Droughts, Floods, the Medieval Warm Period and the Rise and Fall of Civilisations in Central and South America

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Changes in climate together with extreme weather events have played major roles in the history of the pre-Columbian civilizations of Central and South America of the past two millennia, including the period known as the Medieval Warm Period.
Below is a brief narrative of major climate related events in Central and South America, from the start of the Christian era to the 16th Century.

This timespan is necessary both to provide explanation and set Hubert Lamb's Medieval Warm Period (MWP) in a context. All that is below was unknown to Lamb when he wrote 'Climate, History and the Modern World' (1982).

Lamb was the first to use the term Medieval Warm Epoch. Lamb thought that this epoch was at its maximum between 1000 AD–1300 AD, and that it peaked in the 11th Century.

 Lamb worked predominantly from written records (Inevitably Europe has more of these, going back far longer, than all other regions of the world; with a possible exception of China.) and noted that his Medieval Warm Epoch coincided with the West European period known to historians as the High Middle Ages; a period of relative stability, peace and prosperity; in the preceding centuries the opposite had often been the case.

Lamb also connected this warmer, moist (bountiful harvests require plentiful rainfall) period of climate in northwestern Europe to an idea borrowed from his art-historian contemporary, Kenneth Clark. Lamb quotes Clark's description of this period as being "...the first great awakening in European Civilisation" following the collapse of the western Roman Empire in the 5th Century.
(Lamb. CHMW. p171. 1982.) You can read more on what really Lamb wrote here. Today Lamb's Medieval Warm Epoch has become widely known as the Medieval Warm Period (MWP). Of course, history in the Americas had followed a very different trajectory; and would continue to do so until Columbus crossed the Atlantic in 1492 and 'Old' and 'New' worlds finally met. (See also Note at bottom of Mesoamericans page.)

In the Americas, the climatic period 1000-1300 AD saw the culmination of very different changes to its climate. These had begun in the first half of the first millennium; a slow drying that developed over the following centuries and reached its apogee between 900-1300 AD.
In both California's Sierra Nevada mountains and in Patagonia, evidence recovered from the beds of deep lakes (trees that grew for well over a century or two on the lake-floor and died when the waters returned, human artefacts etc) show these lakes had dried out. Analysis provides dates for these epic dry periods: between 892–1112 AD (220 years) and 1209-1350 AD (141 years). There were other periods of great drought elsewhere in N. America between 900-1300 AD. e.g. Image below: mapping a 12th Century drought in N. America.

What is remarkable is how markedly different this drying climate was in North, Central and South America - and the consequent impacts on human societies there - compared to the mild, moist climate that Western Europe experienced during the same
period of time.

The most striking aspect of the period of American climate, between the 2nd and 16th Centuries, is the incidence, extent, prevalence, duration and severity of droughts, throughout the Americas; particularly - but by no means exclusively - over western and central regions of the Americas.

These droughts often lasted for a decade or longer and have been dubbed meagadroughts. Two droughts, in California and Patagonia, each lasted for well over 100 years and have been described as epic droughts.

Because of this notable difference, one researcher (Stine*) has suggested the term Medieval Climatic Anomaly to describe the American climatic experience during the period 1000-1300 AD; so as to differentiate the American experience from  the moist, benign Medieval Warm Period that Lamb had documented as the northwestern European experience
(*Extreme and Persistent Drought in California and Patagonia During Mediaeval Time. S. Stine. Nature. 1994.)
  • In the Andes, lake-bed sediments, glacier cores etc. show three much dryer than average periods between 540–610, 650–760 and 1040–1450 AD. Archaeologists have now demonstrated that drought in the last of these periods (1040–1450 AD) caused the collapse of two major South American pre-Inca civilisations: the Tiwanaku and the Wari in 1100 AD and 1150 AD.  (C. Ortloff et al. Climate and Collapse: Agro-Ecological Perspective on the Decline of the Tiwanaku State. Journal of Field Archaeology. 1993. M. Binford et al. Climate Variation and the Rise and fall of an Andean Civilisation. Quaternary Research. 1997. A 1500 year Record of Tropical Precipitation recorded in Ice Cores from the Quelccaya Ice Cap. L.. Thompson et al. Science. 1985. I. Shimada et al. Cultural Impacts of Severe Droughts in the Prehistoric Andes: Application of a 1,500-year Ice Core Precipitation Record. World Archaeology: Archaeology and Arid Environments. 1991. etc.)
  • Related to the above, a seabed core taken from off the Peruvian coast records: "a period of extreme drought without flooding occurred from 800-1250 AD". (B. Rein et al. A Major Holocene ENSO Anomaly During the Medieval Period. Geophysical Research Letters. 2004.)
  • Yet the usually hyper-arid Atacama desert was considerably wetter than today between 880-1250 AD. (Perennial Stream Discharge in the Hyper-arid Atacama Desert of Northern Chile during the Late Pleistocene. Nester et al. PNAS. 2007.)
  • Amazonia suffered immense drought between 700-800 AD and 1000-1100 AD. (Fire, Climate Change and Biodiversity in Amazonia: a Late Holocene Perspective. M. Bush. Phil. Trans. Royal Society. 2008. Archaeological Evidence for the Impact of Mega-Nino Events on Amazonia during the past two Millennia. B. Meggers. Smithsonian. 1994.)
  • During this same period the tropical eastern Pacific was abnormally cool. (See also below). (Competitive and Cooperative Responses to Climatic Instability in Coastal Southern California. D. Kennett & P. Kennett. American Antiquity. Vol. 65. 2000.)
  • Isotope analysis of a stalagmite from Chilibrillo cave on the Isthmus of Panama revealed a precipitation record. The researchers wrote the results: "show a broad increase (drying) from 180 B.C. to 1310 A.D." – and – "Rainfall anomalies began as early as 550 A.D. in Panama, but the driest conditions occurred between 900 and 1310 A.D.". (M. Lachniet et al. A 1500 year El Nino/Southern Oscillation and Rainfall History for the Isthmus of Panama from a Spelotherm Calcite. Journal of Geophysical Research. 2004.)
There is a connection between all the above; the answer lies in changes to the Pacific's tropical waters that had begun many centuries before, to reach their apogee between 1000-1300 AD.

Most, if not all, of what follows, from both archaeology and from research into past changes to climate in the Americas, was unknown to Hubert Lamb when he was working several decades ago. The more recent past has seen the discovery of much that is new, both about the pre Columbian civilizations and of the climatic events that shaped them; and in some instances destroyed them.

