Query: "The 30 major droughts of the 20th century were likely natural in all respects; and, hence, they are "indicative of what could also happen in the future," as Narisma

Relevant passages:
Drought <=SEP=> A drought is a period of below-average precipitation in a given region , resulting in prolonged shortages in its water supply , whether atmospheric , surface water or ground water . A drought can last for months or years , or may be declared after as few as 15 days . It can have a substantial impact on the ecosystem and agriculture of the affected region and harm to the local economy . Annual dry seasons in the tropics significantly increase the chances of a drought developing and subsequent bush fires . Periods of heat can significantly worsen drought conditions by hastening evaporation of water vapour .   Many plant species , such as those in the family Cactaceae ( or cacti ) , have drought tolerance adaptations like reduced leaf area and waxy cuticles to enhance their ability to tolerate drought . Some others survive dry periods as buried seeds . Semi-permanent drought produces arid biomes such as deserts and grasslands . Prolonged droughts have caused mass migrations and humanitarian crises . Most arid ecosystems have inherently low productivity . The most prolonged drought ever in the world in recorded history occurred in the Atacama Desert in Chile ( 400 Years ) .

Illinois <=SEP=> Illinois ( -LSB- ɪlᵻˈnɔɪ -RSB- ) is a state in the Midwestern region of the United States , achieving statehood in 1818 . It is the 6th-most populous state and 25th-largest state in terms of land area , and is noted as a microcosm of the entire country . The word `` Illinois '' comes from a French rendering of a native Algonquin word . With Chicago in the northeast , small industrial cities and great agricultural productivity in central and northern Illinois , and natural resources like coal , timber , and petroleum in the south , Illinois has a diverse economic base and is a major transportation hub . The Port of Chicago connects the state to other global ports from the Great Lakes , via the Saint Lawrence Seaway , to the Atlantic Ocean , as well as the Great Lakes to the Mississippi River , via the Illinois Waterway . For decades , O'Hare International Airport has been ranked as one of the world 's busiest airports . Illinois has long had a reputation as a bellwether both in social and cultural terms and politics .   Although today the state 's largest population center is around Chicago in the northern part of the state , the state 's European population grew first in the west , with French Canadian colonists who settled along the Mississippi River in the 17th and 18th century , and gave the area the name , `` Pays des Illinois '' or Illinois Country a region that was known as part of New France . After the American Revolutionary War established the United States , American settlers began arriving crossing the Appalachians barrier range in the 1810s via the gaps of the Allegheny to boat building centers in Pittsburgh , from Cumberland , Maryland via the Cumberland Narrows pass to outfit in Brownsville , Pennsylvania , from North Carolina and Virginia via the Cumberland Gap to Kentucky and Tennessee , all on the Ohio River .   With the War of 1812 , the growth of Illinois slowed as both Native Americans and Canadian forces often raided the American Frontier . After the war 's end , the federal government re-established forts such as Fort Dearborn ( in 1816 -- now the site is within Chicago ) and army patrols west of the Mississippi diminished the threat from Amerindian raids , so settlers were able to move into all of Illinois from the eastern and southern emigrant trails .   Mineral finds and timber stands also had spurred immigration -- by the 1810s , the Eastern U.S. had exhausted most timber stands close to the established cities creating a hard felt first energy crisis by the late-1790s , and after 1818 , the industrial revolution was being fueled by new canals such as the Lehigh Canal feeding the furnaces of the rapidly industrializing east coast . Also in 1818 , Illinois achieved statehood and its growth , as yet untroubled by the speed of as yet unrefined railway technology , would be fueled by the new religion of industrialized forward thinking .   After construction of the Erie Canal with increasing traffic and trade through the Great Lakes , Chicago was founded in the 1830s on the banks of the Chicago River , at one of the few natural harbors on southern Lake Michigan . John Deere 's invention of the self-scouring steel plow turned Illinois ' rich prairie into some of the world 's most productive and valuable farmlands , attracting new immigrant farmers from Germany and Sweden . Railroads arose and matured in the 1840s , and soon carried immigrants to new homes in Illinois , as well as being a resource to ship their commodity crops out to markets . Railroads freed most of the land of Illinois and other mid-western states from the tyranny of water transport ; no longer was a location near a river or canal a need to ship bulk goods .   By 1900 , the growth of industrial jobs in the northern cities and coal mining in the central and southern areas attracted a new group of immigrants , from Eastern and Southern Europe . Illinois was an important manufacturing center during both world wars . The Great Migration from the South established a large community of African Americans in Chicago , who created the city 's famous jazz and blues cultures .   Three U.S. presidents have been elected while living in Illinois : Abraham Lincoln , Ulysses S. Grant , and most recently Barack Obama . Additionally , Ronald Reagan , whose political career was based in California , was the only U.S. president born and raised in Illinois . Today , Illinois honors Lincoln with its official state nickname `` The Land of Lincoln , '' which has been displayed on its license plates since 1954 . The Abraham Lincoln Presidential Library and Museum is located in the state capital of Springfield , and the Barack Obama Presidential Center will be completed in Chicago by 2020 .

