University of Wyoming Women’s Nordic Team at Teton Ridge Classic near Alta, Idaho. Photo courtesy of Rachel Watson.
adaptation — Actions taken to help communities and ecosystems better cope with potential negative effects of climate change or take advantage of potential opportunities.
adaptive capacity — The inherent ability of a system (e.g., ecosystem or social system) to adapt to a changing environment; for example, a plant species that can survive a broader range of temperatures has a greater adaptive capacity compared to a plant that can only tolerate a narrow range of temperatures.
air temperature — An objective measure of how hot or cold an object is with reference to some standard value; seasonal variations in temperature result from the latitudinal differences in the amount of solar radiation received at the Earth’s surface and the contrasts in seasonal heating of land and oceans.
annual streamflow — The cumulative quantity of water that discharges through a river or stream for a period of record, in this case a calendar year.
anthropogenic — Originating from human activity.
aquifer — A water-saturated body of permeable rock or alluvial soil that contains or transmits groundwater.
Atlantic Multi-decadal Oscillation (AMO) — A 60- to 80-yr cycle of warm and cold sea-surface temperatures in the North Atlantic Ocean.
atmospheric carbon dioxide (CO2) — The amount of CO2 in Earth’s atmosphere. Although the proportion of Earth’s atmosphere made up by CO2 is small, CO2 is one of the most potent greenhouse gases and directly related to the burning of fossil fuels. Atmospheric carbon dioxide levels in Earth’s atmosphere are at the highest levels in an estimated 3.3 million years and these levels are projected to increase global average temperatures through the greenhouse effect.
atmosphere-ocean interactions, circulation patterns — The atmosphere and ocean are the two large reservoirs of water in the Earth’s hydrologic cycle, and these systems are complexly linked to one another and responsible for the Earth’s weather and climate. Recurring and persistent global-scale interactions between the atmosphere and the ocean are responsible for year-to-year and decadal climate variations in the GYA.
attribution — Identification of a source or cause of something.
average — The value that is found by summing all the numbers in a data set and dividing that sum by the number of values in the set. Average and mean are used interchangeably in this report.
base flow — The portion of streamflow that is not runoff and results from seepage of water from the ground into a stream channel slowly over time. It is the primary source of water in a stream during dry weather.
base period — Used for comparison with future periods as the 1986 through 2005 average. We chose this 20-year base period because a) it captures observed global warming trends and, therefore, is a conservative (warm) baseline; and b) climate model simulations of the historical period end at 2005 and projections of future climate begin in 2006.
basin — A drainage basin or catchment basin is an extent or an area of land where all surface water from rain, melting snow, or ice converges to a single point at a lower elevation, usually the exit of the basin, where the waters join another body of water, such as a river, lake, reservoir, estuary, wetland, sea, or ocean.
biodiversity — The variety of all native living organisms and their various forms and interrelationships.
braided river — A river that consists of a network of small channels separated by islands. The pattern of channels and islands wanders across the landscape as a result of changes in the sediment load and streamflow. Braided rivers typically flow from the terminus of a melting glacier.
chemical bond — A lasting attraction between atoms that enables the formation of chemical compounds.
climate (versus weather) — The difference between weather and climate is a measure of time. Weather represents the conditions of the atmosphere over a short period of time (hours to days), and climate is how the atmosphere "behaves" over relatively long periods of time (decades and beyond).
climate anomalies — The positive or negative difference of a future or past climate measurement compared to that of a defined base period.
climate change — Changes in average weather conditions that persist over multiple decades or longer. Climate change encompasses increases and decreases in temperature, as well as shifts in precipitation (including snowfall), changing risk of certain types of severe weather events, and changes to other features of the climate system.
climate drivers — The suite of physical and chemical changes that affect the global energy balance and force changes in the Earth’s climate; also referred to as climate forcings.
climate model simulation — The process of using a climate model to study the behavior and performance of the climate system under a prescribed set of conditions. Model simulations are used to understand past, present, and future climate. See GCM.
climate system — Describes all the interacting components that create Earth’s climate: the atmosphere (air), hydrosphere (water), the cryosphere (ice and permafrost), lithosphere (Earth’s upper rocky layer), biosphere (living things), and anthroposphere (humans).
climate trend — The long-term trajectory of change in the average climate.
