Drought, Megadrought & Water Stress

What exactly is a drought and how is it measured? What are the causes and effects of this type of water shortage? Why can't scientists predict droughts? Is climate change responsible for an increase in megadroughts around the world? We explain the answers to these questions and more.
Drought causes earth to dry up
Dried mud in Folsom Lake, California, Nov 2015, after record low water levels due to drought. Image: CC0 Public Domain

An estimated 55 million people around the world are affected by droughts every year, and this is set to rise. 1 While droughts are a natural phenomenon, increasingly evidence points towards a link between human induced climate change and the frequency of extreme weather events such as heatwaves, floods and wildfires. By 2025, scientists estimate that half of the world’s population will be living in water-stressed areas. 2

What Is A Drought?

A drought is a natural hazard. It is defined as a period of time when an area or region experiences less precipitation (rain or snowfall) than usual for that area or region. The lack of precipitation causes the soil to dry out, damaging crops and triggering a general water shortage. 3

Droughts and heatwaves often occur together and create a positive climate feedback loop. Drought causes the soil to become dry and absorb more incoming heat. This is because there is less moisture evaporation which has a cooling effect. This means the air above the soil heats up faster. In the process, plants lose moisture by transpiration and eventually shrivel up.

Unlike other sudden weather events like tornadoes, thunderstorms and hurricanes, it can be tricky to pinpoint when a drought has started. This is because droughts don’t make a big entrance, the initial effects are slow and gradual. It can take weeks or even months sometimes for the full effects of inadequate rainfall to become apparent.

The first evidence is usually observed in climographs, graphs which show average monthly temperature and precipitation for a place. The lack of water in streams and reservoirs may also be noticed after a few weeks, though water levels in wells might not reflect a shortage of rainfall for a year or more after a drought begins.

The end of a drought is equally difficult to identify as it can end as quietly as it begins.

Droughts can last for weeks, months or even years.

Economically, droughts are the second most costly weather event after hurricanes.

What Is A Megadrought?

A megadrought is an extreme drought that lasts at least two decades.

A recent study by the University of Queensland, which aimed to see what the world will look like as a result of global warming over the next 20 to 50 years, concluded that megadroughts are tipped to increase. 4

The team engaged in paleoclimatology, going back to the Eemian Period – 129,000 to 116,000 years ago  – in order to find conditions similar to the warming conditions we are experiencing today. At that time, drier conditions lasted sometimes for more than 1000 years, with El Niño events most likely increasing their severity.

Droughts like the 1930s Dust Bowl and other U.S. droughts have historically lasted maybe a decade or less. Research by NASA shows that by 2100, the U.S. is likely to experience a drought similar to those events, but which lasts at least 30 to 35 years. 5. The NASA study used climate models and tree ring records and found that a rise in Earth’s temperature – and not a lack of precipitation – could increase the chance of the U.S. West and Southwest experiencing a megadrought – by more than 80 percent.

NASA Drought Climate Model
Map projects soil moisture in North and Central America by 2100. Brown shading shows soil that is drier than the 20th century average at 30 centimeters below the surface. Blue shadings show wetter-than-average soil at the same depth. Source: NASA’s Earth Observatory

Outside of soaring temperatures, climate forcing is another driver of mega-droughts. This refers to how much of the sun’s energy is trapped by the atmosphere. As we burn more fossil fuels, a greenhouse effect is created which leads to higher rates of evaporation and drier soil. 6

Another factor is La Niña – a weather phenomenon. It can push storms to certain areas causing severe floods, while leaving other areas prone to drought. Scientists note that La Niña events have happened more frequently in recent years. For details, see: El Nino Southern Oscillation (ENSO).

Why Doesn’t Rain End A Drought?

It requires plenty of soaking rainfall to end a drought. One single downpour will not provide lasting relief, especially where soil has hardened and rain water runs off the surface. There needs to be numerous such rains over a period of months, and a return to normal rainfall patterns before a drought can be said to be over.

Can We Predict When A Drought Is Going To Happen?

No. This is because droughts are never the result of a single cause, and climate models are not sophisticated enough to make completely accurate predictions.

