Megadrought
Encyclopedia
A megadrought 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 civilization
s, 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
.
Drought
A drought is an extended period of months or years when a region notes a deficiency in its water supply. Generally, this occurs when a region receives consistently below average precipitation. It can have a substantial impact on the ecosystem and agriculture of the affected region...
lasting two decades or longer. Past megadroughts have been associated with persistent multiyear La Niña
La Niña
La Niña is a coupled ocean-atmosphere phenomenon that is the counterpart of El Niño as part of the broader El Niño-Southern Oscillation climate pattern. During a period of La Niña, the sea surface temperature across the equatorial Eastern Central Pacific Ocean will be lower than normal by 3–5 °C...
conditions (cooler than normal water temperatures in the tropical eastern Pacific Ocean
Pacific Ocean
The Pacific Ocean is the largest of the Earth's oceanic divisions. It extends from the Arctic in the north to the Southern Ocean in the south, bounded by Asia and Australia in the west, and the Americas in the east.At 165.2 million square kilometres in area, this largest division of the World...
). The term megadrought is generally used to describe the length of a drought, and not its severity. In scientific literature
Scientific literature
Scientific literature comprises scientific publications that report original empirical and theoretical work in the natural and social sciences, and within a scientific field is often abbreviated as the literature. Academic publishing is the process of placing the results of one's research into the...
the term is used to describe decades-long droughts or multi-decadal droughts. Multiyear droughts, such as the Dust Bowl
Dust Bowl
The Dust Bowl, or the Dirty Thirties, was a period of severe dust storms causing major ecological and agricultural damage to American and Canadian prairie lands from 1930 to 1936...
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
Population decline
Population decline can refer to the decline in population of any organism, but this article refers to population decline in humans. It is a term usually used to describe any great reduction in a human population...
from pre-drought levels. They are suspected of playing a primary role in the collapse
Societal collapse
Societal collapse broadly includes both quite abrupt societal failures typified by collapses , as well as more extended gradual declines of superpowers...
of several pre-industrial civilization
Civilization
Civilization is a sometimes controversial term that has been used in several related ways. Primarily, the term has been used to refer to the material and instrumental side of human cultures that are complex in terms of technology, science, and division of labor. Such civilizations are generally...
s, including the Anasazi of the North American Southwest, the Khmer Empire
Khmer Empire
The Khmer Empire was one of the most powerful empires in Southeast Asia. The empire, which grew out of the former kingdom of Chenla, at times ruled over and/or vassalized parts of modern-day Laos, Thailand, Vietnam, Burma, and Malaysia. Its greatest legacy is Angkor, the site of the capital city...
of Cambodia, the Mayan
Maya civilization
The Maya is a Mesoamerican civilization, noted for the only known fully developed written language of the pre-Columbian Americas, as well as for its art, architecture, and mathematical and astronomical systems. Initially established during the Pre-Classic period The Maya is a Mesoamerican...
of Mesoamerica, the Tiwanaku
Tiwanaku
Tiwanaku, is an important Pre-Columbian archaeological site in western Bolivia, South America. Tiwanaku is recognized by Andean scholars as one of the most important precursors to the Inca Empire, flourishing as the ritual and administrative capital of a major state power for approximately five...
of Bolivia, and the Yuan Dynasty
Yuan Dynasty
The Yuan Dynasty , or Great Yuan Empire was a ruling dynasty founded by the Mongol leader Kublai Khan, who ruled most of present-day China, all of modern Mongolia and its surrounding areas, lasting officially from 1271 to 1368. It is considered both as a division of the Mongol Empire and as an...
of China. As such they constitute one of the greatest threats to human civilization. The African Sahel
Sahel
The Sahel is the ecoclimatic and biogeographic zone of transition between the Sahara desert in the North and the Sudanian Savannas in the south.It stretches across the North African continent between the Atlantic Ocean and the Red Sea....
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
Medieval Warm Period
The Medieval Warm Period , Medieval Climate Optimum, or Medieval Climatic Anomaly was a time of warm climate in the North Atlantic region, that may also have been related to other climate events around the world during that time, including in China, New Zealand, and other countries lasting from...
.
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 datesRadiocarbon datingRadiocarbon dating is a radiometric dating method that uses the naturally occurring radioisotope carbon-14 to estimate the age of carbon-bearing materials up to about 58,000 to 62,000 years. Raw, i.e. uncalibrated, radiocarbon ages are usually reported in radiocarbon years "Before Present" ,...
, and the tree rings of the same long dead trees can be studied. Such trees have been found in MonoMono LakeMono Lake is a large, shallow saline lake in Mono County, California, formed at least 760,000 years ago as a terminal lake in a basin that has no outlet to the ocean...
and Tenaya lakeTenaya LakeTenaya Lake is a lake in Yosemite National Park, located between Yosemite Valley and Tuolumne Meadows.Tenaya Lake was created by the Tenaya branch of the Tuolumne Glacier as it passed through Tenaya Canyon. The lake is supplied by a network of creeks and springs including Murphy Creek to the...
s in California, Lake BosumtwiLake BosumtwiLake Bosumtwi , situated within an ancient meteorite impact crater, is approximately 8 km across and the only natural lake in Ghana. It is situated about 30 km south-east of Kumasi and is a popular recreational area. There are about 30 villages near the lake, with a combined population of...
in Ghana; and various other lakes.
- DendrochronologyDendrochronologyDendrochronology or tree-ring dating is the scientific method of dating based on the analysis of patterns of tree-rings. Dendrochronology can date the time at which tree rings were formed, in many types of wood, to the exact calendar year...
, 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 pineBristlecone pineThe bristlecone pines are a small group of pine trees that are thought to reach an age far greater than that of any other single living organism known, up to nearly 5,000 years....
trees. The University of ArkansasUniversity of ArkansasThe University of Arkansas is a public, co-educational, land-grant, space-grant, research university. It is classified by the Carnegie Foundation as a research university with very high research activity. It is the flagship campus of the University of Arkansas System and is located in...
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 sampleCore sampleA core sample is a cylindrical section of a naturally occurring substance. Most core samples are obtained by drilling with special drills into the substance, for example sediment or rock, with a hollow steel tube called a core drill. The hole made for the core sample is called the "core hole". A...
s taken at the volcanic calderaCalderaA caldera is a cauldron-like volcanic feature usually formed by the collapse of land following a volcanic eruption, such as the one at Yellowstone National Park in the US. They are sometimes confused with volcanic craters...
in Valles Caldera, New Mexico and other locations. The cores from Valles Caldera go back 550,000 years and show evidence of megadroughts that lasting 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 AtollPalmyra AtollPalmyra Atoll is an essentially unoccupied equatorial Northern Pacific atoll administered as an unorganized incorporated territory by the United States federal government...
. 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.
External links
- 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