Photograph of a field with buildings and an old windmill surrounded by trees without leaves on the horizon. The sky is partly cloudy to the left, stormy with a rainbow to the right.

Climate of the Southwestern United States

Page snapshot: Introduction to the climate of the southwestern United States, including present, past, and future climate.


Topics covered on this page: Present climate of the southwestern U.S.; Present temperature; Present precipitation; Severe weather; Regional climate variation; Past climate of the southwestern U.S.; Paleozoic; Mesozoic; Cenozoic; Future climate of the southwestern U.S.; Resources.

Credits: Most of the text on this page comes from "Climate of the Southwestern US" by Ingrid H. H. Zabel, Judith T. Parrish, and Andrielle N. Swaby, chapter 8 in The Teacher-Friendly Guide to the Earth Science of the Southwestern US, edited by Andrielle N. Swaby, Mark D. Lucas, and Robert M. Ross (published in 2016 by the Paleontological Research Institution; currently out of print). The book was adapted for the web by Elizabeth J. Hermsen and Jonathan R. Hendricks in 2022. Changes include formatting and revisions to the text and images. Credits for individual images are given in figure captions.

Updates: Page last updated May 12, 2022.

Image above: Sun and storm in Weld County, in the Great Plains region of Colorado, 2015. Photo by Lane Pearman (flickr, Creative Commons Attribution 2.0 Generic license, image cropped and resized).

Present climate of the southwestern U.S.

Although much of the Southwest falls within the category of an arid zone, using a single label to describe the Southwest's climate would belie its diversity. The main features that influence the area’s climate are latitude, regional topography, and a low atmospheric moisture content that leads to quick evaporation. For example, parts of the Colorado Rockies experience cool annual temperatures and over 8 meters (25 feet) of snowfall every year, while the dry deserts in southwestern Arizona receive only about 8 centimeters (3 inches) of precipitation a year and can experience as much as a 15°C (60°F) degree temperature difference between night and day.

The location of the Southwest and the topographical extremes across this area strongly influence its weather. The Southwest experiences nearly every variety of extreme weather; heat snaps and cold waves, droughts, floods, blizzards, and even tornados are all considerations for residents of the southwestern states.


Photograph of the Santa Catalina Mountain range with Saguaro cacti in the foreground.

The Santa Catalina Mountains near Tucson, Arizona are surrounded desert in which saguaro cacti grow. The highest point in these mountains has a relief of 1572 meters (5157 feet) over the surrounding landscape, and the mountains are tall enough to receive snowfall. Photograph by Bill Morrow (FlickrCreative Commons Attribution 2.0 Generic license).


Temperature

Average temperatures found in the Southwest tend to decrease northward, which is largely the influence of latitude and elevation. Lower latitudes receive more heat from the sun over the course of a year; for each degree increase in latitude, there is approximately a 1°C (2°F) decrease in temperature. Higher elevations (such as those found in the Rockies and on the Colorado Plateau) are also cooler, with approximately a 1.5°C (3°F) decrease in mean annual temperature for each 300-meter (1000-foot) increase in elevation. The warmest temperatures in the Southwest are found in Arizona and New Mexico, while the coolest are found in Utah and Colorado. The Southwest's overall average high temperature of 19.2°C (66.6°F) and average low of 2.8°C (37.0°F) are indicative of a varied climate, one much less uniform than that found in many other parts of the United States. By comparison, the average high and low temperatures for the entire United States are 17°C (63°F) and 5°C (41°F), respectively.

Another factor besides latitude and elevation that influences temperature in the Southwest is its arid climate. The lack of moisture in the air allows heat trapped in the earth during daylight hours to rapidly radiate away, leading to cool evenings. Thus, each Southwestern state experiences both extreme highs and lows. In New Mexico, for example, the average difference between the daily high and low temperatures ranges from 14° to 19°C (25° to 35°F). Record high temperatures for the Southwest range from 53°C (128°F) in Arizona to 47°C (117°F) in Utah, while record low temperatures range from −56°C (−69°F) in Utah to −40°C (−40°F) in Arizona.


