Page snapshot: Introduction to the climate of the northwest-central United States, including present, past, and future climate.
Topics covered on this page: Present climate of the northwest-central U.S.; Present temperature; Present precipitation; Severe weather; Past climate of the northwest-central U.S.; Paleozoic; Mesozoic; Cenozoic; Future climate of the northwest-central U.S.; Resources.
Credits: Most of the text on this page comes from "Climate of the Northwest Central US" by Ingrid H. H. Zabel and Judith T. Parrish, chapter 9 in The Teacher-Friendly Guide to the Earht Science of the Northwest Central US, edited by Mark D. Lucas, Robert M. Ross, and Andrielle N. Swaby (published in 2015 by the Paleontological Research Institution; currently out of print). The book was adapted for the web by Elizabeth J. Hermsen, Jonathan R. Hendricks, and Ingrid Zabel. Changes include formatting and revisions to the text and images. Credits for individual images are given in figure captions.
Updates: Page last updated Nov. 10, 2022.
Image above: Bison grazing in wintertime at Yellowstone National Park, 2016. Photo by Carol M. Highsmith (Library of Congress Reproduction Number LC-DIG-highsm-35554, Carol M. Highsmith Archive, no known restrictions on publication).
Present climate of the northwest-central U.S.
Due to their diverse topographical features, the northwest-central states encompass a broad range of climates, including subarid steppe in the Great Plains, warm temperate highlands in the Rocky Mountains, and humid continental plains in the eastern Central Lowland. Even individual states can have tremendous diversity; depending on which of the many Köppen system maps you refer to, the state of Idaho alone contains as many as eight different climate types.
The main drivers of climate in the northwest-central U.S. are exposure to Arctic air from Canada in the winter, the lack of large bodies of water nearby (except for Idaho, whose climate is influenced by the Pacific Ocean), and the presence of the Rocky Mountain chain in the west. These mountains block moist Pacific Ocean air from the interior of the continent and create a cold, high altitude zone.
Prairie at Chase Lake National Wildlife Refuge, North Dakota. Photo by Rick Bohn (USFWS Mountain-Prairie on flickr, Creative Commons Attribution-NonCommercial-ShareAlike 2.0 Generic, image resized).
Grand Tetons, Wyoming. Photo by James St. John (flickr, Creative Commons Attribution 2.0 Generic license, image cropped and resized).
Temperatures in the northwest-central U.S. are characterized by seasonal extremes. South Dakota’s temperature, for example, varies between an average low of -14°C (6°F) in January and an average high of 86°F (30°C) in July. Record lows and highs are astonishing: -57°C (-70°F) in Montana in 1954 and 49°C (121°F) in North Dakota in 1936.
Average temperatures in the northwest-central U.S. tend to decrease northward, which is in part influenced by latitude: lower latitudes receive more heat from the sun over the course of a year. The overall warmest temperatures are found in Nebraska, and the coolest are found in North Dakota and parts of Wyoming. The northwest-central states’ overall average high temperature of 14°C (57°F) and average low of 0.7°C (33°F) are indicative of a generally cool climate. By comparison, the average high and low temperatures for the entire United States are 17ºC (63ºF) and 5ºC (41ºF), respectively.
Mean annual temperature for the northwest-central states. Adapted from image by Scenarios for Climate Assessment and Adaptation.
Average Annual Temperatures in the Northwest-central United States
|Idaho||6.9°C (44.4°F)||0.8°C (33.4°F)||14.9°C (58.8°F)|
|Montana||5.9°C (42.7°F)||-0.8°C (30.6°F)||13.3°C (55.9°F)|
|Nebraska||9.3°C (48.8°F)||2.7°C (36.9°F)||17.0°C (62.6°F)|
|North Dakota||4.7°C (40.4°F)||-1.2°C (29.8°F)||11.7°C (53.1°F)|
|South Dakota||7.3°C (45.2°F)||-1.3°C (34.3°F)||14.6°C (58.3°F)|
|Wyoming||5.6°C (42.0°F)||-2.1°C (28.2°F)||13.4°C (56.1°F)|
The northwest-central U.S. is dry compared with many other parts of the United States. In fact, it is so dry that all the states within it except Nebraska rank within the top 10 driest states based on annual precipitation. Precipitation generally tends to decrease to the west across the Rocky Mountains, with an average annual precipitation of 65–90 centimeters (25–35 inches) in the Central Lowland region of the eastern Dakotas and Nebraska, about 25–50 centimeters (10–20 inches) in the Great Plains, and less than 25 centimeters (10 inches) in parts of Wyoming and Idaho. By comparison, the average amount of precipitation for the United States is 85.6 centimeters (33.7 inches).
Mean annual precipitation for the northwest-central states. Adapted from image by Scenarios for Climate Assessment and Adaptation.
Oglala National Grasslands, Nebraska, 2016. Ammchale (Wikimedia Commons, Creative Commons Attribution-ShareAlike 4.0 International license, image cropped and resized).
Pilot Butte, Wyoming, 2012. Photo by James St. John (flickr, Creative Commons Attribution 2.0 Generic license, image cropped and resized).
