Drought

A drought is a period when conditions are drier than normal, lasting from a few days to several years. It is a common climate event in many regions, but climate change—traceable in tree rings since around 1900—has made droughts more severe and harder to predict. Droughts often disrupt ecosystems and farming, hurting local economies. In the tropics, annual dry seasons raise the chance of drought, which in turn increases wildfire risk. Heat waves can worsen drought by speeding up evapotranspiration, drying out forests and vegetation and creating more fuel for fires.

Drought effects fall into three categories: environmental, economic, and social. Environmentally, droughts dry up wetlands, fuel larger and more frequent wildfires, and reduce biodiversity. Economically, they harm agriculture and livestock, leading to food insecurity; they also disrupt forestry, public water supplies, river navigation (due to low water levels), hydropower systems, and human health. Socially and health-wise, droughts expose people to extreme heat, raise food costs, cause stress from failed harvests and water shortages, and increase air pollution from dust and wildfires. Prolonged droughts have triggered mass migrations and humanitarian crises.

Regions with higher drought risks include the Amazon basin, Australia, the Sahel, and India. In 2005, parts of the Amazon experienced their worst drought in a century. A 2008 Australian government report warned that future droughts could be more severe and frequent; the long Australian Millennial drought ended in 2010. The 2020–2022 Horn of Africa drought was longer and more severe than the 2010–2011 one. Historically, people have seen droughts as disasters because they threaten food and society, viewing them as natural events, human-influenced, or caused by supernatural forces.

The IPCC Sixth Assessment Report defines drought simply as “drier than normal conditions”—a moisture deficit compared to average water availability for a given place and season. The National Integrated Drought Information System calls it a precipitation deficiency over a season or more, leading to water shortage. NOAA’s National Weather Service defines it as a moisture deficit that harms people, animals, or vegetation over a large area. Drought is complex and hard to define; by the early 1980s, over 150 definitions existed, reflecting different regions, needs, and scientific approaches.

There are three main drought categories based on where the moisture deficit occurs in the water cycle: meteorological, hydrological, and agricultural or ecological. Meteorological drought comes from a lack of precipitation and usually precedes other types. Hydrological drought involves low runoff, streamflow, reservoir and groundwater levels; it develops slowly because it affects stored water that isn’t replenished. Water management can influence this type—both positively and negatively—and strategic management is key to reducing impacts. For example, around 2007, Kazakhstan received World Bank funds to restore water diverted from the Aral Sea, while its largest lake, Balkhash, faces similar drying risks. Agricultural or ecological drought stresses plants due to evaporation and low soil moisture, and can occur without rainfall changes if poor farming practices, irrigation, or erosion cause water shortfalls. Some organizations add socioeconomic drought, which happens when demand for water exceeds supply due to weather-related shortages—similar to water scarcity.

To monitor drought, several indices have been created for different spatial and temporal scales. These indices must be statistically robust and allow comparisons across locations. Examples include the Palmer drought index.