Drought

General precipitation deficiency

Mechanisms of producing precipitation include convective, stratiform,^[32] and orographic rainfall.^[33] Convective processes involve strong vertical motions that can cause the overturning of the atmosphere in that location within an hour and cause heavy precipitation,^[34] while stratiform processes involve weaker upward motions and less intense precipitation over a longer duration.^[35]

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,^[36] hot conditions which can promote warm core ridging,^[37] and minimal evapotranspiration can worsen drought conditions.

Climate change

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.^[27]

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"^[48]: 1057 and "Increased atmospheric evaporative demand increases plant water stress, leading to agricultural and ecological drought".^[49]: 578

There is a rise of compound warm-season droughts in Europe that are concurrent with an increase in potential evapotranspiration.^[50]

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.^{[52][53]: 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.^[53]: 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.^[53]: 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.^[53]: 1157 Due to global warming droughts are more frequent and intense than in the past.^[54]

Several impacts make their impacts worse. These are increased water demand, population growth and urban expansion in many areas.^[55] Land restoration can help reduce the impact of droughts. One example of this is agroforestry.^[56]

Erosion and human activities

Human activity can directly trigger exacerbating factors such as over-farming, excessive irrigation,^[57] deforestation, and erosion adversely impact the ability of the land to capture and hold water.^[58] In arid climates, the main source of erosion is wind.^[59] 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.^[60]

Environmental and economic impacts

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:

• Alteration of diversity of plant communities, which can have an impact on net primary production and other ecosystem services.^[63]
• Wildfires, such as Australian bushfires and wildfires in the United States, become more common during times of drought and may cause human deaths.^[64]
• Dust Bowls, themselves a sign of erosion, which further erode the landscape
• Dust storms, when drought hits an area suffering from desertification and erosion
• Habitat damage, affecting both terrestrial and aquatic wildlife^[65]
• Snake migration, which results in snake-bites^[66]
• Reduced electricity production due to reduced water-flow through hydroelectric dams^[67]
• Shortages of water for industrial users^[68][69]

Agricultural impacts

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

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.^[27]

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.

Further examples of social and health consequences include:

• Water scarcity, crop failure, famine^[79] and hunger – drought provides too little water to support food crops; malnutrition, dehydration and related diseases
• Mass migration, resulting in internal displacement and international refugees
• Social unrest
• War over natural resources, including water and food
• Cyanotoxin accumulation within food chains and water supply (some of which are among the most potent toxins known to science) can cause cancer with low exposure over the long term.^[80] High levels of microcystin appeared in San Francisco Bay Area salt-water shellfish and fresh-water supplies throughout the state of California in 2016.

Loss of fertile soils

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.^[59]

Amazon basin

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

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

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

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. More than 150 districts in India are drought vulnerable, mostly concentrated in the state of Rajasthan, Gujarat, Madhya Pradesh and its adjoining Chhattisgarh, Uttar Pradesh, northern Karnataka and adjoining Maharashtra of the country.^[16] 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]

North America

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.^[96][97]