1 | Introduction to the Greater Yellowstone Climate Assessment

Cathy Whitlock, Steven Hostetler, and Bryan Shuman


Beartooth Mountains

Beartooth Mountains, a water source for the Yellowstone River. Photo courtesy of Scott Bischke.


The Greater Yellowstone Area (GYA) is one of the last remaining large and nearly intact temperate ecosystems on Earth (Reese 1984; NPSa undated). GYA was originally defined in the 1970s as the Greater Yellowstone Ecosystem, which encompassed the minimum range of the grizzly bear (Schullery 1992). The boundary was enlarged through time and now includes about 22 million acres (8.9 million ha) in northwestern Wyoming, south central Montana, and eastern Idaho. Two national parks, five national forests, three wildlife refuges, 20 counties, and state and private lands lie within the GYA boundary (Figure 1-1). GYA also includes the Wind River Indian Reservation, but the region is the historical home to several Tribal Nations (see box).


Figure 1-1. Map of the Greater Yellowstone Area (GYA) showing the six Hydrologic Unit Code 6 (HUC6) watersheds studied under the Assessment, and including mountain ranges, lakes and major river systems, jurisdictions, and selected towns. The portions of the watersheds within the GYA boundary are studied in this report. (Map created using ArcGIS® software, copyright ESRI and used herein under license.)


Federal lands managed by the US Forest Service, the National Park Service, the Bureau of Land Management, and the US Fish and Wildlife Service amount to about 64% (15.5 million acres [6.27 million ha] or 24,200 square miles [62,700 km2]) of the land within the GYA. The federal lands and their associated wildlife, geologic wonders, and recreational opportunities are considered the GYA’s most valuable economic asset. GYA, and especially the national parks, have long been a place for important scientific discoveries, an inspiration for creativity, and an important national and international stage for fundamental discussions about the interactions of humans and nature (e.g., Keiter and Boyce 1991; Pritchard 1999; Schullery 2004; Quammen 2016).

The Greater Yellowstone Area (GYA) is one of the last remaining large and nearly intact temperate ecosystems on Earth…. GYA, and especially the national parks, have long been a place for important scientific discoveries, an inspiration for creativity, and an important national and international stage for fundamental discussions about the interactions of humans and nature.

Yellowstone National Park, established in 1872 as the world’s first national park, is the heart of the GYA. Grand Teton National Park, created in 1929 and expanded to its present size in 1950, is located south of Yellowstone National Park[1] and is dominated by the rugged Teton Range rising from the valley of Jackson Hole. The Gallatin-Custer, Shoshone, Bridger-Teton, Caribou-Targhee, and Beaverhead-Deerlodge national forests encircle the two national parks and include the highest mountain ranges in the region. The National Elk Refuge, Red Rock Lakes National Wildlife Refuge, and Grays Lake National Wildlife Refuge also lie within GYA.


The People of the GYA

Andrew Marcus, University of Oregon

People have lived in the GYA as far back as 12,500 yr ago (Rasmussen et al. 2014) and actively used the resources of the region for millennia (MacDonald 2012). Today, 27 Tribes are formally recognized to have connections to the lands and resources of the region (NPSb undated), including, but not limited to, several Tribes of Shoshone, Bannock, Lemhi, Niitsitapi/Blackfeet, Nez Perce, Salish, Apsáalooke/Crow, Arapaho, Pend d’Oreille, Kootenai, Gros Ventre, Assiniboine, Sioux, Little Shell, Northern Cheyenne, and Chippewa Cree. The Tribal Nations of the Eastern Shoshone, Northern Arapaho, Apsáalooke/Crow, Northern Cheyenne, Shoshone, and Bannock have reservations in and near the Greater Yellowstone Area. The long-term presence of these Indigenous peoples is apparent across the cultural landscapes of the region, just as their stewardship of the lands is core to the conservation and preservation of natural resources in the region.

GYA is today the fastest-growing rural region in the western US. In 2020, the 20 counties of the GYA had a combined population of nearly 488,000, more than twice the number of residents in 1970 (USCB undated). The recent influx of people and businesses, drawn by the area’s high quality of life, is known as “amenity migration.” Bozeman is the largest city within the GYA boundary, and the fastest growing city of its size in the nation. Most of the region’s smaller cities and towns are also seeing rapid population growth (USCB 2018). At the current rate of growth, Hansen and Phillips (2018) estimated the GYA will have 846,000 residents and over 503,000 homes by 2050.

