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National Register of Historic Places Program:
Archeology Month - Archeology and Climate Change

The National Register of Historic Places is the official list of the Nation's historic places worthy of preservation. Authorized by the National Historic Preservation Act of 1966, the National Park Service's National Register of Historic Places is part of a national program to coordinate and support public and private efforts to identify, evaluate, and protect America's historic and archeological resources.

 

[photo] Photo from wikipedia http://en.wikipedia.org/wiki/File:Mendenhall_Glacier_pano_Aug2011.jpg
Retreating of the Mendenahall Glacier in Alaska.
Photo from wikipedia

 

Climate Change and Archeology in the Southwest

[photo] Eroding shorelines and melting permafrost damage at Bering Landbridge National Preserve (NPS photo). Drought, heat, and high winds have fueled fires in Mesa Verde National Park. The effects of wildfire on cultural resources can be manifold, including damage or destruction from the fire itself, increased erosion resulting
from devegetation, and the staining of rock formations and historic masonry by fire retardants. Here, blackened trees and orange staining can be seen on the mesa above one of Mesa Verde’s alcove sites. Photo: Wikimedia.org

For many places in the ancient Southwest, droughts were not uncommon in the precontact past.  However, by the mid-1100’s CE (Common Era), in the areas surrounding the heavily populated Chaco Canyon and Mesa Verde, we see the cessation of building activity and a massive decline of the regions material culture.  Along with some of the complex factors associated with drought including soil exhaustion, and social unrest, prolonged cases of severe drought are widely seen as one of the main catalysts of population migrations in the past.  As the drought region experienced a decline in populations, other regions higher in altitude such as the Bandelier area and the Chama Valley regions experienced ample population growth.  The story of this tremendous movement of human populations of the past can be told through archeological data.

For more than 12,000 years humans have inhabited the southwestern region of the United States.  This human occupation and other factors have made the Southwest a prominent location for archeological excavations over the last century.  Though our modern climate records only document the last 130 years, new methods of determining climate change across various disciplines have been emerging that provide insight into the lives of southwestern American populations of the past, the environment in which they lived, and the response of those populations to the changing climate.

[photo] NPS photo Bluebonnets (lupine) poking up through mud cracks left behind by the 2008 flood of the Rio Grande near Big Bend National Park
Photo: J. Axel – NPS

In combination with archeological remains, Pueblo oral histories, and dendrochronology ( the study of tree-ring growth patterns and assigning absolute dates to those rings), hints at the factor of climate as playing a role in Southwest migrations of the past.  A.E. Douglas was one of the first to propose such an idea when he discovered that a severe drought coincided with the movement of populations out of the Four Corners region.  He reasoned that these Pueblo populations were heavily dependent on farming and the “great drought” of 1276 CE was the causal factor behind this abandonment.  In contrast to agriculture-based societies, early hunter-gatherer populations did not appear to be greatly affected by the vagaries of weather as their population is believed to have only gradually increased over a span of 6,000 to 7,000 years.  Only after people began to rely on agriculture to sustain the population did climate become a major player in understanding the distribution and movement of people across the Southwest (Gauthier, Civitello, and Allen 10-11).

One way to see the impact of climate change in past agricultural based societies is by an analysis of the role of the irrigation infrastructure that supported them.  Though social benefits of this technological innovation are plainly evident in the short term, for instance through greater crop production, concentrated water management and/or population growth, potential vulnerabilities to climate conditions can accompany the reliance of the irrigation infrastructure over longer periods of time..  The Hohokam-Pima Irrigation Sites National Historic Landmark that are now part of Pueblo Grande Ruin and Irrigation Sites in central and southern Arizona for example, is known for the largest precontact canal irrigation systems in North America.  These canals were used for centuries until the late 1400’s and brought enormous success to agricultural development that allowed the Hohokam to have a more sedentary lifestyle and enabled them to settle in large population centers like Snaketown which is an archaeological site within the Hohokam Pima National Historic Landmark and National Monument. However, the elaborate irrigation system brought about traumatic transformation as well in the wake of prolonged drought.  In this case the transformation was preceded by a long period of social rigidity (Nelson, Hegmon, and et al 197-215).  Some of this ancient population became resistant to the idea of relocation possibly due to their material and labor investments already spent on the irrigation infrastructure.    Because this society was so place-focused, long-term occupation of the Hohokam region began to severely deplete local resources leading to a greater incentive to leave the area behind. 