Map showing the four southwestern states (Arizona, Colorado, New Mexico, and Utah) shaded by color to show temperature.  The coolest regions average from -5 to 1.1 degrees Celsius (21 to 30 degrees Fahrenheit), whereas the warmest are 24.4 to 26.6 degrees Celsius (76 to 80 degrees Fahrenheit).

Average annual temperatures for the southwestern U.S. Image adapted from an image by Scenarios for Climate Assessment and Adaptation, first published in The Teacher-Friendly Guide to the Earth Science of the Southwestern US.


Average Annual Temperatures in the Southwestern United States

State nameAverage TemperatureAverage Low TemperatureAverage High Temperature
Arizona16.0°C (60.8°F)7.6°C (45.7°F)24.5°C (76.1°F)
New Mexico11.9°C (53.2°F)3.0°C (37.4°F)20.3°C (68.5°F)
Utah9.1°C (48.4°F)1.5°C (34.7°F)16.7°C (62.1°F)
Colorado7.5°C (45.5°F)-0.9°C (30.4°F)15.4°C (59.7°F)

Precipitation

The average precipitation for the United States is 85.6 centimeters (33.7 inches). In the Southwest, average precipitation ranges from only 34 centimeters (13.4 inches) in Utah to 39.9 centimeters (15.7 inches) in Colorado, which reflects the area's general aridity. Elevation does, however, play a key role in precipitation received throughout the Southwest. In New Mexico, for example, average annual precipitation ranges from less than 25 centimeters (10 inches) within the Great Plains and Basin and Range regions to more than 50 centimeters (20 inches) at the higher elevations to the northwest. Arizona's highest elevations receive an average of 65 to 76 centimeters (25 to 30 inches), with lower areas in the state’s southwestern portion averaging less than 8 centimeters (3 inches). In Utah, areas below 1200 meters (4000 feet) receive less than 25 centimeters (10 inches) per year, while higher elevations in the Wasatch Mountains receive more than 100 centimeters (40 inches).

Across New Mexico, Arizona, and Utah, summer rains originate from moisture brought into the area from the Gulf of Mexico. Warm, moist air from the south occasionally but infrequently moves into Colorado during the summer. During the winter, moisture travels from the west, as storms from the Pacific Ocean move east. Pacific storms lose most of their moisture as they pass over the Rocky Mountains, so much of the Southwest's winter precipitation falls as snow within the area’s mountainous regions.


Map showing the four southwestern states (Arizona, Colorado, New Mexico, and Utah) shaded by color to show precipitation. The driest regions are dark red and receive less than 25 centimeters (10 inches) of rain annually. In a few places in Colorado, Utah, and New Mexico, 104 to 114 centimeters (41 to 45 inches) of precipitation fall.

Average annual preciptiation for the southwestern U.S. Image adapted from an image by Scenarios for Climate Assessment and Adaptation, first published in The Teacher-Friendly Guide to the Earth Science of the Southwestern US.


Severe Weather

Thunderstorms and tornados

The geography and climate of the southwestern U.S. east of the Rocky Mountains (in other words, in the Great Plains region in Colorado and New Mexico) are nearly ideal for their formation of thunderstorms and tornados, especially in the summer.

Storms form when there is strong convection in the atmosphere. Convection occurs when buoyant warm air rises (moves up) while denser cool air sinks (moves down). Convective mixing forces the moisture in warm air to condense as it comes into contact with cool air, forming vapor (clouds) and precipitation (for example, rain or hail). A strong difference in air temperature at different heights creates instability; the warmer the air near the surface is relative to the air above it, the more potential (stored) energy the warm air has to move up, and the more potential for a storm. The final ingredient is wind. Wind moves the air, promoting mixing. The formation of precipitation also causes electrical charging of particles in the atmosphere, which in turn produces lightning.