The striking differences in climate in this period, between the two continents, is due to the oceans that cover 70% of earth's surface. Western Europe’s climate is principally determined by Atlantic conditions. The Americas have an ocean either side of the continent; the vast Pacific makes its presence felt over the entire continental landmass. (It could also be said that when the Pacific sneezes the whole world notices ~ El Nino and La Nina are those sneezes.)


This is confirmed by similar research into past sea temperatures at a small tropical island of Palmyra, at the mid-point of the tropical Pacific. Isotope analysis of ancient corals shows sea temperatures for the past 1100 years. These coral records show the "central tropical Pacific ranging from relatively cool and dry during the tenth Century to increasingly warmer and wetter climate in the twentieth Century." (El Nino/Southern Oscillation and Tropical Pacific Climate During the Last Millennium. Cobb et al. Nature. 2003.)Yet 12,000 miles away, off the Indonesia coast; researchers analysed a seabed core taken from an area called the West Pacific Warm Pool (aka Indo- Pacific Warm Pool) at the western end of the Pacific, that shows sea-surface temperatures for this period were the exact opposite. "These records resolve a warming trend that began 2000 years ago that culminated in the warmest SST values between 900 and 1500 AD." … "This period of maximum warmth of is followed by cooling sea surface temperatures that extend to the beginning of the 20th Century." (Climate and Hydrographic Variability in the Indo-Pacific Warm Pool During the Last Millenium. A. Newton et al. Geophysical Research Letters. 2006.) 

Oceanographers report that analysis of sediment cores taken from the seabed of the Santa Barbara basin (a sea area of southern California's coast) show that the western North American area of the [eastern] Pacific ocean, for the period 4501300 AD, was one of the "coldest and most unstable marine climatic intervals of the Holocene" and that sea temperatures "dropped to around 1.5 degrees Celsius (2.7 degrees F) cooler than they had been for 8,000 years". (Competitive and Cooperative Responses to Climatic Instability in Coastal Southern California. D. Kennett and P. Kennett. American Antiquity. Vol. 65. 2000.)

This means
 the Pacific entered into a predominantly La Nina state for around 1,000 years (much greater frequency of La Ninas than El Ninos; the past few hundred years saw the opposite.) with warm waters  by Indonesia in the western Pacific, but colder waters extending from the central mid-Pacific to the Americas in the eastern Pacific. (NB: Lamb's suspicions about the Pacific during his Medieval Warm Epoch: "There are hints that this was a cold time generally in and around the wide expanse of the the North Pacific Ocean." (Lamb. CHMW. p171. 1982.) turned out to have had some foundation; but not quite in the way he thought; nor caused by a tilting of the circumpolar vortex as he hypothesised.)

There was a cold eastern tropical Pacific during Lamb's Medieval Warm Period.
The effect that this 450-1300 AD cooling of the eastern side of the tropical Pacific ocean appears to have had on much of the Americas, not only cuts right through the epoch known as Lamb's Medieval Warm Period but continued into the centuries that followed it well into the 16th Century.
This cooling of the tropical eastern Pacific also appears to have determined the majority of these  changes in climate between 450-1300 AD; not just in the Americas but elsewhere in the world too. (Just as La Nina/El Nino events affect the whole world today; not just the Americas.)

Droughts and floods - changes to average precipitation patterns - across the Americas are the type of impact that an ENSO event such as La Nina would be expected to have. (NB: Both El Nino and La Nina both bring droughts and heavy rains; but typically to different regions of the world; impacts can vary between winter and summer. El Nino and La Nina are often described as opposite sides of the same phenomena. The PDO, NAO etc also have considerable influence on earth's climatic variability.)

If sea conditions in the Pacific were in La Nina state, it is reasonable to expect that areas affected by modern La Nina episodes would have had similar conditions to those a La Nina produces today, during Lamb's MWP too. e.g.
  • A La Nina often brings heavy rains to Venezuela  in  South America. Analyses of titanium and iron concentrations from a seabed core from the Cariaco Basin on the Northern Shelf, off Venezuela, shows that higher precipitation (and flooding) was indeed the norm (and within Lamb's Medieval Warm Period), from 1050 to 1250 AD. (Southward Migration of the Intertropical Convergence Zone Through The Holocene. Haug et al. Science. 2001.)
  • Research into lake levels in East Africa (Lakes Naivasha, Victoria, Turkana, Edward, Tanganyika, and Malawi) shows that east Africa had a very dry climate between 1000-1270 AD (Rainfall and Drought in Equatorial East Africa During the Past 1100 years. D. Verschuren et al. Nature, 2000. Lake Level History of Lake Tanganyika, East Africa, for the Past 2500 Years based on Ostracode-inferred Water Depth Reconstruction. S. Alin et al. Paleogeography, Paleoclimatology, Paleoecology. 2003.) La Nina is known to bring drought to much of east Africa.
  • Examination of wave built shingle terraces indicate a permanently full Lake Eyre in Australia during the ninth and tenth centuries (R. Allen. The Australasian Summer Monsoon, Teleconnections, and Flooding in the Lake Eyre Basin. Royal Geographical Society of Australasia. 1985.) La Nina is known to bring heavy rains to this part of Australia. (Flooding in Queensland swells rivers that flow into Lake Eyre.) Lake Eyre has usually been dry ever since, other than when filled by occasional La Nina rains.
  • During a La Nina the West Pacific Warm Pool (aka Indo-Pacific Warm Pool) moves further westwards, this results in stronger summer monsoons and higher rainfall in Southeast Asia and East Asia. Research in Oman, southwest India, the continental shelf off Pakistan and in northeast China suggest a wetter period between approx. 1000-1400 AD, indicating increased summer monsoon rains. (D. Fleitmann et al. Holocene Forcing of the Indian Monsoon Recorded in a Stalagmite from Southern Oman. Science. 2003. A. A. Gupta  et al. 2003: Abrupt Changes in the Asian Southwest Monsoon During the Holocene and Their Links to the North Atlantic Ocean. Nature. 2003. U. von Rad et al. A 5000-yr Record of Climate Change in Varved Sediments from the Oxygen Minimum Zone off Pakistan, north-eastern Arabian Sea. Quat. Res. 1999. Sinha et al. Possible Solar Forcing of Late Holocene Indian Monsoon Rainfall. Transactions of the American Geophysical Union [Fall Meeting paper.] 2005. G. Ren. Pollen Evidence for Increased Summer Rainfall in the Medieval Warm Period at Maili, Northeast China. Geophysical Research.1998.) The Khmer Empire in SE Asia (Cambodia, Thailand, Vietnam etc.) rose to its high point in this time. La Ninas are also associated with cooling temperatures across the tropics. (Currents of Change. M. Glantz. CUP. 2nd edition. 2001.)
  • La Nina brings drought to South China. A lake core study from tropical South China shows a long dry period from 880-1260 AD. (The ‘Mediaeval Warm Period’ Drought Recorded in Lake Huguangyan, Tropical South China. C. Chu et al. The Holocene. 2002. See also: The Great Warming. Chpts. 3 & 12. B. Fagan. Bloomsbury. 2008)

