological Solutions

Engineering solutions for drought mitigation emphasize for capture and storage, alongside technologies that enhance efficiency in use and alternative sourcing. and s store surplus during wet periods for release during deficits, with studies showing that regions with greater storage capacity, such as arid areas in the , avoided up to 13% losses in irrigated crop value during severe droughts compared to those with limited reserves. However, expanded storage can inadvertently exacerbate long-term shortages by enabling higher consumption and , as observed in downstream areas following construction. Sustainable designs, incorporating eco-friendly materials, aim to balance storage benefits against hydrogeological risks. Precision irrigation systems, particularly , deliver water directly to plant roots, achieving efficiencies of 90-95% versus 50-70% for traditional sprinklers, thereby reducing usage by 30-70% in drought-prone . In field trials, subsurface drip systems saved an average of 2.2 acre-feet of water per acre while maintaining or increasing yields for crops like tomatoes. These systems are especially viable in arid zones, minimizing and runoff, though initial installation costs and requirements limit adoption in some developing regions. Desalination of seawater provides a drought-independent supply for coastal areas, with plants like California's Carlsbad facility producing up to 50 million gallons daily to supplement municipal and agricultural needs. Such plants have alleviated scarcity in regions like during the Drought (1997-2009), where six facilities were commissioned to boost freshwater availability. Yet, high demands—often exceeding those of conventional treatment—pose economic and environmental challenges, with critics noting that conservation measures may yield greater returns per unit cost. Genetic engineering enhances crop resilience through targeted modifications, such as inserting the Bacillus subtilis cspB gene into corn, which has commercialized drought-tolerant varieties covering millions of acres in the U.S. since 2013, sustaining yields under water stress without yield penalties in normal conditions. In wheat, CRISPR-Cas9 editing of root growth genes has produced varieties with extended roots accessing deeper soil moisture, improving survival in simulated drought scenarios. These biotechnological approaches, including osmoprotectant enhancements like trehalose accumulation in rice, demonstrate empirical gains in tolerance to drought and salinity, though regulatory hurdles and public skepticism constrain widespread deployment. Cloud seeding, involving the dispersion of into suitable clouds, offers modest precipitation augmentation of 10-15% in targeted watersheds, as evidenced by long-term operations in the U.S. Sierra Nevada that have added seasonal water volumes equivalent to small reservoirs. Effectiveness depends on cloud presence, limiting utility during prolonged dry spells, and randomized trials confirm benefits primarily for enhancement rather than direct drought reversal. Emerging variants, such as drone-delivered seeding, aim to refine precision but remain supplementary to storage and efficiency measures.

Market-Based and Policy Reforms

Market-based approaches to drought mitigation emphasize the establishment of tradable water rights and pricing mechanisms that reflect , enabling efficient reallocation from lower- to higher-value uses during shortages. In regions prone to recurrent droughts, such as Australia's Murray-Darling Basin, the introduction of cap-and-trade systems for water entitlements in the early 2000s allowed farmers to buy and sell allocations, resulting in a 20-30% reduction in water use while maintaining agricultural output through shifts to more efficient practices. These markets facilitated voluntary transfers that conserved an estimated 1,000 gigaliters of water annually by 2010, demonstrating how property rights can incentivize conservation without centralized mandates. Policy reforms supporting these mechanisms often involve unbundling water rights from land ownership and enforcing measurement standards to prevent overuse. Australia's National Water Initiative of 2004, enacted amid the Millennium Drought (1997-2009), separated entitlements from allocations, enabling permanent trading across state boundaries and generating over AUD 2 billion in market value by 2014, which funded infrastructure upgrades and environmental flows. In contrast, 's fragmented water rights system, reliant on prior appropriation doctrines, has limited trading volumes to less than 1% of total supply during droughts, prompting reforms like the 2014 Sustainable Management Act (SGMA), which aims to impose sustainable yields by 2040 through local agency plans incorporating market tools. Critics of prior policies note that subsidized flat-rate in encouraged inefficient , consuming 80% of developed water for , whereas scarcity-based could reduce demand by 10-20% per empirical models. Further reforms include phasing out perverse subsidies that distort allocation, such as those promoting water-intensive crops in arid areas. In the U.S. West, eliminating federal subsidies tied to drought-vulnerable lands could save $1-2 billion annually while signaling true costs to producers, as analyzed in frameworks. Internationally, Chile's 1981 Water Code established private, perpetual water rights with minimal government intervention, leading to market-driven reallocations that mitigated urban shortages during the 2010s by prioritizing municipal over agricultural uses. These examples underscore that effective reforms require secure, verifiable rights and minimal transaction barriers, though implementation faces resistance from entrenched users, as seen in Australia's politically contentious buybacks of over AUD 3 billion in entitlements for environmental restoration by 2020. from these systems indicates markets enhance resilience by dynamically adjusting to hydrological variability, outperforming administrative in conserving resources and minimizing economic losses.

International Aid and Relief Efforts

International aid for drought relief primarily involves coordination by United Nations agencies such as the (WFP) and the (FAO), alongside NGOs like and bilateral donors including the World Bank. These efforts focus on emergency food distribution, water provision, and anticipatory measures like distributing drought-resistant seeds to mitigate impacts before full crises unfold. For instance, in response to recurrent droughts, the UN has developed frameworks such as the Drought Toolbox under the UN Convention to Combat (UNCCD), aiding countries in crafting national drought plans that integrate early warning systems and resilience-building activities. In the , the WFP launched a Regional Drought Response Plan in 2022 targeting , , and , seeking $327 million to assist 13 million people facing severe from the prolonged La Niña-induced drought that began in 2020. This plan emphasized cash transfers, nutritional support, and protection, with FAO complementing efforts through revised rapid response plans aggregating financial needs for agricultural recovery. Similarly, in Ethiopia's 2015-2016 drought affecting 10.2 million people, international donors mobilized over $1.4 billion, primarily via UN-coordinated appeals, enabling aid distribution and temporary water infrastructure, though delivery was hampered by access restrictions and regulatory environments favoring international over local NGOs. Efforts in the , including , , and , have seen World Bank-supported projects enhance amid droughts exacerbated by conflict, with $500 million allocated since 2017 for nutrition-sensitive and early warning systems reaching over 5 million beneficiaries by 2023. The WFP and partners have repeatedly responded to Sahel droughts, such as the 2022 crisis affecting millions, using multidimensional frameworks like the Cadre Harmonisé for forecasting acute food insecurity. Anticipatory humanitarian action, piloted by organizations like , has provided pre-crisis cash and livestock support, potentially reducing response costs by up to 50% compared to reactive aid. Despite short-term life-saving impacts, international drought faces criticisms for fostering dependency and inefficiencies, as often bypasses long-term structural reforms in and , leaving populations vulnerable to recurrent cycles. Reports highlight risks in aid delivery, such as in Somalia's 2022 drought where funds were diverted, underscoring how opaque tendering and weak oversight can inflate costs and reduce effectiveness. While claims of 20-40% aid loss to are overstated "zombie statistics" lacking robust , substantiated cases of mismanagement persist, particularly in conflict zones, eroding donor confidence and recipient . Sustainable outcomes require integrating aid with market-based reforms and local governance improvements rather than perpetual humanitarian interventions.

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