Archeology & Adaptation to Climate Change in Yellowstone
by Staffan Peterson
The effects of climate change may pose the greatest threat to the integrity of natural and cultural resources that Yellowstone National Park (YNP) has ever experienced (NPS 2010). Protection and preservation of these resources requires park managers to understand potential threats using the best available research, and that they act in the long-term public interest. The causes and consequences of climate change, including how climate affects ecosystems and how humans have adapted to climate change, are critical research areas. This article focuses on how probable climate-driven changes in land cover and disturbance regimes may be impacting archeological resources in YNP, and how park managers use science to respond and adapt to emerging challenges.
YNP encompasses over 2.2 million acres in northwest Wyoming, southern Montana, and eastern Idaho, between 5,000 and 11,000 ft. above sea level. The surface geology is primarily volcanic plateaus of Quaternary rhyolitic rock surrounded by Eocene mountains. The park’s highly variable ecosystem is home to diverse flora, fauna, and microorganisms, some of which are uniquely found in Yellowstone (Despain 1990). The climate in the Greater Yellowstone Ecosystem (GYE) includes long cold winters, short cool summers, with precipitation of 10-70 in. annually. Summer droughts help sustain the normal fire regime. Because climate is a determining factor for the presence and distribution of all life, understanding climate change can explain how past peoples adapted to life in Yellowstone over several millennia.
Today, we are faced with a paradox. Evidence of adaptation to past climate change can explain different aspects of past lifeways, but current and future climate change can threaten or erase that very evidence. It is a critical part of the mission of the NPS to protect, preserve, and interpret this record for this and future generations. Given the agency mission of resource preservation, we have to understand and anticipate critical vulnerabilities to meet our preservation mandate, including new threats from a changing environment.
Potential Climate Change in the GYE
The truism that the climate has been changing since the end of the Ice Age does not speak to the fact that the climate has changed significantly faster in just the last few decades than over the prior 12,000 years. Globally, an increase of 6°F-13°F may occur over the next 80 years, or within the lifetime of today’s preschoolers (Collins et al. 2013). These changes can have cascading effects on ecosystems and park resources. For example, a 2°F increase in average annual temperature may result in a 600% increase in area burned each year (Peterson and Littell 2014). Snowpack may decrease by 3-4 in. per year (Chang and Hansen 2015), while spring rains may increase. Effects of climate change can also impact park infrastructure, visitation patterns, and visitor experiences in the park and its resources in unpredictable ways. Parks across the United States are responding to these challenges by developing mitigation and adaptation strategies, including increased monitoring of ongoing impacts, predictive modeling of climate change scenarios (e.g., sea level rise, increased storm surges, flooding, wildfires, and drought), and modeling predicted impacts to vulnerable cultural resources.
In the western U.S., most climate change scenarios suggest higher summer temperatures and earlier spring snowmelt, creating conditions for increased wildland fire frequency and intensity (Flannigan 2006,Littell et al. 2011, Gross 2016,Halofsky et al. 2017). Burned acreage has increased significantly over the past 20 years, and is projected to double by 2040 and triple by 2080. Drought and hotter temperatures also weaken trees, making them more susceptible to infestation by mountain pine beetle, Engelmann spruce beetle, and western spruce budworm. Insect-killed trees provide fuels that further increase the risk of wildland fire. Federal and academic researchers have linked these trends to climate change (e.g., Littell et al. 2016, Loehman 2017).
Climate change studies in the GYE have been published in dozens of articles and books, including a recent issue of this journal (NPS 2015). The study of modern climate change in YNP began in 1992, when Romme and Turner (1992) explored the logical consequences of rising global greenhouse gas emissions on Yellowstone’s ecosystem. They predicted that high elevations would experience upward shifts in the elevations of upper and lower tree lines, and fire regimes would become more severe (Romme and Turner 1992, 2015). Paleontological studies of pollen provide a model for how forest composition and treelines in the GYE changed with climate changes over the last 12 millennia (Whitlock 1993). Thirty years have passed since these early forecasts, and current data support the 1992 study in its broad outlines. Present conditions and near-term projections include changes in temperature, precipitation, humidity, wind speed, sunshine duration, and evaporation (Hartmann et al. 2013), resulting in earlier snow melts, warmer summers, and longer growing and fire seasons (Romme and Turner 2015, Tercek 2015). In YNP, average temperatures over the last three decades are 3°F warmer than in the preceding three decades (Tercek et al. 2015).
