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A megadrought is an exceptionally severe drought, lasting for many years and covering a wide area.

Definition

There is no exact definition of a megadrought.^[2] The term was first used by Connie Woodhouse and Jonathan Overpeck in their 1998 paper, 2000 Years of Drought Variability in the Central United States.^[2]^[3] In this, it referred to two periods of severe drought in the US – one at the end of the 13th century and the other in the middle of the 16th century.^[3] The term was then popularised as a similar severe drought affected the Southwestern US from the year 2000.^[2]

Benjamin Cook suggested that the definition be a drought which is exceptionally severe compared to the weather during the previous 2,000 years.^[2] This was still quite imprecise and so research has suggested quantitative measures based on a Standard Precipitation Index.^[4]

Causes

Past megadroughts in North America have been associated with persistent multiyear La Niña conditions (cooler than normal water temperatures in the tropical eastern Pacific Ocean).^[5]

Impact

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 Ancestral Puebloans of the North American Southwest,^[6] the Khmer Empire of Cambodia,^[7] the Maya of Mesoamerica,^[8] the Tiwanaku of Bolivia,^[9] and the Yuan Dynasty of China.^[10]

The African Sahel region in particular has suffered multiple megadroughts throughout history, with the most recent lasting from approximately 1400 AD to 1750 AD.^[11] North America experienced at least four megadroughts during the Medieval Warm Period.^[12]

Historical evidence

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.^[13]
Dendrochronology, the dating and study of annual rings in trees. The tree-ring data indicate that the Western U.S. states have experienced droughts that lasted ten times longer than anything the modern U.S. has seen. Based upon data derived from 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 1238-year tree-ring based chronology of weather condition in central Mexico by examining core samples taken from living Montezuma Cypress trees.^{[citation needed]}
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 studied and added to the record.^{[citation needed]}
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 Niña conditions.^[14]
During a 200-year mega drought in the Sierra Nevada that lasted from the 9th to the 12th centuries, trees would grow on newly exposed shoreline at Fallen Leaf Lake, then as the lake grew once again, the trees were preserved under cold water.^[15] However, a 2016–2017 expedition by the Undersea Voyager Project found evidence that the ancient trees did not grow there during an ancient drought, but rather slid into the lake during one of the many seismic events that have occurred in the Tahoe Basin since it was formed.^[16]^[17]
The 2000–present southwestern North American megadrought was the driest 22-year period in the region since at least 800. Both 2002 and 2021 were drier than any other years in nearly 300 years and were, respectively, the 11th and 12th driest years between 800 and 2021.^[18] The atmospheric rivers of 2024 delivered some of the wettest climate since 2004.^[19]

References

^ Irina Ivanova (2 June 2022). "California is rationing water amid its worst drought in 1,200 years". CBS News. Retrieved 4 June 2022.
^ ^a ^b ^c ^d James Dinneen (7 October 2022), "What is megadrought? How scientists define extreme water shortages", New Scientist
^ ^a ^b Woodhouse, Connie A.; Overpeck, Jonathan T. (1 December 1998), "2000 Years of Drought Variability in the Central United States", Bulletin of the American Meteorological Society, 79 (12): 2693–2714, Bibcode:1998BAMS...79.2693W, doi:10.1175/1520-0477(1998)079<2693:YODVIT>2.0.CO;2, ISSN 0003-0007
^ Kim, Youngkyu; Kim, Sunmin; Jeong, Hoseong; An, Hyunuk (31 December 2022), "Quantitatively defining megadrought based on drought events in central Chile", Geomatics, Natural Hazards and Risk, 13 (1): 975–992, doi:10.1080/19475705.2022.2060763, ISSN 1947-5705, S2CID 248073159
^ 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. Archived from the original on 20 August 2011. Retrieved 15 August 2011.
^ Richard Stone (12 March 2009). "Tree Rings Tell of Angkor's Dying Days" (PDF). American Association for the Advancement of Science. Archived from the original (PDF) on 14 December 2011. 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. Archived from the original on 9 August 2011. 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". The 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". Quaternary Science Reviews. 30 (1–2): 47–62. Bibcode:2011QSRv...30...47S. doi:10.1016/j.quascirev.2010.10.005. 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 News Scripps 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". The 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" (PDF). 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".
^ "Undersea Voyager Project Returns to Fallen Leaf Lake To Study Ancient Trees". California Diver Magazine. October 2016. Retrieved 17 May 2019.
^ "Home". Undersea Voyager Project. Retrieved 17 May 2019.
^ Williams, A. Park; Cook, Benjamin I.; Smerdon, Jason E. (14 February 2022). "Rapid intensification of the emerging southwestern North American megadrought in 2020–2021". Nature Climate Change. 12 (3): 232–234. Bibcode:2022NatCC..12..232W. doi:10.1038/s41558-022-01290-z. ISSN 1758-678X. S2CID 246815806. Retrieved 15 February 2022.
^ "Flash flooding, mudslides ongoing threat from Santa Barbara to Los Angeles to San Diego". ABC News. 5 February 2024. Retrieved 17 June 2024. a historic atmospheric river storm slams Southern California […] the city's wettest day since December 2004