Drought is defined as a prolonged period when a region experiences a deficiency in precipitation compared to its historical normal, leading to a shortage of water. This shortage can severely affect human populations, agriculture, and local ecosystems. Determining the “longest” drought in history is complex because the answer depends entirely on the timescale used and the definition of what constitutes a drought event. While human records and paleoclimatology can document events lasting decades or centuries, the planet’s geological history contains evidence of arid conditions spanning millions of years. Scientists must therefore distinguish between short-term regional dry spells and vast, deep-time periods of extreme aridity to accurately answer this question.
Establishing Drought Duration and Metrics
Scientists classify and track water deficits across three main categories to measure drought duration and severity. Meteorological drought is the first to occur, defined by a lack of rainfall over a specific period. This leads to agricultural drought, characterized by a lack of soil moisture that affects crops and vegetation health. The final stage is hydrological drought, where the prolonged moisture deficit begins to affect surface and subsurface water supplies, such as streamflow and groundwater reserves.
Researchers rely on natural archives to extend the history of these events far beyond written human records. Paleoclimatology uses proxy data from natural sources to reconstruct past climates. Tree rings, known as dendrochronology, provide annual records of moisture availability, with narrow rings indicating dry years. Sediment cores from lakes and oceans, along with stalagmites found in caves, offer long-term records covering thousands of years. These indicators allow researchers to quantify the duration and intensity of past dry periods.
The Record Holder: Earth’s Longest Documented Drought
The longest documented period of continuous hyper-aridity on Earth is found in the Atacama Desert of Northern Chile. The core region of the Atacama is considered the driest non-polar desert in the world. Geological evidence suggests that this hyper-arid environment has persisted for at least 15 million years, since the middle Miocene epoch.
Scientists use the presence of evaporite formations, which are mineral deposits left behind when water evaporates, as evidence of ancient water conditions. Analysis of sulfate isotopes within these layers indicates a climate shift from semi-arid to hyper-arid conditions that began prior to 9.47 million years ago.
This extended period of dryness is largely caused by a unique geographical feature known as a “double rain shadow.” The Andes Mountains block moisture from the Amazon basin to the east, while the Chilean Coastal Range and the cold Humboldt Current prevent rain-bearing clouds from forming over the coast.
Historical Megadroughts: Examples of Prolonged Aridity
When the focus shifts to periods of aridity relevant to human history, the longest events are termed megadroughts, which are severe dry spells lasting for decades or even centuries. The North American continent experienced several such events during the Medieval Climate Anomaly, profoundly affecting ancient civilizations. Megadroughts lasting multiple decades were a factor in the abandonment of settlements by the ancestral Puebloans in the Southwestern United States.
A more recent example from the same region is the megadrought of the 16th century, which was recorded by tree-ring data as one of the most intense, prolonged, and widespread dry periods in the last 1,200 years. This event lasted for about 40 years in some areas, demonstrating a scale of aridity far exceeding the modern Dust Bowl era.
Another globally significant example is the prolonged dry episode that contributed to the collapse of the Classic Maya civilization in the Yucatán Peninsula. Between 800 and 1000 CE, the region suffered from a series of severe dry periods, with annual rainfall decreasing by an estimated 50% on average. The longest continuous drought event identified during this period lasted for 13 years, placing immense stress on the rain-fed agricultural system of the Maya.
The African Sahel region also provides a record of devastating multi-decadal aridity. A prolonged dry episode began in the late 1960s and lasted until the mid-1990s, severely affecting the region’s ecosystems and economies for approximately 25 years. Evidence from lake sediments suggests that multi-decadal megadroughts have been a common feature of the West African climate over the past 3,000 years.