3-Panel image showing stages of formation in a thunderstorm. Panel 1: Cumulus cloud formation. Panel 2: Mature stage with convective mixing of air. Panel 3: Dissipating stage with reduced air mixing and rainfall.

Stages in the formation of a thunderstorm. Left: Warm air rises. Moisture condenses out of the warm air as it comes into contact with cool air, forming clouds. Center: As warm air rises, cool air sinks. Precipitation forms. Right: As the vertical column of air turns over, with warm air at the top and cool air at the bottom, the storm begins to dissipate. Image by The High Fin Sperm Whale, created from images by NOAA National Weather Service training material (Wikimedia Commons, public domain).


Photograph of a cloudy sky with two funnel clouds (cone-shaped extensions of the clouds) extending towards the ground. On the ground, the landscape is so dark it is nearly black.

Funnel clouds (developing tornadoes) over El Paso County, Colorado, March 29, 2019. Photo by Stefan Klein (Wikimedia Commons, Creative Commons Attribution-ShareAlike 4.0 International license, image cropped and resized)


Typically, a storm blows itself out once the warm air has moved up and the cool air has moved down. Convective mixing stops because the vertical column of air has turned over so that the cool air is at the bottom and the warm air is at the top.

A strong temperature difference at different heights creates instability. The warmer the air near the surface is relative to the air above it, the more potential energy it has to move up. The Great Plains receive warm, moist air moving north from the Gulf of Mexico, and cold, dry air moving in from the Rocky Mountains and the northern U.S. Where these air masses meet, vigorous mixing causes thunderstorms. Because warm air can hold more moisture than cool air can, convective mixing with cool air forces moisture to condense out of warm air as vapor (clouds) and precipitation. This movement of air in different directions is also the reason for the high incidence of powerful tornados that occur along "Tornado Alley" in the Great Plains, which affect eastern New Mexico and especially eastern Colorado.


Map of the United States showing the frequency of tornado watches in individual counties, with a region in the central U.S. known as "Tornado Alley" identified.

Average yearly tornado watches in each county of the United States between 1993 and 2012. Tornado Alley is identified. Map by NOAA (public domain) modified for the [email protected] project.


Regional climate variation

In a broad sense, the Southwest’s climate is mostly dry and hot, with much of the region characterized as arid. Dry conditions are common throughout the Great Plains, Colorado Plateau, and Basin and Range. Cold continental conditions dominate the higher altitudes, especially within the Rocky Mountains. Scattered pockets of drier, Mediterranean temperatures can also be found.


Detailed Koppen climate map of the continental United States.

Köppen climate map of the 48 contiguous states of the continental United States. The first letter of each zone in the key indicates its major classification. A = Tropical (equatorial), B = Arid, C = Temperate (warm temperate), D = Continental (cold), E = polar. Modified from a map by Adam Peterson (Wikimedia CommonsCreative Commons Attribution-ShareAlike 4.0 International license).


Arizona

Arizona's climate is influenced by three main topographical areas: the high Colorado Plateau (about 1520–2130 meters or 5000–7000 feet in elevation), the rugged mountains to the west (2740–3660 meters or 9000–12000 feet high), and the low southwestern mountains with desert valleys (as low as 30 meters or 100 feet above sea level). While the state is generally arid, its high western mountains experience more precipitation each year than the desert southwest and the high northeastern plateau do. The desert experiences large temperature extremes, especially between day and night; daily temperature may change as much as 15°C (60°F) during the driest parts of the year.


Photograph of the Sonoran Desert in Arizona. The photo shows the sun just over a low mountain range, flooding the landscape with yellow to orange sunlight. In the foreground, branch saguaro cacti, short, highly branched cholla cacti, and shrubs dot the arid landscape.

Saguaro and cholla cacti in the Sonoran Desert National Monument, Arizona. The Sonoran Desert is located in southwestern Arizona and adjacent regions of California and Mexico. Photo by Bob Wick, Bureau of Land Management (flickr, public domain).