It looks as if the great droughts and floods in the Americas of this period would have still occurred even if there had been no Medieval Solar Maximum (approx. 1100-1250 AD); and temperature (other than changes to sea temperatures) doesn't appear to be the defining feature in producing these droughts and floods.

NB: Causes of drought. (1) One of the driest deserts in the world is in Antarctica. Another is the Atacama Desert on Chile's Pacific coast. In neither case is extreme warmth the cause of their aridity. It is a mistake to automatically associate increased incidence of drought with an increase in warmth. (2) The Classic Period Maya droughts, for example, appear to have occurred during a period of cooler, dry conditions produced by a shift in the position of the North Atlantic High. (R.B. Gill. The Great Maya Droughts. Water, Life, and Death. UNMP. 2000. See Chpt. 6 for explanation.) Archaeologists investigating the collapse of the Maya (9th/10th C) and Tula (12th C) believe these occurred during periods of cold, drought and famine; these appear connected in Mexico. See also below.

As well as these peculiar El Nino Southern Oscillation (ENSO) conditions, is also likely that other major climate variability systems such as the North Atlantic Oscillation (NAO), Atlantic Multidecadal Oscillation (AMO) Pacific Decadal Oscillation (PDO) and Arctic Oscillation (AO) also contributed to this climatic regime shift. (NB: PMV - above - = Pacific Multidecadal Variability.)

If the NAO is in a positive - or warm phase - it produces a large pressure difference between the Azores and Iceland that results in stronger winds crossing the Atlantic travelling in a north-easterly direction. These winds bring heat from the tropical ocean leading to warm and wet winters in north-western Europe and the eastern USA. The positive mode of the NAO also results in conditions that are colder and drier than average over southern Europe, Mediterranean and North African regions with below average precipitation over southern and central Europe. The NAO positive phase also produces cold and dry winters in northern Canada and Greenland.

There is evidence of tropical sea temperatures starting to rise in the in the western tropical Atlantic (e.g. Sargasso Sea) - before the MWP - and cooling thereafter. This will have affected the behaviour of the NAO. Shifts between the NAO’s positive and negative phase are also likely to have contributed to many of the changes in climate during this period. (There is also a known link between the NAO and conditions in the North Pacific. This part of the Pacific holds the Pacific Decadal Oscillation, or PDO.)
Various periods of severe droughts (e.g. the Maya droughts) within this long drying period have also been discovered; due to complex interactions between the earth's oceans, their oscillations of warm and cold waters and resulting interactions with earth's atmosphere.

It is likely that differences between the resulting High & Low pressure areas, produced by these different regions of cool and warm seas, and the oscillations of warm and cooler waters within them, together with the resulting changes to atmospheric and precipitation patterns, combined to produce these epic pan American droughts that characterise the period. Changes in sea temperatures also played a part in ending them. The coral records from the central tropical Pacific show that temperatures there had warmed by around 1400 AD; core records from the western (Indonesian side) tropical Pacific showed they cooled. The eastern tropical Pacific waters slowly began to return to a warmer state.

1300 AD may mark the start of this transition point, accounting for both the abrupt change in Europe's climate that Lamb noticed starting at around 1300 AD: a "cold, or cooling stormy climate" (Lamb. CHMW. p194. 1982.); and the Pacific's "1300 event"; the name researcher P. Nunn gave to a period of increased storminess in the eastern Pacific, that caused major disruptions to island communities. (The A.D. 1300 Event in the Pacific Basin. P. Nunn. The Geographical Review. 2007.) This 1300 AD event is consistent with a switch from persistent La Nina conditions to an increase in the frequencies of El Ninos.

(NB: I will return to all the above again when I cover N. American for this period. I also hope to address this cold Medieval eastern tropical Pacific anomaly (and the warm tropical western Atlantic) as a separate section. Then, amongst other things, a look at Asia. Bookmark this site.)

Though the Aztec, Inca and Maya are quite well known; other civilisations mentioned in the text below (those of the pre-Inca civilisations of South America in particular) may be new to you. I have attempted to provide enough information to help you form a mental map, so as to locate them in time and place; both here and on other pages. There are also two further pages of information about the pre Columbian civilisations (see also page-links at top left) of Central America and South America in order to proved more information about these peoples and climatic events. I also provide an account of the great Maya droughts and the abandonment of their cities.

To set the scene: The Mesoamericans were severely tested by a progressively drying climate throughout the first Millennium and into the second. Droughts and famines played a major role in the rise and fall several major civilisations - Teotihuacan & Cholola (7th/8th Centuries), the Classic Maya (9th/10th Centuries), Tula (12th Century) - by placing great strains on their ability to produce enough food to support their populations, provide enough water for them to drink, by bringing waves of desperate migrants fleeing droughts and famines from other areas and causing local conflicts over resources. At times drought may have been the background music, at other times it took centre-stage as the leading protagonist in events.

As well as the evidence for the great Maya droughts, evidence also comes from further north in Mexico. Studies of twenty-one lake cores from Lake Patzcuaro in Michoacan indicate that agriculture had come to an end around 750 AD. Geochemical analysis indicates a severe drought occurred between 750-850 AD. In the Zacapu Basin diatoms record a rapid drop in lake levels around 850 AD. A sediment core from La Piscina de Yuriria shows farming activity between 300-950 AD from the presence of Zea mays pollen in the sediments; the pollen amounts drop sharply after 950 AD indicating a decrease in agriculture; pinon pine pollen decreases whilst juniper increases (Junipers grow in semi-arid lands in the Americas.) The researchers believe the lake here completely dried out for a period.