Visitor numbers to the region have increased enormously in recent years. Yellowstone National Park visitation increased by 85% from 1970 to 2015, with nearly 4 million people entering the park every year since 2015 (NPSb undated). Similar increases in visitation have occurred in Grand Teton National Park. Skier days have risen by 5% per yr in the three commercial ski areas of the region. Angler days on the Madison River have tripled from 1984-2016 (Hansen and Phillips 2018).

The region’s economy has undergone a massive transition over the past 50 yr (Marcus et al. forthcoming). In 1970, agriculture, mining, and oil and gas development made up nearly 30% of labor earnings; they now account for less than 8%. The service sector now provides more than 50% of the income in 11 of the 20 GYA counties; these jobs include work associated with tourism and recreation and high-wage jobs in architecture, engineering, software development, and legal and medical services. Non-labor income from investments and retirement is more than 50% of total income in five of the counties centered around Yellowstone National Park and, in total, is equal to labor income in the region. Jobs with federal, state, county, and local governments and public universities provide more than 20% of the total income in ten of the 20 counties. Across the whole region, the single largest employer is retail trade, followed by accommodation and food services, health care services, and construction. The counties that include the towns of Jackson WY, Cody WY, Livingston MT, and Gardiner MT are more dependent on travel and tourism than other counties in the region, reflecting the importance of Yellowstone National Park to the local economies.

Developed lands, which include agriculture, exurban, suburban/urban, and commercial/industrial areas as well as roads and buffers, comprise about one-third of the GYA (Hansen and Phillips 2018). Cattle and associated hay production dominate the agricultural landscape through most of the region, although production of wheat, barley, potatoes, and vegetables are the primary crops in the Snake River Plain of Idaho. Wyoming has significant earnings in the oil and gas industry, and large active mines still operate in all the GYA states.

The potential impacts of climate change in the GYA are inextricably linked to those caused by rapid population growth and dramatic economic change. Suburban and exurban sprawl, increased demand for water as water supplies diminish, changing wildlife habitats, and myriad other climate- and population-driven changes will challenge public and private efforts to maintain resilient ecosystems and communities in the coming decades.



Little Bighorn anniversary

Little Big Horn anniversary, Crow Agency, Montana. Photo courtesy of Crystal C-Bearing.


In recent decades, climate assessments have been conducted at many geographic and jurisdictional scales. Internationally, the Intergovernmental Panel on Climate Change (IPCC) completed climate assessments in 1990, 1996, 2001, 2007, 2014, and, most recently, in 2018 (IPCC 2018). In the United States, congressionally mandated national climate assessments were undertaken in 2000, 2009, 2014, and 2017 (USGCRP 2017). Some states, including Montana, have produced state-focused climate assessments, and communities have undertaken local ones. These assessments examine trends and projections of future climate change, usually through the 21st century.

Climate assessments at all scales draw on the best science available at the time of writing to evaluate the state of climate change and its observed and potential impacts. Given their generally nontechnical presentation of information, assessments have been fundamental in increasing awareness and understanding of climate change. The 2017 Montana Climate Assessment (Whitlock et al. 2017), for example, addresses potential climate change impacts on the state’s water, forests, and agriculture and has been used by diverse stakeholders across Montana for a wide range of planning efforts and decision-making.

The borders of the GYA cross three states, plus multiple agency jurisdictions and land ownerships. For this reason, the Greater Yellowstone Climate Assessment is a regional assessment. The decision to take a regional focus is motivated by a body of literature that indicates the impacts of climate change should be evaluated across the entire Yellowstone ecosystem (e.g., Romme and Turner 1991; Bartlein et al. 1997; Saunders et al. 2011; Al-Chokhachy et al. 2013; Monahan and Fisichelli 2014; Chang and Hansen 2015).

The Greater Yellowstone Climate Assessment (“the Assessment”) is planned as a multi-phase effort, one that will analyze and communicate climate change and its potential impacts across a variety of sectors. The overarching goals of the Assessment are to a) present understandable, science-based, and geographically specific information about the potential impacts of climate change on the people and resources of the region; and b) provide a foundation of knowledge that helps the region prepare for and respond to climate changes occurring within the 21st century.

