by Bill Robinson
Planet Earth is a very dynamic place. Over its roughly 4-billion-year history, its climate has changed dramatically on numerous occasions, with some of those changes causing mass extinctions of many of the dominant life forms at the time. The current climate change is no different, save for two things: 1. It has been caused by humans. 2. It’s happening very rapidly.
The following tells of a potent (but virtually unknown) climate change that began and ended in a 300-year timeframe. At first, it benefitted the people and led to population growth and stability. But when it changed again at the end of that period, the two largest American civilizations north of Mexico had ceased to exist.
Until the middle of the 20th century, very little was known about the early history of the distribution of maize. The European invaders, and most historians, generally assumed that maize had always existed throughout the continent. That view, however, broke down quickly when archeologists, anthropologists, and ethnobotanists began examining abandoned sites of ancient peoples, people who were the ancestors of the current peoples and tribes (Bellwood 2001, 30:181-207). Early studies conducted during the first years of the twentieth century revealed much about the movements of maize from its birthplace in Mesoamerica into the rest of the North American continent. Two things were learned that had everyone scratching their heads. First, it was discovered that maize had started moving north from Mesoamerican at a much later date than was previously thought. Second, once it started moving, it traveled much faster and much further than was thought possible.
Most of the large and elaborate archeological sites north of the Aztec Empire, like Casas Grandes in Mexico, and Mesa Verde, Chaco Canyon, and Cahokia in the US, were barely in their infancy in 700 CE. Although there is archeological evidence that a small amount of maize was being eaten, grown, and traded at that time, most of the food supply in those locations came from gathering wild plants and hunting (Price and Baker 2006, 190-191). Yet, evidence dating from 1050 CE to 1350 CE at those same sites, shows that by 1100 maize had become the principle food at all four locations.
In addition, the growth of the population had been extraordinary. Cahokia, near present-day St. Louis, had grown to become the largest city in what is now the United States. So much maize was grown that surplus was traded throughout the Trans-Mississippi region and beyond. There is considerable speculation in academic circles that maize carried from Cahokia may have provided the seed stock for the rapid ascendancy of maize growing in all of eastern North America. Maize seed acquired from Cahokia may, in fact, have been the ancestor of the maize that the Pilgrims reportedly started using in 1621. What it illustrates is this: After more than 4000 years of being a regional Mesoamerican cultivar, maize suddenly (and curiously) spread all over North America, except for the extreme northern regions. What is remarkable is that this expansion occurred very rapidly, perhaps even as fast as two or three hundred years. So, what could have caused this to happen?
A Tale of Two Cities…The Incredibly Rapid Spread of Maize
The Pueblo Bonito great house and plaza at Chaco Canyon, New Mexico, and the hundred-foot-tall Monk’s Mound at Cahokia Illinois, directly across the Mississippi River from modern St. Louis, will never be taken for twins separated at birth. Yet, despite 1000 miles between them and totally dissimilar climates, they have some startling similarities:
Both became major urban centers in the 10th century, and both were completely abandoned by the 14th century, within a few decades of each other. Both cities began, then thrived, and finally expired within a short historical lifespan – 900 CE to 1300 CE. At their apex, in the middle of the eleventh century (at about the same time the Norman Conquerors were attacking the “semi-naked savages” of the British Isles), populations at both locations became enormous. The urban center of Cahokia has been estimated at more than 40,000 inhabitants, at the time the largest city in North America north of Mexico. Cahokia’s suburbs and outlying villages, stretching for hundreds of miles in every direction, added thousands more to the population. In the west, the Chaco Canyon Culture spread out over the entire 50,000 square miles of the Colorado Plateau and San Juan Basin. It included over 20,000 satellite villages, or farmsteads, and its total population is estimated at having been 100 to 125 thousand (Stuart and Gauthier [1981] 1988, 70).