Lake-level reconstructions for three basins in the central Mexican Highlands indicate that the most severe climatic episode was a period of marked aridity between 550-1100 AD in Lakes Patzcuaro, Hoya San Nicholas de Parangueo, La Piscina de Yuriria and Zacapu. In the highlands archaeologists believe the human habitations were abandoned. Many of the new settlements of this time were built in defensive positions. B. Leyden reports a drying phase at cenote San Jose Chulchaca after 900 AD; Bradburg et al report falling lake-levels at Laguna de Cocos. One of the researchers in Michoacan, concluded that the very rapid drying at Zapapu: "seems to reflect a drying of the region as a whole, the most pronounced of the whole Holocene." S. Metcalfe et al. The Palaeoliminological Record of Environmental Change: Examples from the Arid Frontier of Mesoamerica. In Environmental Change in Drylands. Ed: A. Millington. J. Wiley & Sons. 1995. (NB: For brevity's sake I have placed other references into the Sources and Reading List.)

As well as the evidence from hydrologists, geologists etc.; archaeologists have concluded from their researches that drought played a role, if not the key role, in the development, and on some occasions also the ending, of several Mesoamerican civilisations. Archaeologist R. Diehl believes this was predominantly a time of cold droughts in Mexico.  Mayan archaeologist R.B. Gill has argued this is due to a far southward movement of the North Atlantic High that brings cold, dry conditions to this part of the world, along with a failure of rains. (See also note at bottom of the Maya droughts page.) There is now considerable evidence for long droughts, in both North and South America, during this period.

In the Maya lowlands, in northern
Guatemala, a great drought slowly developed between 125210 AD. This was a very long dry spell (85 years) of slowly decreasing precipitation. There may have been intervals of rainfall, but the long term trend would have been of lower water tables and lessened irrigation. Within that time there were a number of shorter multi-year droughts of greater severity. In around 150 AD, the powerful Maya city of El Mirador was abandoned along with a number of others on the lowlands. (An earlier, serious, drought period has also been identified occurring between 475 BC250 BC.)

Further south, in Peru, between 100– 550 AD, the Moche people established the Mochica state. This was located in the Moche and Lambayeque river valleys spreading out across the coastal plains. (On a map of present day northern Peru - just to the south of its border with Ecuador.)

In 550 AD there was a massive deluge onto the Peruvian coastal plains. Swollen rivers poured out of the surrounding hill valleys onto the plains, forming a tsunami covering the plains under 50 feet (approx. 15m)  of water and mud.

These floods were then followed by a prolonged 32 year drought, between 562594 AD, causing the rivers flowing down from the Andean mountains to dry up.

The Moche fields were then covered by wind-blown sand and they suffered year-on-year food shortages and famines.
The surviving Moche peoples relocated their capital to the Pampa Grande plains. In 750 AD another great flood struck their cities of Pampa Grande  and  Galindo. The people deserted those cities too and their civilisation collapsed. (See also the Chimu in the 11th Century: below.)

The Moche people appear to have burnt their temples before leaving, suggesting their faith in their priests had been shattered. They broke into small settlements, that later appear to have fought each other for resources such as food and land.

The same sequence of a massive deluge, followed by rivers drying up also appears to have happened in the 6th Century - to the Wari peoples, located on the Peruvian plains further to the south. The Wari moved to higher ground in the mountains, and so closer to the source of water.

By creating fields on steep sided valley slopes (e.g. photo on left) they established the foundations for an empire that governed most of highland and coastal Peru. The Wari built a number of major cities, among them  Pikillacta, Cerro Baul and Jincamocco.

For around 400 years the Wari and the Tiwanaku empires (see below) each ruled a huge area of South America. Evidence suggests co-operation over water access where their territories met. (See also 11th Century.)

On the Gulf Coast of Mexico, in around 500 AD, the significant city state of El Pital, went into sudden decline following major flooding. Such flooding is associated with tropical storms. (On a map of modern day Mexico – its roughly to the south-west of modern day Monterrey.)

Archaeologists note a major fall in both population and regional influence. The floods are likely to have destroyed El Pital's irrigation systems etc. It  is likely that famine  and disease followed. A city that previously had only limited influence became ascendant - El Tajin. (See 1100 AD below.) El Pital was later conquered by peoples associated with El Tajin.

Sediment layers of material deposited by flooding in the Nautla river valley have been dated and used to compare the severity of different floods. They show that the sediment layer created by the 1995 El Nino flood is 10-15 cm [4-6 inches] thick. The layers for the floods in 500 AD and 1100 AD were each over 100 cm [40 inches] thick. Evidence for these flood events also appears in sediment cores, taken from from the seabed offshore of the Gulf coast. It appears there have been historic megafloods.

Archaeological evidence also shows that around 500 AD a great disruption occurred in the Amazonian basin suggesting a period of major migration. (Archaeological evidence for the impact of mega-Nino events on Amazonia during the past two Millennia. B. Meggers 1994.) NB: There certainly is evidence for droughts, but it looks as if these are due to conditions that caused the other droughts.

was founded in central Mexico in the first Century AD, and in its time was one of the largest cities in the Western hemisphere.

This was Mexico’s equivalent of Egypt's Luxor.
Teotihuacan was a highly planned city, laid out on a grid covering 8 square miles with 600 pyramids, two of which were vast.

At it’s height the city’s population was between one and two hundred thousand; its power and influence extended across Mexico and into the Maya Yucatan.
Teotihuacan reached the heights of its power in the fourth Century, but went into decline around 530590.

This coincides with the period of Mayan abandonments known as the Hiatus; whilst in Peru, between 563–594, there was a coastal drought so severe the Moche were forced to relocate their capital city. NB; refer also to Note regarding the Hiatus at bottom of the Maya Droughts page.
The founders of Teotihuacan in Central Mexico (The name  City of the Gods  comes from the later Aztecs) were unlucky in their timing. Analysis from lake-sediment cores, changes in pollen types found in the soils etc. show the Central Mexican region became increasingly arid between 50 AD1,000 AD.  Lake sediment core studies show the region also had periodic cycles of severe droughts in this period, some droughts lasting for ten years or more.

Teotihuacan's farmers would have struggled in the face of drought to maintain supplies to a population that reached 150,000 - 200,000 at its height. Food shortages may have led to social unrest. It appears people began to leave.

Epidemics of disease may also have taken their toll; during a dry season, without rains to wash away human waste, any standing water can become contaminated, leading to parasitic and intestinal infections (Typhoid fever, cholera, poliomyelitis etc.). Archaeologists studying skeletons from the time of its late history found that average life expectancy in the city fell as low as 16 years. [Life and Death in the Ancient City of Teotihuacan. R. Storey. University of Alabama Press. 1992.]

