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Drought: the story on HearLore | HearLore
— Ch. 1 · Defining The Drought Phenomenon —
Drought.
~8 min read · Ch. 1 of 7
A drought is simply a period of drier-than-normal conditions. This definition comes from the IPCC Sixth Assessment Report published in 2021. It describes a moisture deficit relative to average water availability at a specific location and season. By the early 1980s, over 150 different definitions for drought had already been published. These varied definitions reflect differences across regions, needs, and disciplinary approaches. A multi-agency partnership called the National Integrated Drought Information System defines it as a deficiency of precipitation over an extended period. That period usually lasts a season or more and results in a water shortage. The National Weather Service office of NOAA adds that adverse impacts on people, animals, or vegetation must occur over a sizeable area. Drought is a complex phenomenon relating to the absence of water which makes monitoring difficult. Three major categories exist based on where in the water cycle the moisture deficit occurs. Meteorological drought happens due to a lack of precipitation. Hydrological drought relates to low runoff, streamflow, and reservoir storage levels. Agricultural or ecological drought causes plant stress through evaporation and low soil moisture. Some organizations add a fourth category known as socioeconomic drought. This occurs when demand for an economic good exceeds supply due to weather-related water shortfalls.
Global Monitoring And Indices
Scientists use several indices to quantify and monitor drought at different spatial and temporal scales. The Palmer drought index serves as a regional tool commonly used for monitoring events and studying areal extent. It uses precipitation and temperature data to study moisture supply and demand via a simple water balance model. Another key metric is the Standardized Precipitation Index computed solely based on precipitation. The World Meteorological Organization recommends this index for identifying meteorological droughts across different climates. The Keetch-Byram Drought Index calculates risk based on rainfall, air temperature, and other meteorological factors. A multiscalar index called SPEI accounts for increased atmospheric evaporative demand during precipitation deficits. Vegetation condition indices like VDI and VHI compute health using normalized difference vegetation index data. High-resolution information helps assess spatial and temporal changes in duration and severity at finer scales. Using multiple indices with different datasets manages drought better than relying on a single source. This approach is particularly vital in regions where data availability is limited such as Africa and South America. Careful monitoring of moisture levels can also predict increased risks for wildfires. These tools support the development of site-specific adaptation measures for vulnerable communities worldwide.
Drivers Of Dryness
Natural mechanisms produce precipitation through convective, stratiform, or orographic processes. Convective processes involve strong vertical motions that overturn the atmosphere within an hour causing heavy rain. Stratiform processes involve weaker upward motions producing less intense precipitation over longer durations. Droughts occur mainly in areas where normal rainfall levels are already low. If these factors do not support sufficient precipitation volumes reaching the surface, drought results. A high level of reflected sunlight and above average prevalence of high pressure systems can trigger dry spells. Winds carrying continental rather than oceanic air masses prevent thunderstorm activity from developing. Feedback mechanisms like local arid air and hot conditions promote warm core ridging which worsens drought. Within the tropics distinct wet and dry seasons emerge due to movement of the Intertropical Convergence Zone. The dry season greatly increases drought occurrence characterized by low humidity and drying watering holes. Grazing animals like zebras and elephants migrate due to lack of water searching for fertile lands. Bushfires become common because plants lack water and fire spreads easily. Periods of warmth quicken fruit production but increase evaporation worsening drought conditions. The El Niño, Southern Oscillation phenomenon plays a significant role in regional drought patterns. La Niña events generally associate with drier hotter conditions exacerbating drought in California and the Southwestern United States. Meteorological scientists observed La Niñas becoming more frequent over time. Conversely El Niño events cause drier weather in parts of the Amazon River Basin and Central America.
Environmental And Economic Toll
Drought is one of the most complex natural hazards causing devastating impacts on environment economy and society. Environmental effects include lower surface and subterranean water levels and decreased flow levels below minimum thresholds. Amphibian life faces direct danger when flows drop too low. Increased pollution of surface water occurs alongside the drying out of wetlands. More and larger wildfires consume forests while higher deflation intensity erodes soil. Loss of biodiversity affects both terrestrial and aquatic wildlife populations. Drought-induced tree mortality lacks representation in most climate models treating forests as land carbon sinks. Economic losses result from lower agricultural forest game and fishing output. Higher food-production costs strain household budgets globally. Lower energy-production levels occur in hydroelectric plants due to reduced water flow. Depleted water tourism and transport revenue create financial gaps for local economies. Problems arise with water supply for municipal economies and technological processes in metallurgy mining and chemical industries. Wildfires such as Australian bushfires and those in the United States become common during drought times. These fires may cause human deaths and destroy habitats affecting both terrestrial and aquatic wildlife. Dust storms appear when drought hits areas suffering from desertification and erosion. Reduced electricity production results from diminished water-flow through hydroelectric dams. Shortages of water affect industrial users including paper wood foodstuff and chemical sectors.