Colorado

Colorado has a generally cool and continental climate with low humidity. The climate of the eastern plains is fairly uniform, with hot, windy summers and thunderstorms. The state's highest temperatures occur in the northeastern plains, where they can exceed 46°C (115°F). Moving westward, Colorado's foothills and mountainous areas experience an overall cooler climate and higher levels of precipitation. Here, the state’s varied topography leads to wide changes in climactic conditions that occur across short distances. For example, the difference in annual mean temperature between Pikes Peak (4302 meters or 14,114 feet) and Las Animas (1188 meters or 3898 feet), only 145 kilometers (90 miles) to the southeast, is equivalent to that between Iceland and southern Florida! Precipitation also varies widely. Cumbres in the San Juan Mountains receives nearly 7.6 meters (300 inches) of snowfall annually, while Manassa, less than 50 kilometers (30 miles) away in the San Luis Valley, receives only about 63 centimeters (25 inches) of snow a year.


Photograph of a lake surrounded by mountain peaks in Rocky Mountain National Park, Colorado. The nearest slope is covered with conifers. More distant peaks have no vegetation and a light dusting of snow. In the foreground is the surface of a lake.

Bear Lake and Glacier Gorge, Rocky Mountain National Park, Colorado, 2011. The elevation of Bear Lake is about 2880 meters (9450 feet). Photo by Daniel Mayer (Wikimedia Commons, Creative Commons Attribution-ShareAlike 3.0 Unported license, image resized).


New Mexico

In New Mexico, climate is characterized by arid, semiarid, or continental conditions, with light precipitation, low humidity, and abundant sunshine. As in Arizona, the desert experiences a large range of temperature on a daily basis. The state's mountainous areas, however, have climate characteristics that more closely follow those found in the Colorado Rockies. Summer rains fall almost entirely during brief but intense thunderstorms on the Great Plains, although the occasional hurricane in the Gulf of Mexico may push heavier precipitation inland. Winter is the driest season in New Mexico, because precipitation from eastward-traveling Pacific storms is left behind in the western mountains of Arizona and Utah.


Photograph of Las Cruces, New Mexico. On the horizon, a mountain range rises with the city on a flat plain in front of it. Nearest the camera is a dry landscape dotted with plants separated by patches of bare ground.

Las Cruces, New Mexico, 2006. Las Cruces is located in the Basin and Range region of New Mexico. Photo by Richard Stephen Haynes (Wikimedia Commons, Creative Commons Attribution-ShareAlike 4.0 International license, image resized).


Utah

Utah’s distance from both the Pacific Ocean and the Gulf of Mexico prevents heavy precipitation, and much of the state is typically sunny year-round, with light to moderate winds. Changes in atmospheric pressure during the late fall and winter can lead to an accumulation of haze. Light precipitation travels eastward over the Sierra Nevada and Cascade mountains after dropping heavy snowfall in areas of high elevation. Because high mountains to the west and north act as a barrier to cold Arctic air masses, most areas of Utah rarely experience temperatures below freezing or prolonged periods of extreme cold.


Photograph showing Salt Lake City, barely visible in the distance due to a haze caused by smog.

Smog (haze caused by air pollution) over Salt Lake City, Utah, 2016. Photo by Eltiempo10 (Wikimedia Commons, Creative Commons Attribution-ShareAlike 4.0 International license, image resized).

Past climate of the southwestern U.S.

This section covers the climate of the southwestern U.S. through the Phanerozoic, from about 541 million years ago to the recent. For the climate on early Earth prior to 541 million years ago, see the Introduction to Climate section.


Introduction to Climate

Paleozoic (541 to 252 million years ago)

Cambrian to Ordovician (541 to 444 million years ago)

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, and what would become much of the southwestern U.S. was located near the Tropic of Capricorn. Shallow seas invaded the continent, ultimately covering the whole area until the late Carboniferous. During this time, the only exposed areas were islands in western Colorado and parts of New Mexico. There is a rich marine fossil record from the areas between these islands. 