This first volume of the Assessment focuses on climate and water: both are essential components of a healthy ecosystem, and that changes in either will impact ecosystem services (e.g., clean air and water, fish, wildlife, forests) that GYA communities and economies depend upon.

The specific goals of Greater Yellowstone Climate Assessment—Past, Present, and Future Climate Change in Greater Yellowstone’s Watersheds are to

  • provide information on past, present, and potential future climate change and the potential impacts on water resources across the GYA and within major GYA watersheds;

  • include the perspective of diverse stakeholders on climate change in the GYA, as summarized by a series of listening sessions in 2020 that highlight areas of concern; and

  • point out key knowledge gaps in the science and monitoring.

In the Assessment, we draw on the science expertise of partner universities, federal and state agencies, and non-governmental organizations, including Montana State University (Montana Institute on Ecosystems), University of Wyoming, Boise State University, US Geological Survey, Yellowstone and Grand Teton national parks, and Henry’s Fork Foundation. Support for the project comes from Montana State University, University of Wyoming, US Geological Survey, Greater Yellowstone Coordinating Committee, and Greater Yellowstone Coalition.

In addition to its technical contributions, the Assessment includes a summary report of an ongoing, concerted effort to understand the concerns of citizens and communities of the GYA with respect to current and projected climate change in the region. The effort to listen and engage the region’s constituency is being led by a team from the Greater Yellowstone Coalition, the Greater Yellowstone Coordinating Committee, National Park Service, the universities and extension services, and the Tribes in Wyoming, Idaho, and Montana.

The Geography of the Greater Yellowstone Area

The unique landscape of the GYA is characterized by mountain ranges and intermountain valleys that are the product of geologic uplift and faulting, volcanic activity, and glaciation (Figure 1-1). The mountain ranges include peaks that are nearly 14,000 ft (4300 m) in elevation (Table 1-1). The volcanic plateaus of Yellowstone National Park range from 8000-9000 ft (2400-2700 m) elevation and provide the setting for Yellowstone Lake, the largest lake above 7000 ft (2100 m) in North America. Jackson Hole and other river valleys in the region are bounded by active geologic faults where periodic earthquakes occur. The low-lying Snake River Plain of eastern Idaho is underlain by volcanic rocks and intersects with the southwest margin of GYA.


Three of the nation’s largest river systems—the Missouri-Mississippi, the Colorado, and the Columbia—have headwaters in the GYA (Figure 1-2). Two-thirds of water originating in the GYA reaches the Missouri River by one of two routes: from the Madison and Gallatin rivers, which combine with the Jefferson River to form the Missouri River, and from the Yellowstone River, which drains the central GYA and joins the Missouri River in western North Dakota. The Snake River flows through Jackson Hole and joins with the Columbia River in eastern Washington. The Green River originates at Green River Lakes in the Wind River Range and adds water from the Gros Ventre and Wyoming ranges before it joins the Colorado River in southern Utah.

The geology, soils, topography, and climate of the GYA support a diverse range of vegetation types (Despain 1990; Whitlock 1993). In general, sagebrush (Artemisia tridentata) steppe and grassland predominate dry landscapes below 5900 ft (1800 m) elevation; conifer forests grow in wetter and cooler locations from 5900-9500 ft (1800-2900 m) elevation, and alpine tundra predominates above 9500 ft (2900 m) elevation. The composition of conifer forests is largely determined by gradients of temperature and precipitation that vary with elevation. Rocky Mountain and Utah juniper (Juniperus scopulorum, J. osteospermum), ponderosa pine (Pinus ponderosa), and limber pine (Pinus flexilis) predominate in drier low-elevation forests. Mid-elevation forests support Douglas-fir (Pseudotsuga menziesii) and lodgepole pine (Pinus contorta), and the cooler and wetter upper range forests are composed of Engelmann spruce (Picea engelmannii), whitebark pine (Pinus albicaulis), and subalpine fir (Abies lasiocarpa). Based on the geologic record, the current distribution of plant species in the GYA will be short-lived. Just as species shifted their range in elevation and latitude in response to past climate changes, so will they shift in the future.

