KEY MESSAGES Annual average temperatures, including daily minimums, maximums, and averages, have risen across the state between 1950 and 2015. The increases range between 2.0-3.0°F (1.1-1.7°C) during this period. [high agreement, robust evidence]   Winter and spring in Montana have experienced the most warming. Average temperatures during these seasons have risen by 3.9°F (2.2°C) between 1950 and 2015. [high agreement, robust evidence]   Montana’s growing season length is increasing due to the earlier onset of spring and more extended summers; we are also experiencing more warm days and fewer cool nights. From 1951-2010, the growing season increased by 12 days. In addition, the annual number of warm days has increased by 2.0% and the annual number of cool nights has decreased by 4.6% over this period. [high agreement, robust evidence]   Despite no historical changes in average annual precipitation between 1950 and 2015, there have been changes in average seasonal precipitation over the same period. Average winter precipitation has decreased by 0.9 inches (2.3 cm), which can mostly be attributed to natural variability and an increase in El Niño events, especially in the western and central parts of the state. A significant increase in spring precipitation (1.3-2.0 inches [3.3-5.1 cm]) has also occurred during this period for the eastern portion of the state. [moderate agreement, robust evidence]   The state of Montana is projected to continue to warm in all geographic locations, seasons, and under all emission scenarios throughout the 21st century. By mid century, Montana temperatures are projected to increase by approximately 4.5-6.0°F (2.5-3.3°C) depending on the emission scenario. By the end-of-century, Montana temperatures are projected to increase 5.6-9.8°F (3.1-5.4°C) depending on the emission scenario. These state-level changes are larger than the average changes projected globally and nationally. [high agreement, robust evidence]   The number of days in a year when daily temperature exceeds 90°F (32°C) and the number of frost-free days are expected to increase across the state and in both emission scenarios studied. Increases in the number of days above 90°F (32°C) are expected to be greatest in the eastern part of the state. Increases in the number of frost-free days are expected to be greatest in the western part of the state. [high agreement, robust evidence]   Across the state, precipitation is projected to increase in winter, spring, and fall; precipitation is projected to decrease in summer. The largest increases are expected to occur during spring in the southern part of the state. The largest decreases are expected to occur during summer in the central and southern parts of the state. [moderate agreement, moderate evidence]

 

Climate Change Defined

The US Global Change Research Program (USGCRP undated) defines climate change as follows: “Changes in average weather conditions that persist over multiple decades or longer. Climate change encompasses both increases and decreases in temperature, as well as shifts in precipitation, changing risk of certain types of severe weather events, and changes to other features of the climate system.”

 

This chapter focuses on three areas: 

1. providing a baseline summary of climate and climate change for Montana—with a focus on changes in temperature, precipitation, and extreme events—including reviewing the fundamentals of climate change science;
2. reviewing historical trends in Montana’s climate, and what those trends reveal about how our climate has changed in the past century, changes that are potentially attributable to world-wide increases in greenhouse gases; and
3. considering what today’s best available climate models project regarding Montana’s future, and how certain we can be in those projections.

This chapter serves as a foundation for the Montana Climate Assessment, providing information on present-day climate and climate terminology, past climate trends, and future climate projections. This foundation then serves as the basis for analyzing three key sectors of Montana—water, forests, and agriculture—considered in the other chapters of this assessment. In the sections below, we introduce the climate science and discuss important fundamental processes that determine whether climate remains constant or changes.

 

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CLIMATE CHANGE ASSESSMENTS

 

A growing awareness of our changing global climate since the 1950s has led to a substantial body of research. For example, the National Academy of Sciences (NAS 2011) report, American’s Climate Choices, stated:

 

Climate change is occurring, is very likely caused primarily by human activities, and poses significant risks to humans and the environment. These risks indicate a pressing need for substantial action to limit the magnitude of climate change and to prepare for adapting to its impacts.

 

In 1990, the United Nations tasked the Intergovernmental Panel on Climate Change (IPCC, see sidebar) with assessing existing research on climate change. Since then, five IPCC assessments have increased our scientific understanding of, and certainty about, global climate change. As described later in this chapter, the assessments have incorporated increasingly sophisticated models and analyses that consider both natural and human contributions to changes in our climate system.

In its most recent Fifth Assessment Report, the IPCC raised the likelihood of changes in several global climate events to “virtually certain” (i.e., 99-100% likelihood). Examples of these events include: more frequent hot days, less frequent cold days, reductions in permafrost, and sea-level rise (IPCC 2014).

 

	What is the IPCC? The Intergovernmental Panel on Climate Change is the leading international body for the assessment of climate change. It was established in 1988 by the United Nations Environment Programme and the World Meteorological Organization, and subsequently endorsed by the United Nations General Assembly. The goal of the IPCC is to provide the world with a clear scientific view on the current state of knowledge in climate change and its potential environmental and socioeconomic impacts.

 

Recently, the third National Climate Assessment, produced in collaboration with the US Global Change Research Program, provided further insight into the anticipated climate changes for the conterminous US. The National Climate Assessment (NCA 2014) states:

 

Evidence for changes in Earth’s climate can be found from the top of the atmosphere to the depths of the oceans. Researchers from around the world have compiled this evidence using satellites, weather balloons, thermometers at surface stations, and many other types of observing systems that monitor the Earth’s weather and climate. The sum total of this evidence tells an unambiguous story: the planet is warming. 

 

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FUTURE PROJECTIONS

 

Global Climate Modeling

Projecting future climate on a global scale requires modeling many intricate relationships between the land, ocean, and atmosphere. Many global climate and Earth system models exist, each varying in complexity, capabilities, and limitations.

Consider one of the simplest forms of a model used for future projections, a linear regression model (Figure 2-8). With this model, researchers would plot a climate variable (e.g., temperature) over time, draw a best-fit, straight line through the data, and then extend the line into the future. That line, then, provides a means of projecting future conditions. Whether or not those projections are valid is a separate question. For example, the model may be based on false assumptions: the relationship may a) not be constant through time, b) not include outside influences such as human interventions (e.g., policy regulations), and c) not consider system feedbacks that might enhance or dampen the relationship being modeled.

 

While the linear regression model provides an instructive visual aid for considering modeling, it is too simple for looking at climate changes, in which the interactions are complex and often nonlinear. For example, if temperatures rises, evaporation is expected to increase. At the same time, increasing temperatures increase the atmosphere’s capacity to hold water. Water is a greenhouse gas so more water in the atmosphere means the atmosphere can absorb more heat…thus driving more evaporation. What seemed a simple relationship has changed (possibly dramatically) because of this feedback between temperature, evaporation, and the water-holding capacity of the atmosphere. 