Shifts in the type and location of land cover, hydrology, and fire disturbance regimes will likely impact cultural resources in destructive and irreversible ways. Distinct and serious threats to the record of our collective past exist in the form of climate driven drought, desertification, erosion, flooding, and other environmental impacts (Curry 2009). High elevation ice is being lost at significant rates globally. Ice cores from Mt. Kilimanjaro show that these ice fields are on the verge of disappearing entirely (Thompson et al. 2009). Glacier National Park is projected to lose many of its iconic glaciers within 12 years (Hall and Fagre 2003, USGS 2017). Similar changes are occurring or are projected for high elevations from Alaska to the Andes. Impacts to coastal wetlands are highly vulnerable to climate change in the form of sea-level rise and coastal subsidence.
Climate change impacts may mean changes to many aspects of the resources of Yellowstone. Federal agencies have in recent years begun efforts to anticipate and prepare for new impacts to the cultural and natural resources they are charged with protecting (Melnick 2015; NPS 2010, 2016). These efforts broadly seek to understand climate variability, analyze and quantify resource vulnerabilities, develop and implement adaptation plans, and measure and communicate success. Current research can serve as a tool for identifying a range of management options available for anticipating and mitigating impacts to these critical resources.
Evidence of the Human Past in Yellowstone
Cultural resources, broadly conceived, are things that help us remember the human past and help shape our current identities. They include tangible items such as buildings, historic districts, archeological sites, and artifacts, and intangibles such as ceremonial and traditional observances and values. This article focuses on archeological resources—a subset of Yellowstone’s cultural resources. Over 1,900 archeological sites have been documented in the park, an astounding number given that less than 3% of the park has yet been surveyed. Archeological resources in the GYE represent almost 11,000 years of continuous human presence. Sites from all periods have been recorded, in all kinds of physiographic settings. Native American tribes historically connected to YNP include the Arapaho, Assiniboine, Bannock, Blackfeet, Cheyenne, Chippewa, Comanche, Crow, Flathead, Kiowa, Lakota, Nez Perce, Salish, Sioux, and Shoshone (Nabokov and Loendorf 2002). Artifacts and sites from the prehistoric and historic periods have been documented in diverse settings, including along rivers and lakeshores, on islands, in hydrothermal areas, and on mountain tops.
The European American presence in Yellowstone began in the early 1800s, with fur trappers and prospectors and by the mid-1800s, with military and scientific expeditions. The park was established in 1872, adding to the continuously evolving record of the human imprint on the land. Historic period sites dating from the earliest days of the western expansion of the United States to the modern era include wagon roads, camps, military facilities, trails, and all manner of infrastructure related to the creation and continuous use of the park. Detailed information on the archeology of Yellowstone is available in various books and journals (Reynolds and Johnson 2003, Johnson 2010, Livers 2012, MacDonald and Hale 2012, MacDonald 2018).
Hundreds of archeological studies in YNP have provided insight on this vast history. These studies are primarily undertaken in order to meet legal mandates to preserve park resources ahead of infrastructure projects that support the needs of four million visitors each year. Others are tied to assessing potential or actual effects of natural processes on archeological sites, such as wildfire or erosion. For example, due to long-term upwelling in the Yellowstone magma reservoir, Yellowstone Lake is experiencing changes in wave action that can erode shorelines where sites are located. In order to assess that impact, the park undertook a four-year total survey of the shoreline, documenting numerous sites, some of which are being actively impacted by erosion.
