- 1. Overview
- 2. Etymology
- 3. Cultural Impact
Drought
A drought is a period characterized by drier-than-normal conditions, a phenomenon that can persist for days, months, or even years. This environmental condition significantly impacts ecosystems, agriculture, and local economies, often leading to severe disruptions in water supply, increased wildfire risks, and substantial economic losses. The effects of drought are exacerbated by climate change, which alters the water cycle and increases the frequency and severity of such events.
Definition
Drought is defined as a deficiency of precipitation over an extended period, resulting in a water shortage. The IPCC Sixth Assessment Report describes it as “drier than normal conditions,” indicating a moisture deficit relative to average water availability at a given location and season. This definition underscores the complexity of drought as a phenomenon that is challenging to monitor and define due to its multifaceted nature.
Categories of Drought
Droughts are categorized based on where the moisture deficit occurs in the water cycle:
- Meteorological Drought: Occurs due to a lack of precipitation. This type of drought usually precedes other kinds of drought and can lead to prolonged dry conditions.
- Hydrological Drought: Related to low runoff, streamflow, and reservoir and groundwater storage. This type of drought tends to develop more slowly due to the involvement of stored water.
- Agricultural or Ecological Drought: Causes plant stress due to a combination of evaporation and low soil moisture. This can lead to significant impacts on crop production and ecosystem health.
- Socioeconomic Drought: Occurs when the demand for an economic good exceeds supply due to a weather-related shortfall in water supply. This concept is similar to water scarcity.
Causes of Drought
General Precipitation Deficiency
Droughts primarily occur in areas where normal levels of rainfall are inherently low. Factors such as high levels of reflected sunlight, prevalence of high-pressure systems, and winds carrying continental air masses can prevent or restrict the development of thunderstorm activity or rainfall over a region. Once a drought sets in, feedback mechanisms like local arid air, hot conditions, and minimal evapotranspiration can exacerbate the situation.
Dry Season
Within the tropics, distinct wet and dry seasons emerge due to the movement of the Intertropical Convergence Zone or Monsoon trough. The dry season significantly increases the occurrence of droughts and is characterized by low humidity, drying up of watering holes, and increased bushfire risks. This season forces many grazing animals to migrate in search of more fertile lands.
El Niño–Southern Oscillation (ENSO)
The El Niño–Southern Oscillation (ENSO) phenomenon plays a significant role in drought occurrence. 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, exacerbating drought in regions like California and the Southwestern United States. Conversely, El Niño events can cause drier and hotter weather in parts of the Amazon River Basin, Colombia, and Central America.
Climate Change
Climate change is a major driver of increased drought frequency and severity. The rise in global temperatures and atmospheric evaporative demand has led to more severe drought events. The IPCC Sixth Assessment Report highlights that warming over land drives an increase in atmospheric evaporative demand, leading to more severe drought conditions and increased plant water stress.
Vegetation Changes and Human Activities
Human activities such as over-farming, excessive irrigation, deforestation, and erosion adversely impact the ability of the land to capture and hold water. These activities can trigger exacerbating factors that worsen drought conditions. For example, wind erosion is more severe in arid areas and during times of drought, leading to significant soil loss.
Impacts of Drought
Environmental and Economic Impacts
Droughts have profound environmental and economic impacts, including:
- Environmental Effects: Lower surface and subterranean water levels, increased pollution of surface water, drying out of wetlands, more and larger wildfires, higher deflation intensity, loss of biodiversity, and worse health of trees.
- Economic Losses: Lower agricultural, forest, game, and fishing output, higher food-production costs, lower energy-production levels in hydro plants, losses caused by depleted water tourism and transport revenue, and disruption of water supplies for the energy sector and technological processes.
Agricultural Impacts
Droughts can cause land degradation and loss of soil moisture, resulting in the destruction of cropland productivity. This can lead to diminished crop growth or yield productions and carrying capacity for livestock. Water stress affects plant development and quality in various ways, including poor germination, impaired seedling development, and reduced photosynthetic activity.
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. Droughts can also cause limitations of water supplies, increased water pollution levels, high food costs, and stress caused by failed harvests. Reduced water quality can occur because lower water flows reduce dilution of pollutants and increase contamination of remaining water sources.
Regions Particularly Affected
Amazon Basin
In 2005, parts of the Amazon basin experienced the worst drought in 100 years. This drought response, coupled with the effects of deforestation on regional climate, is pushing the rainforest towards a “tipping point” where it could irreversibly start to die, with catastrophic consequences for the world’s climate.
Australia
Australia has faced severe droughts, such as the 1997–2009 Millennium Drought, which led to a water supply crisis across much of the country. The long Australian Millennial drought broke in 2010, but the country could experience more severe droughts in the future.
East Africa
East Africa, including countries like Ethiopia, Kenya, and Somalia, has a diverse climate with considerable variability in seasonal rainfall. The region has experienced frequent hydrological extremes, such as droughts and floods, which harm the already vulnerable population suffering from severe poverty and economic turmoil.
Himalayan River Basins
Approximately 2.4 billion people live in the drainage basin of the Himalayan rivers. Drought in India affecting the Ganges is of particular concern, as it provides drinking water and agricultural irrigation for more than 500 million people. Retreating glaciers could threaten the food and water supply of 2 billion people worldwide.
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, is also affected by droughts. The region could experience more severe droughts in the future due to climate change.
Protection, Mitigation, and Relief
Strategies for drought protection or mitigation include:
- Dams: Many dams and their associated reservoirs supply additional water in times of drought.
- Cloud Seeding: A form of intentional weather modification to induce rainfall.
- Land Use: Carefully planned crop rotation can help 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, stormwater recovery, or seawater desalination.
History
Throughout history, humans have viewed droughts as disasters due to their impact on food availability and 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. Hunter-gatherer migrations in 9,500 BC Chile have been linked to the phenomenon, as has the exodus of early humans out of Africa and into the rest of the world around 135,000 years ago.