At both locations the primary growth dynamic was trade. Chaco’s trade network reached out from Pueblo Bonito to the local farmsteads and beyond: to south-central Canada in the north, Mexico and the Sea of Cortez in the south, California’s Imperial Valley in the west, and to the central plains to the east. Cahokia’s trade routes extended both up and down the Mississippi. They also reached as far as Florida to the southeast, and Pennsylvania and New Jersey to the northeast. Remarkably, the principal commodity traded by both cities was maize, a commodity that had been barely known at those locations just a few centuries earlier. The massive four-story “great house” at Pueblo Bonito is now believed to have been a maize granary, not a dwelling as was first thought. This granary was duplicated in at least 100 other locations throughout the Colorado Plateau.
Was this all a coincidence? For many years, anthropologists, archeologists, botanists, and historians wondered: Were the stunning similarities between these two huge and widely separated cultures mere serendipity, or were there unknown causes that would explain them? Quite simply, the short answer to this question turned out to be “climate change.” In 1965 a little-known British academic with an even lesser-known title of “paleo climatologist”, published an obscure research paper that brought everything into focus. Hubert H. Lamb, who later founded the Climatic Research Unit of the School of Environmental Sciences at the University of East Anglia in the UK, dropped his bombshell into a shark tank of international skeptics. His research was based on an impressive array of historical documents going back six centuries. He used government and church records of weather, temperature, and rainfall. To those he added published historical accounts, family accounts of weather events, and family letters from relatives serving abroad in the military. What Lamb discovered was evidence that, in the not-so-distant past, the world had experienced an unusual climate change. According to his paper, there was significant evidence suggesting that in the last third of the last millennia, much of the earth had experienced a notably warmer and more stable climate lasting almost 300 years (Lamb, H. 1965).
At first the European scientific community rejected Lamb’s theory because it lacked “instrumental evidence.” Weather recording “instruments,” they argued, didn’t exist prior to 1880. However, biologists and paleobotanists became keenly interested in Lamb’s work. They had been studying ancient climates for years without instrumental data. They used what are called “multi-proxy reconstructions”. Biological “proxy” data consists of things like tree rings, core samples of arctic ice, geologic stratigraphy, pollen embedded in fossils, lake and ocean sediments, and cave deposits. In 2009 Michael E. Mann, at Pennsylvania State University, conducted another climate study of the same time frame that Lamb had researched. Mann’s research used the latest developments in proxy techniques. His evidence completely supported and verified Lamb’s historical evidence. Then, at almost the same time, David E. Stuart, an American anthropologist/archeologist, novelist, and Associate Provost Emeritus at the University of New Mexico, provided further support based on field studies at Chaco: Chacoan society might have withered had climate patterns not become more favorable, just in the nick of time, at about A.D. 1000. Thus, began the Chacoan Millennium. For 130 years, the crucial midsummer rains fell more reliably than they had ever fallen before—or have fallen since. Now more tasseling cornfields in more localities received the gift of rain each summer than ever before. (Price, V.B.; Morrow, B.H. 196).
Hubert Lamb named his discovery the Medieval Warm Period, or MWP for short. As confirmed by Michael Mann, it lasted 300 years and ended when it was replaced by another world climate period that was radically different. Lamb and Mann not only confirmed the existence of the MWP, but they also pinpointed its dates, 950 CE to 1250 CE, almost the same dates that anthropologists agree to mark the rise and fall of Chaco and Cahokia (Mann, M.E. et al, 2009). The MWP brought warmer temperatures, consistent weather patterns, and regular rainfall to most of North America. Suddenly, Lamb’s discovery made the remarkable parallels in the histories of Chaco Canyon and Cahokia look less like a coincidence and more like what it really was: a significant change in our climate that led to population growth coupled with widespread monocropping. When the climate changed again to unstable and cooler, humans experienced crop failure, starvation, dislocation, and social disruption.
With milder temperatures and consistent rainfall, the new conditions of the MWP turned out to be excellent for growing maize. The growing season became longer, which offered a number of advantages. In the American Southwest, for example, desert-adapted maize varieties with short growing seasons enabled farmers to grow two crops in a single season. This is called “double cropping.” The longer growing season also allowed maize to be grown much further north, even as far north as central Canada. Furthermore, new varieties developed rapidly as maize adapted itself to modest regional climates and elevation differences.