The decrease in precipitation is due in large part to rain shadow effects from mountain ranges located west of as well as within the northwest-central region. Rain shadows occur when moist air moves eastward with the prevailing winds, and is pushed upward and cools when it encounters a mountain chain. Water vapor condenses from this cool air and falls as rain or snow on the western side of the mountain. The air that continues to move east over the mountains is now much drier, and as it moves down the eastern side of the mountain range it warms, promoting evaporation.
The rain shadow effect occurs when moisture-laden air rises up the windward side of a mountain, only to release this moisture as precipitation due to cooling and condensation. Once the air reaches the leeward side, it warms due to compression, promoting evaporation and a lack of precipitation. Diagram by Wade Greenberg-Brand, originally published in the Teacher Friendly Guide to Earth Science series, modified for Earth@Home.
The mountainous Continental Divide, which runs through western Montana, creates a rain shadow effect that contributes to the aridity of the plains and badlands in the eastern part of the state. Nebraska’s semi-arid west and fairly uniform average temperatures are moderated by dry, warm rain shadow winds blowing eastward from the Rocky Mountains.
Exceptions to the westward drying trend are found in the mountainous parts of northwestern Wyoming and Montana, and in northern Idaho, where average annual precipitation is typically 101 to 127 centimeters (40 to 50 inches), demonstrating the impact of moisture carried inland from the Pacific Ocean. Idaho’s climate is strongly moderated by the Pacific Ocean, even though the state lies nearly 560 kilometers (350 miles) from the coast. In the winter, humidity from the ocean creates heavy cloud cover and precipitation that helps to moderate temperature.
Continental Divide, Elk Park Pass, Montana, 2010. Elevation is about 1941 meters (6368 feet). Photo by James St. John (flickr, Creative Commons Attribution 2.0 Generic license, image cropped and resized).
Coeur d'Alene, northern Idaho, 2009. Photo by bs4173 (Wikimedia Commons, Creative Commons Attribution-ShareAlike 3.0 Unported license, image resized).
The northwest-central U.S. is sparsely populated, with fewer than seven million people. Weather hazards are a concern for communities and for agriculture. When the area experiences severe drought, as Wyoming did from 1999 to 2004, it is affected by costly losses to food and water supplies and grazing land for livestock, as well as soil erosion, wildfire damage, and poor air quality. The Red River in North Dakota is highly susceptible to flooding, and since it runs through Fargo and Grand Forks, the populations and infrastructure of those cities are put at risk during floods. In the winter, cold brought on by Arctic air masses entering the area can damage livestock and crops. Nebraska, located in a corridor known as Tornado Alley, commonly experiences violent thunderstorms and tornados in spring and summer.
Flooding on the Red River, Grand Forks, North Dakota, 1997. Photo by Michael Rieger/FEMA (U.S. National Archives identifier 5696639, unrestricted use).
Tornado, Pender, Nebraska, June 2014. Photo by Brent Koops (NOAA Photo Library on flickr, Creative Commons Attribution 2.0 Generic license).
Polar jet stream and winter storms
Harsh winter storms are a fact of life in the northwest-central U.S. These storms are carried in by the polar jet stream, which typically falls near or over the area, especially in the winter. Jet streams—more than one exists—are narrow bands of fast moving air high above Earth's surface. The Earth’s rotation drives these rivers of air and causes them to blow from west to east. Jet streams are typically found between 6 and 13 kilometers (4 and 8 miles) above the surface and can move at speeds tens to hundreds of kilometers (miles) per hour.
"The Polar Jet Stream." Video description: Animation of the Earth showing the north polar jet stream. The video starts with a view centered on the north pole, with the jet stream encircling it in a sinuous wave. The Earth then turns to center on North America, showing the jet stream taking an ever-changing, sinuous route over Canada and the northern United States. Video by NASA Scientific Visualization Studio (via YouTube).
Because jet streams separate warm and cold air masses, their movements influence the weather. Polar jet streams are typically found between 50° and 60° North and South latitude, and subtropical jet streams are typically found around 30° North and South latitude. As the boundaries between hot and cold air are sharpest in the winter months, jet streams are stronger in the winter.
In the northwest-central states, the polar jet stream strongly impacts the area’s weather. Blizzards with high winds, high snowfalls, and low visibility are common. They are brought on by cold air masses known as the Alberta Low from the north and the Colorado Low from the south. Snow fences are used to keep snow from drifting over roadways (snow fences can also be used to capture snow as a water source). Spring storms are common, and heavy downpours can lead to flash flooding. Rain coupled with rapid snowmelt in the spring is a common source of flooding in the Rocky Mountain region’s river basins.
Video of blizzard conditions in South Dakota, December 24, 2020. Video description: Scenes from a South Dakota blizzard, December 2020. The first part of the video shows the view from the windshield of a vehicle driving in near whiteout conditions with blowing snow. The second part of the video shows the view from a window of a home, with blowing snow falling in a neighborhood. "Frightening drive through South Dakota blizzard" by Reuters (via YouTube).
Snow fence in Divide County, North Dakota, 1942. Photo by John Vachon (Library of Congress Reproduction Number LC-USF34-064802-D, U.S. Farm Security Administration/Office of War Information, no known restrictions).