Linear models do not account for such nonlinear relationships. Instead, climate scientists account for nonlinearity through computer simulations that describe the physical and chemical interactions between the land, oceans, and atmosphere. These simulations, which project climate change into the future, are called general circulation models (GCMs; see sidebar)

	General Circulation Models General circulation models (GCMs) help us project future climate conditions. They are the most advanced tools currently available for simulating the response of the global climate system—including processes in the atmosphere, ocean, cryosphere, and land surface—to increasing greenhouse gas concentrations.  GCMs depict the climate using a 3-D grid over the globe, typically having a horizontal resolution of between 250 and 600 km (160 and 370 miles), 10-20 vertical layers in the atmosphere and sometimes as many as 30 layers in the oceans. Their resolution is quite coarse. Thus, impacts at the scale of a region, for example for Montana, require downscaling the results from the global model to a finer spatial grid (discussed later) (text adapted from IPCC 2013b).

 

Because of the complexities involved, climate scientists rarely rely on a single model, but instead use an ensemble (or suite) of models. Each model in an ensemble represents a single description of future climate based on specific initial conditions and assumptions. The use of multiple models helps scientists explore the variability of future projections (i.e., how certain are we about the projection) and incorporate the strengths, as well as uncertainties, of multiple approaches.

For the work of the Montana Climate Assessment, we employed an ensemble from the fifth iteration of the Coupled Model Intercomparison Project (CMIP5), which includes up to 42 GCMs depending on the experiment conducted (CMIP5 undated). The World Climate Research Program describes CMIP as “a standard experimental protocol for studying the output” of GCMs (CMIP undated). It provides a means of validating, comparing, documenting, and accessing diverse climate model results. The CMIP project dates back to 1995, with the fifth iteration (CMIP5) starting in 2008 and providing climate data for the latest IPCC Fifth Assessment Report (Stocker et al. 2013).

We employed 20 individual GCMs from the CMIP5 project for the Montana Climate Assessment ensemble, chosen because they provide daily outputs and a range of important climate variables.⁹ For this first Montana Climate Assessment, we are only using climate variables of temperature and precipitation (later assessment may evaluate other important variables such as wind and relative humidity).

The benefits of using CMIP5 data are that each model in the ensemble a) has been rigorously evaluated, and b) uses the same standard socioeconomic trajectories—known as Representative Concentration Pathways (RCPs)—to describe future greenhouse gas emissions. RCPs are future greenhouse gas concentration scenarios.

Four RCP scenarios are available in CMIP5: RCP2.6, RCP4.5, RCP6.0, and RCP8.5. The number after RCP represents the increase in radiative forcing in watts/m² by the year 2100. Higher radiative forcing values are associated with larger amounts of trapped heat in the atmosphere due to increased greenhouse gas emissions (see sidebar). Simply stated, higher RCP values are typically associated with greater greenhouse gas emissions and therefore greater potential for climate change. Each RCP scenario makes different assumptions about future energy sources, population growth, economic activities, and technological advancements, as follows:

• RCP2.6.—The peak-and-decline scenario assumes greenhouse gas emissions peak between 2010-2020 and then decline by the end-of-century, leading to a radiative forcing of 2.6 watts/m². It assumes greenhouse gas emissions are substantially reduced over time (Van Vuuren et al. 2011). 
• RCP4.5.—The stabilization scenario where technological advancements and strategies lead to a peak in greenhouse gas emissions at about 2040 followed by a decline (Clarke et al. 2007). We explore the RCP4.5 scenario in this assessment, and the United Nations Paris Agreement of 2016 curbs emissions at a level between RCP2.6 and RCP4.5.
• RCP6.0.—A second stabilization scenario, but in this pathway greenhouse gas emissions peak at 2080 and stabilization is not achieved until after 2100 (Fujino et al. 2006). 
• RCP8.5.—The business-as-usual emission scenario where greenhouse gas emissions increase throughout the 21^st century (Riahi et al. 2007, 2009), based on the assumption that society is largely unsuccessful in curbing those emissions. We use the RCP8.5 scenario, in which greenhouse gases steadily rise, and note that this pathway best matches current trends. 

 

	Representative Concentration Pathways Representative Concentration Pathways (RCPs) make different assumptions about energy sources, population growth, economic activities, and technological advancements. Scientists run general circulations models against these scenarios to project future climate conditions, including atmospheric carbon concentrations. For this Montana Climate Assessment, we considered the stabilization (RCP4.5) and business-as-usual (RCP8.5) emission pathways.  This graph illustrates the different atmospheric CO2 concentrations associated with each Representative Concentration Pathways. For example, if we continue our carbon emissions at the current rate (i.e., the business-as-usual [RCP8.5] emission scenario), the atmospheric CO2 concentration will be roughly 700 ppm by 2075 (IPCC 2014).

 

For the Montana Climate Assessment, we explore the RCP4.5 and RCP8.5 scenarios only. We do not include RCP6.0 or RCP2.6 in our assessment for several reasons. RCP6.0 overlaps with RCP4.5 in the first half of the century and provides intermediate values between RCP4.5 and RCP8.5 at the end of the century. Additionally, RCP2.6 is becoming less and less realistic as society continues with business as usual regarding greenhouse gas emissions. For the remainder of the chapter, we will regularly refer to RCP4.5 and RCP8.5 as the stabilization and business-as-usual emission scenarios, respectively. 

Due to their complexity and global extent, GCMs can be computationally intensive. Thus, scientists often make climate projections at coarse spatial resolution where each projected data point is an average value of a grid cell that measures hundreds of miles (kilometers) across. 

For areas where the terrain and land cover are relatively homogenous (e.g., an expanse of the Great Plains), such coarse grid cells may be adequate to capture important climate processes. But in areas with complex landscapes like Montana, data points so widely spaced are inadequate to reflect variability in terrain and vegetation and their influence on climate. A 100 mile (161 km) grid, for example, might not capture the climate effects of a small mountain range rising out of the eastern Montana plains or the climate differences between mountain summits and valleys in western Montana where temperature and precipitation vary greatly.

To capture such important terrain characteristics, scientist take the coarse-resolution output from a GCM and statistically attribute the results from those models to smaller regions at higher resolution (e.g., grid points at 1 mile rather than 100 mile apart). This process, called downscaling, more accurately represents climate across smaller, more complex landscapes, including Montana.

For this climate assessment, we used a statistical downscaling method called the Multivariate Adaptive Constructive Analogs.¹⁰ By using a downscaled dataset—rather than the original output from the ensemble of GCMs—we gained the ability to evaluate temperature and precipitation at relatively high resolution statewide before conveying the results at the climate division scale. Additionally, we were able to aggregate data points within each of Montana’s seven climate divisions (Figure 2-3), and look at Montana’s climate future in different geographic areas. Aggregating to the climate-division level minimizes the potential for false precision by reporting results at spatial scales that better represent underlying climate processes. 