NB: A tree-ring and lake bed sediment core study found that in the 8th Century AD a great drought extended from the northern Great Plains of N. America, across the southwestern United States, and into central Mexico and the Yucatan peninsula. In Colorado and New Mexico severe drought is shown there for A.D. 735-765. (Stahl et al. 2002.)

Teotihuacan would have been affected by this drought; by the direct impacts on its agriculture and reservoirs and by the waves of migrants fleeing their homelands to the north. (The final act in Teotihuacan's history is dated at 750 AD.)

Teotihuacan wasn't completely abandoned; but its population and influence became negligible by the mid eighth century. It was repopulated in a later period by a different ethnic group, probably Nahuatl speakers like the Toltecs and Aztecs.

 By 600 AD Teotihuacan had lost much of its influence in Mexico. The final act was in around 750 AD when the central buildings of the state and priesthood were burnt and vandalised; possibly in an internal revolt, or by the invading Toltecs.
The collapse of Teotihuacan, during a period of drought, left a power vacuum that several city states attempted to fill, over the course of the next few hundred years.
The Maya seem to have had significant influence on two: Xochicalco and Cacaxtla. Other city states were Cantona, El Zapotal and Alta Vista.
On the Gulf coast, the longest lasting was the Totonac city of El Tanjin.

In central Mexico it was the Toltec's (who migrated into the area  at the time of Teoihuacan's fall) that became Teoihuacan's successors, founding an influential state. They founded a number of cities, including Tula (aka Tolla).

The Toltec civilisation collapsed from the 1100’s onwards, again during a period of severe drought and warfare.

Tula’s final moment is dated at sometime around 1170 AD, when it was invaded by nomadic Chichimecs, who were fleeing drought to the north (Researcher R. Diehl believes this was a time of cold, drought and famine.) and the captured Toltec king put to death.

Apparently, the invaders then found there wasn’t enough food to feed their forces, many warriors died. The surviving Toltecs dispersed across Mexico.

The Toltec were a major influence on the Aztecs; who openly admired Toltec cities and copied both Toltec and Teotihuacan architecture and artistic styles. (See section on pre Columbian civilisations of Central America for more on the Toltecs - and drought in this part of Mexico.)

Between 750 through to 1025 AD, another prolonged period of droughts contributed to the collapse of Classic Maya cities, that had flourished on the Yucatan peninsula and the southern lowlands (On a modern day map, the area adjacent to Belize.) of Yucatan and Guatemala.

As with the drought that led to the abandonment of El Mirador in 150 AD, this was a very long dry spell of decreasing precipitation. 

Within this period there were a number of shorter multi year (3 - 9 year) droughts of greater severity.

These were centred upon 760 AD, 810 AD, 860 AD, and 910 AD. The drought  centred on 910 AD lasted for six years.

In a letter to the scientific journal  Nature the scientists that conducted the isotope and geochemical analysis of a lake bed core from
Lake Chichancanab wrote: “The interval between 1,300 and 1,100 yr BP (AD 800 – 1,000) was the driest of the middle to late Holocene epoch, and coincided with the collapse of Classic Maya civilisation.(Hodell et. al., Nature. 375. June 1995.)

Yet more evidence for the Maya droughts comes from analysis of a stalagmite from Chilibrillo cave in the Isthmus of Panama. As stalagmites are built up
slowly by calcite and water dripping onto them, they record both a physical (more rain = thicker bands) and geochemical record of precipitation that can be precisely analysed and dated using isotopes.

This stalagmite study shows two very dry periods at the time of the pre Classic abandonments. A third dry period at 560-630 AD coincides with the period called the Hiatus, whilst: "Two pronounced dry episodes in Panama between ~ 770  and 1010 A.D. coincide with the Classic Maya collapse [Webster*, 2002] from 750-950 AD.(* David Webster is another noted Maya historian, author of: The Fall Of The Ancient Maya. Thames and Hudson. 2002.)

The researchers also noted that their isotope results "show a broad increase (drying) from 180 B.C. to 1310 A.D."
in southern Central America. Their results: "also show pronounced hydrologic anomalies during Medieval time, particularly during the 1100-1200 A.D. "High Medieval" [Bradley et al. 2003] when western European temperatures were anomalously high. Rainfall anomalies began as early as 550 A.D. in Panama, but the driest conditions occurred between 900 and 1310 A.D.". (M. Lachniet et al. A 1500 year El Nino/Southern Oscillation and rainfall history for the Isthmus of Panama from a Spelotherm calcite. Journal of Geophysical Research. 2004.)

It’s estimated that at least two-thirds of the dozens of Mayan cities and towns (cities mentioned below were simply the largest) in the southern lowlands were abandoned during this period. There were three great phases of collapse.

760810. In the western lowlands, where groundwater is scarce, and both agriculture and reservoirs are dependent on rainfall, the major cities of Yaxchilan, Palenque, Aguateca, Calakmul and Edzna were abandoned.

811-860 AD. In the south-east lowlands, where freshwater lagoons had acted as natural reservoirs, the cities of Copan and Caracol were abandoned.

910 AD. On the Yucatan peninsula Uaxactun, Chichen Itzan, Izimte  Tikal together with those few cities remaining in the central lowlands collapsed. The Yucatan had probably survived until this period by using water from underground limestone caves and sink-holes. (Yucatan's famous Cenotes.) But they couldn’t bring up enough water enough to support agriculture.

Archaeologist Peter Harrison* discovered human remains from the time of this collapse that showed both evidence of burning and human tooth marks, that could only have been caused by cannibalism. (*The Long Summer. B. Fagin. Granta. 2004.)

Maya authority, Michael Coe*, simply describes this last period of collapse as “the Maya apocalypse”. (*The Maya. M. Coe. Thames & Hudson. 2005.)

It is thought a majority of the population (that numbered millions) of the central lowlands simply died of hunger, thirst and disease. The social structures and hierarchies of these Mayan city states collapsed; the people that survived continued to live amongst the ruins of their cities, but now in shacks. 