The polar vortex
The polar vortex is a pattern of winds around the North Pole, including the polar jet stream. In the winters of 2013– 2014 and 2014–2015, the polar vortex shifted southward, bringing unusual weather patterns to much of North America. Weaker polar vortices can occur when weather near the pole is warmer than usual, and a weak polar vortex allows for a wandering jet stream. Some climate scientists believe the unusual winters of recent years are explained by natural variations, while others suggest that they could be driven due to decreases in sea ice and faster increases in arctic temperatures when compared to areas at lower latitudes.
"NASA's AIRS Sees Polar Vortex Behind U.S. Big Chill." Video description: Video shows an animation of a globe with a view of North America. Color coding indicates temperature in degrees Kelvin. The polar vortex is indicated by dark blue to purple, and reaches as far south as the northern United States, especially the northwest-central and Great Lakes regions. Video by airsnews (via YouTube).
Past climate of the northwest-central U.S.
In this Guide we divide the northwest-central states into five regions, but it is possible to more generally recognize two broad areas of strikingly different geology: the Cordilleran (Idaho, western Montana, and western Wyoming; Regions 3–5) and the Great Plains (North and South Dakota, the rest of Wyoming and Montana, and Nebraska; Regions 1 and 2). The main difference between the two areas is that the Cordilleran area has been subjected to mountain building, while the Plains area has remained tectonically quiet.
Paleozoic (541 to 252 million years ago)
Cambrian to Ordovician (541 to 444 million years ago)
Throughout most of the Paleozoic, the Northwest Central was part of a large passive margin that formed when Rodinia broke up, and major changes in deposition there were related to changes in climate and sea level. With the start of the Paleozoic era, climates across the world were warm, and North America was located in the low and warmer latitudes of the Southern Hemisphere. As the Cambrian progressed, North America moved northward; by about 480 million years ago, what would become the northwest-central region was located just above the equator. Cambrian fossils reveal that most of the area was covered by warm, shallow seas during this time.
Trilobites from Cambrian rocks of Yellowstone National Park (Idaho-Montana-Wyoming region). Left: Pieces of Wilbernia walcotti, Cambrian Snowy Range Formation. Right: Pygidium of Ehmania walcotti. Left photo of USNM 35227 and right photo of USNM 90667 by Megan Norr (Yellowstone National Park on flickr, public domain).
An echinoderm (Gogia palmeri) from the Cambrian Lead Bell Shale, Bear Lake County, Idaho. Photo of YPM IP 530453 and YPM IP 530455 by Jessica Utrup, 2016 (Yale Peabody Museum of Natural History on GBIF, CC0 1.0 Universal/public domain dedication).
The Earth went through another ice age from 460 to 430 million years ago, and although sea level dropped during this event, North America’s position near the equator kept its climate relatively warm. The change in sea level meant that the environment of the Plains area fluctuated from shallow marine, to brackish, to freshwater, and back. Farther west, in what is now the northern Rocky Mountains, the environment mostly alternated between shallow and deeper marine.
Ordovician rocks in Idaho contain abundant fossils of graptolites, which are thought to have floated in the open ocean, and thus indicate deeper waters than those implied by bottom-dwelling brachiopods, corals, cephalopods, and other fossils common in other Paleozoic rocks. One of the characteristics of these warm, shallow sea deposits is that they often alternated between limestone, sandstone, and mudstone; reefs were not common at this time.
The globe about 485 million years ago, near the Cambrian-Ordovician boundary. Note that the southwestern region of the U.S. is covered by a shallow sea. Reconstruction created using basemap from the PALEOMAP PaleoAtlas for GPlates and the PaleoData Plotter Program, PALEOMAP Project by C. R. Scotese (2016); map annotations by Jonathan R. Hendricks for PRI's Earth@Home project (CC BY-NC-SA 4.0 license).
Silurian to Devonian (444 to 359 million years ago)
A major interruption in this overall picture occurred during the Devonian period, when the huge Bakken oil formation that underlies parts of Montana and North Dakota formed. This oil-rich rock is part of a larger complex of such deposits that covered not just this area but large areas farther east as well. The richness of the organic matter indicates a sea that was highly productive, with such abundant planktonic life that the organic matter from the dead organisms took up all the oxygen in the water, allowing the rest to remain undecayed and preserved in the sediments.
Core from the Bakken Formation. Source: USGS.
Carboniferous to Permian (359 to 252 million years ago)
During the late Paleozoic, the sea gradually began to withdraw, and the Plains area became terrestrial. The sea still flooded parts of the northern Cordilleran area—this time farther west, in southeastern Idaho—and became exceptionally productive during the Permian. This is evidenced by the large deposits of phosphorite, a rock mined for fertilizer in Idaho, Wyoming, and Montana. By the late Carboniferous, North America had collided with Gondwana, leading to the formation of Pangaea. Pangaea was a supercontinent composed of nearly all the landmass on Earth.