The 20-downscaled GCMs in CMIP5 were evaluated at two future time periods: 1) mid century (2040–2069) and 2) end-of-century (2070–2099). Thirty-year averages of these future projections were then compared to a historical (1971–2000) 30-year average, which results in a projected difference, or change, from historical conditions. We make those projections using the stabilization (RCP4.5) and business-as-usual (RCP8.5) emission scenarios described previously (see sidebar). These future projections were then compared to the historical trends in Montana to reveal the major climate-associated changes that Montana is likely to experience in the future.

 

	Modeling Montana’s Climate Future To derive the climate projections for this assessment, we employed 20 general circulation models to consider two scenarios of global carbon emissions: one where atmospheric greenhouse gases are stabilized by the end of the century and the other where it grows on its current path (the stabilization [RCP4.5] and business-as-usual [RCP8.5] emission scenarios, respectively).    Model output summary from the 20 GCMs that compares projected changes in temperature and precipitation for the state of Montana for stabilization (RCP4.5 – blue symbols) and business-as-usual (RCP8.5 – red symbols) emission scenarios between (A) mid century (2040-2069), and (B) end-of-century (2070-2099).   As shown in the figures above, we forecast Montana’s future climate for two periods: mid century and end-of-century. In brief: All models and scenarios show increasing annual temperatures, while most models also show increasing annual precipitation. The business-as-usual emission scenario consistently projects warmer temperatures and generally wetter conditions than the stabilization emission scenario. The end-of-century period also projects warmer temperatures but similar precipitation change to the mid-century projections. This finding suggests that temperatures will continue to warm throughout the century, but precipitation changes may level off in the latter half of the century.

 

Temperature projections

Below we provide projections for various aspects of Montana’s future temperature based on our modeling analysis. These projections are for the stabilization (RCP4.5) and business-as-usual (RCP8.5) emission scenarios and for two periods: mid century (2040-2069) and end-of-century (2070-2099).

We discuss a subset of our modeling results here, including a) temperature projections reported by the median values of the 20 GCM ensemble and b) figures that include maps and graphs that represent the median value and distribution of values observed for temperature across the 20 GCMs. 

An ensemble minimum, maximum, and percent agreement are also provided parenthetically. The percent agreement represents the number of GCMs that project the same sign of change (i.e., positive or negative) as the median value. For example, if the median value is positive and 18 out of 20 models also project positive change, then the percent agreement would be 100 x 18/20 = 90%. This simple calculation helps convey the uncertainty in the projections. 

 

Average annual temperatures 

Average annual temperatures increase in the mid-century and end-of-century projections for both stabilization and business-as-usual emission scenarios (Figures 2-9, 2-10). In the mid-century projection, most of the state has increases of about 4.5°F (2.5°C) for the stabilization emission scenario and 6.0°F (3.3°C) for the business-as-usual emission scenario. For end-of-century, statewide temperature increases by about 5.6°F (3.1°C) for the stabilization emission scenario and 9.8°F (5.4 °C) for the business-as-usual emission scenario. Although small differences exist between climate divisions, the general magnitude of these changes is consistent across the state for both emission scenarios and both time periods.

• Mid-century projection specifics.—Average annual temperatures increase by mid century in both emission scenarios (Figures 2-9 and 2-10). In the stabilization emission scenario, most of the state is projected to have increases of about 4.5°F (2.5°C) (minimum: 2.7°F [1.5°C], maximum: 6.1°F [3.4°C], percent agreement: 100%). The business-as-usual emission scenario projects larger increases in temperature of about 6.0°F (3.3°C) (minimum: 4.0°F [2.2°C], maximum: 8.2°F [4.6°C], model agreement: 100%). While small discrepancies exist between climate divisions, in general the magnitude of these changes is consistent across the state in both emission scenarios.
• End-of-century projection specifics.—Average annual temperatures increase by about 5.6°F (3.1°C) (minimum: 3.6°F [2.0°C], maximum: 7.7°F [4.3°C], percent agreement: 100%) in the stabilization emission scenario and by about 9.8°F (5.4°C) (minimum: 6.6°F [3.7°C], maximum: 12.9°F [7.2°C], percent agreement: 100%) in the business-as-usual emission scenario (Figure 2-9).

 

Figure 2-9. Graphs showing the minimum, maximum, and median temperature increases (°F) projected for each climate division in both stabilization (RCP4.5) and business-as-usual (RCP8.5) emission scenarios. The top row shows mid-century (2040-2069) projections and the bottom row shows end-of-century (2070-2099) projections. The outline of each box is determined by model agreement on the sign of the change (positive or negative). A black outline means there is >=80% model agreement and a red outline means that there is <80% model agreement. In this case, all models indicated the direction of the temperature trend at an agreement of greater than 80%.

 

Average daily minimum and maximum temperatures

Average daily minimum and maximum temperatures increase in the mid-century and end-of-century projections for both stabilization and business-as-usual emission scenarios (Figure 2-10 shows output for annual average daily maximum temperature). The degree of change is similar to that found for the average annual temperatures. In end-of-century projections, summers have the largest increases in average temperature: 6.5°F (3.6°C) for the stabilization emission scenario, 11.8°F (6.6°C) for the business-as-usual emission scenario.

• Mid-century projection specifics.—Average daily minimum and maximum temperatures change in a manner similar to the average annual projected increases (again for both RCP scenarios).
• End-of-century projection specifics.—Average daily minimum and maximum temperatures increase by similar magnitudes to average annual daily temperatures for both emission scenarios. Summer months have the largest projected increase in average temperature. In the stabilization emission scenario, summer temperatures increase by 6.5°F (3.6°C) (minimum: 3.2°F [1.8°C], maximum: 9.1°F [5.1°C], percent agreement: 100%) and in the business-as-usual emission scenario, summer temperatures increase by about 11.8°F (6.6°C) (minimum: 8.0°F [4.4°C], maximum: 15.2°F [8.4°C], percent agreement: 100%).

 

 

Mid-century and End-of-century

Figure 2-10. The projected increase in annual average daily maximum temperature (°F) for each climate division in Montana for the periods 2049-2069 and 2070-2099 for (A) stabilization (RCP4.5) and (B) business-as-usual (RCP8.5) emission scenarios.

 

Average monthly temperatures

Average monthly temperatures are projected to increase across all climate divisions by mid century (2040-2069) and for both stabilization and business-as-usual emission scenarios (Figure 2-11). Average monthly temperatures in summer and winter generally show larger projected increases than those in spring and fall. In the business-as-usual emission scenario, August has the largest projected change across all climate divisions. 

 

Figure 2-11. The projected monthly increase in average temperature (°F) for each climate division in Montana in the mid-century projections (2040-2069) for the (A) stabilization (RCP4.5) and (B) business-as-usual (RCP8.5) emission scenarios.

