
June 01st 2009 print

Michael Kile

The Aztec Solution



Aztec culture has attracted scrutiny ever since Hernan Cortéz and his conquistadors entered King Montezuma’s palace in the lagoon city of Tenochtitlan (now Mexico City) on November 8, 1519, triggering the destruction of an entire civilisation in only two years.

There has been, however, no investigation into whether Aztec sacrificial rituals might have influenced climate change in Middle America. Did they affect regional temperature and rainfall patterns? Did they improve crop yields in ways unknown to modern science?

Recent research by Mexico’s Institute Nacional de Antropologia e Historia and international climate experts is producing new data that could answer these controversial questions. But why has the subject been neglected for over half a century? 

First, cultural sensitivities and entrenched superstition discouraged study of Aztec human sacrifice. Second, there was a lack of evidence. While pictorial codices and other accounts of Aztec society exist, it took the sensational rediscovery last year of missing sections of the Codex Cihuacoatl (circa 1520) to revive academic interest.

Third, climate research has been refining its knowledge over the past two decades. The World Climate Research Programme (WCRP)—and its Climate Variability and Predictability (CLIVAR) model—only recently acquired the capacity to “simulate and project climate with unprecedented accuracy”[1]. With CLIVAR, reconstruction of past Middle American climates with greater precision has been possible. It has allowed complex time series simulation and use of eclectic proxies for temperature and other variables where empirical data is lacking.

The new Codex evidence is reviewed here with reference to Aztec cosmology, sacrificial rituals, sacred tonalamatl (divinatory) calendars, CLIVAR modelling and Mexico’s mammoth discoveries. Implications for global warming mitigation are discussed and recommendations made for the urgent attention of international agencies and governments.


When the Spanish conquistadors arrived in the Valley of Mexico in 1519, they saw Mexico-Tenochtitlan, the capital of the Aztec empire, rising out of an immense lagoon known as Metzliapan, or “lake of the moon”. (Mexico means “in the middle of the lake of the moon”.)

At its centre was the coatepantli, a cluster of monumental buildings dedicated to deities Aztecs believed controlled climate, weather and their fate. It was dominated by the teocalli, a thirty-metre-high flat-topped pyramid with two sanctuaries on top. One was for Tlaloc, god of rain and fertility; the other for Huitzilopochtli, god of the sun and strife.

Facing the Great Teocalli was the temple of Echecatl, god of winds. The adjacent courtyard was used for tlachtli, a ritual ball game where the ball symbolised the sun. On the right was the Tzompantli (“hairy skulls”), a huge rack displaying skulls of sacrificial victims. (Cortez’s companions counted 136,000 of them.)

Sacred Sacrifice

Aztecs believed their cultural continuity depended on sacrifice: “All life exists because of the gods. Their sacrifices gave us life and sustenance. We must repay them through sacrifice.” Central to their apocalyptic worldview was a sense of sacred obligation, possibly similar to our concern for the environment. The Aztec word for debt repayment, nextlahualli, was also a metaphor for human sacrifice. Without constant sacrifices the Aztecs believed the sun would become “angry”, temperatures would rise, corn yields decline and their world would be threatened with imminent destruction.

All life had an animating spirit or tonalli. Everything was tonacayotl, a manifestation of the gods. Everything came from them. Human blood hosted tonalli, hence there was an insatiable divine “hunger for the heart” of sacrificial victims.

According to Bernardino de Sahagun (1540–85), victims were taken to the top of a teocalli and laid on a stone slab. The abdomen was sliced open with a ceremonial flint knife. The still-beating heart was pulled out by priests. It was placed in a bowl held by a statue of the honoured god and the body was thrown down the temple stairs. The Aztecs sacrificed 80,400 prisoners in just four days during their 1487 re-consecration of Tenochtitlan’s Great Teocalli.