Earth 300 million years ago, during the end of the Carboniferous Period (Pennsylvanian). Pangaea was completed when North America finally collided with Gondwana. Reconstruction created using basemap from the PALEOMAP PaleoAtlas for GPlates and the PaleoData Plotter Program, PALEOMAP Project by C. R. Scotese (2016); map annotations by Jonathan R. Hendricks & Elizabeth J. Hermsen for PRI's Earth@Home project (CC BY-NC-SA 4.0 license).
Tooth whorl of a shark (Helicoprion), Permian Phosphoria Formation, Conda Mine, Idaho. Photo of USNM V 22577 by Michael Brett-Surman (Smithsonian National Museum of Natural History, public domain).
Mesozoic (252 to 66 million years ago)
Triassic (252 to 201 million years ago)
Around 220 million years ago the northwest-central states moved north from the equator. By this time, the sea had withdrawn completely from the area. Sediments suggest that the Triassic climate in the northwest-central U.S. was warm. Initially arid, it gradually shifted to a humid climate with abundant, seasonal rainfall. The climate resembled that of modern India, where monsoons soak the land in the summer and completely dry out in the winter. At the very end of the Triassic, climate once again became arid.
Some Triassic rocks now found in the northwest-central were not, however, part of the North American continent at that time. Triassic rocks in western Idaho include tropical reefs, and the fossils found in them (such as corals and brachiopods) are not similar to fossils found in the rest of North America. Many parts of the western U.S. originated as microcontinents (also called terranes) that drifted in during the process of subduction at the continent’s active plate margin and accreted to North America as they collided with it. The Triassic reef deposits in Idaho rode in on one such microcontinent.
An outcrop of Triassic sandstone (Chugwater Formation) near Thermopolis, Wyoming. Photograph by James St. John (Flickr; Creative Commons Attribution 2.0 Generic license).
Jurassic (201 to 145 million years ago)
Pangaea began to break up during the Jurassic, rifting apart into continents that would drift toward their modern-day positions. The supercontinent was split by spreading along the mid-Atlantic ridge, initiating the formation of the Atlantic Ocean. The Jurassic remained warm, but gradually became wetter, this time without the strong seasonality of the Triassic. The region was ruled by dinosaurs, and some of the most famous Jurassic dinosaur localities in North America are found in the northwest-central states, such as Como Bluff in Wyoming.
By this time, mountain building (the Laramide Orogeny) was underway. The Black Hills were uplifted and sediment was deposited from both west and east. Ancient metamorphic rocks of the continental core were uplifted and eventually exposed in the Black Hills and even farther west.
"Dippy" (Diplodocus carnegii) from the Jurassic Morrison Formation, Como Bluff, Wyoming. Specimen on display at the Carnegie Museum in Pittsburgh, Pennsylvania. Photo by ScottRoberAnselmo (Wikimedia Commons, Creative Commons Attribution-ShareAlike 3.0 Unported license, image cropped and resized).
Earth 150 million years ago, near the end of the Jurassic Period. North America and Europe are part of Laurasia, and South America and Africa are part of Gondwana. Notice that North America has separated from Africa and there is a spreading center in the Central Atlantic Ocean. Reconstruction created using Paleomap (by C. Scotese) for GPlates. Reconstruction created using basemap from the PALEOMAP PaleoAtlas for GPlates and the PaleoData Plotter Program, PALEOMAP Project by C. R. Scotese (2016); map annotations by Jonathan R. Hendricks and Elizabeth J. Hermsen for PRI's Earth@Home project (CC BY-NC-SA 4.0 license).
Cretaceous (145 to 66 million years ago)
As a result of displacement due to continental rifting and seafloor spreading, sea level throughout the Cretaceous was much higher than it is today. Global temperatures during the Cretaceous were very warm, as much as 10°C (18°F) above those at present. There was likely little or no glacial ice anywhere on Earth, and temperatures were highest in lower latitudes.
Shallow seaways spread over many of the continents, including South America, Africa, Eurasia, and North America. In the middle Cretaceous, oceans covered most of the Southwest, with the exception of parts of Arizona and New Mexico. By the start of the Late Cretaceous, this inland sea, called the Western Interior Seaway, divided North America in two. Cretaceous fossils from the northwest-central states, especially the Great Plains region, show that the seaway supported sharks, rays, mosasaurs (large marine reptiles), and giant turtles, while crocodiles and dinosaurs were abundant on land. This seaway was also productive, although most of its organic-rich rocks lie just south of the northwest-central states.
Extent of the Western Interior Seaway during the Cretaceous Period. Image from Cretaceous Atlas of Ancient Life: Western Interior Seaway (Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International license).
A heteromorph ammonoid (Emperoceras beecheri), Late Cretaceous Pierre Shale, Custer County, South Dakota. Photo of Yale Peabody Museum of Natural History specimen YPM 52828 (Cretaceous Atlas of Ancient Life, Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International license).
A mosasaur (Platycarpus ictericus) from the Cretaceous of South Dakota. Left: Full specimen. Right: Detail of skull. Specimen on display in the Naturalis Biodiversity Center, Leiden, The Netherlands. Photos of RGM.401860 by Naturalis Biodiversity Center (on GBIF, CC0 1.0 Universal/public domain dedication).
