This Assessment of climate and water in the Greater Yellowstone Area (GYA) shows that climate trends and variability that have been part of the GYA’s past will continue to be part of its future. Past climate trends are evident from a variety of geological and paleontological data sets in the region as described in Chapter 2. During the last glaciation (22,000-13,000 yr ago), the GYA was 5-7°F (3-4°C) colder than the pre-industrial period (1850-1900). The glacial period was terminated by a warming trend that led to rapid glacial recession and forest colonization. By the early Holocene (11,500-7000 yr ago), the climate was up to 3.8°F (2°C) warmer than the pre-industrial period. Climate variability in the GYA has also occurred in the past. For example, there have been dramatic fluctuations between wet and dry periods in the last 1000 yr. The last 20 yr (2001-2020) stands out as the warmest period of at least the last 20,000 yr in the GYA, and probably longer. Atmospheric greenhouse gases (GHGs) have not been at the current level for the last 3.3 million years.
Past climate changes were caused by natural climate drivers (e.g., Milankovitch cycles, changes in atmospheric composition, volcanic activity, solar output, and atmosphere-ocean circulation). In addition to the consequences of natural drivers, the climate of recent decades has been warming as a result of human-caused emissions and attendant increases in GHGs. Based on weather station data, the GYA has warmed on average by 2.3°F (1.3°C) since 1950 (see Chapter 3). This warming has resulted in a growing season that now is 2 weeks longer than it was in the 1950s, and below 8000 ft annual snowfall has declined by 25% (nearly 24 inches), including by 96% in September. The rapid warming that marks the end of winter now occurs in February to March, instead of March to April as it did in 1950. Melting of the snowpack is also occurring earlier in the year, and peak annual stream runoff now occurs on average 8 days earlier than it did in 1950.
Based on weather station data, the GYA has warmed on average by 2.3°F (1.3°C) since 1950 (see Chapter 3). This warming has resulted in a growing season that now is 2 weeks longer than it was in the 1950s, and below 8000 ft annual snowfall has declined by 25% (nearly 24 inches), including by 96% in September. The rapid warming that marks the end of winter now occurs in February to March, instead of March to April as it did in 1950. Melting of the snowpack is also occurring earlier in the year, and peak annual stream runoff now occurs on average 8 days earlier than it did in 1950.
The magnitude and rate of projected future warming are determined by the amount of GHG emissions into the atmosphere. We based the Assessment on two of the internationally used scenarios of future GHG emissions, Representative Concentration Pathways 4.5 and 8.5 (RCP4.5 and RCP8.5). RCP4.5 is an intermediate scenario in which the rate of emissions is curtailed and stabilizes by 2080; RCP8.5 is an upper bound scenario in which emissions continue to increase through the end of century (see Chapter 4). These two pathways differ in their related projections of GYA’s climate future. By the end of century, temperatures in the GYA could range from 5-6°F (2.8-3.3°C) warmer than our 1986-2005 base period under RCP4.5, to as much as 10-11°F (5.6-6.1°C) under RCP8.5 (Figure 9-1; see Chapter 5). Over the next 20 yr (2021-2040), the projected warming of 2.5-2.9°F (1.4-1.6°C) under RCP4.5 and RCP8.5, respectively, is about the same as occurred between 1950 and 2005. After 2040, the projected rate of warming until the end of century will be about twice that of the 1950-2005 period under RCP4.5 and nearly five times greater under RCP8.5. In both cases, temperature increases will bring warmer days and nights, warmer winters, and hotter summers in the coming decades. These warmer conditions will affect water supplies, natural and managed ecosystems, economies, and human and community well-being in the GYA.
[T]emperature increases will bring warmer days and nights, warmer winters, and hotter summers in the coming decades. These warmer conditions will affect water supplies, natural and managed ecosystems, economies, and human and community well-being in the GYA.
Annual temperature and precipitation in the GYA have varied over the last 120 yr with a substantial range of year-to-year variability and extended periods that were drier or wetter than average and colder or warmer than average (see Chapter 2). Climate models project rising temperatures through the 21st century (see Chapter 5) (Figure 9-1) accompanied by slight increases in precipitation (see Chapter 6) (Figure 9-2). As a result, more winter precipitation will fall as rain instead of snow and the amount of water stored annually in snowpack will decline (Figure 9-3). Snowmelt and runoff will occur earlier in spring, and higher evapotranspiration and reduced runoff will amplify water shortages in summer (see Chapter 7).
[Through the 21st century] more winter precipitation will fall as rain instead of snow and the amount of water stored annually in snowpack will decline (Figure 9-3). Snowmelt and runoff will occur earlier in spring, and higher evapotranspiration and reduced runoff will create water shortages in summer (see Chapter 7).
Based on our current understanding of the impacts of past climate change, the consequences of future climate change in the GYA will likely include:
large-scale ecological changes;
changes in seasonal water availability for communities, agriculture, and recreation;
warmer water temperatures combined with lower streamflow; and
more large wildfires than have occurred historically.