 The few cities that remained went into decline, they may also have fought over resources. The drought was later followed by rains; the forest began to reclaim the fields. Maya power was now limited to the few surviving Maya cities in the Yucatan peninsula; last to collapse, most able to later attempt to rebuild a few centuries later; though they never matched their former strength & achievements. Read more about the Maya droughts here

In Mexico's southwest, in the Valley of Oaxaca the Zapotec state also collapsed around 900 AD, their capital city of Monte Alban was abandoned. The region had attracted large numbers of Mixtec migrants who were becoming the dominant force in the region through conquest. This period was characterised by migration and warfare, a situation seen elsewhere in valleys that supported large populations and described by one researcher as a process of 'Balkanisation'. This was also a period of notable dryness. A study into historic lake levels in the Mexican state of Michoacan, one of the researchers concluded that the very rapid drying at Lake Zacapu: "seems to reflect a drying of the region as a whole, the most pronounced of the whole Holocene." S. Metcalfe et al. The Palaeoliminological Record of Environmental Change: Examples from the Arid Frontier of Mesoamerica. In Environmental Change in Drylands. Ed: A. Millington. J. Wiley & Sons. 1995. It is thought that some lakes may have dried out.

In the high Andean Plateau (the Altiplano) of what is now Peru and Bolivia, the pre-Inca city state of Tiwanaku was established in AD 500, close to Lake Titicaca. It developed complex irrigation systems for farms spread out over a large area. Farming supported a society numbering 170,000 people and Tiwanaku dominated the Peruvian Andes and plains.

Sediment cores from the lake and ice cores from the Quelccaya glacier reveal that the Altiplano had two wetter than normal periods between 610650 and 7601040 AD. These periods were separated by three much dryer than normal periods between 540610, 650760 and 10401450 AD.

From 1040 AD onwards the Altiplano was stricken by much reduced precipitation. This led to reduced spring snow-melt and gradually lowered the water table and Lake Titicaca
’s levels. The dryer air prevented evening mists forming (these helped prevent frost damage) from water that had evaporated from irrigation channels.
It has been possible to reconstruct lake and precipitation (snow, rain, dew) levels from sediment cores and trees remains* taken from the lake bed (* the trees grew when lake levels were low and died when they rose again) and from high Andes glacier cores that show precipitation changes over past centuries.

Between 1100–1150 AD there was a severe drought. By 1100 AD the lake level had already fallen by 36ft (11 metres). The Tiwanaku state’s sophisticated irrigation system failed completely at around 1100 AD. The Tiwanakus' cities and fields were abandoned.

It appears from the remains that the people who were left simply had to walk away, to become dispersed subsistence farmers. Although the worst of the drought ended in 1150, low levels of precipitation remained for another 350 years.

Note: Research - by S. Stein* - into the water levels at Mono Lake in the Sierra Nevada has identified a epic period of severe, persistent drought lasting from 8921112 AD (220 years), and also from research in Patagonia, (from dead southern beech trees recovered from the beds of Alake Argentino, Lake Cardiel & Lake Ghio) a similar drought of equal length there; this strongly suggests that atmospheric conditions were being disrupted by conditions in the Pacific. (*Extreme and Persistent Drought in California and Patagonia During Mediaeval Time. S. Stine. Nature. 1994.)

  • The period, 8921112 AD, corresponds closely with the period of droughts in the Andes that caused the collapse of the Tiwanaku and Wari civilizations: 1040-1150 AD. A second epic period of drought occurred again in both Sierra Nevada and Patagonia between 1209-1350 AD (140 years).
  • In the Amazon basin a researcher analysed data from 300 studies of layers of charcoal, found at various depths of the forest floor, and found that the periods when forest fire frequencies, and therefore prolonged periods of drought, were at there greatest were between 700-800 AD and 1000-1100 AD. (Fire, Climate Change and Biodiversity in Amazonia: a Late Holocene Perspective. M. Bush. Phil. Trans. Royal Society. 2008.)
  • Interestingly, tree-ring analysis from northern Patagonia shows a cold period for 900-1070 AD (within the first period of epic drought), followed by a warm period between 1080-1250 AD (within the second period of epic drought), which was followed by another cold period, that  then became cooler and wetter up to 1660 AD.
  • Temperature doesn't appear to have been a determining factor in these droughts occurrence; droughts happened in both cool and warm periods. (NB. The dates for the warm period above, 1080-1250 AD, closely match the dates for the Medieval Solar Maximum that took place at approx.1100-1250 AD.) Villalba reported droughts between 1280-1450 AD, 1570-1650 AD  and 1770-1820 AD; Villalba also reported that glaciers had advanced between 1270-1380 AD and 1520-1670 AD; within these drought periods . (Tree Ring and Glacial Evidence for the Medieval Warm Epoch and the Little Ice Age in Southern South America. R. Villalba. Climatic Change. 1993.)
  • Another study of glacial advanced in the North Patagonian icefield showed that Soler glacier advanced between 1222-1342 (within the epic drought period). This suggests atmospheric reorganisations had brought moisture to Patagonia during these droughts, but it was diverted to high mountain areas, leaving lower areas parched. (NB. The Soler glacier is in N. Patagonia and on the eastern side of the Andes. I'm curious as whether more moisture was now arriving from the Atlantic than the Pacific?)
  • Drought appears to have extended down most of the western side of South America to reach Tierra del Fuego, the southern-most tip of South America. Analysis of peat, spores, pollen etc from a bog there (Mauquoy. 2004.) shows a period of low water tables between 960-1022 AD, followed by a wetter period between 1030-1100 AD.
  • The hyper-arid Atacama desert was considerably wetter than today between 880-1250 AD. (Perennial Stream Discharge in the Hyper-arid Atacama Desert of Northern Chile during the Latest Pleistocene. Nester et al. PNAS. 2007.) 
  • A study of peat and macrofossils from glacial moraines in Tierra del Fuego indicates that glaciers advanced between 1010-1275 AD (Late Holocene Glacier Variations in the Cordillera Darwin, Tierra del Fuego, Chile. Kuylenstierna et al. The Holocene. 1996.)
  • A muti-proxy study of Laguna Potrok Aike shows the steppe regions of southern Patagonia became extremely dry from 1230–1410 AD. (Climatically Induced Lake Level Changes During the Last Two Millennia as Reflected in Sediments of Laguna Potrok Aike, Southern Patagonia. Haberzettl et al. Journal of Palaeoclimatology 2005.) 

  • In Argentina a humid, warm climate developed over the eastern plains (essentially the Pampas) with recession of the Andean glaciers and expansion of wetlands. In the western region greater aridity occurred, rivers and wetlands contracted during the MWP [No dates given]. Once the MWP ended the situation reversed itself. (M. Cioccale. Climatic Fluctuations in the Central Region of Argentina in the last 1000 years. Quaternary International. 1999.)