However, by monitoring rainfall, river flow and soil moisture, as well as historical information on El Niño and La Niña weather patterns, scientists can make a judgement call on the likelihood of a drought or flood occurring in a certain area.

How Are Droughts Measured?

Climatologists have drought monitoring tools, but due to the limitations of each system, data from all sources is often combined to create a more reliable forecast and ranking system. These tools include:

Palmer Drought Severity Index (PDSI)

The Palmer Drought Severity Index (PDSI) is used to measure drought severity. It uses temperature and physical water measurements to rank how moist the soil is. The index spans from -10 (dry) to +10 (wet), although agencies like NOAA typically show a range of -4 to +4. As it uses temperature data, it can capture basic data on the effects of global warming on evapotranspiration.

PDSI does not provide any early warning system. Additionally, it is not accurate in mountainous areas because it only accounts for rain, and not snow, as precipitation. PDSI is often used by the American government to determine when to begin drought relief.

Standardized Precipitation Index (SPI)

The Standardized Precipitation Index (SPI) is often used to supplement PDSI data. Less sophisticated than PDSI, it only measures precipitation. The advantage is, SPI can identify droughts many months earlier.

The Evaporative Demand Drought Index (EDDI)

This is an experimental drought monitoring tool. EDDI can serve as an early warning tool for flash droughts and sustained droughts by capturing the precursor signals of water stress. It examines the atmospheric evaporative demand (E0), also known as ‘the thirst of the atmosphere’ for a given location.

EDDI Drought Categories
EDDI categories – chart of North America drought risk. Brown areas are most at risk, and blue are least at risk. Source: NOAA 2021.

Did You Know?

Scientists to who study climate, including droughts, are called climatologists. These scientists record precipitation and temperature levels of an area, and average this over 30 years to determine a location’s ‘normal’ climate.

Paleoclimatologists are scientists who study planet Earth’s climate before written records exist. They study the lithosphere, ancient ice, soil and lake beds for clues of what the climate was like thousands and millions of years ago. One method of studying paleoclimate climate is by observing tree rings. Each ring represents a year the tree was alive. The distance between each ring shows how much the tree grew that year. The further apart the ring, the more it grew. During times of drought, the rings will be closer together.

Hydrologists are scientists who study the water cycle – how water moves through the environment. They use stream gauges to measure how much water is flowing through rivers and streams; and sensors and tape measure to monitor ground level water. They measure snowpacks on mountains, the snow that melts in the spring and flows into streams and rivers. This combined information is fed into a computer climate model to better understand droughts.

The 4 Types of Droughts

The scientific community and American Meteorological Society, have defined 4 types of drought, as follows:

1. Meteorological Drought

This is your classic drought, a long dry period with below-normal levels of rainfall. Conditions are possibly aggravated by hot temperatures causing high evapotranspiration rates.  7  Meteorological drought usually precedes the other kinds of drought.

2. Agriculture Drought

This type of drought mostly concerns farmers and horticulturists. Agricultural drought occurs when crops become affected by water shortage.

It happens when there is a water shortage at a time in a crop’s growth cycle, when it needs the water most. If there is a water shortage earlier in the crop’s cycle, when it needs less water, this does not qualify as a drought.

Sometimes the term agricultural drought is viewed as controversial because they feel it points the finger unfairly at Mother Nature. The view is, farmers must accrue some portion of blame as they decide which crops to grow, and they decide on land use management, fertilizers, soil care and production technology.

Methods for measuring agricultural droughts include: Standardised Precipitation-Evapotranspiration Index (SPEI), Vegetation Health Index (VHI), Scaled Drought Condition Index (SDCI) and Temperature Vegetation Dryness Index (TVDI). 8

3. Hydrological Drought

Hydrological drought occurs when the amount of rain or snow in an area drops for an extended period of time, leading to low water levels, especially in streams, reservoirs and groundwater levels. This only usually occurs after many months of meteorological drought.

Although all droughts are caused by a deficiency of precipitation, hydrologists are more concerned with how this deficiency plays out through the hydrologic system. (See also: Hydrosphere.) It can impact hydropower electricity generation as well as water storage systems used for additional purposes such as flood control, crop irrigation, wildlife and recreation.