Human Health And Social Crisis
The most negative impacts of drought for humans include crop failure leading to food crisis and famine. Malnutrition dehydration and related diseases threaten lives directly exposed to excessive heat waves. Water scarcity limits supplies increasing contamination of remaining sources like cyanotoxin accumulation within food chains. High levels of microcystin appeared in San Francisco Bay Area salt-water shellfish and fresh-water supplies throughout California in 2016. Mass migration results from internal displacement and international refugee flows driven by failed harvests. Social unrest often follows periods of prolonged dryness and resource scarcity. War over natural resources including water and food becomes a risk in vulnerable regions. Severe drought has been noted to cause unrest preceding political upheaval in some cases. Subsistence farmers are more likely to migrate because they lack alternative food sources. Areas with populations depending on water sources as major food sources face higher vulnerability to famine. People displaced by a drought in Somalia arrived at a camp in Dolo Ado Ethiopia in 2011. The gap between developed and developing countries widens due to these combined factors. Reduced water quality occurs because lower flows reduce dilution of pollutants increasing contamination of remaining sources. These conditions explain why droughts operate as a factor increasing the gap between nations.
Regional Vulnerability Hotspots
In 2005 parts of the Amazon basin experienced the worst drought in one hundred years. A 2006 article reported that the forest in its present form could survive only three years of drought. Scientists at the Brazilian National Institute of Amazonian Research argue this response pushes the rainforest toward a tipping point. It would irreversibly start dying if deforestation effects combine with climate change. The combination increases drying effect of dead trees fueling forest fires according to WWF warnings from March 2996. Australia faced the 1997, 2009 Millennium Drought leading to a water supply crisis across much of the country. Many desalination plants were built for the first time as a result. By far the largest part of Australia is desert or semi-arid lands known as the outback. Regular burning by human settlers arriving about 50,000 years ago may have prevented monsoons reaching interior regions. An expert panel warned in June 2008 of long term severe ecological damage for the whole Murray-Darling basin. Australian environmentalist Tim Flannery predicted Perth in Western Australia could become the world's first ghost metropolis without sufficient water. East Africa including Ethiopia Eritrea Kenya Somalia South Sudan Sudan Tanzania and Uganda has diverse climates ranging from hot dry to cooler wet highlands. Below-average rainfall occurred for six consecutive rainy seasons in the Horn of Africa during 2020, 2023. This led to the third longest and most widespread drought on record with dire implications for food security.
Historical Disasters And Cultural Views
Throughout history humans viewed droughts as disasters due to impact on food availability and society. Drought is among earliest documented climatic events present in Epic of Gilgamesh and tied to Biblical story of Joseph. Hunter-gatherer migrations in 9,500 BC Chile linked to phenomenon while early humans exited Africa around 135,000 years ago. Beliefs shaped by cultural factors include local knowledge perceptions values beliefs and religion. In some places drought interpreted as work of supernatural forces globally people more likely explain natural events like drought through supernatural means than social phenomena. Historically rituals used attempt prevent or avert drought ranging dances scapegoating human sacrifices. Many ancient practices now matter folklore while others still practiced today. In areas limited understanding scientific basis beliefs reflect indigenous power spirits Christian philosophies seeing drought divine punishment. Creationism curricula sometimes give religious explanations denying evolution human agency affecting climate change occurring. The 4.2-kiloyear event megadrought took place Africa Asia between 5,000 and 4,000 years ago linked collapse Old Kingdom Egypt Akkadian Empire Mesopotamia Liangzhu culture lower Yangtze River area Indus Valley Civilization. Longest recorded drought started 400 years ago Atacama Desert Chile still continues today. Classic Maya collapse between 7th and 9th centuries possibly contributed factor drought. Year-long unprecedented European heat drought occurred 1540 with eleven months without rain temperatures 5, 7°C above average 20th century. India killing between 250,000 and 3.25 million in 1900 due to drought. Over 5 million perished from starvation Soviet Union 1921, 22 combined severe drought war effects. Northwest China resulting over 3 million deaths by famine during 1928, 1930.
What is the definition of drought according to the IPCC Sixth Assessment Report published in 2021?
The IPCC Sixth Assessment Report published in 2021 defines a drought as simply a period of drier-than-normal conditions. This definition describes a moisture deficit relative to average water availability at a specific location and season.
How many different definitions for drought had been published by the early 1980s?
By the early 1980s, over 150 different definitions for drought had already been published. These varied definitions reflect differences across regions, needs, and disciplinary approaches.
Which index does the World Meteorological Organization recommend for identifying meteorological droughts across different climates?
The World Meteorological Organization recommends the Standardized Precipitation Index for identifying meteorological droughts across different climates. This metric is computed solely based on precipitation data.
When did parts of the Amazon basin experience the worst drought in one hundred years?
In 2005 parts of the Amazon basin experienced the worst drought in one hundred years. A 2006 article reported that the forest in its present form could survive only three years of drought.
What were the human death toll figures for the Soviet Union famine caused by severe drought during 1921 and 1922 combined with war effects?
Over 5 million people perished from starvation in the Soviet Union during 1921 and 1922 due to combined severe drought and war effects. Northwest China resulted in over 3 million deaths by famine during 1928 and 1930.