The above - particularly the rainfall over the Atacama and a humid climate developing in the previously dry eastern Pampas, yet droughts occurring elsewhere - suggests a major re-arrangement of airstreams carrying moisture had occurred.

The same epic drought that caused the abandonment of Tiwaniku also brought an end to the Wari empire in the Peruvian highlands. Greatly reduced rainfall and snow melt failed to irrigate the mountainside fields.

The Wari were forced to abandoned them. Archaeology reveals that construction work in their capital Pikillacta stopped during the 11th century, then that numerous doorways were sealed up. It had been completely abandoned by 1155 AD.

Only the Wari's mountain fortress of Cerro Baul remained inhabited, but with a much reduced population.

In 1100 AD, down on the northern Peruvian coastal plain in the Moche valley, two Chimu cities – their capital Chan Chan and the city of Pacatnamu, 100 miles to the north, were almost washed away by a massive flood. This flood also destroyed the complex system of irrigation canals that sustained the Chimu state’s agriculture.The Chimu people piled wood against the remains of their temples and set them alight; then they abandoned the cities.
These floods were widespread across the coastal plains and significantly damaged two cities of the neighbouring Sican State: Chotuna and Batan Grande. The Sican established a new capital at Tucume.
Flood levels in valleys during this event reached 60ft (18 metres) up the slopes; the worst flood of the 20th century only reached 26ft (8 meters). (Natural Hazards. E. Bryant. CUP. 2004.)

The Tiwanaku’s successors were to be the Chimu, down on the coastal plains. The Wari’s Peruvian highland successors were to be the Inca. A study of sedge pollens* (Chepstow-Lusty. 2003, 2009.) from a lake basin in the Cuzco region of Peru (the Inca heartland) shows the long dry spell for this area of the Andean highlands lasting from 900 AD-1800 AD, whilst warm conditions continued from 1100 AD right into the fifteen hundreds. (* Sedge expanded onto the lake-floor when water-levels were low, but died when levels rose again. Their pollens are left in sediment layers on the lake bed.)
Archaeological evidence shows that these conditions encouraged the Inca to farm at higher altitudes from 1150 AD onwards, so as to take advantage of water from glacial run-off. They also expanded forestry by planting several varieties of trees, both for timber and possibly also to help reduce soil loss by stabilising slopes. Two species of trees Casuarina and Alnus are nitrogen fixing trees, used as a traditional native response to low crop productivity and the need to rehabilitate abandoned fields following drought. (Putting the Rise of the Inca Empire in a Climatic and Land Management Context. A. Chepstow-Lusty et al. Climate of the Past Discussions. 2009. Also Ambio article.) NB. Over-exploitation by the Spanish severely reduced the numbers of these trees from the 16th Century onwards.

There appear to have been regional differences in the climatic effects of this drought period, across the long, high Andes; North Patagonia entered a long cool and dry period from 1250 AD onwards; whilst Peru experienced warm dry conditions into the fifteen hundreds.
 (See also above.)
Archaeological evidence from tributaries of the river Amazon shows that also around 1100 AD, a great disruption occurred in the Amazonian basin, suggesting a period of mass migration.(Archaeological evidence for the impact of mega-Nino events on Amazonia during the past two Millennia. B. Meggers 1994.)

Analysis of dated charcoal layers in the Rio Negro Basin in the upper Amazon show widespread forest fires for 1100 AD [Natural Hazards. E. Bryant. CUP. 2004.], suggesting prolonged drought for that part of
Amazonia. Similar layers were recorded for 500 AD, 1200 AD and 1500 AD.

In Mexico, on the Gulf Coast, the powerful city state civilisation of El Tajin was  also suddenly abandoned in 1100 AD; its people migrated southwards; some may have contributed to the re-population of Maya cities in coastal areas of Yucatan. (On a map of modern day Mexico El Tajin is roughly to the west of modern day Monterrey.)

El Tajin had grown to dominate the majority of the North-central are of the Gulf coast following the collapse of El Pital in 500 AD due to floods. Such flooding is associated with tropical storms.
Archaeology shows that both El Pital and El Tajin had agricultural based economies; that had collapsed following a massive flood episode. Populations fell as people migrated elsewhere.

Sediment layers of material, deposited by flooding in the Nautla river estuary in Veracruz (the EL Tajin area) over the centuries have been analysed and dated. These have been used to compare the severity of different floods.

These show that the sediment layer created by the 1995 El Nino flood was 10-15 cm thick. The layers for two floods in 500 AD and 1100 AD were each over 100 cm thick.
  The 1100 AD flood marked the collapse of El Tajin and the end of the period on the Gulf coast known as Classic Veracruz.

Archaeological evidence shows that around 1200 AD a great disruption again occurred in the Amazon basin; suggesting yet another period of major migration. (Archaeological evidence for the impact of mega-Nino events on Amazonia during the past two Millennia. B. Meggers 1994.)

Following the decline and collapse of the Tiwanaku and Wari empires another culture, the Chiribaya, began to expand. The Chiribaya lived on the Peruvian coastal plains, but much further south. (On a modern map just north of the border with Chile.)

The Chiribaya developed several sizeable settlements on the plains and in the hills rising from them. The most important towns were El Yaral, Chiribaya Alta, Chiribaya Baja and San Geronimo.

Many of their field were on high ground to protect them from flooding. But in 1350 AD a major El Nino event caused such massive rains and flooding that the towns were all but destroyed and field and irrigation systems devastated. Archaeologists estimate that 80% of the population died from the flooding and the starvation that followed.

Aztec chronicles record great snowfalls and frosts (The Aztec Historia chichimeca tells of a 'catarro pestilencial' (pestilence) that came during the unusually cold weather of 1454, from which many people died.) between 1447–1450 AD, followed by a great drought between 1450–1454 AD. The cold had also destroyed the Aztec empire’s annual harvest in Central Mexico.

The drought was recorded as being so severe that Aztec records say that even birds were reported to have collapsed onto the ground through thirst and: "some people simply made holes in the ground and crawled into them to await death and when it came they were devoured by buzzards because there was no-one to bury them." (From: The Great Maya Droughts. R. B. Gill.)

The Aztecs had maize reserves for times of famine, but they were soon overwhelmed.  With no food or water thousands fled from the Valley of Mexico.
The Codex Ramirez reports that: many - including regiments of warriors - sold themselves or their children as slaves to rival states on the coast.