Methods for measuring hydrological droughts include: Standardized Precipitation Index (SPI) and Streamflow Drought Index (SDI) 9

4. Socioeconomic Drought

A socioeconomic drought is where a water shortage cannot meet human needs, causing adverse effects on society, the economy and environment. It is linked to an increase in foods prices, and depending on how severe the shortage – can cause malnutrition and famine, migration, resettlement, unemployment and conflict.

One study showed that drought raised the risk of rioting between 10 and 50 percent. 10

Socioeconomic droughts are closely linked to the Anthropocene Epoch and the influence of man-made climate change. This is because even a slight climate change can cause significant change in the water cycle and influence water supply. 11

Socioeconomic droughts are measured using the Multivariate Standardized Reliability and Resilience Index (MSRRI) and SocioEconomic Drought Index (SEDI).

Causes of Drought

1. Natural Weather Patterns

Droughts have occurred throughout history, and are a natural phenomenon triggered by cyclical weather patterns.

Many studies have demonstrated a close link to drought and weather patterns such as EI Niño–Southern Oscillation (ENSO), the East Asian Summer Monsoon (EASM), Atlantic Oscillation (AO) and the Pacific North American Pattern (PNA). 12 13

Other weather cycles have an influence too. The Madden-Julian Oscillation can amplify the impact of an El Niño or La Niña event, while other important regional weather cycles include the Indian Ocean Dipole and the Southern Annular Mode, also known as the Antarctic oscillation (AAO).

2. Change in Circulation Patterns

Our changing climate is altering large-scale atmospheric circulation patterns, shifting storm tracks off their typical paths. This, in turn, can magnify weather extremes.

El Niño and La Niña

Both these two weather patterns are unpredictable and keep scientists guessing.

El Niño is a natural weather cycle that affects winds, air pressure, temperatures and cloud formation across the tropical Pacific. It is associated with warmer waters and altered storm patterns that can cause droughts in Indonesia, Australia and northeastern South America.

La Niña is the counterpart to El Niño and is associated with drier-than-normal conditions in parts of North and South America. La Niña typically has a cooling effect on global temperatures, but this has been offset by the heat trapped in our atmosphere by greenhouse gases. 14

Two of the worst droughts in the history of the United States — the 1930s Dust Bowl and the 1988 drought in the Midwest — were associated with the effects of La Niña. 15 Scientists note that La Niña events have happened more frequently in recent years.

Although not believed to be directly affected by climate change, recent studies show that unless we reduce our use of fossil fuels, the effects of El Niño and La Niña are likely to intensify around the world.

According to the World Meteorological Organization (WMO) Secretary General, “El Niño and La Niña are major, naturally occurring drivers of the Earth’s climate system. But all naturally occurring climate events now take place in against a background of human-induced climate change which is exacerbating extreme weather and affecting the water cycle. ” 14

3. Ocean Temperature

Ocean temperatures largely dictate global weather patterns, including dry and wet conditions on land, and even tiny temperature fluctuations can have huge ripple effects on climate systems.

The latest research indicates that, since 1970, oceans have been absorbing more than 90 percent of the thermal energy created by global warming, while only 4 percent has been absorbed by the atmosphere and the land. 16 This is reflected in the increasing duration and frequency of marine heatwaves throughout the tropics.

Ocean warming is associated with a cascade of disasters – particularly, extreme weather patterns on land, including drought. 17.
See also: How Do Oceans Influence Climate Change?

4. Humans

While there have always been droughts, the consequences of droughts in recent years have been exacerbated by increased populations of people and grazing animals relying on land for food and water.

Deforestation and other poor land-use practices, such as intensive farming, can diminish soil quality and reduce the land’s ability to absorb and retain water. As a result, soil dries out faster – inducing desertification and agricultural drought.

Excess water demand is another stress. One study found that human water consumption had doubled over the past 50 years, intensifying hydrological drought worldwide. 18

Furthermore, urbanization, changes in land-use and population growth are putting increasing pressure on access to water.