Trade also benefitted from the milder climate. Travel was easier, and shorter winters left more time for travel. Residents of Pueblo Bonito, at Chaco Canyon, quickly noticed that food supplies influenced by regional weather differences from year to year could be evened out. By using a very sophisticated system of warehousing and trade, a region having a bad crop year could replenish its supplies at the giant Pueblo Bonito warehouse. The warehouse would be refilled by trading for maize in regions with good crops. Though not much maize was grown in or around Pueblo Bonito, the residents became proficient at making fine pottery and cookware. This pottery became the medium of exchange used to move the maize from region to region and to increase the overall health and prosperity of the whole Colorado Plateau. Most anthropologists believe that the trading of maize was the underlying cause of Chaco’s incredibly rapid growth.
For slightly different reasons, maize was also the underlying cause of Cahokia’s even more incredible growth. Cahokia was in the fertile bottomlands of the Mississippi River. Unlike Pueblo Bonito, Cahokia was surrounded by the most productive maize farms on earth. This allowed the city to provide food for a truly huge population…and still have plenty of maize left over for trade. Yet despite the speed and extent of growth in both metropolises, by 1350 both had ceased to exist ( Munoz: Sissel; Schroeder; Williams Geology (2014) 42 (6): 499-502).
So, what happened?
The short answer to this question is the same as the answer to our earlier question about the beginnings of the two cities: climate change. The climate conditions supporting the MWP began to collapse at about 1250. They were replaced by less hospitable conditions that led to a much longer period we now call The Little Ice Age. This was not a true ice age with sheets of glacial ice covering North America, but it was measurably cooler. In the Colorado Plateau, it caused prolonged periods of drought that proved the undoing of the Chaco Culture. Cahokia suffered even more complicated effects. Massive periodic flooding of the Mississippi River, punctuated by extended drought and related social unrest, wiped out many city structures and most of the farms (Benson, Pauketat, Cook, 2009 American Antiquity 74 (3): 467–483). The floods left sanitation problems and disease in their wake. Amazingly, maize survived throughout most of its new range. Its volume of production was reduced, to be sure, but its extraordinary genetic adaptability allowed it to adjust to the new conditions. Despite dramatically different new climate conditions, many varieties of maize continued to proliferate. Its range also expanded. By 1492, the year when Columbus arrived in the Caribbean, almost all Native Americans living below the 60th parallel relied on maize. The only exceptions in the west were the Pacific coastal tribes, where the sea was the principal food provider.
Maize has since become the largest agricultural crop in the United States, though very little of it ends up providing meals for Native Americans, or the general population. 450 years of European colonial appropriation and product “modification” have taken most of the crop off our dinner plates. Instead, it goes into the gas tanks of automobiles and factories, and to feedlots all over the world. Much of the North American crop consists of genetically modified (GMO) varieties. 40% is used to make ethanol, and another 40% goes to feed livestock and to produce products like starch, corn oil, and an impressive array of industrial chemicals. Yet only 20% is grown for comestible products like “on-the-cob,” cornmeal, masa, masa harina, tortillas, and chips.
Climate change, as most of today’s climatologists can attest, is happening again, as atmospheric pollution continues unabated temperatures are rising at an alarming rate. Water shortages and political water distribution decisions are endemic throughout the Southwest. Urban populations and corporate manufacturers seem to be able to get all the water they want. But small farmers and Native Americans must struggle and negotiate for every drop. The summers of 2023 and 2024 were the hottest on record, WORLDWIDE. In 2023 Arizona had 76 consecutive days where temperatures were 100 degrees or hotter. Then, in 2024, when Arizonans were hopeful of relief, the State had 117 consecutive days of 100+. That’s a huge increase. It’s hard to ignore these numbers. Perhaps we should be looking at the Chaco and Cahokia Cultures to remind ourselves of the physical and cultural disruptions that will occur if we continue to downplay the cries of our neighbors, and casually ignore the human-caused growing pains of our Mother Earth. The Episcopal Diocese and The Council for Creation Care would like your input. The Comment Section below is open for business.