A ceratopsian or horned dinosaur (Brachyceratops montanensis), Cretaceous Two Medicine Formation, Glacier County, Montana). Photo of USNM V 7591_15 by James Di Loreto (National Museum of Natural History, Smithsonian Institution, public domain).
In the latest Cretaceous, sea level dropped again and the western northwest-central became a broad coastal plain that hosted lush forests, abundant dinosaurs, and large swamps. By the end of the Cretaceous, uplift to the west was great enough that the resulting hills shed large amounts of sand and gravel in an easterly direction, pushing the shoreline eastward until sediment (combined with a worldwide drop in sea level) filled the area formerly occupied by the Western Interior Seaway. At the close of the Mesozoic, global climate—although warmer than today—was cooler than at the start of the era.
Sue theTyrannosaurus rex on display at the Field Museum of Natural History in Chicago, Illinois. Photo by Evolutionnumber9 (Wikimedia Commons, Creative Commons Attribution-ShareAlike 4.0 International license, image cropped and resized).
Edmontosaurs annectens from the Cretaceous, South Dakota. Specimen on display at the Carnegie Museum of Natural History (Pittsburgh, Pennsylvania). Specimen on display in the Naturalis Biodiversity Center, Leiden, The Netherlands. Photos of RGM.401870 by Naturalis Biodiversity Center (on GBIF, CC0 1.0 Universal/public domain dedication).
End-Cretaceous extinction (66 million years ago)
At the very end of the Cretaceous, the Gulf Coast experienced an enormous disruption when a large asteroid or bolide collided with Earth in what is now the northern Yucatán Peninsula in Mexico. The impact vaporized both water and rock, blocking out sunlight for weeks to years, which led to a collapse of photosynthesis and food webs on land and in the oceans. The event devastated the northwest-central U.S.
Map of the modern Yucatán Peninsula region showing the location of the Chicxulub impact crater. The inset image is a shaded relief image that shows the edge of the crater on the Yucatán Peninsula with sinkholes in the rock surrounding it. Map made by Elizabeth J. Hermsen using Simplemappr and modified in Photoshop. Inset image from the NASA Jet Propulsion Laboratory (PIA03397).
A butte near Jordan, Montana, that preserves the Z-coal marking the Cretaceous-Paleogene (K-Pg) boundary. Layers in the Cretaceous Hell Creek Formation are labeled Khc, whereas the overlying Paleogene Union Formation is labeled PgF. Source: Figure 21 from Fowler (2020) Geosciences 10(11) (Creative Commons Attribution 4.0 International license).
Cenozoic (66 million years ago to present)
Paleogene to Neogene (66 to 2.6 million years ago)
Following that event, the climate may have cooled briefly (as suggested, for example, by an abundance of ferns), but it soon rebounded to a warmer state, and continued to warm into the Eocene. Around 60 million years ago, much of the northwest-central U.S. had a milder climate than it does today, and it was even subtropical in some areas. Dinosaurs gave way to mammals, and forests with ferns, palms, and dawn redwoods provided food for browsers.
The climate was wet enough in the Eocene to support large lakes in Wyoming, although these lakes occasionally dried out. The lakes supported an abundant diversity of fish and other organisms that today are exquisitely preserved as the famous Green River Formation fossils.
Branchlets and leaves of dawn redwood (Metasequoia occidentalis) from the Paleocene Golden Valley Formation, Oliver County, North Dakota. This genus of tree is native only to China today. Photo of YPM PB 028230 by Division of Paleobotany, Yale Peabody Museum (Yale Peabody Museum of Natural History on GBIF, CC0 1.0 Universal/public domain dedication).
The size and location of various lakes in which the Green River Formation sediments were deposited during the Eocene epoch. Maps modified from maps by Wade Greenberg-Brand, originally published in The Teacher-Friendly Guide to the Earth Science of the Southwestern US, after figure 3 in L. Grande (2013) The Lost World of Fossil Lake.
Slab showing mass mortality of Knightia eocaena, a freshwater fish similar to a herring, Fossil Lake, Wyoming. Photo by National Park Service (public domain).
By the early Cenozoic, the continents had approached their modern configuration, and India began to collide with Asia to form the Himalayas. The formation of the Himalayas had a significant impact on global climate, with the newly exposed rock serving as a sink to take up atmospheric CO2. With the reduction of this greenhouse gas, global temperatures cooled. Antarctica moved south, and by 30 million years ago, temperatures were low enough that glaciers began to grow on its mountains.
Studies of ancient soils show that parts of Montana, Nebraska, and Wyoming went through several periods of warm, wet climate between 35 and 4 million years ago, although overall the climate became drier. Grasses evolved during the Miocene as climate became drier. Miocene rocks in Nebraska support some of the most amazing sites for fossil mammals known anywhere.
A fossil rhino (Menoceras) bone bed, Miocene Harrison Formation, Nebraska. Specimen on display at the Carnegie Museum, Pittsburgh, Pennsylvania. Photo by James St. John (flickr, Creative Commons Attribution 2.0 Generic license, image resized).