climate variability — Refers to short-term departures from the average or mean state of the climate (note that here we are referring to climate variations that are longer than individual weather events, spanning seasons or years).
climatology/climatological — The scientific study of regional and global climates.
cold days — The annual count of days where daily minimum temperature drops below 32°F (0°C).
cold spell — A sequence of 6 or more days in which the daily maximum temperature is below the 10th percentile of daily minimum temperature for a 5-day running window.
confidence interval— An estimate computed from the statistics of the observed data to propose a range of plausible values for an unknown parameter (for example, the mean). The interval has an associated confidence level that the true parameter is in the proposed range. Most commonly, a 95% confidence level is used.
confined aquifer — An aquifer with layers of impermeable material both above and below the aquifer, causing it to be under pressure so that when the aquifer is penetrated by a well, the water will rise above the top of the aquifer.
direct effect — A primary impact to a system from shifts in climate conditions (e.g., temperature and precipitation), such as direct mortality to species from increased heat extremes.
disturbance regime — The frequency, severity, and pattern of events that disrupt an ecosystem or community; for example, a forest’s fire disturbance regime may be the historical pattern of frequent, low-intensity fires.
drivers (climate) — The suite of physical and chemical changes that affect the global energy balance and force changes in the Earth’s climate; also referred to as climate forcings.
drought — A prolonged period of dryness relative to long-term average conditions. The climatological community defines four types of drought: 1) meteorological drought occurs when unusually dry weather patterns persist over an area from days to months; 2) hydrological drought refers to low-water supply and usually occurs after many months of meteorological drought; 3) agricultural drought occurs when low soil moisture limits survival and production of crops and grazing lands; and 4) socioeconomic drought reflects the economic and social impact of a combination of hydrological and agricultural drought. In this report, we use the term drought, without distinguishing the type, but unless otherwise noted, we are referring to meteorological or hydrological drought.
dry spell — Maximum number of consecutive days/yr with daily precipitation amount of less than a trace (<1 mm).
Earth system — Refers to Earth´s interacting physical, chemical, and biological processes. The system consists of a) the land, oceans, cryosphere, and atmosphere; b) the planet's natural cycles (e.g., the carbon, water, nitrogen, and other chemical cycles); and c) deep Earth processes.
ecosystem — The complex of living organisms, their physical environment, and all their interrelations in a particular place.
El Niño — See El Niño-Southern Oscillation.
El Niño-Southern Oscillation (ENSO) — A periodic variation in wind and sea-surface temperature patterns that affects global weather; El Niño (warming phase where sea-surface temperatures in the eastern Pacific Ocean warm) generally means warmer (and sometimes slightly drier) winter conditions in the GYA. In contrast, La Niña (cooling phase) often means cooler (and sometimes wetter) winters for the GYA. The two phases each last approximately 6-18 months, and oscillate between the two phases approximately every 3-4 yr.
ephemeral stream — A stream that flows only briefly during and following a period of rainfall in the immediate locality.
evaporation — The change of a liquid into a vapor at a temperature below the boiling point. Evaporation takes place in all forms of liquid water, from water bodies to raindrops.
evapotranspiration — The combined process of evaporation from open ground and plant transpiration, one of the most important processes in the hydrologic cycle. Evapotranspiration is analyzed in two ways, as potential evapotranspiration, which is a measure of how much evapotranspiration would occur with unlimited water availability, and actual evapotranspiration, which is how much evapotranspiration occurs under given moisture conditions. Actual evapotranspiration is determined by water availability, meteorological conditions, the amount of land cover, and plant type. Transpiration from vegetation is affected factors such as leaf area, physiology, and rooting depth.
flood — An overflowing of a large amount of water beyond its normal confines, especially over what is normally dry land.
flood plain — An area of low-lying ground adjacent to a river, formed mainly of river sediments and subject to flooding.
forcings — See drivers (some authors use the word forcings instead of drivers; for this report we will generally use the latter).
geologic fault — A fracture or zone of fractures between two blocks of bedrock. Faults cause blocks to move relative to each other; rapid movement comes in the form of an earthquake.
glacial periods — An interval in geologic history, lasting thousands of years and marked by colder temperatures, when polar and mountain ice sheets were unusually extensive across the Earth’s surface.