 

The number of days above 90°F (32°C)

The number of annual days where maximum temperatures are above 90°F (32°C) increases across all climate divisions in both mid-century and end-of-century projections and for both stabilization and business-as-usual emission scenarios (Figures 2-12, 2-13). Large differences in the magnitude of change exist, however, among the climate divisions. For example, in mid-century projections using the business-as-usual emission scenario, the northwestern part of the state shows increases of about 11 days with temperatures above 90°F (32°C), while the eastern parts of the state have increases of about 33 days. Similarly, in end-of-century projections based on the business-as-usual emission scenario, the northwestern part of the state shows an increase of about 34 days, while the eastern parts of the state have an increase of about 54 days above 90°F (32°C).

• Mid-century projection specifics.—The number of annual days at mid century where maximum temperatures are above 90°F (32°C) increases across all climate divisions and both emission scenarios (Figure 2-12, 2-13). Large differences in the magnitude of change exist, however, among the climate divisions. These differences are likely due, in part, to variability in moisture availability among the climate divisions and the energy it takes to evaporate this moisture (i.e., latent heat). In the stabilization emission scenario, the northwestern and north central climate divisions have increases of about 5.0 days (minimum: 1.5 days, maximum: 12.0 days, percent agreement: 100%); while the number of days in both eastern and south central climate divisions of the state increase by about 25.0 days (minimum: 6.0 days, maximum: 36.0 days, percent agreement: 100%). Similar spatial patterns exist for the business-as-usual emission scenario, but the magnitudes of change increase along with the ranges of the ensemble minimums and maximums. In the northwestern and north central climate divisions of the state, increases of about 11 days are projected (minimum: 1.5 days, maximum: 25.0 days, percent agreement: 100%); in the south central and both eastern climate divisions increases are projected to be about 33.0 days (minimum: 11 days, maximum: 44.0 days, percent agreement: 100%).
• End-of-century projection specifics.—The number of days where maximum temperatures exceed 90°F (32°C) by the end-of-century continues to increase across the state in both emission scenarios, with 100% model agreement. The spatial pattern in the end-of-century projection is similar to that of the mid-century one (Figures 2-12, 2-13). For the stabilization emission scenario, the number of days/yr exceeding 90°F (32°C) increases in the northwestern and north central regions by about 8.5 days (minimum: 1.7 days, maximum: 22.0 days, percent agreement: 100%), while in the southern and eastern parts of the state, it increases by about 29.0 days (minimum: 11.0 days, maximum: 43.0 days, percent agreement: 100%). For the business-as-usual emission scenario, the number of days exceeding 90°F (32°C) in the northwestern and north central parts of the state increases by about 34.0 days (minimum: 9.5 days, maximum: 58.0 days, percent agreement: 100%), while in the southern and eastern parts of the state, it increases by about 54.0 days (minimum: 26.0 days, maximum: 70.0 days, percent agreement: 100%). 

 

 

Mid-century and End-of-century

Figure 2-12. The projected increases in number of days above 90°F (32°C) for each climate division in Montana over two periods 2040-2069 and 2070-2099 for (A) stabilization (RCP4.5) and (B) business-as-usual (RCP8.5) emission scenarios.

 

Figure 2-13. Graphs showing the increase in the number of days per year above 90°F (32°C) projected for each climate division in both stabilization (RCP4.5) and business-as-usual (RCP8.5) emission scenarios. The top row shows mid-century projections (2040-2069) and the bottom row shows end-of-century projections (2070-2099). The outline of each box is determined by model agreement on the sign of the change (positive or negative). A black outline means there is ≥80% model agreement and a red outline means that there is <80% model agreement. In this case, all models indicated the direction of the trend for days above 90°F (32°C) at an agreement of greater than 80%.

 

Number of days where minimum temperatures are above 32°F (0°C)

The number of days/yr where minimum temperatures exceed 32°F (0°C; i.e., frost-free days) also increases across all climate divisions in both mid- and end-of-century projections and for both stabilization and business-as-usual emission scenarios (Figures 2-14, 2-15). While varying considerably across the state, projected changes are substantial. For example, in the mid-century projections with the stabilization emission scenario, frost-free days increase by about 30 days in the western part of the state and by 23 days in the eastern part of the state. Similar patterns exist for end-of-century projections: in the business-as-usual emission scenario, frost-free days increase by about 70 days in the western part of the state and by about 55 days in the eastern part of the state. 

• Mid-century projection specifics.—The number of days/yr where minimum temperatures are above 32°F (0°C; i.e., frost-free days) increases across all climate divisions and both emission scenarios (Figures 2-14, 2-15). In the stabilization emission scenario, frost-free days increase by 30.0 days in the western part of the state (minimum: 9.0 days, maximum: 51.0 days, percent agreement: 100%) and by 23.0 days in the eastern part of the state (minimum: 10.0 days, maximum: 43.0 days, percent agreement: 100%). In the business-as-usual emission scenario, frost-free days increase by 41.0 days in the western part of the state (minimum: 17.0 days, maximum: 68.0 days, percent agreement: 100%) and by 32.0 days in the eastern part of the state (minimum: 15.0 days, maximum: 63.0 days, percent agreement: 100%).
• End-of-century projection specifics.—The number of days/yr where minimum temperatures are above 32°F (0°C; i.e., frost-free days) continues to increase in the end-of-century projections across all climate divisions and for both emission scenarios, with 100% model agreement. Again, similar spatial patterns exist between the mid-century and end-of-century projections (Figures 2-14, 2-15). In the stabilization emission scenario, frost-free days increase by 41.0 days in the western part of the state (minimum: 18.0 days, maximum: 66.0 days, percent agreement: 100%), and by 30.0 days in the eastern part of the state (minimum: 14.0 days, maximum: 60.0 days, percent agreement: 100%). In the business-as-usual emission scenario, frost-free days increase by 70.0 days in the western part of the state (minimum: 36.0 days, maximum: 110.0 days, percent agreement: 100%), and by 55.0 days in the eastern part of the state (minimum: 26.0 days, maximum: 100.0 days, percent agreement: 100%).

Mid-century and End-of-century

Figure 2-14. The projected change in the number of frost-free days for each climate division in Montana over two periods 2040-2069 and 2070-2099 for (A) stabilization (RCP4.5) and (B) business-as-usual (RCP8.5) emission scenarios.

 

 

Figure 2-15. Graphs showing the increases in frost-free days/yr projected for each climate division in both stabilization (RCP4.5) and business-as-usual (RCP8.5) emission scenarios. The top row shows mid-century projections (2040-2069) and the bottom row shows end-of-century projections (2070-2099). The outline of each box is determined by model agreement on the sign of the change (positive or negative). A black outline means there is >=80% model agreement and a red outline means that there is <80% model agreement. In this case, all models indicated the direction of the trend of frost-free days at an agreement of greater than 80%.

 

Summary

In general, there is high model agreement and low uncertainty that temperatures and associated temperature metrics will increase both by mid century and end-of-century. For both periods, annual and seasonal temperature averages, the number of days/yr with extreme heat, and the overall length of the growing season are projected to increase. Differences exist in projections for the stabilization and business-as-usual emission scenarios, with the former consistently showing lower magnitudes of change than the latter. Many of the trends and spatial patterns seen in the mid-century projections are extended and exacerbated in the end-of-century projections. The range of model outputs also increases for end-of-century projections, suggesting that the magnitude of change becomes more uncertain in the models further out in time.