Approximately 3.5 million years ago, glacial ice began to form over the Arctic Ocean and on the northern parts of North America and Eurasia. A major contributing factor to this event was a geological change that occurred far to the south. The Central American Isthmus, which today makes up most of Panama and Costa Rica, rose out of the ocean at approximately this time, formed by undersea volcanoes. The new dry-land isthmus blocked the warm ocean currents that had been flowing east-to-west from the Atlantic to the Pacific for more than 100 million years, diverting them into the Gulf of Mexico and ultimately into the western Atlantic Gulf Stream. The strengthened Gulf Stream carried more warm, moist air with it into the northern Atlantic, which caused increased snowfall in high latitudes, leading to accelerating cooling.
Map of the Gulf of Mexico region before the closure of the Isthmus of Panama. Before the Isthmus closed, the Atlantic and Pacific oceans were connected. Map modified from a map by Chiche Ojeda (Wikimedia Commons, Creative Commons Attribution-ShareAlike 4.0 International license, image cropped and modified).
Maps showing the progressive closure of the Isthmus of Panama at 20 million years ago (A) and 15 million years ago (B). Dark gray is land, white and light gray are submerged areas. Modified from Figure 11 in Kirby et al. (2011) PLoS ONE 3(7): e2791 (Creative Commons Attribution license, image reorganized and resized).
Quaternary (2.6 million years ago to present)
Eventually, a sheet of sea ice formed over the Arctic, and ice sheets spread over northern Asia, Europe, and North America, signaling the start of the most recent ice age. Since 800,000 years ago, an equilibrium has been reached between warming and cooling, with the ice caps growing and retreating primarily due to the influence of astronomical forces (i.e., the combined gravitational effects of the Earth, Sun, moon, and planets). During the ice sheet’s maximum extent, it reached into Montana, the Dakotas, and Nebraska.
Extent of glaciation over North America at the Last Glacial Maximum. Image adapted from map by the USGS (public domain).
Aerial photo of the Prarie Pothole Region, Waubay Wetland Management District, South Dakota. The "potholes" are kettle lakes formed by chunks of ice left by a continental glacier. Photo by USFWS (public domain).
The portions of the northwest-central U.S. that were not covered by ice experienced a variety of cold climates and abundant lakes. These lakes were also related to two very large flooding events, among the largest floods on Earth.
The first flood event was the Bonneville megaflood: melting glaciers fed the waters of ancient Lake Bonneville (the remains of which are today the Great Salt Lake), which broke through a dam of loose sediment and rapidly drained northward through southern Idaho, along what is now the Snake River, all the way to northern Idaho.
The second flood event was a series of floods that occurred when the ice sheet alternately blocked and retreated from what is now the Clark Fork River in northwestern Montana and northern Idaho. When the river was blocked, an enormous lake built behind the ice dam, and when the ice dam failed, the water was released catastrophically. Although the floods mostly affected central Washington, large ripples from the intense flow are preserved both near Missoula, Montana, and just downstream from where the ice dammed the river in northern Idaho.
Between 13,000 and 8500 years ago, fossil evidence shows that spruce and aspen forests grew in areas of North Dakota that are now warmer, drier, and covered with prairie. Idaho became more humid and warmer than it was during the last glacial maximum.
Pleistocene Lake Bonneville. Left: Lake Bonneville's maximal extent during the Pleistocene. Right: Graph of the lake's changing level over time. Modified from illustrations by Wade Greenberg-Brand originally published in The Teacher-Friendly Guide to the Earth Science of the Southwestern US.
Map showing the extent of glacial Lake Missoula and the Channeled Scablands. Map modified from a map by Wade Greenberg-Brand, adapted from image by the Montana Natural History Center, originally published in The Teacher-Friendly Guide to the Earth Science of the Northwest Central US.
Future climate of the northwest-central U.S.
The northwest-central U.S. contributes to climate change, although its total greenhouse gas emissions are lower than those of other areas of the United States. The population of any industrialized and particularly wealthy country produces pollution; the majority of these emissions come from the use of petroleum. The 6.5 million residents of the northwest-central U.S. use electricity, transportation, and products that come from carbon-rich fossil fuels. Burning fossil fuels releases carbon into the atmosphere, which warms the Earth.
Of the northwest-central states, Wyoming emits the most greenhouse gases, releasing about 56 million metric tons of energy-related carbon dioxide in 2020. By contrast, the highest greenhouse gas-emitting state in the nation is Texas, which released more than 624 million metric tons of CO2 in 2020. Idaho and South Dakota were among the ten lowest carbon emitters in the nation in 2020, each producing less than 20 million metric tons of CO2. The northwest-central region’s low total emissions profile is related to its low population. For example, Wyoming has fewer than 600,000 residents; in 2020 it emitted over 96 metric tons of CO2 per capita, the highest in the nation, while Texas, with a population of more than 28 million, emitted only about 21 metric tons of CO2 per capita.
Dave Johnston Power Plant, a coal-fired power plant in Wyoming, photographed in 2006. Photo by Greg Goebel (flickr, Creative Commons Attribute-ShareAlike 2.0 Generic).
A gas flare (nature gas being burned off) in the Bakken Formation, North Dakota. Natural gas (methane, CH4) is a powerful greenhouse gas. Photo by Joshua Doubek (Wikimedia Commons, Creative Commons Attribution-ShareAlike 3.0 Unported license, image cropped and resized).