[photo] Eroding shorelines and melting permafrost damage at Bering Landbridge National Preserve (NPS photo). Excavated Canal
Photograph from NHL file

A much smaller scale irrigation system compared to that found within the Hohokam-Pima Irrigation Sites National Historic Landmark was found in the Mimbres region roughly around the same time (650- 1450 CE).  With the Mimbres populations on the rise, previously established investments in infrastructure, and a focus on flood plain farming, residential patterning became focused around this infrastructure.  Over the course of hundreds of years the Mimbres experienced the depletion of soil, plant, and animal resources that exacerbated the impact of a period of extremely low precipitation around 1130 CE which coincided with the depopulation from nearly all the large villages in the area (Nelson, Hegmon, and et al 197-215).  Even though the dispersion of the population may have been more gradual than formerly predicted, the people of the Mimbres area relocated due to the vulnerabilities created by environmental and social pressures.

People of the past may have been reluctant to leave an area after considerable investments in the infrastructure that framed their way of life. For instance, in Chaco Canyon the labor force imported over 200,000 timber trunks from at least 50 miles away in order to create the 1000 year old pueblos, including Pueblo Bonito, which we can still see today at the Chaco Culture National Historical Park.  In 1040 CE these ancestral pueblos were abandoned but one can imagine how hard it must have been to leave after dedicating so much work to their creation.  Agricultural studies of the Chaco Region indicate that the maize and beans that can survive with little water could only support a population of about 2200 people.  The production of irrigation agriculture made such growth possible, however long-term population growth may have eventually outstripped the productive capacity of local resources leading to food shortages in the area.  Consequently, the Chaco population probably had to abandon their homes because life could no longer be supported. 

As a part of a developing society we unintentionally play a role in creating social vulnerabilities. Some of these vulnerabilities manifest during the occurrence of environmental disturbances when the climates’ power over human beings and their success with technology is clearly exhibited like within the Chaco, Hohokam and Mimbres examples noted above.  This vulnerability lies in both the nature and magnitude of hazards in the environment and in people’s response to those changes.  The natural world is an infinite and complex place (or as some would call it a body of knowledge), yet the only way to explore what is to be learned is by our interaction with it.  This dynamic and interactive relationship between humans and nature is bounded to our use of technology as we, and the environment, come to create the landscape in which we live.  Though we may have some idea of what to predict with the advent of new technologies, we should always expect a degree of uncertainty as we continue to experience the unintended consequence in a world we are still coming to understand. 

References:

Nelson, Margaret C., Michelle Hegmon, et al. "Long-Term Vulnerability and Resilience: Three Examples from Archaeological Study in the Southwestern United States and Northern Mexico." Surviving Sudden Environmental Change. Ed. Jago Cooper and Ed. Payson Sheets. Boulder: University Press of Colorado , 2012. 197-215. Print.

Gauthier, Rory P., Jamie A. Civitello, and Craig D. Allen. "What Happens When the Rains Don't Come? Climate Change, Southwest Migrations, and Where We Go From Here." Vanishing Treasures 2010 Year-End Report. Ed. Virginia Salazar-Halfmoon and Ed. Randy Skeirik2010. 10-11. Print.

Williams, James, C.. "Understanding the Place of Humans in Nature." The Illusory Boundary: Environment and Technology in History. Ed. Martin Reuss and Ed. Stephen H. Cutcliffe. Charlottesville and London: University of Virginia Press, 2010. 9-25. Print.

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