# Drought: A Common Environmental Disaster
## 2 Historical Droughs  
In the past, droughs were frequently studied by scientists to put current droughs in context. Scientists must study paleoclimatology, the study of the atmosphere of prehistoric Earth, because current data from thermometers and rain gauges only go back around 100 to 150 years [11; 36]. Tree rings, sediments found in lakes and oceans, ice cores, archaeological features and artefacts are all used to acquire paleoclimatic data. This enables scientists to gain a better grasp of climate patterns, including droughs that occurred millions of years ago [37; 38; 39]. Droughs that are severe and last for a long time are an unavoidable feature of natural climate cycles, according to paleoclimatic data. There have been several long-term droughs in North America, each with severe consequences [37]. Droughs are thought to have contributed to the fall of the Ancestral Puebloans in the Southwest around the 13 th century, as well as the central and lower Mississippi societies during the 14 th and 16 th centuries [11]. The Great Drought was a climatic interval during the Holocene Epoch that affected much of what is now western America and had a significant impact on the region's vegetation, animals and prehistoric Native American cultures. The Holocene began 11,700 years ago and is still going on today [37; 38]. The Great Drought affected an area that stretched from what is now Oregon to southern California and east to what is now eastern Texas; dendrochronology, or tree-ring investigations, show that it began in the year 1276 and lasted until the year 1299. The Great Drought had a particularly negative impact on the Ancestral Pueblo (Anasazi) and Hohokam peoples, whose agricultural economies had enabled them to construct densely populated settlements by this time [37; 38]. They were forced to quit their towns and spread throughout the region after years of crop failure. Nomadic peoples were affected by parallel declines in wild food sources, and social disturbances are likely to have occurred as nomads and former farmers competed for the limited resources that survived under extremely dry conditions [38]. The Great Drought was one of the numerous severe droughs that struck the same region over the last three millennia. The Fairbank Drought of 500 BC and the Whitewater Drought of AD 300 are two other droughs that have been identified. Notably, all of these dates appear to be linked to key shifts in North and Central American cultures [38]. In Africa, between 1400 and 1750, the Sahel region of Africa suffered a dry period that drastically affected the environment. For example, the water level in Ghana's Lake Bosumtwi dropped to the point where a complete forest developed on the lake's edge [11; 40]. The tops of trees growing out of the lake, which is now more than 15 m (50 feet) deep, may still be seen today. By studying Earth's drought history, scientists have demonstrated that catastrophic droughs are a recurring component of nature's cycle [40]. Droughs in the recent century have been catastrophic, but they are modest in comparison to the severity of prior droughs that lasted more than a century. In addition, many severe and long-lasting droughs have occurred in Africa, including the 1999-2002 drought in northwest Africa, droughs in western Africa (Sahel) in the 1970s and 1980s, droughs in eastern Africa (Horn of Africa) in 2010-2011, and droughs in southern and southeastern Africa in 2001-2003, to name a few [11; 40]. Before the twentieth century, the available (albeit limited) evidence demonstrates the occurrence of many extreme and multi-year droughs during each century, with the most prolonged and intense droughs occurring in the Sahel and equatorial eastern Africa [40; 41]. Many physical mechanisms, such as the El Nio-Southern Oscillation (ENSO), sea surface temperature (SST) and land-atmosphere interaction, are complex and highly variable, making drought monitoring and forecasting a demanding task [40; 41].