Short answers, pulled from the story.
Drought is a period of drier-than-normal conditions, defined by the IPCC Sixth Assessment Report as a moisture deficit relative to the average water availability at a given location and season. The National Weather Service defines it as a deficiency of moisture that results in adverse impacts on people, animals, or vegetation over a sizeable area. By the early 1980s, over 150 definitions of drought had already been published.
There are three major categories of drought: meteorological drought, caused by prolonged below-average precipitation; hydrological drought, which occurs when aquifers, lakes, and reservoirs fall below normal thresholds; and agricultural or ecological drought, which arises from low soil moisture and high evaporation affecting plant growth. A fourth category, socioeconomic drought, occurs when demand for water as an economic resource exceeds weather-related supply.
The Food and Agriculture Organization of the United Nations calculated that droughts caused US$278 billion of damage between 1991 and 2023. Economic losses span agriculture, fishing, forestry, hydropower, river transport, and municipal water systems.
The longest drought in recorded history began 400 years ago in the Atacama Desert in Chile and is still ongoing. The 4.2-kiloyear event, a megadrought between 5,000 and 4,000 years ago, has been linked to the collapse of the Old Kingdom in Egypt, the Akkadian Empire, and the Indus Valley Civilization. In 1540 Central Europe, a drought lasting eleven months with temperatures 5-7 degrees Celsius above 20th-century averages is described as the worst drought of the millennium.
Dendrochronological studies date the increased extremity and unpredictability of drought back to 1900. The IPCC Sixth Assessment Report states that warming over land drives an increase in atmospheric evaporative demand and in the severity of drought events. Higher temperatures, greater climate variability, land use change, and increased water demand have made droughts both more common and harder to predict.
Regions with elevated drought risk include the Amazon basin, Australia, the Sahel, and India. In 2005, parts of the Amazon basin experienced the worst drought in 100 years. The Horn of Africa suffered below-average rainfall for six consecutive rainy seasons between 2020 and 2023, the third longest and most widespread drought on record. Approximately 2.4 billion people live in the drainage basin of the Himalayan rivers, where retreating glaciers pose a long-term water security threat.