These events appear to have deeply shaken Aztec rulers. They had sophisticated irrigation systems, astrolonomical observatories to attempt to predict the weather and reservoirs. But the unseasonal frosts and cold followed by severe, prolonged drought may have taken them to the brink of collapse. When rainfall and agriculture had resumed, the Aztecs responded by massively increasing the number of human sacrifices to their rain god Tlaloc. It is thought that hundreds of thousands of people were sacrificed. (See also Note 'Human Sacrifice', bottom of Peoples of Mesoamerica page and Note 4: Maya Post-Classic Collapse on Maya Droughts page.)

Further to the south in the Yucatan, also in 1450 AD, there was another sudden phase where a
number of Mayan cities were abruptly abandoned. (Archaeologist R.B. Gill’s period of Post Classic abandonment’s.) This drought appears to have affected a wide geographic region; tree-ring studies also show extreme drought in Central Mexico between 1450-1454.
NB: The switch from heavy precipitation to drought is indicative of an El Nino/La Nina event. Nile flood records [Nile flood records are a good indicator of an El Nino event, as El Ninos are noted for reducing rainfall in the Ethiopian highlands, where the Blue Nile starts.] for 1450/51 show the lowest levels for around 300 years. The N. American Mississippi region was also affected by a major drought between 1449-1458 AD; this caused the abandonment of many major native American settlements. There were also several major volcanic eruptions at around that time; such eruptions are known to cool the atmosphere. Hernan Cortes brought an end to the Aztec empire in 1521 AD, and began the period of Spanish conquest and rule. (NB: The Inca Empire fell later, following the invasion of Spanish forces led by Francisco Pizarro, in 1533 AD.) The smallpox the Spanish brought with them caused a massive epidemic amongst the native Mexicans. It’s thought that 5-8 million people may have died. The Mesoamericans had no immunity to the diseases the Europeans brought with them; although Europeans could still die from infection, many had also acquired some basic immunity over the previous millennia from periodic contact with infections; the Aztecs and other pre Columbians had none. (See Note 4 below.)

From 1540 another megadrought affected much of North America and Mexico, lasting several decades. Tree-ring evidence from various locations, together with Spanish historical records, document an extremely long period - 1545-1576 AD - [31 years] of persistent, severe drought that affected much of the southern parts of America and all of Mexico during this time. This is now known as the 16th century megadrought. Droughts, together with poor living conditions, combined with a disease epidemic to produce massively high death rates amongst the native peoples.

During this period of drought epidemics of an indigenous disease known as ‘Cocoliztli (Nahuatl for 'pest' or  pestilence) began to make an appearance. It was a hemorrhagic fever, transmitted by rats and exacerbated by a drought conditions followed by rains.

From descriptions of its symptoms, written in 1576 by the former personal physician of King Phillip of Spain, Cocoliztli’s symptoms (‘Large nodules behind the ears, often invading the neck and face… profuse bleeding from nose, eyes and mouth, it caused death within 3-4 days.’) are similar to haemorrhagic fevers such as
Ebola and Dengue.

Cocoliztli was widespread in 1545 and again in 1576 AD. Cocoliztli is estimated to have killed vast numbers (possibly over 50%, or 5 million) of the native population of Mexico. (Source:
Large Epidemics of Haemorrhagic Fever in Mexico 1545-1815. R Acuna-Soto. American Journal of Tropical Medicine and Hygiene. 62. 2000.) See also article here.

Note 1: It is thought the disease, Cocoliztli, had previously been limited to some isolated highland areas amongst rats. Drought caused rats to congregate around limited food sources and spread it amongst themselves through aggressive behaviour (i.e. bite wounds). By such vigorous mixing of the virus genes, at some point the virus mutated into a form that could infect humans. When wetter conditions returned the small rodent (mice) population dramatically increased, as did the rat population that preyed on them. The mice and rats dispersed and passed Cocoliztli onto humans working in fields.

Such viruses can be spread by human contact with fecal matter, urine, saliva etc from infected rodents, as well as by fleas infected by ingested blood from infected rodents; the fleas can go on to bite animals, including humans. Inevitably, rodents are attracted to food stores etc near human settlements; fleas feed from animal blood and like warmth. (Viruses kills the fleas too, but not before they've had some time to transmit the disease.) Rodents are known to be hosts to many viruses and diseases that can be passed onto humans.

Ten lesser Cocoliztli epidemics returned in 1559, 1566, 1587, 1592, 1601, 1604, 1606, 1613, 1624, and 1642 AD.

Note 2: It is thought that Bubonic plague originates in a similar way, mutating from a rodent virus to one that could infect humans; following food shortages resulting from serious flooding or drought. It’s thought that past major Bubonic plague epidemics have begun this way; originating, variously, in central Asia, China and Africa

The Bubonic plague epidemic we know today as The Black Death refers to a bubonic plague epidemic that killed an estimated 50% of Europe's' population in the 14th Century (there were also smaller recurrences throughout the centuries that followed); but there have been several other major Bubonic plague epidemics, across the world - throughout human history.

In the 20th Century several similar types of viruses (Sin Nombre) were identified amongst rodent populations in isolated areas of America. So far these haven’t mutated into a virus that could infect humans, but the potential is there.

Note 3: Many human diseases have origins in diseases that originated from animals. Through millennia of close contact people and animals were exposed to each others microbes; over time mutation allowed animal diseases to become ones that could also afflict humans; e.g. bovine-rinderpest became human measles.

Note 4: Smallpox originates from an animal virus; probably from cattle, horses or camels, all of which have similar viruses. (These animals are not indigenous to the Americas, hence the pre Columbian people had no experience of, or immunity to, smallpox.) Smallpox has an incubation period of around 12 days, during which time the sufferer may not realise they are infected. It was transmitted during face-to-face contact, by touching contaminated bodily fluids, clothing etc. Occasionally it could be inhaled if in an enclosed space with a sufferer. People that survived smallpox became immune to it; its morality rate varies. Conquistadors brought smallpox with them to the Americas (as did the later English and French to the north) and it spread ahead of them like an invisible army. Smallpox is thought to have reduced the population of the Inca Empire by half. In North America it decimated the native peoples. Smallpox has now been eradicated though a worldwide vaccination program.

Note 5: Scores of shorter (1-3 year) droughts and floods have been recorded between the Spanish conquest and the present day, as well as major flood events. Droughts and floods in Central and South America are known to be often related to El Nino/La Nina events.

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