global climate models (GCMs) — Numerical models based on the long-known physics that govern the circulation of the atmosphere and oceans. GCMs were originally derived from weather prediction models and have progressively become more complex and comprehensive to be capable of simulating the Earth system. They now account for physical processes in the atmosphere, ocean, cryosphere, and land surface. GCMs are the most advanced tools currently available for simulating the response of the global climate system to increasing greenhouse gas concentrations.
global warming — An increase in Earth’s surface air temperatures averaged over the globe over a decade or longer. Increases in global average temperatures do not mean the same amount of increase everywhere on Earth, nor that temperatures in a given year will be warmer than the year before (which represents weather, not climate). More simply: Global warming is used to describe a gradual increase in the average temperature of the Earth's atmosphere and its oceans.
greenhouse effect — The Earth’s energy balance is driven by solar radiation that is absorbed by land and oceans at the Earth’s surface and radiated back to the atmosphere as heat. Greenhouse gas molecules, like carbon dioxide (CO2), have chemical bond structures that trap and reradiate some of the heat from the Earth’s surface that otherwise would escape back to space.
greenhouse gas (GHG) — A gas in Earth’s atmosphere that absorbs and then re-radiates heat from the Earth and thereby affects global temperatures. The primary greenhouse gases in Earth's atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, and ozone. Earth relies on the warming effect of greenhouse gases to sustain life, but increases in greenhouse gases, particularly carbon dioxide from the burning of fossil fuels, have increased average global temperatures over historical norms.
greenhouse gas emissions — The discharge of greenhouse gases, such as carbon dioxide, methane, nitrous oxide and various halogenated hydrocarbons, into the atmosphere. Combustion of fossil fuels, agricultural activities, and industrial practices contribute to the emissions of greenhouse gases.
groundwater — Water held underground in the soil or in pores and crevices in rock.
growing degree days — A weather-based indicator for assessing crop development. It is a calculation used by crop producers that is a measure of heat accumulation used to predict plant and pest development rates such as the date that a crop reaches maturity.
Holocene — The current geologic epoch that began approximately 11,650 yr before present after the last glacial period.
hot days — Percentage of time when daily maximum temperature >90th percentile.
hydrograph — A hydrograph is a graph showing the rate of flow (discharge) versus time past a specific point in a river, or other channel or conduit carrying flow. The rate of flow is typically expressed as cubic feet per second, CFS, or ft3/s (the metric unit is m3/s).
hydrologic cycle — The sequence of conditions through which water passes from vapor in the atmosphere to precipitation upon land or water surfaces and ultimately back into the atmosphere as a result of evaporation and transpiration.
Hydrologic Unit Code (HUC) — A hierarchical classification developed in the 1980s by the USGS that subdivides the country’s river basins and watersheds into regions, subregions, and smaller units.
hydrology — The study of water, generally focused on the distribution of water and its interaction with the land surface and underlying soils and rocks.
ice ages — An ice age is a long period of reduced atmospheric greenhouse gases and low temperature of the Earth’s surface and atmosphere, resulting in the presence or expansion of continental and polar ice sheets and mountain glaciers. Ice ages, like that of the last 2.65 million years, include glacial as well as interglacial periods, as a result of Milankovitch variations in the Earth’s orbit and axial tilt and natural changes in greenhouse gas concentrations in the atmosphere.
indirect effect — A secondary impact to a system from a change that was caused by shifting climate conditions, such as increased fire frequency, which is a result of drier conditions caused by an increase in temperature.
infiltration — The movement of water from the land surface into the soil.
interception — The capture of precipitation above the ground surface, for example, by vegetation or buildings.
interglacial periods — An interval of warmer climate lasting thousands of years that separates glacial periods within an ice age.
IPCC — The Intergovernmental Panel on Climate Change was created in 1988 by the World Meteorological Organization and the United Nations Environment Program. The IPCC provides regular assessments of the scientific basis of climate change, its impacts and future risks, and options for adaptation and mitigation.
irrigation — Application of water to soil for the purpose of plant production.
La Niña — See El Niño-Southern Oscillation.
Little Ice Age — A period of cooling that occurred from about 1550-1850 after the Medieval Climate Anomaly. The Little Ice Age was not a true ice age, although glaciers became active in the highest elevations of the Rocky Mountains.
LOESS fit — A statistical method for fitting a smooth curve to a scatter plot of two variables, such as temperature and time. The acronym is derived imperfectly from a description of the process: locally weighted scatter plot smoothing or, alternatively, locally weighted smoothing.