Based on the geologic record, the current distribution of plant species in the GYA will be short-lived. Just as species have shifted their range in elevation and latitude in response to past climate changes, so will they shift in the future.




Figure 1-2. Two views—continental (above) and regional (below)—of major river systems that have headwaters in the Greater Yellowstone Area (GYA). (Image credits: Atlas of Yellowstone [Marcus et al. 2012]).


The HUC6 Watersheds in the GYA

In the 1980s, the United States Geological Survey (USGS) developed a hierarchical classification—the Hydrologic Unit Code (HUC) system—that subdivides the country’s river basins and watersheds into regions, subregions, and smaller units (Seaber et al. 1987; NRCS 2007; USGS undated). The HUC system divides land based on the physical properties of rivers and tributaries and is thus independent of political boundaries and ownership. We use the HUC system for the Greater Yellowstone Climate Assessment because the impact of climate change on GYA rivers can be better studied for individual watersheds than inside artificially defined borders (e.g., state or national park boundaries).

In the Assessment, we focus on six river basins that meet the definition of HUC level 6 (HUC6), also considered a subregion in USGS parlance. The area and elevation data in the following HUC descriptions are based on the 4-km (2.5-mi) resolution map shown in Figure 1-3:

  • Missouri Headwaters (area: 6526 square miles [16,898 km2]; 21% of the GYA area) includes the Gallatin, Madison, Ruby and Upper Red Rock river watersheds. Elevation ranges from 4100-10,000 ft (1250-3050 m), with a mean elevation of 6900 ft (2100 m). The subregion supports the northern Centennial Range, the Ruby Range, the Madison Range, and the western side of the Gallatin Range. The city of Bozeman, and towns of Belgrade, Big Sky, and Ennis, Montana are in this HUC.

  • Upper Yellowstone (area: 7791 square miles [20,178 km2]; 23% of the GYA area) includes the Upper Yellowstone, which originates in Bridger-Teton National Forest, with the added tributaries of the Shields and Stillwater river watersheds. Elevation ranges from 4200-11,150 ft (1280-3400 m), with a mean elevation of 9850 ft (3000 m). The subregion includes the Absaroka Range, including the Beartooth Mountains, the Crazy Mountains, and the east side of the Gallatin Range and Bridger Range. The Montana towns of Livingston and Red Lodge are in this HUC.

  • Big Horn (area: 5395 square miles [13,973 km2]; 10% of the GYA area) includes the Big Horn, North Platte, Clarks Fork, Shoshone, and Upper Wind river watersheds. Elevation ranges from 5250-12,139 ft (1600-3700 m), with a mean elevation of 8700 ft (2650 m). The region includes the Absaroka Range, the Owl Creek Range, and the north slope of the Wind River Range. Cody, Wyoming, is in this HUC, and Lander is near the border.

  • Upper Green (area: 3486 square miles [9029 km2]; 17% of the GYA area) includes parts of the Upper Green, Upper Bear, Lower Bear, and the New Fork river watersheds. Elevation ranges from 6700-12,300 ft (2040-3750 m), with a mean elevation of 8400 ft (2560 m). The subregion extends from the south side of the Wind River Range to the Wyoming Range. Pinedale, Wyoming, is in this HUC.

  • Snake Headwaters (area: 5772 square miles [14,591 km2]; 14% of the GYA area) includes the Upper Snake River, Gros Ventre, Grays-Hoback, Salt, and Palisades river watersheds. Elevation ranges from 4840-9680 ft (1475-2950 m), with a mean of 6500 ft (1980 m). Jackson, Wyoming, is the largest community in this HUC. This region includes Grand Teton National Park, with the east side of the Teton Range, the Gros Ventre Range, and Wyoming Range.

  • Upper Snake (area: 4969 square miles [12,870 km2]; 16% of the GYA area) includes Henrys, Teton, and Upper Beaver-Camas river watersheds. Elevation ranges from 5250-10,732 ft (1600-3271 m), with a mean elevation of 7790 ft (2374 m). This is the lowest elevation HUC6 and includes the eastern end of the Snake River Plain. It is bound by the west side of the Teton Range and the south side of the Centennial Range. Driggs, Idaho, is in this HUC.