Reservoirs play an important role in water supply – not only for electricity generation via hydropower and marine energy – but also in managing water supply and demand. Currently, reservoirs provide about 70 percent of freshwater in the world. 19

Where Do Droughts Occur?

Areas in every inhabited continent experience water stress. 

The whole continent of Australia for example, is indicated as extremely high-risk area for water stress by 2040. This was borne out by the Australian bushfires of 2019-2020, which caused the worst blazes ever recorded.

Satellite images posted online from NASA’s Earth Observatory, showed trails of smoke from the Australian fires reaching as far as South America. The fires were caused by two decades of drought across much of the continent, and by the convergence of several regional climatic variables.

In the United States, droughts are most likely to occur in the Midwest and the South. According to the United States Drought Monitor, the states of Oklahoma, Montana and Iowa are ranked as the most vulnerable states. By 2040 however, much of the country is forecast to have a high risk of water stress.

Even in ‘cold climates’ in the Arctic circle, rising temperatures are drying out forests and melting permafrost, triggering enormous forest fires. The Arctic fires of 2019, covered more than 5 million square kilometres (larger than the size of the European Union), of central and northern Asia, blocking sunlight and impacting air quality.  20

Is Climate Change Causing More Droughts?

In 2013, the Intergovernmental Panel on Climate Change (IPCC) did not see a global trend toward increasing dryness or drought across the world.

However, since then, as global temperatures have unequivocally become hotter and hotter, that view is changing.

It is now generally accepted that global warming is impacting drought in two ways: Rising temperatures on Earth generally make wet regions wetter and dry regions drier. 21

In wetter regions, warm air absorbs more water vapor, leading to larger rain events. But in more arid regions, warmer temperatures mean water evaporates more quickly, leaving drought conditions.

Using the newest generation of climate models called CMIP6, Researchers at the American Geophysical Union found that drought intensity and duration was directly linked to levels of greenhouse gases in the troposphere (lower atmosphere). This data will inform the next IPCC assessment report on climate change. 22 23

What Are The Effects of Drought?

Humanitarian Crisis

Drought and water scarcity are interconnected phenomena. This deadly combination can trigger famine, migration and displacement as well as conflict over diminishing resources. According to UN-OCHA, in 2017 drought led to the worst humanitarian crisis since the Second World War, when 20 million people across the Middle East and Africa came to the brink of starvation.

According to the United Nations, 40 percent of the world’s population will be affected by water scarcity by 2030 and 700 million people are at risk of being displaced as a result of drought.

In developed countries, the consequences tend to be less severe but can include lower crop yields, higher food prices, water rationing and hosepipe bans.

Crop Failure and Other Dangers

Drought causes significant crop yield reductions. This is worrying, at a time when we need to increase yield to meet future population growth. Drought can also have a significant impact on the economy. For example, in the first year of a severe drought from 2007 to 2009, the U.S. Southeast lost over $1.3 billion due to destruction of major crops such as corn, cotton, wheat, soybean and hay. 24

Drought can cause soil desiccation, increasing soil ‘blowability’ and sandstorms. Gradually this leads to decreased soil productivity and desertification.

Droughts, as we have seen can also provide a substantial increase in risk of wildfires; threatening humans and wildlife. Whole ecosystems are under threat.

Drought Mitigation

As water is a finite resource which needs to serve more and more people, ensuring a reliable supply is crucial to human survival and sustainable progress.

Drought mitigation encompasses a large number of actions, which can be grouped into three broad categories: (1) increasing water supply (2) reducing water demand and (3) minimizing the impact of drought.

The following are climate mitigation examples in each category:

1. Increasing Water Supply

  • Increase water storage capacity
  • Locate new potential resources such as aqueducts and canals
  • Encourage small water collection/harvesting
  • Desalination of saline and brackish
  • Better recycling of wastewater
  • Establishing sponge cities to mange urban expansion

2. Reducing Water Demand

Agriculture is the biggest user of water. While the shift to biofuels as a renewable energy is generally welcomed, their production could demand as much water as fossil fuels. 25 There has also been a shift in global diet away from starch food towards more water-intensive meat and dairy. These trends are not helping to reduce water demand.