MACAv2-METDATA (Multivariate Adaptive Constructed Analogs version 2) — This data set, used for projections made in this report, includes 20 GCMS that were statistically downscaled to a 4 km by 4 km (2.5 mile by 2.5 mile) grid using the Multivariate Adaptive Constructed Analogs method. The MACAv2-METDATA data were also used in the Montana Climate Assessment.
mean — See average.
median — The middle value when a data set is ordered from least to greatest.
Medieval Climate Anomaly — A period of warming that occurred from about 800 to 1300 when summers were slightly warmer than the pre-industrial period. This period was characterized by decade-long droughts that brought more fires, lower streamflow, establishment of trees above present tree line, and even a near-century hiatus of geyser activity at Old Faithful.
megadrought — A prolonged and intensive drought lasting decades.
microclimate — The local climate of a given site or habitat varying in size from a tiny crevice to a large land area. Microclimate is usually, however, characterized by considerable uniformity of climate over the site involved and relatively local when compared to its enveloping macroclimate. The differences generally stem from local climate factors such as elevation and exposure.
Milankovitch cycles — The collective effects of changes in the Earth’s movements on its climate over thousands of years. The term is named for Serbian geophysicist and astronomer Milutin Milanković, who in the 1920s, hypothesized that variations in the Earth’s orbit and axial tilt were cyclical and determined the amount of solar radiation reaching the Earth. This orbital forcing strongly influences long-term Earth climate patterns.
mitigation — Efforts to reduce greenhouse gas emissions to, or increase carbon storage from, the atmosphere as a means to reduce the magnitude and speed of onset of climate change.
model — A physical or mathematical representation of a process that can be used to predict some aspect of the process.
model spread — The maximum and minimum values for the 20 models used in the average or ensemble mean.
moraine — A mass of rocks and sediment carried down and deposited by a glacier, typically as ridges at its edges or extremity.
oscillation — A recurring cyclical pattern in global or regional climate that often occurs on decadal to sub-decadal timescales. Climate oscillations that influence the GYA’s climate are the El Niño-Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO).
Pacific Decadal Oscillation (PDO) — A periodic variation in sea-surface temperatures that is similar to El Niño-Southern Oscillation but has a much longer duration (approximately 20-30 yr). When the PDO is in the same phase as El Niño-Southern Oscillation, weather effects are more pronounced.
Palmer Drought Severity Index (PDSI) — A standard measure of drought that combines temperature or potential evapotranspiration and precipitation data to quantify dryness or wetness relative to average or normal conditions. The PDSI describes soil moisture conditions (generally the top meter of soil).
peak flow — The point of the hydrograph that has the highest flow over a given period of record (e.g., annual peak flow is the largest flow during a given year).
permeability — A measure of the ability of a porous material (often, a rock or an unconsolidated material) to allow fluids to pass through it.
Pliocene — The geologic epoch that extends from 2.58 to 5.33 million years ago, when the climate was warmer than present and CO2 levels were equal to present day.
precipitation — The quantity of water (solid or liquid) falling to the Earth’s surface at a specific place over a given period. Like temperature, precipitation varies from season to season and place to place depending on atmospheric and oceanic circulation.
pre-industrial — The reference period 1850-1900, which is used to represent temperature before the 20th century rise of greenhouse gases in the atmosphere.
radiative forcing — The difference between the amount of sunlight absorbed by the Earth versus the energy radiated back to space. Greenhouse gases in the atmosphere, particularly carbon dioxide, increase the amount of radiative forcing, which is measured in units of watts/m2. The laws of physics require that average global temperatures increase with increased radiative forcing.
rangeland — Land on which the historical climax plant community is predominantly grasses, grasslike plants, forbs, or shrubs. This includes lands re-vegetated naturally or artificially when routine management of the vegetation is accomplished through manipulation of grazing. Rangelands include natural grasslands, savannas, shrublands, most deserts, tundra, alpine communities, coastal marshes, and wet meadows.
rate of change (temperature, precipitation) — The amount of change in a climate variable over a defined period of time (e.g., oF warming per decade).
