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. 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. 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. The term was then popularised as a similar severe drought affected the Southwestern US from the year 2000. Benjamin Cook suggested that the definition be a drought which is exceptionally severe compared to the weather during the previous 2,000 years. This was still quite imprecise and so research has suggested quantitative measures based on a Standard Precipitation Index. 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). 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 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. 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. 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. 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