by Kimi Eisele
When people visit Tumacácori National Historical Park, off Highway 19 between Tucson and Nogales, AZ, they often arrive knowing very little, says Anita Badertscher, the park’s chief of interpretation and education.
“They’ve just seen the brown sign on the highway, they don’t know what it is,” she says. The park preserves three Spanish colonial missions in the Santa Cruz River valley of Southern Arizona and shares their history and influence on indigenous communities of the region.
But when they wander through the old earthen structures, Badertscher says, “They feel the history here. They feel the presence of the people. They feel the peacefulness and the sacredness of the place.”
Badertscher attributes this, in part, to the adobe construction of the mission San José de Tumacácori and two nearby structures, Los Santos Ángeles de Guevavi, and San Cayetano de Calabazas. “You’re looking at the bare bones of the structure. The humbleness of the material is part of it. There’s a contrast. It’s this impressive structure, but it’s just dirt.
Earth has long been the building material of choice for people living in dry, arid climates. In the Southwest, the Hohokam built structures of puddled adobe, and the Spanish later used adobe mud blocks.
“It was always about choosing the right clay content and the right sand source and mixing them so the material would stand up,” said Sharlot Hart, an archaeologist with the Southern Arizona Office of the National Park Service.
But today, keeping historical adobe structures standing has become a challenge given increased climate variability in the region. After Tumacácori archaeologists reported adobe collapse in the wake of several sustained rainfalls, Hart and others knew it was time to start looking more closely at the material and its response to climate.
On the far east side of Tucson, a few hundred feet from the Desert Research Learning Center, a series of uniform adobe walls rise up from ground, each nearly four feet tall and three feet wide.
“We lovingly call it adobe-henge,” says Hart.
The walls, 20 in total, are the site of an interdisciplinary experiment by park researchers to understand the impacts of climate change on adobe construction and eventually, on various kinds of treatment to help preserve it, says Hart, one of the project’s lead researchers.
To build the site, the research team brought in adobe experts David Yubeta and another builder, Ray Madril, who worked with a team of volunteers to build the walls.
In partnership with the USDA Agricultural Research Service, they then used a rain simulator to measure impacts of precipitation events on the walls. They measured 30-minute rain events, at intensities equivalent to one-year, 25-year, and 100-year storm events, based on actual storms measured at Tumacácori.
Initial results from the project showed a highly significant relationship between rain intensity and rate of adobe loss, says Kara Raymond, a hydrologist with the southern Arizona Office of the National Park Service, and one of the project researchers.
“The really intense 30-minute storms caused a lot of physical damage to the walls. This was across the board with the three things we were looking at—material loss, depth eroded, and surface area affected. The impact was actually exponential,” Raymond says.
These results weren’t entirely unexpected, but they were much more significant than what the team had anticipated, Hart says. “It was the kind of data set you long your whole career for. We were getting something like a six percent loss with a 100-year-storm event in 30 minutes. That’s kind of scary.”
Data from the intense 30-minute storms did not show a significant amount moisture entering into the wall, Raymond says, but a second round of simulations looked at the impacts of low intensity storms occurring over a longer duration. These tests showed more moisture build-up in the walls, Raymond says, matching what actually happened at Tumacácori, where four durational rain events over ten years resulted in significant damage to structures.
Of the earth, for the earth
Bob Vint, a Tucson architect who has worked to preserve prehistoric cliff dwellings and colonial missions, says extreme wet, heat, and cold—or “climate weirding,” as he calls it—will continue to impact adobe and other forms. “Without a doubt historical architecture will experience a more rapid deterioration in those conditions. There’s no way around it.”
Adobe is particularly susceptible, he says. “Cycles of wetting and drying is what wears on adobe. If walls get saturated at the base they can slump and fall. That’s how adobe fails. Water rises up from below by capillary action.”
It’s called “basal coving” in the trade, Vint says. “If you go into the old barrio a lot of old buildings have erosion at the base. People will make repairs with modern material, cement block, or concrete, but that forces moisture up higher. Cement holds moisture and doesn’t let it evaporate,” he says.
It’s ironic that adobe may be the material most threatened by climate change, but also the material that may best address challenges associated with it, Vint says. “Adobe will keep you at about 75 night and day, even if 110 degrees by day and 60 at night.”
A good example of thermal regulation is within the three-feet-thick adobe walls of the convento, or east wing, of the Mission San Xavier del Bac, just south of Tucson, where he serves as preservation architect. “The space has no mechanical ventilation at all yet it’s comfortable all year round. The Spanish Franciscans and Tohono O’odham work force understood that you had to build an architecture to keep you comfortable. To this day it still functions the way it was originally intended,” he says.
Mud adobe also holds a high amount of “embodied energy,” Vint says. Embodied energy refers to the amount of energy required to create a given material. He sites work by University of Kassel’s Gernot Minke, which has compared the energy expenditure of building materials to show that steel requires 54,950 kilowatt hours per cubic meter (kWh/m3) to produce, compared 550 kWh/m3 for timber, 1100 kWh/m3 for solid brick, 500 kWh/m3 for concrete, and 5-10 kWh/m3 for earth.