In the late Ordovician (about 460 to 430 million years ago), the Earth fell into another brief but intense ice age. Glaciers covered most of the world's southern landmasses, which were located over the South Pole. This led to global cooling and dropping global sea levels. North America's position near the equator kept its climate relatively warm. Ordovician deposits across the Southwest indicate warm, shallow seas rich in invertebrate life. Shelly sandstones in Utah represent vast tidal flats.


Illustration of the Earth near the Cambrian-Ordovician boundary, about 485 million years ago.

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 [email protected] project (CC BY-NC-SA 4.0 license).


2-panel figure showing photographs of trilobites from the Cambrian Bright Angel Shale, Grand Canyon. Panel 1: specimen with a single whole trilobite and several trilobite fragments; the whole trilobite has no spines or ornamentation. Panel 2: Specimen of a single whole trilobite; this trilobite looks similar to the complete trilobite in the first panel.

Cambrian trilobites from the Bright Angel Shale (Tonto Group), Grand Canyon National Park, Arizona. Left: Trilobites identified as Dolichometoppus productus and Alokistocare altheaRight: Dolichometoppus productusLeft photo and right photo by NPS/Michael Quinn (Grand Canyon National Park via flickr, Creative Commons Attribution 2.0 Generic license, images cropped and resized).


Photograph of a crinoid from the Ordovician of Utah. The specimen appears to preserve only the arms, with the ends of the arms on the left and the bases on the right. The arms are made up of a series of segments or plates. The scale bar is in millimeters (specimen probably around 2 to 3 centimeters long).

A crinoid (Ibexocrinus lepton) from the Ordovician Kanosh Shale, Millard County, Utah. Photo of USNM PAL 165239 by Crinoid Type Project (Smithsonian National Museum of Natural History, public domain).


Silurian to Devonian (444 to 359 million years ago)

In the Silurian and Devonian (430 to 359 million years ago), North America moved north across the equator, and the cycle of warming and cooling was repeated yet again. Glaciation in the Southern Hemisphere occurred during the late Devonian, while the supercontinent Gondwana was located over the South Pole, and intensified during the early Carboniferous. At this time, the Southwest was still submerged. The oceans between Gondwana and North America began to close.

Carboniferous (359 to 299 million years ago)

In the early Carboniferous (Mississippian), ice capped the South Pole and began to expand northward. Much of the Southwest became an archipelago of warm shallow seaways and uplifted islands, with terrestrial swampy forests and shallow sea floors populated by bivalves, brachiopods, arthropods, corals, and fish.


Photograph of a chunk of dark brown rock containing large rugose corals from the Mississippian of Utah. The corals are off-white in color, solitary, randomly oriented, and horn-shaped. A 3 centimeter scale bar is on one corner of the rock.

Rugose corals or horn corals (Turbophyllum) from the Mississippian Great Blue Limestone, Cache Canyon, northern Utah, near the border between the Basin and Range and Rocky Mountain physiographic provinces. Photos of YPM IP 529539 by Jessica Utrup, 2015 (Yale Peabody Museum of Natural History/YPMCC0 1.0 Universal/Public Domain Dedication, via GBIF.org).


By the late Carboniferous, North America had collided with Gondwana, leading to the formation of Pangaea—a supercontinent composed of nearly all the landmass on Earth. Although the mountain building that occurred during this event was mostly far to the east, the Southwest was influenced by both fluctuating sea levels and a few significant tectonic changes.