Energy production has been changing rapidly in the U.S., and this is apparent in changes in greenhouse gas emissions in the northwest-central region. As recently as 1990, Montana was estimated to be a net carbon sink, with carbon sequestered in its forests and soils. By 2005, it had become a net carbon emitter, and carbon emissions in other northwest-central states have increased as well. Over the period from 2000 to 2011, Nebraska experienced a 25% increase in the amount of CO2 it emitted—the greatest absolute increase in the country—due to an increasing amount of fossil fuel-related energy production. In the early to mid-2010s, emissions began to trend down in several northwest-central states.
Time series graph of energy-related carbon dioxide emissions from northwest-central states, from 1970 to 2020. Figure by Ingrid Zabel for PRI's Earth@Home project (CC BY-NC-SA 4.0) using data from eia.gov.
Nebraska City Station near Nebraska City, Nebraska. This coal-burning power plant has a capacity of more than 1350 MW. Photo by Ammodramus (Wikimedia Commons, CC0 1.0 Universal/public domain dedication).
On the other hand, many northwest-central states are also making changes to reduce human impact on the climate. Boise, Idaho, Big Sky, Montana, and Jackson, Wyoming, are just a few locations that have adopted the 2030 Challenge, an effort by cities to reduce fossil fuel use in buildings so that both new and renovated buildings will qualify as carbon neutral by the year 2030. Additionally, many states are stepping up their use and production of renewable energy. In 2021 Idaho ranked fourth in the nation for renewable energy production, most of which it generates from hydroelectricity. Idaho is also one of only a small number of states that generates electricity on a utility scale from geothermal energy. In 2021 South Dakota got over 80% of its electricity from wind and hydropower, and for the first time ever wind energy overtook hydropower as the main type of renewable in-state electricity generation.
Boise, Idaho, 2021. Photo by Jyoni Shuler (Wikimedia Commons, Creative Commons Attribution-ShareAlike 4.0 International license, image resized).
Hungry Horse Dam (a hydroelectric dam), Montana, 2013. Photo by Tonyglen14 (flickr, Creative Commons Attribution 2.0 Generic license, image resized).
Trends and predictions
Studies show that the northwest-central states' climate is changing right now and that change has accelerated in the latter part of the 20th century. These changes include the following:
- During the 20th century, the average annual temperature of the northwest-central U.S. as a whole increased by 0.9°C (1.6°F). North Dakota’s average temperature increased 1.9°C (3.4°F) during the last 130 years, the fastest increase in the U.S.
- Soils in Nebraska have become warm enough to plant corn one to three weeks earlier in the 2000s compared to in the 1990s.
- Springtime snowmelts in Wyoming in 1990 were flowing four days earlier than in 1950.
- The Ogalalla Aquifer, which provides fresh water to most of Nebraska, has been depleted by more than 40% in some areas, thanks to years of decreased rainfall.
- The bull trout, an endangered freshwater fish native to northwestern North America, is estimated to have lost 11% of its stream habitat in Idaho’s Boise River Basin due to an increase in water temperature.
- In the last century, annual precipitation has increased by up to 20% in South Dakota.
- In 1850, Montana’s Glacier National Park contained an estimated 150 glaciers. In 2015, only 26 glaciers remained. Models predict that all of them will have disappeared by 2030.
Original caption: "This map shows changes in Ogallala water levels from before the aquifer was tapped to 2015. Declining levels are red or orange, and rising levels are blue." Map from Climate.gov (NOAA).
A person attaching a tag to a bull trout, Boise River, Idaho. Photo by Bureau of Reclamation (flickr, Creative Commons Attribution-ShareAlike 2.0 Generic license, image cropped).
Boulder Glacier in 1910 and 2007, Glacier National Park, Montana. Photos by Morton Elrond/University of Montana Collection (1910) and Fagre/Pederson/USGS (2007). Source: USGS (public domain).
Climate models predict that the northwest-central region’s climate will continue to warm; the average annual temperature in most of the area is expected to rise by 3°C to over 6°C (6°F to over 10°F) by the end of the 21st century. These increased temperatures lead to a whole host of other effects, including drier soils from greater evaporation and increased likelihood of drought and fires. In Montana, for example, the annual amount of wildfire-prone land area is predicted to increase by nearly 400% by the end of the century.
Agriculture is a huge industry in the northwest-central U.S., especially in the Great Plains and Central Lowland. To the advantage of soybean and corn growers in Nebraska, warmer temperatures and increased precipitation have helped bring on longer growing seasons. Warmer temperatures, however, also make it easier for insect pests to overwinter and produce more generations. The European corn borer, a devastating pest found in the central and eastern U.S., produces more generations in warmer parts of the country. As the Great Plains and Central Lowland warm, one can expect three or four generations of these pests annually in regions that previously had only one or two.
A European corn borer (Ostrinia nubilalis). Photograph of Keith Weller, USDA-ARS (Wikimedia Commons, public domain).