Regardless of uncertainties, the GCMs show full agreement regarding the direction of change: temperatures will be increasing. 

 

Precipitation projections

Below we provide projections of Montana’s future precipitation based on our modeling efforts. Those projections are for the stabilization and business-as-usual emission scenarios and for two periods: mid century (2040-2069) and end-of-century (2070-2099).

We discuss a subset of our precipitation modeling results here, including a) precipitation projections reported by the median values of the 20 GCM ensemble and b) figures that include maps and graphs that represent the median value and distribution of values observed for precipitation across the 20 GCMs. Special consideration is required for interpretations of precipitation changes in Montana’s complex terrain. Precipitation increases drastically with elevation such as that found in northwest Montana. Here, median values do not characterize the potential for spatial variability that exists within these regions.

An ensemble minimum, maximum, and percent agreement are also provided parenthetically. As with our temperature analysis, the percent agreement concerning the precipitation trends is based on the number of GCMs that project the same sign of change (i.e., positive or negative) as the median value. For example, if the median value is positive and 18 out of 20 models also project positive change, then the percent agreement would be 100 x 18/20 = 90%. This simple calculation helps convey the uncertainty in the projections. For some variables both the absolute change and the percent change from historical is calculated. 

 

Average annual precipitation

Average annual precipitation increases across the state in both mid-century and end-of-century projections for both emission scenarios (Figures 2-16, 2-17, 2-18). For the mid-century projection using the stabilization emission scenario, increases of about 1.3 inch/yr (3.3 cm/yr) occur in the northwestern and north central climate divisions and about 0.9 inch/yr (2.3 cm/yr) in the southwestern, central, and eastern climate divisions. For the business-as-usual emission scenario in the mid-century projection, average annual precipitation increases by about 2.0 inch/yr (5.1 cm/yr) in the western half of the state, and about 1.8 inch/yr (4.6 cm/yr) in the eastern half of the state. The GCMs used in the ensemble show large differences in their end-of-century projections, but there is high agreement in the positive direction of change. 

• Mid-century projection specifics.—Average annual precipitation increases by mid century across the state for both emission scenarios, with moderately high agreement among models (Figures 2-16, 2-17, 2-18). In the stabilization scenario, increases of about 1.3 inch/yr (3.3 cm/yr) and 5.0% (minimum: -0.5 inch/yr [-1.3 cm/yr], -1.1%; maximum: 3.2 inch/yr [8.1 cm/yr], 14.0%; percent agreement: 85%) are projected in the northwestern parts of the state. In the southern and eastern parts of the state, increases of about 0.9 inch/yr (2.3 cm/yr) and 6.5% are projected (minimum: -1.2 inch/yr [-3.0 cm/yr], -6.0%; maximum: 2.5 inch/yr [6.4 cm/yr], 18.0%; percent agreement: 85%). In the business-as-usual emission scenario, average annual precipitation increase by about 1.6 inch/yr (4.1 cm/yr) and 6.5% in the northwestern parts of the state (minimum: -0.2 inch/yr [-0.51 cm/yr], -1.0%; maximum: 4.4 inch/yr [11.2 cm/yr], 17.0%; percent agreement: 90%), and by about 1.2 inch/yr (3.0 cm/yr) and 10% in the southern and eastern parts of the state (minimum: -0.5 inch/yr [-1.3 cm/yr], -3.5%; maximum: 2.9 inch/yr [7.4 cm/yr], 22.0%; percent agreement: 85%).
• End-of-century projection specifics.—Average annual precipitation is projected to increase through the end-of-century for both emission scenarios (Figures 2-16, 2-17, 2-18). The GCMs used in the ensemble show large differences in their end-of-century projections, but there is high agreement in the positive direction of change. In the stabilization emission scenario, average annual precipitation increases in the northwestern climate division by about 2.2 inch/yr (5.6 cm/yr) and 7.3% (minimum: -1.2 inch/yr [-3.0 cm/yr], -4.5%; maximum: 3.6 inch/yr [9.1 cm/yr], 12.9%; percent agreement: 85%), and by about 1.1 inch/yr (2.8 cm/yr) and 8.0% in the two eastern climate divisions (minimum: -0.5 inch/yr [-1.3 cm/yr], -4.5%; maximum: 3.0 inch/yr [7.6 cm/yr], 18.0%; percent agreement: 85%). In the business-as-usual emission scenario, average annual precipitation is projected to increase by slightly more than in the stabilization emission scenario, although the range of model projections also increases. In the western half of the state, annual precipitation increases by about 2.0 inch/yr (5.1 cm/yr) and 10.0% (minimum: 0.4 inch/yr [1.0 cm/yr], 1.3%; maximum: 5.5 inch/yr [14.0 cm/yr], 28.0%; percent agreement: 100%), and in the eastern half of the state annual precipitation increases by about 1.8 inch/yr (4.6 cm/yr) and 14.0% (minimum: -0.2 inch/yr [-0.5 cm/yr], -1.0%; maximum: 3.6 inch/yr [9.1 cm/yr], 26.0%; percent agreement: 95%). 

 

Mid-century and End-of-century

Figure 2-16. The projected change in annual precipitation (inches) for each climate division in Montana over two periods 2040-2069 and 2070-2099 for (A) stabilization (RCP4.5) and (B) business-as-usual (RCP8.5) emission scenarios.

 

 

Figure 2-17. Graphs showing annual precipitation change (in inches) projected for each climate division in both stabilization (RCP4.5) and business-as-usual (RCP8.5) emission scenarios. The top row shows mid-century projections (2040-2069) and the bottom row shows end-of-century projections (2070-2099). The outline of each box is determined by model agreement on the sign of the change (positive or negative). A black outline means there is >=80% model agreement and a red outline means that there is <80% model agreement. In this case, all models indicated the direction of the annual precipitation trend at an agreement of greater than 80%.

 

 

Figure 2-18. Graphs showing the minimum, maximum, and median percent changes in annual precipitation projected for each climate division in both stabilization (RCP4.5) and business-as-usual (RCP8.5) emission scenarios. The top row shows mid-century projections (2040-2069) and the bottom row shows end-of-century projections (2070-2099). The outline of each box is determined by model agreement on the sign of the change (positive or negative). A black outline means there is >=80% model agreement and a red outline means that there is <80% model agreement. In this case, all models indicated the direction of the precipitation trend at an agreement of greater than 80%.

 

 

Interannual variability

Interannual variability (i.e., the amount precipitation changes from year to year) is also projected to increase slightly across the state by mid century and end-of-century for both emission scenarios (Figure 2-19). The increase could be attributed to wet years getting wetter, dry years getting drier, or some combination of both.