The climate remained warm, despite large southern ice sheets, but it had grown much drier. Thick salt deposits accumulated in the northwestern Four Corners area as the seas evaporated. Where the land was exposed, deposits of dust (loess) accumulated and were blown across much of the Southwest. In southern New Mexico and Arizona, shallow marine deposits, laid down when the ice in Gondwana retreated and sea level rose, alternate with layers of dust blown in when the ice in Gondwana advanced and sea level fell. Loess is often, though not exclusively, associated with dry areas around glaciers. One controversial hypothesis proposes that an area of western Colorado—one of the islands that dotted the early Carboniferous sea—was, in fact, glaciated.


Reconstruction of Earth 300 million years ago showing Gondwana glaciation and Central Pangean Mountains.

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 [email protected] project (CC BY-NC-SA 4.0 license).


Photograph of the Onion Creek salt diapir in Utah. The diapir looks like an outcropping of light gray rock between an outcrop of red rock to the right and an outcrop of darker gray rock to the left.

Onion Creek salt diapir, a salt dome exposed at the surface at Fisher Towers, Utah. This salt is part of the Late Carboniferous (Pennsylvanian) Paradox Formation. Photo by Archaeopoda (Wikimedia CommonsCreative Commons Attribution-ShareAlike 3.0 Unported license, image cropped, resized, and labeled).


Permian (299 to 252 million years agoo)

During the Permian, shallow marine waters gave way to lowland coastal areas across portions of the Southwest. Extensive Permian deposits throughout the Southwest are home to a host of fossils, including terrestrial amphibians, reptiles, and synapsids. The climate was drier than that of the Carboniferous, and mudflats with salt and gypsum formed across the Southwestern states. Sand dunes started to become widespread.

A shift in plant type to those better adapted to drier conditions further suggests a change in climate during the Permian. A large, low-latitude desert formed along Pangaea's western margin, generating extensive dune deposits. By the end of the Permian, the southern ice sheets had disappeared.


Close-up photograph of a piece of gypsum rock from New Mexico. The rock shows thin layers the alternate in color from off-white to dark brown.

Layers of gypsum, an evaporate, from the Permian Castile Formation, Eddy County, New Mexico. Photo by James St. John (flickr, Creative Commons Attribution 2.0 Generic license).


Photograph of a fossil conifer branch from the Permian Hermit Shale of Arizona. The fossil is preserved in a medium-orange rock on which the impression of a branch with lateral branches given off on its right side are preserved. Impressions of scale or needle leaves are also present.

Branches and leaves of an ancient conifer (Walchia dawsonii), Permian Hermit Shale, Arizona. Photo of USNM P 38052 by Frederic Cochard (Smithsonian National Museum of Natural History, public domain).


Mesozoic (252 to 66 million years ago)

Triassic to Jurassic (252 to 145 million years ago)

As the Triassic period began, the Southwest moved north from the equator. The world warmed, and would stay warm through the Mesozoic. The continued growth of Pangaea created an intense monsoonal climate, similar to that of Asia today, that affected large parts of the continent.

As Pangaea reached its greatest size during the early Triassic, the monsoon’s intensity increased, and the vast dune deserts of the late Permian were replaced by rivers and floodplains. Soils associated with these floodplains testify to the extreme seasonality of rainfall during that time. The monsoon's intensity waned by the early Jurassic, and the rivers and floodplains were replaced by even larger deserts.

The Southwest's Triassic to Jurassic dune deposits are some of the most extensive in the world, and the dune field that existed during the Jurassic may be the largest in Earth history. These deposits, including the Navajo Sandstone, are responsible for spectacular scenery in the national parks and recreation areas of northernmost Arizona and southern Utah. Despite the area’s arid climate, the dunes were surprisingly full of life, particularly in southeastern Utah. Here, oases with large trees, large colonies of burrowing animals, and reptile trackways punctuated the otherwise dry and sandy landscape. These oases were fed by groundwater that originated in the higher country of what is now western Colorado.

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. Later in the Jurassic, the climate became more moderate; dune fields were replaced by rivers and floodplains populated by a rich dinosaur fauna (exemplified by the Morrison Formation) and large trees along rivers, streams, and grasslands.