We note that historical and projected changes in GYA temperature are less dramatic than changes in other parts of the United States, a result of the GYA’s relatively high elevation. For example, the modest (2.3°F [1.3°C]) warming since 1950 in the GYA is close to the US average (2.2°F [1.2°C]) (NOAA undated), and the number of days of extreme heat (>90°F [32°C]) projected for the future is far less than other parts of the country and limited to lower elevations in the GYA. In addition, the average amount of snowpack has declined since 1950 in the GYA but less so than in other mountain regions. Snowpack in the future will continue to decline in the coming decades with warming temperatures, but the losses will be less than in the southern and central Rocky Mountains (USGCRP 2017).
As illustrated by the differences in the projected warming under the RCP4.5 and RCP8.5 scenarios (Figure 9-1), the trajectory of future climate change in the GYA can be altered by reducing human emissions of GHGs at a global scale. A shared goal among Nations is to reach net carbon neutrality (i.e., the release of carbon to the atmosphere is equal to or less than the amount of carbon removed from the atmosphere) by mid century (IPCC 2018), which would achieve a level of warming in the GYA more or less consistent with that of RCP4.5. If GHGs continue unabated at the current rate, however, the resulting warming would be more similar to RCP8.5.
The magnitude of changes in either the RCP4.5 or RCP8.5 scenario will require people in the GYA—whether living in urban or rural locations—to adapt to climate change. Interviews with stakeholders in the region (see Chapter 8) reveal that they are concerned about reliable water supplies and the protection of native species. Communities, especially those far from services, would benefit by planning for the social and economic impacts of potentially more floods in spring, longer periods of reduced water availability in summer, and more wildfires in the future. Conservation specialists should consider the ecological consequences of climate change that will impact native species distributions, abundance, and behavior.
Communities, especially those far from services, would benefit by planning for the social and economic impacts of potentially more floods in spring, longer periods of reduced water availability in summer, and more wildfires in the future. Conservation specialists should consider the ecological consequences of climate change that will impact native species distributions, abundance, and behavior.
While it is known with high certainty that humans are largely responsible for global warming over the past 150 yr (IPCC 2013; USGCRP 2017), our understanding of climate change and the underlying science continues to evolve and improve (see Chapter 4). By synthesizing the best-available science, climate assessments, like this one, provide a shared knowledge base for evaluating the scope of change and identifying solutions at a regional scale. For this reason, it is important that climate assessments are updated regularly to include new scientific information and to convey that information in ways that are useful for the public, planners, and resource managers. This Assessment, which provides an overview of the potential impacts of climate change in GYA watersheds, is intended as a starting point for future assessments on related topics, including impacts on water, fish and wildlife, local economies and communities, and human health in the GYA.
We conclude this report by identifying some of the important gaps in our scientific understanding of climate change in the GYA. We also highlight some climate adaptation needs for resource managers and communities in the region. These lists are not exhaustive and are intended only to highlight issues we believe deserve attention in future assessments and planning efforts.
Science and Monitoring Needs
Provide regular updates of the Greater Yellowstone Climate Assessment, incorporating the latest climate projections consistent with those developed at the national and international level.
Develop and apply more detailed models of snow processes, groundwater, surface water, and ecosystem and human water demand to refine our understanding of water and water use in the GYA. Modeling potentially complex local changes in water supply, demand, and their interactions will require improved representations of the underlying processes in each watershed.
Maintain and expand monitoring of snow, streams, lakes, and wetlands within GYA watersheds. Currently, weather stations and streamgages are unevenly distributed in the GYA, few water bodies and wetlands are monitored, particularly at high elevations, and water demand for ecosystems and for human use and consumption is poorly measured.
Quantify the connections between climate change, the carbon cycle, urbanization, agricultural practices, and biodiversity in the GYA. This information will help identify opportunities to maintain valued ecosystem qualities and services, sustain essential economic and cultural uses, and increase carbon storage on natural and managed lands.
Continue to expand monitoring efforts of fish and wildlife to improve our understanding of their changing behavior, disease, and distribution in response to climate change.
Continue to improve our understanding of the linkages between long-term trends in fire climate and short-term fire weather and fuel conditions so that we can better project fire activity.
Support studies of forest health, including the impact of climate change on insect outbreaks, wildfire activity, drought-caused mortality, and carbon storage to guide appropriate management planning.
Quantify how climate change in the GYA will affect vital ecosystem services, including air quality, water quality and quantity, food, timber, and biodiversity.
Climate Adaptation and Related Needs
Expand efforts to engage regional stakeholders on the topic of climate change through listening sessions and other exchanges that help find common ground for effective watershed and community planning. Establish effective ways to share information from new scientific studies and from monitoring and evaluation efforts so that it is available to all stakeholders in a timely way.
Work with communities and water management districts to identify the local consequences of climate change, as a step toward developing implementing adaptation plans. On Tribal lands, sustaining traditional subsistence, ceremonial, and medicinal resources is also important. Identify cross-jurisdictional challenges early in the process, so that planning efforts are effective and efficient.
Develop a list of at-risk habitats and specific indicators of ecological and human health to be studied and monitored to help resource managers maintain a robust baseline for measuring change and assessing the effectiveness of adaptation measures.
Evaluate the effects of projected climate change on the economies of the GYA: tourism and recreation, hunting and fishing, agriculture and forestry, and mineral and energy resource extraction, as part of a sustained Assessment effort.
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