Figure 2-19. Graphs showing the interannual variability of precipitation projected for each climate division in both stabilization (RCP4.5) and business-as-usual (RCP8.5) emission scenarios. The top row shows mid-century projections (2040-2069) and the bottom row shows for end-of-century projections (2070-2099). The outline of each box is determined by model agreement on the sign of the change (positive or negative). A black outline means there is ≥80% model agreement and a red outline means that there is <80% model agreement.

 

Monthly and seasonal change in average precipitation

While annual increases in precipitation are projected across the state with moderately high model agreement, the monthly and seasonal projections vary. In mid-century projections, winter, spring, and fall increase in monthly precipitation for both emission scenarios, with spring experiencing the largest increases (e.g., 0.4 inch/month [1.0 cm/month] for the business-as-usual emission scenario; Figure 2-23). Summers, however, are projected to decrease by about 0.1 inch/month (0.3 cm/month) in both emission scenarios (model agreement, however, is fairly low for these projections). For end-of-century projections, the same trends are seen for increasing precipitation in winter, spring, and fall and decreasing precipitation in summer. The magnitude of change is similar to that of mid-century projections. 

Figure 2-20. Projected monthly change in average precipitation (inches) for each climate division in Montana in the mid-century projections (2040-2069) for (A) stabilization (RCP4.5) and (B) business-as-usual (RCP8.5) emission scenarios.

Figure 2-21. The projected monthly change in average precipitation (inches) for each climate division in Montana in the end-of-century projections (2070-2099) for (A) stabilization (RCP4.5) and (B) business-as-usual (RCP8.5) emission scenarios.

 

 

• Mid-century projection specifics.—Although annual precipitation increases across the state with moderately high model agreement, the monthly and seasonal projections vary somewhat. Winter, spring, and fall increase in monthly precipitation for both emission scenarios, with the largest increases in spring (Figure 2-20). For the stabilization emission scenario, spring months increase by about 0.2 inch/month (0.5 cm/month) (minimum: -0.1 inch/month [-0.3 cm/month], maximum: 0.8 inch/month [2.0 cm/month], percent agreement: 85%). In the business-as-usual emission scenario, spring months increase by 0.4 inch/month (1.0 cm/month) (minimum: 0.0 inch/month [0 cm/month], maximum: 1.0 inch/month [2.5 cm/month], percent agreement: 95%). Summer months, however, show decreasing precipitation for both scenarios, although model agreement is fairly low in the projections. For the both the stabilization and business-as-usual emission scenarios, summer precipitation decreases by -0.1 inch/month (-0.3 cm/month) (minimum: -0.4 inch/month [-1.0 cm/month], maximum: 0.5 inch/month [1.3 cm/month], percent agreement: 65%). 
• Figure 2-21. The projected monthly change in average precipitation (inches) for each climate division in Montana in the end-of-century projections (2070-2099) for (A) stabilization (RCP4.5) and (B) business-as-usual (RCP8.5) emission scenarios.

 

Projected change in consecutive dry days

To assess changes in the frequency of dry events, we determined the annual number of dry days (defined as days when precipitation is less than 0.01 inch [0.03 cm]), then calculated the maximum number of consecutive dry days/yr averaged over the 30-year periods of interest. In general, in both mid- and end-of-century projections, we found a modest increase statewide in consecutive dry days—generally less than 0.5 days—for both emission scenarios (Figures 2-22, 2-23). Low model agreement exists and the range of projections from the ensemble of GCMs is wide, both suggesting high uncertainty in these projections.

• Mid-century projection specifics.—In general, consecutive dry days show a modest increase (i.e., less than 0.5 days); however, model agreement is low (approximately 60%; where 50% would mean complete disagreement among models) in both emission scenarios. 
• End-of-century projection specifics.—In end-of-century projections, changes in consecutive dry days/yr remain positive, but the increase is small (generally less than 0.5 days) with low model agreement (approximate 60%). This result is consistent across both emission scenarios. The range of projections from the ensemble of models is wide; however, minimum and maximum values are projected to increase by about -2.5 days and 4.0 days, respectively. This large range, in addition to the low model agreement, suggests high uncertainty in these projections. 

 

Mid-century and End-of-century

Figure 2-22. The projected change in the number of consecutive dry days (<0.1 inch [0.3 cm] of precipitation) for each climate division in Montana over two periods 2040-2069 and 2070-2099 for (A) stabilization (RCP4.5) and (B) business-as-usual (RCP8.5) emission scenarios.

 

 

Figure 2-23. Graphs showing the number of consecutive dry days in a year projected for each climate division in both stabilization (RCP4.5) and business-as-usual (RCP8.5) emission scenarios. The top row shows mid-century projections (2040-2069) and the bottom row shows end-of-century projections (2070-2099). The outline of each box is determined by model agreement on the sign of the change (positive or negative). A black outline means there is ≥80% model agreement and a red outline means that there is <80% model agreement. In the case of consecutive dry days, there was less than 80% agreement across the models for all climate divisions.

 

Projected change in wet days

To evaluate changes in wet events, we calculated the number of days/yr where precipitation is greater than 1.0 inch (2.5 cm) and average those values over the period of interest (Figures 2-24). 

• Mid-century projection specifics.—Very modest changes in the number of wet events (i.e., less than 0.5 days) is projected for both emission scenarios. This time, however, model agreement is high that these small changes will occur (approximately 90%).

• End-of-century projection specifics.—Very high model agreement (approximately 100%) exists that the number of days/yr with precipitation above 1.0 inch (2.5 cm) will increase, although the magnitude of change is still small (less than 1.0 day). The northwestern climate division is projected to have the largest changes in this metric for both emission scenarios, reaching almost a 1.0 day increase of over 1.0 inch (2.5 cm) of precipitation for the period from 2070 to 2099. The range of model output is higher in the business-as-usual emission scenario.

Figure 2-24. Graphs showing the increase in the number of wet days/yr projected for each climate division in both stabilization (RCP4.5) and business-as-usual (RCP8.5) emission scenarios. The top row shows projections for mid century (2040-2069) and the bottom row shows projections for end-of-century (2070-2099). The outline of each box is determined by model agreement on the sign of the change (positive or negative). A black outline means there is ≥80% model agreement and a red outline indicates <80% model agreement. Model agreement for the trend of wet days each year was greater than 80%, except for the northeastern climate division.

 

Summary

In mid-century and end-of-century projections, average annual precipitation and variability increase across the state, as does winter, spring, and fall precipitation. Summers, however, show slight decreases in precipitation. The projections suggest little change in the annual frequency of dry and wet events, although there is high uncertainty in the case of wet events. Similar analysis using different metrics for the larger region surrounding Montana indicates an even larger potential (30%) for more days of extreme precipitation (NCA 2014). Overall, the differences in precipitation resulting from the different emission scenarios (i.e., stabilization versus business-as-usual) are small when compared to the impact of the emission scenarios on the temperature projections. Uncertainty in the projections generally increases the further out in time (i.e., in the end-of-century projections), as well as for the higher business-as-usual emission scenario.