Another major pest affected by the warming climate is the mountain pine beetle (Dendroctonus ponderosae), which has been devastating pine forests throughout the Pacific Northwest and Canada, and is now spreading west into Montana, Wyoming, and the Dakotas. The beetle’s numbers can spiral out of control due to warmer temperatures, which extend the breeding season and generate fewer cold-related die-offs in the insect population.
So far, 36 million hectares (88 million acres) of pine forest have been affected, with a 70–90% tree mortality rate. The death of these trees will have a significant impact on the forests’ ability to sequester carbon; researchers have estimated that the die-offs in Canada alone will have caused the release of 270 million metric tons of CO2 into the atmosphere by 2020.
Video showing Lansat photos documenting mountain pine beetle (bark beetle) damage in the Black Hills, South Dakota. Note: The video includes narration. "Image of the week - beetle damage in the Black Hills." USGS on YouTube.
Future precipitation and water supply
Water supply is a critical issue in the northwest-central U.S., and communities will need to adapt to changes in precipitation, snowmelt, and runoff as the climate changes. Models project less future summer precipitation and more future winter precipitation. In Idaho, it is likely that precipitation will increasingly fall as rain rather than snow, and snow in the mountains will melt earlier in the spring. This could strain the water supply in the warm season.
Because higher temperatures mean greater evaporation and warmer air can hold more water, precipitation will occur in greater amounts at a time. During the cooler spring this will lead to flooding; during hotter summers, droughts will become more frequent. These drier summers and wetter winters and springs could have significant adverse impacts. Drier summer days and higher temperatures will amplify evaporation, increasing the risk of desertification and affecting natural ecosystems as well as increasing pressure on the water supplies for agriculture and cities.
An example of what could happen more frequently in the future happened in Yellowstone National Park in June 2022. Heavy rains fell before snowpack had completely melted, causing flooding that washed out roadways and damaged other infrastructure in the park. The U.S. Geological Survey Yellowstone Volcano Observatory called flooding in parts of the Yellowstone region in June 2022 "literally unprecedented."
U.S. drought map for September 1, 2022. As of August 2022, much of the western U.S. was experiencing drought conditions, including parts of Idaho, Montana, Nebraska, South Dakota, and Wyoming. Map from U.S. Drought Monitor.
The North Entrance Road to Yellowstone National Park was washing out during flooding in June 2022. Photo by Jacob W. Frank, National Park Service (Yellowstone National Park on flickr, pubic domain).
Video description: The first part of this video shows scenes of Yellowstone National Park during a flood event on June 13, 2022, including swollen rivers, washed out roadways, and erosion in various areas. The second park of the video shows scenes of damage after floodwater receded, including damaged roads and erosion. This video has no narration. "Minute Out In It: Flood Event June 13, 2022," National Park Service/Jacob W. Frank.
Future severe weather events
The causes of specific weather events such as tornados and severe thunderstorms are incredibly complex, although climate change has enhanced some correlated factors, such as increased wind speed and an unstable atmosphere. Higher atmospheric moisture content has also been correlated with an increased incidence of tornados and winter storms. Although climate change is predicted to enhance the intensity of severe weather, there is currently no way to calculate what effect climate change will have on the frequency of specific storm events. For example, we might see more powerful tornados, but we do not know if we will see more of them.
A tornado near LaGrange, Wyoming, 2009. Photo by Dr. Mike Coniglio, NOAA NSSL (NOAA photo library on flickr, Creative Commons Attribution 2.0 Generic license).
Climate change and wildfire risk
The risk of dangerous wildfires is currently very high in parts of the northwest-central U.S.. The reasons for this are complex and involve a combination factors. Weather conditions, particularly hot, dry weather and wind that spreads flames, contribute significantly to the ignition and growth of wildfires. Other elements involved in the ignition and growth of fires and the risks they pose to people living in the northwest-central U.S. include (but are not limited to) forest management practices, development patterns, and human behavior (intentionally or unintentionally starting fires).
As average temperatures rise and the northwest-central states becomes drier with a longer annual fire season (season conducive to the ignition and spread of wildfires), the number and intensity of wildfires is expected to increase.
Wildfire risk map for the United States. Source: FEMA National Risk Index.
An airplane dropping fire retardant on the Ross Fork Fire, Idaho, 2022. Photo by Austin Catlin, BLM (National Interagency Fire Center on flickr, Creative Commons Attribution 2.0 Generic license).
Moose Fire, Idaho, 2022. Photo by Marilynn Davis (National Interagency Fire Center on flickr, public domain).
Resources from the Paleontological Research Institution
Zabel, I.H.H., D. Haas, and R.M. Ross. 2017. The Teacher-Friendly Guide to Climate Change. Paleontological Research Institution Special Publication 53, Ithaca, NY, 294 pp. Find updated, digital versions of the chapters of this book on Earth@Home Climate.
Allmon, W. D., T. A. Smrecak, and R. M. Ross. 2010. Climate change—past, present & future: a very short guide. Paleontological Research Institution Special Publication 38, Ithaca, NY, 200 pp.
Earth@Home Climate: https://climate.earthathome.org/
Earth@Home: Quick guides & FAQ: Climate and Energy: https://earthathome.org/quick-faqs/#climate
Earth@Home: Here on Earth: Introduction to Climate: https://earthathome.org/hoe/climate/