Still other work is conducted for research purposes by university partners, most recently by the University of Montana and the University of Wyoming. For example, an ongoing study of sites related to the Nez Perce Flight of 1877 in Yellowstone discovered numerous sites connected to that event and yielded fascinating insights on some aspects of the war (Horton and Eakin, this issue). Research on prehistoric bear hunting (Ciani 2014), obsidian use (Park 2010, Doss and Bleichroth 2012), game drives and sheep traps (Eakin 2009), wickiups (Eakin 2009, White and White 2012), tipi rings (Livers 2012), 19th century wagon roads, mining, and tourist developments (Corbin and Russell 2010, Flather 2003) all incorporate archeological information and have greatly enriched our understanding of how people have used YNP.
As native peoples used this land for nearly 11,000 years, the vast majority of archeological sites documented are from the prehistoric period. These Native American sites include short-term or seasonal camps, trails, tipi rings, obsidian and chert quarries, vision quest sites, wikiups, game drives, and other places where people worshipped, hunted, gathered plants, made tools, fished, traveled, or otherwise conducted a myriad of day-to-day tasks.
Archeology is not the sole source of information on human use of the park. Native American traditional knowledge and historical documents are rich sources of information. The best narratives of the past are based on many types of information, each leveraging the strengths of the others. However, the further back we look, the less there is to go on. For the majority of the prehistoric and historic periods, archeology is a primary source of information on the park’s complex human presence.
Climate Impacts Life
While the climate has always been changing, how climate change affects people and their ways of life is less well understood. Researchers in paleoclimatology, paleontology, geomorphology, demography, and archeology have collaborated to develop a broad outline of ways in which people have interacted with changing climates. The findings indicate complex interactions between climate and lifeways, operating at different spatial and temporal scales.
Around 13,000 years ago, glaciers up to one mile thick began to melt off of mountains in the GYE. Within 1,500 years, people began to enter YNP, evidenced by distinctive large spear points made of obsidian from YNP’s Obsidian Cliff found in the environs of YNP. Excavations at sites in the GYE show hunters in this period had a diverse subsistence base, with a focus on bison. Between 8,000 and 5,000 years ago, the climate became drier and hotter. Analysis of ancient pollen recovered from lake beds in the southern part of Yellowstone indicate peak dryness, much dryer than today, occurred around 7,000 years ago (Whitlock 1993). Stone tools from this period are much more abundant than those from the preceding period, however, archeological features dating from this period, such as hearths; are rarer and more ephemeral, suggesting long-term use of the area decreased. Animal bones recovered from this period point to a relative decrease in bison hunting. The drier, hotter climate may have led to poorer forage, resulting in smaller bison herds.
Beginning about 5,000 years ago, the climate cooled and became wetter. Changes in the human use of the area are correlated with this climatic shift. For example, increases in the number of artifacts and features indicate substantially increased use of Yellowstone between 3,000 and 1,500 years ago, and continue to increase all the way up to the 19th century. Thus, the long-term perspective that only archeological evidence can provide, points to a strong relationship between climate change and human ways of life. Evidence also points to how major changes in future climate patterns could impact modern people.
Climate Change Impacts on Archeological Resources
Modern scientific archeology investigates not just artifacts, but the environmental context in which they are found. Climate change can impact the environment in which these resources exist, and through which we understand and manage these resources in complex and often poorly understood ways. It is as much a fallacy to assume we can preserve these resources but not the environments where they exist, as it is to assume we can manage wildlife, but not wildlife habitat. Two major sources of climate change driven threats— wildland fire and the melting of ancient ice—are explored below.
The area impacted by wildfire is predicted to increase in the GYE with global warming. Empirical statistical models and process-based simulations agree almost universally. The relationship most subject to change is between drought and fire, and this effect is being recorded at multiple scales.