Better Farming Practices

  • Save water by more precision irrigation
  • Grow less water demanding crops
  • Practice soil conservation to combat erosion, soil degradation and surface runoff. This includes established practices such as crop rotation, terracing, water retention and detention as well as windbreaks and erosion-control structures

Other industries also come under the spot light. For example, fossil fuel production, including fracking (techniques used for the extraction of oil and gas) is very water-intensive. There needs to be more support for less water-intensive renewable energy which produces little or no greenhouse gases and does not consume water.

3. Minimizing The Impact of Drought

This can be achieved by providing:

  • Drinking water supplies to affected people, livestock and wildlife
  • Public aid and compensation
  • Tax relief
  • Food and feed programs
  • Fire control programs.

Additional Reading

Drought Characteristics and Propagation in the Semiarid Heihe River Basin in Northwestern China. An investigation of meteorological and hydrological drought characteristics and propagation using the standardized precipitation index (SPI) and standardized streamflow index (SSI).Feng Ma et al. Jan 2019.

Assessing future socioeconomic drought events under a changing climate over the Pearl River basin in South China. Variable Infiltration Capacity (VIC) model is used to simulate the streamflow in the Pearl River basin during the period of 2020–2099. Suning Liu et al. August 2020.

Evidence of wet-dry cycles and mega-droughts in the Eemian climate of southeast Australia. Hamish McGowan et al. October 2020.

References

  1. World Health Organization. []
  2. United Nations, Drinking Water. []
  3. National Geographic on Droughts. []
  4. “Expect More Megadroughts.” October 2020. []
  5. “Megadroughts the Future of the U.S. “. NASA. 2015 []
  6. “Climate Change Could Revive Medieval Megadroughts in U.S. Southwest.” Columbia University. []
  7. “A new global database of meteorological drought events from 1951 to 2016.” Spinoni, Jonathan et al.  Journal of hydrology. Regional studies vol. 22 (2019). []
  8. “An Agricultural Drought Index for Assessing Droughts Using a Water Balance Method.” College of Geo-exploration Science and Technology, Jilin University. Yijing Cao et al. May 2019. []
  9. “Meteorological Drought, Hydrological Drought, and NDVI in the Heihe River Basin, Northwest China: Evolution and Propagation.” Fanglei Zhong et al. 2019. []
  10. “Water scarcity and rioting: Disaggregated evidence from Sub-Saharan Africa.” Journal of Environmental Economics and Management, 2017. []
  11. “Is the cold region in Northeast China still getting warmer under climate change impact?” Z.Q. Zhou et al. 2020. []
  12. “An empirical seasonal prediction model of the East Asian summer monsoon using ENSO and NAO.” Wu Z, Wang B, Li J, Jin F. 2009. []
  13. “Boreal summer convection oscillation over the Indo-Western Pacific and its relationship with the East Asian summer monsoon.” Li Y et al. 2013. []
  14. WMO: “La Niña has Developed.” October 2020 [][]
  15. National Geographic. Drought: Below-average precipitation affects the amount of moisture in soil as well as the amount of water in streams, rivers, lakes, and groundwater. []
  16. “2018 Continues Record Global Ocean Warming.” Cheng, L., Zhu, J., Abraham, J. et al. 2019. []
  17. “Sea temperature changes contributing to droughts.” University of Exeter 2017. []
  18. “Human water consumption intensifies hydrological drought worldwide.” Yoshihide Wada et al. 2013. Environomental Research Letters. []
  19. “Assessing socio-economic drought evolution characteristics and their possible meteorological driving force.” Menglong Zhao et al. Feb 2019. []
  20. World Meteorological Organization. August 2019. []
  21. Drought and Climate Change: Center for Climate and Energy Solutions. []
  22. ” Robust Future Changes in Meteorological Drought in CMIP6 Projections Despite Uncertainty in Precipitation.” Anna M. Ukkola et al. May 2020. []
  23. “Future drought characteristics through a multi-model ensemble from CMIP6 over South Asia.” Jianqing Zhai et al. 2020. []
  24. Effects of Drought: North Carolina Climate Office. []
  25. UN Water, Food and Energy []
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