A megadrought (or mega-drought) is a prolonged drought lasting two decades or longer. Past megadroughts have been associated with persistent multiyear La Niña conditions (cooler than normal water temperatures in the tropical eastern Pacific Ocean). The term megadrought is generally used to describe the length of a drought, and not its severity. In scientific literature the term is used to describe decades-long droughts or multi-decadal droughts. Multiyear droughts, such as the Dust Bowl drought of the 1930s, are generally not described as megadroughts even though they are of a long duration. In popular literature multiyear or even single year droughts are occasionally described as megadroughts based upon their severity, the economic damage they inflict or other criteria; but this is the exception and not the rule.
Megadroughts have historically led to the mass migration of humans away from drought affected lands, resulting in a significant population decline from pre-drought levels. They are suspected of playing a primary role in the collapse of several pre-industrial civilizations, including the Anasazi of the North American Southwest, the Khmer Empire of Cambodia, the Mayan of Mesoamerica, the Tiwanaku of Bolivia, and the Yuan Dynasty of China. As such they constitute one of the greatest threats to human civilization. The African Sahel region in particular has suffered multiple megadroughts throughout history, with the most recent lasting from approximately 1400 AD to 1750 AD. North America experienced at least four megadroughts during the Medieval Warm Period.
There are several sources for establishing the past occurrence and frequency of megadroughts, including:
- When megadroughts occur, lakes dry up and trees and other plants grow in the dry lake beds. When the drought ends the lakes refill, when this happens the trees are submerged and die. In some locations these trees have remained preserved and can be studied giving accurate radio-carbon dates, and the tree rings of the same long dead trees can be studied. Such trees have been found in Mono and Tenaya lakes in California, Lake Bosumtwi in Ghana; and various other lakes.
- Dendrochronology, the dating and study of annual rings in trees. The tree-ring data indicate that the Western states have experienced droughts that lasted ten times longer than anything the modern U.S. has seen. Based on annual tree rings, NOAA has recorded patterns of drought covering most of the U.S. for every year since 1700. Certain species of trees have given evidence over a longer period, in particular Montezuma Cypress and Bristlecone pine trees. The University of Arkansas has produced a 1,238-year tree-ring based chronology of weather condition in central Mexico by examining core samples taken from living Montezuma Cypress trees.
- Sediment core samples taken at the volcanic caldera in Valles Caldera, New Mexico and other locations. The cores from Valles Caldera go back 550,000 years and show evidence of megadroughts that lasted as long as 1,000 years during the mid-Pleistocene Epoch during which summer rains were almost non-existent. Plant and pollen remains found in core samples from the bottom of lakes have been also been studied and added to the record.
- Fossil corals on Palmyra Atoll. Using the relationship between tropical Pacific sea surface temperatures and the oxygen isotope ratio in living corals to convert fossil coral records into sea surface temperatures. This has been used to establish the occurrence and frequency of La Nina conditions.
- During a 200 year mega drought in the Sierra Nevada that lasted from the 9th and 12th centuries, trees would grow on newly exposed shoreline at Fallen leaf lake, than as the lake grew once again, the trees were preserved under cold water 
- Richard Seager, Celine Herweijer and Ed Cook (2011). "The characteristics and likely causes of the Medieval megadroughts in North America". Lamont–Doherty Earth Observatory of Columbia University. Retrieved 15 August 2011. "Consequently, despite considerable limitations of the proxy evidence, to date it does support the idea that, during medieval times, the global hydroclimate tended towards what we would now call a La Niña-like state."
- Bob Varmette (4 August 2011). "Megadroughts". Fort Stockton Pioneer. Retrieved 15 August 2011.
- Richard Stone (12 March 2009). "Tree Rings Tell of Angkor’s Dying Days". American Association for the Advancement of Science. Retrieved 15 August 2011. "New findings suggest that a decades-long drought at about the time the kingdom began fading away in the 14th century may have been a major culprit. Evidence for a megadrought comes from centuries-old conifers that survived the Angkor era."
- Melissa Lutz Blouin (3 February 2011). "Trees Tell of MesoAmerican MegaDroughts". Fulbright College of Arts and Sciences University of Arkansas. Retrieved 15 August 2011. "This far-reaching rainfall chronology also provides the first independent confirmation of the so-called Terminal Classic drought, a megadrought some anthropologists relate to the collapse of the Mayan civilization."
- William K. Stevens (19 July 1994). "Severe Ancient Droughts: A Warning to California". New York Times. Retrieved 15 August 2011. "In medieval times the California droughts coincided roughly with a warmer climate in Europe, which allowed the Vikings to colonize Greenland and vineyards to grow in England, and with a severe dry period in South America, which caused the collapse of that continent's most advanced pre-Inca empire, the rich and powerful state of Tiwanaku, other recent studies have found."
- Ashish Sinhaa (January 2011). "A global context for megadroughts in monsoon Asia during the past millennium". ScienceDirect/Elsevier. Retrieved 15 August 2011. "Although the relationship between climate and societal change is complex and not necessarily deterministic, the widespread societal changes across monsoon Asia between the mid 13th to 15th centuries, which include famines and significant political reorganization within India ([Dando, 1980], [Pant et al., 1993] and [Maharatna, 1996]), the collapse of the Yuan dynasty in China (Zhang et al., 2008); Rajarata civilization in Sri Lanka (Indrapala, 1971), and the Khmer civilization of Angkor Wat fame in Cambodia (Buckley et al., 2010), strongly suggest that the MMDs may have played a major contributing role in shaping these societal changes."
- Catherine Brahic (16 April 2009). "Africa trapped in mega-drought cycle". New Scientist. Retrieved 15 August 2011. "As well as the periodic droughts lasting decades, there was evidence that the Sahel region has undergone several droughts lasting a century or more....The most recent mega-drought was just 500 years ago, spanning 1400 to 1750 and coinciding with Europe's Little Ice Age."
- Jim Erickson (11 October 2004). "Tree rings reveal 'megadroughts'". Deseret NewsScripps Howard News Service. Retrieved 15 August 2011. "The new record sheds light on a drought-prone 400-year period between A.D. 900 and 1300. It is punctuated by four decades-long, regionwide megadroughts centered on the years 936, 1034, 1150 and 1253."
- William K. Stevens (19 July 1994). "Severe Ancient Droughts: A Warning to California". New York Times. Retrieved 15 August 2011. "The evidence for the big droughts comes from an analysis of the trunks of trees that grew in the dry beds of lakes, swamps and rivers in and adjacent to the Sierra Nevada, but died when the droughts ended and the water levels rose. Immersion in water has preserved the trunks over the centuries."
- Edward R. Cook, Richard Seager, Richard R. Heim, Jr., Russell S. Vose, Celine Herweijer, Connie Woodhouse. "Megadroughts in North America: Placing IPCC Projections of Hydroclimatic Change in a Long-Term Paleoclimate Context". Lamont–Doherty Earth Observatory of Columbia University / Journal of Quaternary Science. Retrieved 15 August 2011. "Marine coral records from the core ENSO region of the tropical Pacific also support the concept of decadal and longer ENSO variability during the last millennium (Cobb et al., 2003), with some indication that the MCA period experienced persistent La Niña-like SST conditions that would be drought-inducing over North America."
- Perrin Ireland (13 February 2013). "The Alien World of Deepwater Research".
- Global Drought Monitor Current worldwide drought conditions
- US Drought Monitor Current U.S. drought conditions
- Persistent drought in North America: a climate modeling and paleoclimate perspective Lamont-Doherty Earth Observatory of Columbia University