As modern officials try to assess the risk global warming might present to the American Southwest, they're paying a lot of attention to what scientists say about how climate changes affected the region's ancient past.
Archaeologist Kristen Kuckelman has spent many years digging in the ruins of ancient farming villages on the Colorado Plateau and analyzing the artifacts and specimens she takes from them.
The people who lived in these ancient villages, which are known as pueblos, were part of a large culture that thrived for several hundred years in the high desert plain that covers parts of modern Utah, New Mexico, Colorado and Arizona. Archaeologists call them Anasazi, or Ancient Pueblo People. One of the best known of their pueblos is in Mesa Verde National Park in Colorado.
But sometime in the late 1200s, the Anasazi abandoned all of their pueblos. And for more than a hundred years, archaeologists have been perplexed about why.
Kuckelman thinks she may have found the answer. The pueblo people didn't leave a written record, but she believes they left clues behind in the villages she's studying.
"I think we're finally really making some important inroads into answering that question," says Kuckelman, who works for Crow Canyon, a nonprofit archaeological center in southwestern Colorado. "It had to have been a pretty important reason (for them) to leave and never come back."
Kuckelman thinks the reason was climate change. A major drought hit the area in the 1270s. Kuckelman says her research from one of the villages, Sand Canyon Pueblo, shows that the drought destroyed the people's ability to grow corn to feed themselves and their turkey flocks. They were forced to revert to hunting and gathering.
She figured this out by analyzing their garbage. According to Kuckelman, the people tossed the remains of their meals and ash from their hearths into pits outside the walls of their kivas, their round stone homes. She compared refuse from the early days of the settlement to what was left in the kivas when the residents moved away.
"I wasn't particularly looking for anything," Kuckelman says. "At some point, it just sort of jumped out of the data at me."
It was turkey bones, lots of them, in the garbage from the earliest days of the pueblo.
"The percentage of turkey bones is 96 percent of the identified animal bones — 96 percent! That's huge," Kuckelman exclaims.
But of the bones left over from the final days of the pueblo, only 13 percent were from turkeys. Most were from wild game.
"That can't be accidental," she says. "It has to mean something. And what it means is, when they first moved here, they were eating primarily turkey. And at they very end they weren't. So why?"
This and other evidence convinced Kuckelman that the corn crops that fed the ancient farmers and their turkeys failed because of the drought. Sometime after 1277, Sand Canyon Pueblo was attacked, and at least 32 people were killed.
"And no one lived in the village after the attack occurred," Kuckelman says.
She believes other villages' corn crops failed, too. There wasn't enough wild food to go around, so people were hungry, and that led to violence.
Kuckelman puts it simply: "The system fell apart."
Kristen Kuckelman couldn't have come up with her theory if it weren't for the work of scientists who study the growth rings of trees. The rings reveal important secrets about the distant past, such as when kivas were built, and when the great drought hit. That information enabled Kuckelman to link the collapse of the pueblo's crops and turkey flocks to the Great Drought of the 1270s.
A few hundred miles northeast of where Kuckelman is digging, University of Arizona Climate Science Professor Connie Woodhouse was doing her summer field work, high in the Rocky Mountains.
Her team uses a chainsaw to cut slices of the remnants of old dead trees to take back to the lab for analysis.
To sample live trees, Woodhouse drills into one with a hand tool, called a tree borer, and extracts a sample of the tree's rings the size and shape of a drinking straw. It shows distinct sections, each of which holds information about the past.
"The wide rings are wet years and the narrow rings are dry years," Woodhouse says, "And when you put together the information from the tree rings into what we called the tree ring chronology, it gives you a picture of the moisture in that area where we collected it."
Woodhouse and her colleagues recently presented an alarming picture of the ancient history of the Colorado River. They sampled the oldest trees they could find — dead and alive — and used them to estimate stream flows all the way back to the year 762. Their results show that the droughts over the last hundred years weren't as severe or as long as earlier droughts. And in fact, the first part of the 20th century was unusually wet.
"Not only was it wet in the context of 100 years, but there was not a wet period like that for at least 400 years," Woodhouse says.
That has major ramifications for modern people who rely on the Colorado River for water. The laws that are used to divvy up the river assume that the extremely wet period was normal.
Woodhouse says the lesson from the tree rings is that longer dry spells, like the one that chased the pueblo farmers from their villages, could return.
Some experts believe they already have.
Eric Kuhn, the chief water manager for western Colorado, stands at the convergence of the Colorado River and the Roaring Fork, a major tributary. Kuhn says the rivers are flowing at only 70 percent of their normal rate. Other tributaries of the Colorado are flowing at only 40 percent.
It is the seventh dry year the river has had in the past eight years.
"The water community is all hoping that this is a drought," Kuhn says. "Drought implies things will return to normal someday."
But he doesn't think they will. Kuhn believes the lower river levels over the last several years represent a new, drier normal. And because of climate change, he expects the river to become even drier in the future.
With that in mind, Kuhn thinks that the tens of millions of people who rely on the Colorado River to irrigate their fields, water their lawns and fill their bathtubs, should take the experiences of the ancient pueblo farmers seriously.
"They obviously didn't have our technology. They didn't have Hoover Dam and Glen Canyon Dam. And when there was a change in the climate, they could not adapt to it," he says.
Kuhn notes that while modern society has a lot of technology, it also has a thirst for the Colorado that is greater than its supply. He believes that what happened to the ancients could still happen today.
"We're used to a certain amount of water," he says. "If that changes just a little bit, it's going to cause some big disruptions in how we deal with life here in the southwest."
Kuhn predicts homeowners will get used to cactus gardens instead of lawns, cities will buy water rights from farmers, and irrigated agriculture — a way of life in the southwest for a century — will disappear, just like the pueblo farmers' turkey flocks.
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