Sahel drought <=SEP=> The Sahel has long experienced a series of historic droughts , dating back to at least the 17th century . The Sahel region is a climate zone sandwiched between the African Savannah grasslands to the south and the Sahara desert to the north , across West and Central Africa . While the frequency of drought in the region is thought to have increased from the end of the 19th century , three long droughts have had dramatic environmental and societal effects upon the Sahel nations . Famine followed severe droughts in the 1910s , the 1940s , and the 1960s , 1970s and 1980s , although a partial recovery occurred from 1975-80 . The most recent drought occurred in 2012 .   While at least one particularly severe drought has been confirmed each century since the 17th century , the frequency and severity of recent Sahelian droughts stands out . Famine and dislocation on a massive scale -- from 1968 to 1974 and again in the early and mid-1980s -- was blamed on two spikes in the severity of the 1960-1980s drought period . From the late 1960s to early 1980s famine killed 100,000 people , left 750,000 dependent on food aid , and affected most of the Sahel 's 50 million people . The economies , agriculture , livestock and human populations of much of Mauritania , Mali , Chad , Niger and Burkina Faso ( known as Upper Volta during the time of the drought ) were severely impacted . As disruptive as the droughts of the late 20th century were , evidence of past droughts recorded in Ghanaian lake sediments suggest that multi-decadal megadroughts were common in West Africa over the past 3,000 years and that several droughts lasted far longer and were far more severe .   Since the 1980s , summer rainfall in the Sahel has been increasing ; this has been associated with an increase in vegetation , forming what has been called a ` greening ' of the Sahel . The observed increase in rainfall is accounted for by enhancements in the African easterly jet , which is known to induce wet anomalies . A 2011 study found that the positional shifts in the African easterly jet and African easterly waves accompanied the northward migration of the Sahel rainband .

Effects of global warming <=SEP=> The effects of global warming are the environmental and social changes caused ( directly or indirectly ) by human emissions of greenhouse gases . There is a scientific consensus that climate change is occurring , and that human activities are the primary driver . Many impacts of climate change have already been observed , including glacier retreat , changes in the timing of seasonal events ( e.g. , earlier flowering of plants ) , and changes in agricultural productivity .   Future effects of climate change will vary depending on climate change policies and social development . The two main policies to address climate change are reducing human greenhouse gas emissions ( climate change mitigation ) and adapting to the impacts of climate change . Geoengineering is another policy option .   Near-term climate change policies could significantly affect long-term climate change impacts . Stringent mitigation policies might be able to limit global warming ( in 2100 ) to around 2 ° C or below , relative to pre-industrial levels . Without mitigation , increased energy demand and extensive use of fossil fuels might lead to global warming of around 4 ° C. Higher magnitudes of global warming would be more difficult to adapt to , and would increase the risk of negative impacts .

United States <=SEP=> The United States of America ( USA ) , commonly known as the United States ( U.S. ) or America , is a constitutional federal republic composed of 50 states , a federal district , five major self-governing territories , and various possessions .  Forty-eight of the fifty states and the federal district are contiguous and located in North America between Canada and Mexico . The state of Alaska is in the northwest corner of North America , bordered by Canada to the east and across the Bering Strait from Russia to the west . The state of Hawaii is an archipelago in the mid-Pacific Ocean . The U.S. territories are scattered about the Pacific Ocean and the Caribbean Sea . Nine time zones are covered . The geography , climate and wildlife of the country are extremely diverse .   At 3.8 million square miles ( 9.8 million km2 ) and with over 324 million people , the United States is the world 's third - or fourth-largest country by total area , third-largest by land area , and the third-most populous . It is one of the world 's most ethnically diverse and multicultural nations , and is home to the world 's largest immigrant population . The capital is Washington , D.C. , and the largest city is New York City ; nine other major metropolitan areas -- each with at least 4.5 million inhabitants and the largest having more than 13 million people -- are Los Angeles , Chicago , Dallas , Houston , Philadelphia , Miami , Atlanta , Boston , and San Francisco .   Paleo-Indians migrated from Asia to the North American mainland at least 15,000 years ago . European colonization began in the 16th century . The United States emerged from 13 British colonies along the East Coast . Numerous disputes between Great Britain and the colonies following the Seven Years ' War led to the American Revolution , which began in 1775 . On July 4 , 1776 , during the course of the American Revolutionary War , the colonies unanimously adopted the Declaration of Independence . The war ended in 1783 with recognition of the independence of the United States by Great Britain , representing the first successful war of independence against a European power . The current constitution was adopted in 1788 , after the Articles of Confederation , adopted in 1781 , were felt to have provided inadequate federal powers . The first ten amendments , collectively named the Bill of Rights , were ratified in 1791 and designed to guarantee many fundamental civil liberties .   The United States embarked on a vigorous expansion across North America throughout the 19th century , displacing Native American tribes , acquiring new territories , and gradually admitting new states until it spanned the continent by 1848 . During the second half of the 19th century , the American Civil War led to the end of legal slavery in the country . By the end of that century , the United States extended into the Pacific Ocean , and its economy , driven in large part by the Industrial Revolution , began to soar . The Spanish -- American War and confirmed the country 's status as a global military power . The United States emerged from as a global superpower , the first country to develop nuclear weapons , the only country to use them in warfare , and a permanent member of the United Nations Security Council . The end of the Cold War and the dissolution of the Soviet Union in 1991 left the United States as the world 's sole superpower . The U.S. is a founding member of the United Nations , World Bank , International Monetary Fund , Organization of American States ( OAS ) , and other international organizations .   The United States is a highly developed country , with the world 's largest economy by nominal GDP and second-largest economy by PPP . Though its population is only 4.3 % of the world total , Americans hold nearly 40 % of the total wealth in the world . The United States ranks among the highest in several measures of socioeconomic performance , including average wage , human development , per capita GDP , and productivity per person . While the U.S. economy is considered post-industrial , characterized by the dominance of services and knowledge economy , the manufacturing sector remains the second-largest in the world . Accounting for approximately a quarter of global GDP and a third of global military spending , the United States is the world 's foremost economic and military power . The United States is a prominent political and cultural force internationally , and a leader in scientific research and technological innovations .