Most of our HUC6 watersheds include part of a main stem river (e.g., a segment of the Yellowstone River or Snake River) that is fed by smaller tributaries (designated as HUC8). In the case of the Snake Headwater and Upper Snake subregions, there is no single main stem river, but rather a set of intermediate-sized smaller rivers.



Figure 1-3. Topography of the Greater Yellowstone Area (GYA, dark outline). The names of the six Hydrologic Unit Code 6 (HUC6) watersheds addressed in this report are labeled. The topography is represented by 4-km (2.5-mile) grid cells, which is also the resolution of the climate and hydrology data in the report. Note that small areas of several other HUC6 watersheds are within the boundary of the GYA. For example, the Upper Missouri north of Belgrade MT and the Lower Bear south of the Snake Headwaters, and the North Platte south of Lander WY. For this study, we combined the smaller HUCs with the appropriate neighboring larger HUCs.


Structure of the Assessment

The Greater Yellowstone Climate Assessment—Past, Present, and Future Climate Change in Greater Yellowstone’s Watersheds is divided into nine chapters. Following this Introduction, in Chapter 2 we present basic concepts of climate and hydrologic change, summarize past climate changes in the GYA over the last 20,000 yr based on the geologic record, and explain the natural and anthropogenic drivers of climate change. In Chapter 3, we examine observed 20th- and early 21st-century changes and trends in climate and water in the GYA based on weather and streamgaging station measurements. In Chapter 4, we provide an overview of the scientific methods used to develop projections of future changes in climate and water. In Chapters 5, 6, and 7, we present 21st-century projections of air temperature, precipitation, and water, respectively, with focuses on climate variables that are relevant to ecosystems, agriculture, winter recreation, and energy use. In Chapter 8, we offer some of the results of interviews with residents in the Greater Yellowstone Area, including their concerns for the future. In Chapter 9, we identify knowledge gaps and outline the next steps in the assessment process. The report also contains a glossary and several appendices that provide additional details for some chapters and include technical information about the data and methods used in the Assessment.

We begin Chapters 2, 3, 5, 6, 7, and 8 with key messages of the chapter’s information. These messages are accompanied by a statement of confidence by the chapter authors. Confidence levels are based on the authors’ judgment following the approach used by the Intergovernmental Panel on Climate Change’s (IPCC’s) Fifth Assessment Report (IPCC 2014). The greater the evidence, agreement, and statistical significance, the higher the level of author confidence in the certainty of the key message (Table 1-2).


The authors of Chapters 2 rate their confidence in the observed data, with evidence of change as limited, medium, or robust, depending on the type, amount, and quality of the scientific information supporting the finding. These authors rate agreement as the consistency of the evidence (low, medium, or high) among scientific publications. The authors of Chapter 3 combine their confidence statement into a single net confidence rating.

In Chapters 5-7, the authors rate the confidence of projected climate and hydrologic changes from climate and water balance models. Consistent with the MCA (Whitlock et al. 2017), the authors report the number of models out of 20 that agree on the sign (positive or negative) of the median value of the future change. For example, if the median value is positive and 18 out of 20 models project positive change, then the percent agreement is 100 ×18/20 = 90%. In addition, the authors follow the IPCC (Meehl et al. 2007) and report the signal‑to‑noise ratios (SNRs). The SNR is the ratio of the mean change in a climate variable (signal) to the standard deviation of the 20 models comprising the mean (noise). SNRs greater than one (SNR >1) are used to establish when a projected climate change emerges over the 21st century (Hawkins and Sutton 2012) and provide additional support for confidence in the change. The categories for assigning model confidence are also based on guidance from the IPCC AR5 (Fifth IPCC Assessment Report) (Mastrandrea et al. 2010):

  • high confidence—greater than 80% model agreement (more than 16 of the 20 models) with added confidence from SNR greater than 1;

  • medium confidence—60 to 80% model agreement with or without SNR greater than 1;

  • low confidence—less than 60% model agreement SNR less than 1.

These assignments of confidence on model-based results are specific to the projections in this Assessment.



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[1] Yellowstone and Grand Teton national parks are connected by another unit of the national park system, the 23,000 acre (9300 ha) John D. Rockefeller, Jr. Memorial Parkway.



Elk in the Beartooth Mountains. Photo credit Bryan Shuman.