“By these calculations, earth is 5,500 times more efficient than steel,” Vint says.
One main drawback to adobe is its cost, Vint says. While still common in Mexico, in the United States adobe has largely become a building material for the wealthy because the labor involved is so expensive. And since fewer and fewer people are skilled at making it, it drives up the price.
Vint believes preserving adobe is environmentally responsible. “It has a lot of advantages as a building material and in that sense, it has a future. But addressing its susceptibility to extreme weather will be critical,” he says.
Climate futures in the Southwest
While broad climate models predict less annual rainfall in the Southwest, they also project an increase in extreme precipitation, consistent with general warming trends, Raymond says, though add the data sets are still conflicting.
Research sites maintained in Southeast Arizona by the USDA Agricultural Research Service don’t offer conclusive results yet, but a 2019 study did find an increase in the intensity of rainfall.
Michael Crimmins, a climate scientist and professor of environmental science at the University of Arizona, says the data is still murky when it comes to precipitation and climate change variability in the region. Some evidence points to continued extremes.
“In the Southwest, climate is enormously variable in the historic record. This place will get blasted with deep droughts and every monsoon season will roll the dice. In terms of heat, the temperatures are definitely getting warmer and warmer and warmer, but in terms of precipitation it’s so ‘noisy’ down here it’s hard to see a clear climate change signal.”
Raymond says hurricanes may drive multi-day storms that yield a lot of rain over time. “While they don’t happen every year, they can be some of the most damaging.”
One projection, for example, shows that hurricanes from the northeastern Pacific Ocean may increase 338 percent by the late 21st century. “That’s over three times what we see now,” she says. “These are the kind of events we experience here in southern Arizona as remnants of tropical storms, which bring days of rain.”
This could mean vulnerability for adobe structures. “If you have more rain you have more problems,” Yubeta says. “Adobe wears one inch every year, but if it loses a roof and is exposed to rain and wind, the elements attack the walls so it goes faster.”
New treatments for old walls
Yubeta understands the embodied energy of adobe firsthand. He’s been working with the material since boyhood, when he lived with his grandmother in an adobe home in South Tucson and helped family members with annual maintenance.
Later, he later worked to preserve adobe structures for the National Park Service, a job he held for over 25 years. He recalls his first official job site, at Tumacácori, when he felt the material was more than just dirt. “It was almost spiritual to me. I felt its strength, I felt it being alive, the material,” Yubeta said.
Yubeta thinks of adobe itself as a non-renewable resource. “Sure, you can make more but once you lose historical adobe you’ve lost it forever.”
The mission of the National Park Service is to preserve places and historic sites for future generations. When it comes to preserving historical structures, that can be tricky. While the owner of an adobe home can perform regular maintenance and apply modern technology to keep a structure standing, park archaeologists and preservationists must consider strategies for maintaining the historical and cultural integrity of a site.
Indeed, adobe walls themselves hold history. “Whenever adobe was made, especially in the Spanish contact era, people would use trash pit material as part of the mix,” Hart said. “So you’ll get artifacts in the wall like arrowheads and food refuse and bones.”
All that history is worth preserving. “We don’t want that original fabric to deteriorate anymore because it has so much potential for information,” she said.
Strategies used in the past often offer examples of what not to do. “Covering adobe walls in cement is not preserving them at all. They melt inside,” Hart says.
And while a logical solution might be to put roofs over historical structures, it’s not always that simple. One of the mission sites near Tumacácori, Calabazas, has extensive wear. The Arizona Historical Society maintained the building until 1991, and in the early 1960s, covered the structure with a roof. “When you put a cover over adobe, it creates a different wind pattern within the walls that can erode them,” Yubeta says.
Given the dearth of published research on adobe preservation, Hart says learning from traditional practitioners like Yubeta can help inform historical methods and their potential for the future.
“David [Yubeta] is always saying your cap is the lifeblood of your wall. I think if it was up to him, you would never use amendments in the preservation of material. What you would do is have a really nice thick mud cap as a sacrificial layer and maybe plaster with a sacrificial plaster. But the Park Service has seen that as a problem because that means your cyclical maintenance time is huge. You have to do that every year.”
While all National Park Service adobe structures have some sort of preservation treatment, Hart says the walls at the “adobe-henge” test site are all bare adobe. The next set of experiments will look at testing various treatments for preserving the walls, including using an adhesive polymer called rhoplex, adding a thick mud cap, or using various kinds of natural plasters.
“If we can use these 20 walls to show the difference in just leaving a wall open to having a rhoplex coat to having a mud cap to having a coat fortified with cactus juice, we’ll be able to learn a lot,” Hart says.
 Minke, Gernot “Earth as a Building Material” Out of Earth, First National Conference on Earth Buildings Center for Earthen Architecture, University of Plymouth, UK 1994.
ClimateLore is a series of investigations and stories about the impacts of climate crisis on culture and heritage and climate resilience in folk and indigenous communities of the Greater Southwest and Northwest Mexico. It is funded, in part, by Arizona Humanities.