Representative Concentration Pathways (RCPs) — Plausible pathways (scenarios) of future greenhouse gas emissions based on assumptions about societal choices, population growth, energy use, existing and future technology, and land-use change resulting in a range of concentrations in the atmosphere. RCPs are used in climate models to project future climate. In this Assessment we focus on RCP4.5 and RCP8.5. These scenarios represent a future with an increase in radiative forcing of 4.5 or 8.5 watts/m2, respectively. RCP4.5 assumes greenhouse gas emissions peak at mid century, and then decline, and RCP8.5 scenario assumes continued high greenhouse gas emissions through the end of the century.
resilience — In ecology, the capacity of an ecosystem to respond to a disturbance or perturbation by resisting damage and recovering quickly.
resistance — In ecology, the property of populations or communities to remain essentially unchanged when subject to disturbance. Sensitivity is the inverse of resistance.
runoff — Water available from precipitation and snowmelt.
shallow aquifer — Typically (but not always) the shallowest aquifer at a given location is unconfined, meaning it does not have a confining rock layer (an aquitard or aquiclude) between it and the surface. The term perched refers to groundwater accumulating above a low-permeability unit or strata, such as a clay layer.
signal-to-noise ratio (SNR) — As used in the Assessment, 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) establish when a projected climate change emerges over the 21st century and provide additional support for confidence in the change.
SNOTEL — Short for “snow teleometry,” these are an automated system of snowpack and other climate sensors operated by the Natural Resources Conservation Service.
snowfall and snowpack — Two related terms that represent the amount and fate of solid winter precipitation. Snowfall is the amount of snow measured as it accumulates during a storm. It is measured in terms of the depth and amount of water it contains. In mountainous and relatively dry areas like the GYA, 10 inches (25 cm) or more of snow is needed to create 1 inch (2.5 cm) of water when melted. Snowpack is the amount of snow that accumulates and persists on the ground. It also is measured by both depth (snow depth) and the amount of water (called snow water equivalent or SWE) available when snowpack melts.
snow water equivalent (SWE) — A common snowpack measurement that is the amount of liquid water contained within the snowpack.
soil moisture — A measure of the quantity of water contained in soil. Soil moisture is a key variable in controlling the exchange of water and energy between the land surface and the atmosphere through evaporation and evapotranspiration.
solar activity, solar output — The sum of all variable and short-lived disturbances on the sun, such as sunspot, prominences, and solar flares. These disturbances affect the amount of solar radiation emitted from the sun, which is termed its solar output.
solar radiation — The energy emitted from the sun in the form of electromagnetic waves, including visible and ultraviolet light and infrared radiation. Usually referenced at the Earth surface where it drives the surface energy and water balances.
storage — The volume of water contained in snowpack, glaciers, drainage basins, aquifers, soil zones, lakes, reservoirs, or irrigation projects.
streamflow (sometimes called discharge or channel runoff) — The amount of water moving within a river, measured by the volume of water passing a point in a given time. Streamflow is measured at gaging stations in units of cubic feet per second or cubic meters per second. In the GYA, streamflow is strongly controlled by the seasonality of runoff from snowmelt.
sublimation — The transition of a substance directly from the solid to the gas state, without passing through the liquid state.
teleconnection — A connection between meteorological events that occur a long distance apart, such as sea-surface temperatures in the Pacific Ocean affecting winter temperatures in the GYA. Also referred to as climate oscillations or patterns of climate variability.
transpiration — The passage of water through a plant from the roots through the vascular system to the atmosphere.
trends — The general direction in which something is developing or changing.
unconfined aquifer — A groundwater aquifer is said to be unconfined when its upper surface (water table) is open to the atmosphere through permeable material.
vapor pressure deficit — A measure of the atmosphere’s drying capacity based on temperature and relative humidity. Drying capacity (high deficits) affects transpiration from plants, as well as fuel dryness, the latter being a major factor in wildfire occurrence and extent.
warm nights — Percentage of time when daily minimum temperature is greater than 90th percentile of measurements.
warm spell — A sequence of 6 or more days in which the daily maximum temperature exceeds the 90th percentile of daily maximum temperature for a 5-day running window.
water quality — The chemical, physical, biological, and radiological characteristics of water. It is a measure of the condition of water relative to the requirements of one or more biotic species and/or to any human need or purpose.
watershed — An area characterized by all direct runoff being conveyed to the same outlet. Similar terms include basin, sub-watershed, drainage basin, catchment, and catch basin.
weather versus climate — see climate versus weather.
wet spell — Maximum number of consecutive days per year with daily precipitation amount at least a trace (0.04 inches [1 mm]).