Cortéz (1523) described a typical sacrifice in his Letters:

They have a most horrid and abominable custom which truly ought to be punished and which until now we have seen in no other part, and this is that, whenever they wish to ask something of the idols, in order that their plea may find more acceptance, they take many girls and boys and even adults, and in the presence of these idols they open their chests while they are still alive and take out their hearts and entrails and burn them before the idols, offering the smoke as sacrifice. Some of us have seen this, and they say it is the most terrible and frightful thing they have ever witnessed.[2]

Sacrifices were made on specific days. There were eighteen festivities annually, one for each Aztec month. Sacrifices were made at all of them. Each god required a different kind of victim: young women were drowned for Xilonen; children were sacrificed to Tláloc; Nahuatl-speaking prisoners to Huitzilopochtli, and so on.

Aztec ritual revolved around both agricultural and sacred calendars: a 365-day xiuhpohualli (year count) agricultural cycle based on the sun; and a 260-day tonalpohualli (day count) ritual cycle. The two cycles formed a fifty-two-year “century”, which was to them literally a life cycle. Because of the fear that all order would collapse without appropriate rituals, the end of each “century” required a huge New Fire ceremony. Many human sacrifices had to be made to ensure the sun’s reappearance the next morning.

Attempts have been made to explain the prevalence of human sacrifice in Middle America. Sacrificial ceremonies clearly had important political and religious functions. Aztecs used them as a deterrent against enemies. Fallen conquistadors were often sacrificed, sometimes skinned alive, their heads placed in the Tzompantli.

Some scholars argue cannibalism was the main driver. For them, the aristocracy valued human flesh as a source of protein and merely designed rituals to validate this habit. For others, such as Lloyd deMause, the urge to sacrifice was an unconscious response to “traumatogenic modes of childrearing”.

The new Codex evidence, however, suggests another rationale for such a pervasive ritual. Human sacrifice was an ancient mitigation strategy to neutralise the threat of dangerous climate change and risks of rising temperature, declining precipitation and poor crop yields.

Codex Cihuacoatl

The Codex Cihuacoatl, also known as Codex Borbonicus, is held at the Bibliothèque de l’Assemblée Nationale in Paris, which now occupies the original Palais Bourbon. Named after the goddess Cihuacoatl, it was written by Aztec priests shortly before the Spanish invasion.

Cihuacoatl (“snake woman”) was an Aztec motherhood and fertility goddess. She helped Quetzalcoatl (“feathered serpent”), the sky and creator god, to create humankind by grinding up bones from previous ages and mixing it with his blood. She is often shown as a fierce skull-faced old woman carrying spears and a shield.

The Codex is divided into three sections. The first section contains one of the most intricate surviving divinatory calendars (tonalamatl). The second shows (in order) the dates of the first days of each solar year in the fifty-two-year cycle. These days are correlated with Nine Lords of the Night. The third (now complete) section describes rituals, especially those at the New Fire ceremony marking the end of each cycle.

The Codex is a single 14.2-metre sheet of amatl “paper” with forty accordion-folded pages. Each page represents one of the twenty trecena (thirteen-day periods) in the tonalpohualli (260-day) year. Most of each page depicts a ruling deity or deities, with the remainder taken up with thirteen-day signs and thirteen other glyphs and deities. Aztec priests used these symbols to create horoscopes and divine the future. For example, the thirteenth trecena depicts the goddess Tlazolteotl. She is shown upper left wearing a flayed skin, giving birth to Cinteotl. The thirteen day-signs of this trecena are in a bottom row and begin with 1 (Earthquake), 2 (Flint/Knife), 3 (Rain), and so on.

Discovery of the Codex’s missing first and last two pages in 2008, together with a hitherto unknown part of the second section, in the French National Assembly Library archives, is now attracting international attention. There are two reasons for this interest. The rediscovered trecena contain the most comprehensive Aztec data set yet found. They detail annual sacrificial numbers over the entire empire and relate them to crop harvests—predominantly maize, wheat and barley. The data set extends over 300 years, from the Early to Late Post-Classic Period.

Second, the remarkable second section contains nine depictions of sacrificial rituals involving mammoths. It establishes for the first time that Aztec awareness of climate change and its relationship to solar irradiance extends much further back than previously thought, possibly several thousand years.