Reconstruction of the globe in the late Jurassic about 150 million years ago showing the formation of a spreading center in the Central Atlantic Ocean.

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 GPlatesReconstruction 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 [email protected] project (CC BY-NC-SA 4.0 license).


Photograph of a rock feature called "The Wave" exposed in Navajo Sandstone in Arizona.

The Wave, a series of intersecting U-shaped troughs eroded into Jurassic Navajo Sandstone within the Paria Canyon-Vermilion Cliffs Wilderness, Arizona. The cycling layers in the sandstone represent changes in the direction of prevailing winds as large sand dunes migrated across the desert. Photograph by Julia Manzerova (FlickrCreative Commons Attribution-NonCommercial-ShareAlike 2.0 Generic license; image resized).


Photograph of a trail of three-toed dinosaur tracks on a slab in the Navajo Sandstone of Glen Canyon National Recreation Area, Arizona and Utah.

Ornithopod-type tracks, Powell Fossil Track Block Tracksite, Jurassic Navajo Sandstone, Glen Canyon National Recreation Area, Arizona and Utah. Photo source: National Park Service (public domain).


Photograph of a juvenile sauropod skeleton in a death pose partially embedded in rock. The sauropod has a long neck, a small head and is quadrapedal (walks on all fours). The tail is arched upward.

Skeleton of a juvenile Camarosaurs lentus, a type of sauropod, from the Carnegie Quarry, Jurassic Morrison Formation, Dinosaur National Monument, Utah and Colorado. Photo by James St. John (flickrCreative Commons Attribution 2.0 Generic license, image cropped and resized).


Photograph of a petrified log from Escalante Petrified Forest in the Jurassic Morrison Formation of Utah. The log looks like a weathered pieces of wood. It is light brown or beige in color.

Petrified log at Escalante Petrified Forest State Park, Jurassic Morrison Formation, Garfield County, Utah. Photo by James St. John (flickrCreative Commons Attribution 2.0 Generic license, image cropped and resized).


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; the water was rich with mosasaurs, giant clams, and other marine life. 

Map of the Western Interior Seaway. The map showing the seaway extending across North America from the Gulf of Mexico to the Arctic Ocean. Laramidia occurs to the west of the seaway and Appalachia to the east. The Hudson Seaway branches off the Western Interior Seaway to the northeast, covering modern-day Manitoba and Hudson Bay.

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).


Photographs of an ammonoid from the Late Cretaceous of eastern Colorado. The ammonoid has a spiral shell that is curved around itself in a single plain. The fossil is off-while in color. It is shown in four views, one from each side, and one from the narrow ends.

Fossil ammonoid (Nigericeras scotti) from the Late Cretaceous Greenhorn Limestone, Baca County, Colorado. Photo of USNM 166396 from the Cretaceous Atlas of Ancient Life (Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International license, image cropped).


In the latest Cretaceous, sea level dropped again and the western Southwest 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. As the continents moved closer to their modern positions, the Southwest experienced a hot and humid tropical climate. At the close of the Mesozoic, global climate—although warmer than today—was cooler than at the start of the era.


2-panel figure showing photos of Cretaceous plant fossils from the San Juan Basin of northern New Mexico. Panel 1: A petrified tree stump. Panel 2: Impression of a palm leaf, showing plicate structure. In other words, the leaf looks like it is folded like a folding fan.

Fossil plants, Late Cretaceous Fruitland Formation, San Juan Basin, New Mexico. Left: A petrified stump. Right: Sabalites, a palm leaf. Left image and right image by NickLongrich (Wikimedia Commons, Creative Commons Attribution-ShareAlike 4.0 International license, images cropped and resized).


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 Southwest, shifting a densely forested landscape to one primarily covered with fast-growing herbs and ferns.


Map showing the location of the Chicxulub crater on the Yucatan Peninsula and a detail showing the edge of the crater.

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).