 

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CONCLUSIONS

 

The analysis presented in this chapter shows that Montana has warmed—up to 2.7°F (1.5°C) annually as averaged across the state—since 1950. Seasonally, that warming has been greatest in winter (3.9°F [2.2°C]) and spring (2.6°F [1.4°C]). Montana’s number of frost days has decreased by 12 days since 1951. Statewide, average annual precipitation did not change between 1950 and 2015, although variations caused by global climate oscillations, such as El Niño events, explain some of the historical precipitation variability in parts of the state.

With this historical context, we considered Montana’s future under two potential greenhouse gas emission scenarios. Using those scenarios, we employed standard modeling techniques available to climate scientists today—ensembles of general circulation models—and projected Montana’s climate over the next century. Our analyses focused on projecting the possible range of temperature and precipitation amounts in Montana, under our chosen greenhouse gas emission scenarios.

While the model results varied, one message is imminently clear: Montana in the coming century will be a warmer place.

 

One thing is clear: Montana in the coming century will be a warmer place.

 

In Table 2-6 we provide a summary of the work done and described in this chapter (plus in accompanying appendices). In summary, Montana is projected to continue to warm in all geographic locations, seasons, and under all emission scenarios throughout the 21^st century. By mid century, Montana temperatures are projected to increase by up to 6°F (3°C); by the end of the century, temperatures will increase by up to 9.8°F (5.4°C) (both projections depend on the particular carbon emission scenario [i.e., RCP], and these numbers are based on the business-as-usual [RCP8.5] scenario). Projections show that we could have up to 70 more frost-free days at the end of the century. Likewise, frequency of extreme heat will increase. In eastern Montana, for example, we may have as many as 54 days/yr in which maximum temperatures exceed 90°F (32°C). 

In mid- and end-of-century projections, average annual precipitation and variability increase across the state, as do winter, spring, and fall precipitation. Summer months, however, show small decreases in precipitation. Current projections suggest little change in the frequency of dry and wet events, although projections in the former case show high uncertainty. 

Montanans must be prepared for projected increases in temperature in the future. Because of its interior location, Montana has warmed more over the last 65 yr than the national average, and it will experience greater warming than most parts of the country in the future, particularly when compared to states in coastal regions. Key to the concern is that coming temperature changes will be larger in magnitude and occur more rapidly than any time since our 1889 declaration of statehood (and, to be sure, well before).

 

Montana’s average annual temperature is projected to increase through the end-of-century for all models, all emission scenarios, and in all geographic locations. 

 

Changes in temperature and precipitation associated with climate change will undoubtedly impact Montana’s water resources, forestry, and agriculture. These changes will have direct impacts on all Montanans, as we explore in subsequent chapters of this assessment.

 

Table 2-6. Summary of climate metrics described in this chapter.

Climate Metric—	Trend and future scenario
Atmospheric CO2 concentrations	Global atmospheric carbon dioxide concentrations have increased over 100 ppm since Montana statehood and are projected to increase under both future scenarios considered here.
Average temperature	Since 1950, average statewide temperatures have increased by 0.5°F/decade (0.3°C/decade), with greatest warming in spring; projected to increase by 3-7°F (1.7-3.9°C) by mid century, with greatest warming in summer and winter and in the southeast.
Maximum temperatures	Maximum temperatures have increased most in spring and are projected to increase 3-8°F (1.7-4.4°C) by mid century, with greatest increases in August and in the southeast.
Days above 90°F (32°C)	Extreme heat days are projected to increase by 5-35 additional days by mid century, with greatest increases in the northeast and south.
Minimum temperatures	Minimum temperatures have increased most in winter and spring and are projected to increase 3-7°F (1.7-3.9°C) by mid century, with greatest increases in January and in the southeast.
Frost-free days	Frost-free days are projected to increase by 24-44 days by mid century, particularly in the west.
Average precipitation	Statewide precipitation has decreased in winter ( 0.14 inches/decade [-0.36 cm/decade]) since 1950, but no significant change has occurred in annual mean precipitation, probably because of very slight increases in spring and fall precipitation. Precipitation is projected to increase, primarily in spring (0.2-0.7 inches [0.5-1.8 cm]) in the northwest; a slight statewide decrease in summer precipitation and increased year-to-year variability of precipitation are projected, as well.
Number of consecutive dry days	Little projected change, with a maximum increase of 3 days to -3 days under the most severe scenario by end of the century. However, increased variability in precipitation suggests potential for more severe droughts, particularly in connection with climate oscillations.
Number of consecutive wet days	No substantial change projected.

 

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LITERATURE CITED

 

Abatzoglou JT. 2013. Development of gridded surface meteorological data for ecological applications and modelling. International Journal of Climatology 33:121–31.

Abatzoglou JT, Brown TJ. 2012. A comparison of statistical downscaling methods suited for wildfire applications. International Journal of Climatology 32:772–80.

Cayan DR, Dettinger MD, Kammerdiener SA, Caprio JM, Peterson DH. 2001. Changes in the onset of spring in the western United States. Bulletin of the American Meteorological Society 82:399–415.

Clarke L, Edmonds J, Jacoby H, Pitcher H, Reilly J, Richels R. 2007. Scenarios of greenhouse gas emissions and atmospheric concentrations. Sub-Report 2.1a of synthesis and assessment product 2.1 by the US Climate Change Science Program and the Subcommittee on Global Change Research. Washington DC: Department of Energy, Office of Biological & Environmental Research. 164 p. Available online http://www.climatescience.gov/Library/sap/sap2-1/finalreport/default.htm. Accessed 2017 May 10.

CLIMDEX. [undated]. CLIMDEX—datasets for indices of climate extremes [website]. Available online http://climdex.org/. Accessed 2017 Mar 6.

[CMIP5] Coupled Model Intercomparison Project. [undated]. Overview [website]. Available online http://cmip-pcmdi.llnl.gov/. Accessed 2017 Mar 18.

Dang H, Gillett NP, Weaver AJ, Zwiers FW. 2007. Climate change detection over different land surface vegetation classes. International Journal of Climatology 27:211–20.

Davy R, Esau I. 2016. Differences in the efficacy of climate forcings explained by variations in atmospheric boundary layer depth. Nature Communications 7:11690. doi:10.1038/ncomms11690.

Dettinger MD, Cayan DR. 1995. Large-scale atmospheric forcing of recent trends toward early snowmelt runoff in California. Journal of Climate 8:606–23.

Dightman RA. 1963. -70°F in Montana. Weatherwise 16:272-3. 

Forster P, Ramaswamy V, Artaxo P, Berntsen T, Betts R, Fahey DW, Haywood J, and 45 more. 2007. Changes in atmospheric constituents and in radiative forcing [chapter]. In: Solomon S, Qin D, Manning M, Chen Z, Marquis M, Averyt KB, Tignor M, Miller HL, editors. Climate change 2007: the physical science basis. Cambridge UK: Cambridge University Press. p 129-234.