CLIVAR Modelling

Statistical climate reconstruction techniques using the CLIVAR model produced accurate temperature anomalies for Middle America and Valley of Mexico region for various time periods. Several independent proxy data sets, including the invaluable new Codex data on sacrificial frequencies and corn yield, were used to derive estimates of regional spatial temperature covariance patterns (corrected for atmospheric absorption) and amplitude changes in them through time. A modified principal component analysis technique was used to optimise this combination of spatial and temporal information. Verification statistics obtained from data subsets confirmed the validity of the CLIVAR reconstructions.


There were other intriguing outcomes too. For example, young male sacrifices were statistically the largest percentile and tended to drive cooler weather, while female sacrifice produced warmer weather. The meteorological outcomes of child and animal sacrifice were less clear, and probably masked by the adult groups.

Recent research by David Lobell and Christopher Field concluded that wheat, maize and barley yields decline with increased temperatures. Annual global temperatures have increased by about 0.04 degrees since 1980, with even larger changes in some regions. This has had a negative impact on crop yields and produced serious losses.

Mexico’s Mammoths

It is not widely known that mammoth skeletons and artefacts were discovered in 1952 near Tepexpan, thirty kilometres north-east of Mexico City. According to Wormington’s Ancient Man in North America (1957), the discovery “provided incontrovertible proof of the contemporaneity of men and mammoths in the Valley of Mexico”. The Direccion de Prehistoria of the Institute Nacional de Antropologia e Historia identified the bones as those of imperial mammoths (Mammuthus (archidiskodori) imperator Leidy). They were imbedded in an Upper Pleistocene lake deposit of fine green mud. The formation was dated at between 7000 and 10,000 years by geological methods. 

In mid-1954, the Direccion de Prehistoria excavated a second fossil mammoth of the same species nearby. There was evidence of butchering. The skull was smashed, probably to extract the brains. Many bones showed cuts and grooves made by stone tools. The inscriptions on them are the earliest known depictions of solar sacrifice. Their extinction, then, was not caused solely by habitat disruption due to rapid climate change at the Pleistocene Holocene transition. Human predation for sacrificial rites was important too.

David Nogués-Bravo and his team used mathematical modelling to examine the importance of each of these factors. They predicted climate and species distribution at different times—126,000, 42,000, 30,000, 21,000 and 6000 years ago—and performed temperature and rainfall simulations. The results suggest mammoths suffered a catastrophic loss of habitat, with the species 6000 years ago relegated to just 10 per cent of the habitat available 37,000 years earlier, when glaciers were at their peak.

In Nogués-Bravo’s optimistic case for mammoth numbers 6000 years ago, humans would only have had to kill one mammoth each every three years to push the species to extinction. There was a marked increase in mammoth mortality rates through time. Mammoth sacrifices therefore also seem to have affected solar irradiance and modulated warming, at least until species extinction. From that point, however, the temperature rose rapidly, melting the vast North American ice sheet and raising sea level worldwide. The Aztecs, then, were continuing rites that had been practised for thousands of years. 

Age of Sol 









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[1] The World Climate Research Programme is a global research group sponsored by the International Council for Science, World Meteorological Organization and Intergovernmental Oceanographic Commission. The WCRP’s objectives are to determine climate predictability and the effect of human activity on climate for the UN Framework Convention on Climate Change. 

[2]Cortés, Hernán, 2005, (1523), p. 26. ”Y tienen otra cosa horrible y abominable y digna de ser punida que hasta hoy no habíamos visto en ninguna parte, y es que todas las veces que alguna cosa quieren pedir a sus ídolos para que más acepten su petición, toman muchas niñas y niños y aun hombre y mujeres de mayor edad, y en presencia de aquellos ídolos los abren vivos por los pechos y les sacan el corazón y las entrañas, y queman las dichas entrañas y corazones delante de los ídolos, y ofreciéndolos en sacrificio aquel humo. Esto habemos visto algunos de nosotros, y los que lo han visto dicen que es la más cruda y espantosa cosa de ver que jamás han visto”.