Over 80% of YNP is now forested (Despain 1990). Fire evolved as a critical part of the forest ecology of YNP and continues to be so today. The normal fire season begins after deadfall dries out after the spring melt and summer rains decrease. By July, humidity drops and increasing “dry” lightning strikes create fire starts that can grow rapidly in the dry air. High temperature wind-driven fires burn both the forest understory and crown vegetation during this time (NPS 2015). As temperatures decrease and precipitation increases in September, the wildfire season ends (Marcus et al. 2012). The longer, warmer summers predicted by some models would alter the normal fire regime by creating bigger, hotter fires. Wildland fires are characterized as surface fires that burn surface vegetation, ground fires that burn buried fuels like forest duff, and crown fires that burn up into the forest canopy (Fuller 1991). All three create risk to archeological resources in distinct ways (figure 1).
Wildland fires can impact archeological resources both directly and indirectly. Through post-fire observation and field experiments, archeologists learned how fire impacts diverse types of artifacts (Winthrop 2015). At more than 572°F, obsidian artifacts will bubble, crack, or even melt. A more subtle impact is heat alteration of the artifact surface such that it cannot be dated using the obsidian hydration method. Above 662°F, chert artifacts can fracture, develop fine cracks, shatter, and change in color. Sandstone, bone, or shell artifacts can undergo a range of effects from breaking apart to complete destruction (Winthrop 2015). In Yellowstone, the massive 1988 fires burned over much of the Obsidian Cliff National Historic Landmark with intensely hot crown fires. Post-fire observations by archeologists detected probable fresh fracturing, oxidation, and disintegration of materials at nearly all of the 59 archeological features associated with the cliff (Davis et al. 1995).
The most susceptible artifacts are those on or near the surface (figure 2 and 3). While most fires have minimal impacts below 5.9 in., organic remains such as pollen can be affected. More destructive impacts are the tipping of crown fire burned trees, completely disturbing soil over a large area, or when burning roots carry fire below the surface. When rains or winds follow, sheet or gully erosion can move or bury artifacts, altering their original context; and hearths or midden layers can be destroyed. An indirect effect is the exposure of sites previously concealed by vegetation, creating risk for illegal collecting of artifacts (figure 4).
A little known class of features present in the GYE are Native American wooden structures. Wikiups, or conical timber lodges, and their remnants have been recorded in dozens of locations in the GYE. These small, tipi-like log structures are of varying ages, with some probably being prehistoric. Some may be simple expedient shelters, while others may be associated with ceremonial activities (White and White 2012). Game drives, or sheep traps consisting of rock and brush arranged in fence-like lines up to 200 yards long, were designed to control the movement of sheep or other ungulates across the landscape to areas where they could be easily hunted (Eakin 2009, Lee and Puseman 2017). These rare feature types, documented in the Absaroka Mountains in the eastern part of the park, are highly vulnerable to destruction by wildland fire (figure 5).
Finally, fire fighting itself can impact archeological resources. The heroic efforts of wildland firefighters are often followed up by a little known but critical effort to assess and repair both direct effects of the fire and impacts from the firefighting response. Natural and cultural resource specialists survey burned areas to document the severity and extent of any damage from the fire or firefighting efforts on park resources. In YNP, burned areas are typically left to restore on their own, but tracks from firefighting vehicles or fire lines dug into the ground are typically repaired. Damage to archeological resources is documented and at-risk sites are recommended for follow-up investigations, including surface collection of artifacts or targeted excavations. Federal wildland fire policy is to deploy specially trained resource advisors alongside fire fighting crews with the goal of minimizing impacts to sensitive areas from fire lines and fire fighting vehicles, and to assess the condition of resources after the fire is over. Often that can involve documenting impacts and recommending ways to mitigate damages, such as recovering at-risk artifacts for long-term curation or restoring soil cover in affected areas.
When assessing any new site impacts, it is important to note that many sites may have already experienced impacts from human or natural causes. To better understand old versus new impacts, YNP archeologists monitor the condition of sites on a recurring basis, providing baseline data useful in identifying new impacts.