Fujino J, Nair R, Kainuma M, Masui T, Matsuoka Y. 2006. Multi-gas mitigation analysis on stabilization scenarios using Aim Global Model. Energy Journal 27:343–53.

Global Climate Observing System. [undated]. GCOS essential climate variables [website]. Available online http://www.wmo.int/pages/prog/gcos/index.php?name=EssentialClimateVariables. Accessed 2017 Mar 6.

Guttman NB, Quayle RG. 1996. A historical perspective of US climate divisions. Bulletin of the American Meteorological Society 77:293–303.

[IPCC] Intergovernmental Panel on Climate Change. 2013a. In: Stocker TF, Qin D, Plattner G-K, Tignor M, Allen SK, Boschung J, Nauels A, Xia Y, Bex V, Midgley PM, editors. Climate Change 2013: the physical science basis. Contribution of working group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge UK and New York NY, USA: Cambridge University Press. 1535 p.

[IPCC] Intergovernmental Panel on Climate Change. 2013b. Data distribution center—what is a GCM? [webpage] Available online http://www.ipcc-data.org/guidelines/pages/gcm_guide.html. Accessed 2017 Mar 6.

[IPCC] Intergovernmental Panel on Climate Change. 2014. In: Pachauri RK, Meyer LA, editors. Climate Change 2014: synthesis report; contribution of working groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Geneva Switzerland: IPCC. 151 p.

Karl T, Koss WJ. 1984. Regional and national monthly, seasonal, and annual temperature weighted by area, 1895-983. Historical Climatology Series 4-3. Asheville NC: National Climatic Data Center. 38 p.

Leathers DJ, Brent Y, Palecki MA. 1991. The Pacific/North American teleconnection pattern and United States climate. Part I: Regional temperature and precipitation associations. Journal of Climate 4:517–28.

Liebmann B, Dole RM, Jones C, Bladé I, Allured D. 2010. Influence of choice of time period on global surface temperature trend estimates. Bulletin of American Meteorological Society 91:1485-91. doi:org/10.1175/2010BAMS3030.1.

[MACA] Multivariate adaptive constructed analogs. [undated]. MACA statistically downscaled 

climate data from CMIP5 [website]. Available online http://maca.northwestknowledge.net/. Accessed 2016 Oct 16.

Mann ME, Bradley RS, Hughes MK. 1999. Northern Hemisphere temperatures during the past millennium: inferences, uncertainties, and limitations. Geophysical Research Letters 26 (6):759–62.

Mantua NJ, Hare SR. 2002. The Pacific Decadal Oscillation. Journal of Oceanography 58(1):35–44.

Mote PW, Hamlet AF, Clark MP, Lettenmaier DP. 2005. Declining mountain snowpack in western North America. Bulletin of the American Meteorological Society 86(1):39–49.

[NAS] National Academy of Sciences. 2011. America’s climate choices. Washington DC: The National Academies Press. 134 p.

[NAS] National Academy of Sciences. [undated]. Climate change at the National Academies [website]. Available online https://nas-sites.org/americasclimatechoices/more-resources-on-climate-c.... Accessed 2017 Mar 6.

[NASA] National Aeronautics and Space Administration. [undated]. NASA data [website]. Available online https://mynasadata.larc.nasa.gov/glossary/earths-energy-budget-2/. Accessed 2017 Mar 6.

[NCA] National Climate Assessment. 2014. In: Melillo JM, Richmond T, Yohe GW, editors. Climate change impacts in the United States: the third national climate assessment. Washington DC: US Global Change Research Program. 841 p. doi:10.7930/J0Z31WJ2. 

[NOAAa] National Oceanic and Atmospheric Administration. [undated]. US climate divisions [website]. Available online https://www.ncdc.noaa.gov/monitoring-references/maps/us-climate-division.... Accessed 2017 Mar 6.

[NOAAb] National Oceanic and Atmospheric Administration. [undated]. Climate normals [website]. Available online https://www.ncdc.noaa.gov/data-access/land-based-station-data/land-based.... Accessed 2017 Mar 6.

[NOAAc] National Oceaic and Atmorspheric Administration. [undated]. Climate at a glance [website]. Available online: www.ncdc.noaa.gov/cag/. Accessed 2017 Mar 6.

 [NWSa] National Weather Service. [undated]. Climate prediction center—El Niño and La Niña winter features over North America [website]. Available online http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/ensocycle/nawi.... Accessed 2017 Mar 6.

[NWSb] National Weather Service. [undated]. Montana El Niño precipitation and temperature rankings [website]. Available online http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/ensostuff/stat.... Accessed 2017 Mar 6.

Oyler JW, Ballantyne A, Jencso K, Sweet M, Running SW. 2015. Creating a topoclimatic daily air temperature dataset for the conterminous United States using homogenized station data and remotely sensed land skin temperature. International Journal of Climatology 35:2258–79.

Riahi K, Grübler A, Nakicenovic N. 2007. Scenarios of long-term socio-economic and environmental development under climate stabilization. Technological Forecasting and Social Change 74(7):887–935.

Stewart IT, Cayan DR, Dettinger MD. 2005. Changes toward earlier streamflow timing across western North America. Journal of Climate 18:1136–55.

Stocker TF, Qin D, Plattner G-K, Tignor M, Allen SK, Boschung J, Nauels A, Xia Y, Bex V, Midgley PM, editors. 2013. Climate change 2013: the physical science basis. Contribution of working group I to the fifth assessment report of the Intergovern¬mental Panel on Climate Change. Cambridge UK and New York NY: Cambridge University Press. 1535 p.

[USGCRP] US Global Change Research Program. [undated]. Glossary [website]. Available online http://www.globalchange.gov/climate-change/glossary. Accessed 2017 Mar 6.

van Vuuren DP, Edmonds J, Kainuma M, Riahi K, Thomson A, Hibbard K, Hurtt GC, Kram T, Krey V, Lamarque J-F, Masui T, Meinshausen M, Nakicenovic N, Smith SJ, Rose SK. 2011. The representative concentration pathways: an overview. Climatic Change 109:5. doi:10.1007/s10584-011-0148-z.

Vose RS, Applequist S, Durre I, Menne MJ, Williams CN, Fenimore C, Gleason K, Arndt D. 2014. Improved historical temperature and precipitation time series for US climate divisions. Journal of Applied Meteorology and Climatology 53(5):1232-51. Available online http://dx.doi.org/10.1175/JAMC-D-13-0248.1. Accessed 2017 May 10.

Wilhite DA. 2000. Drought as a natural hazard: concepts and definitions [chapter]. In: Wilhite DA, editor. Drought: a global assessment, Vol. I. London: Routledge. p 3–18.

 

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