Photograph of the Cretaceous-Paleogene boundary near Trinidad, Colorado. The boundary shows a thick, dark-colored later beneath a tan-colored outcrop.

The Cretaceous-Paleogene boundary at Trinidad Lake State Park, Las Animas County, Colorado. Photo by Jeffrey Beall (Wikimedia CommonsCreative Commons Attribution 4.0 International license, image resized).


Cenozoic (66 million years ago to present)

Paleogene to Neogene (66 to 2.6 million years ago)

After the end-Cretaceous bolide impact, the climate may have cooled briefly, but it soon rebounded to a warmer state. During the Paleocene to Eocene, the Southwest’s climate was warm and wet, and large mammals roamed the forested landscape. Large lakes covered parts of northern Utah and Colorado. Volcanic activity was strong. 


2-Panel image showing fossil cycads from the Paleocene Castle Rock flora of Colorado. Panel 1: Leaves of a seedling. The leaves appear to be in an opposite pair. Each is pinnately compound with four leaflets. Panel 2: Large rock slab preserving a flush of mature cycad leaves. Unfortunately, the pinnately compound leaves do not stand out against the background, so the rock appears mostly a mixture of orange and gray regions.

Fossils of a cycad (Dioonopsis praespinulosa) from the Paleocene Castle Rock Flora, Colorado. Cycads are a group of seed plants that look superficially similar to palms, but are not closely related to them and do not produce flowers. Left (1): Leaves of a seedling. Scale bar = 1 centimeter (about 0.4 inches). Right (2): Crown of leaves from a mature plant. The white arrow is pointing to one of the leaflets of a compound leaf. Scale bar = 5 centimeters (about 2 inches). Source: Figure 1 from Erdei et al. (2019) Biology Letters 15: 20190114 (Creative Commons Attribution 4.0 International license, images cropped, reconfigured, resized, and relabeled). 


Maps of Wyoming, Colorado, and Utah at four different times in the Paleocene and Eocene showing the evolution of the Green River Formation lakes. At 56 million years ago, a small Lake Uinta occurs in central Utah. At about 52 million years ago, Lake Uinta occurs in northeastern Utah and northwestern Colorado, Fossil Lake occurs on the Utah-Wyoming border, and Lake Gosiute occurs in southwestern Wyoming. At about 50 million years ago, Lake Uinta is still present in Utah and Colorado, Lake Gosiute has enlarge, and Fossil Lake is gone. At about 45 million years ago, only a small remnant of Lake Uinta occurs, mostly in northeastern Utah.

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.


Roadcut exposing rocks of the Green River Formation in Utah. The rocks are horizontally layered in thin beds that range from buff-colored to greenish-gray to dark gray.

Roadcut exposing lake sediments of the Eocene Green River Formation, Duchesne County, Utah. Photo by Kenneth Carpenter (Wikimedia CommonsCreative Commons Attribution-ShareAlike 4.0 International license, image cropped and resized).


In the late Eocene, the Earth began to cool, and global temperatures fell sharply at the boundary between the Eocene and Oligocene epochs (approximately 35 million years ago), due in part to the separation of South America’s southern tip from Antarctica. Global temperatures fell further in the late Miocene thanks to the formation of the Himalayas.

Volcanic activity intensified in the Southwest, and the Basin and Range region began to form, leading to the topography that is seen in those areas today (i.e., low valleys alternating with high mountain ranges). While most of the evidence for cooling at the Eocene-Oligocene boundary comes from the deep sea, fossil mammals in the Rocky Mountains show clear evidence of a change from forests to grasslands, which is associated with global cooling.


Image

Shiprock, a volcanic monadnock in San Juan County, New Mexico, rises roughly 483 meters (1583 feet) above the desert plain. Shiprock is part of the San Juan volcanic field and dates to the Oligocene (about 27 million years ago). Photograph by "Cathy" (FlickrCreative Commons Attribution-NonCommerical 2.0 Generic license).


2-panel image showing the ancient horse Mesohippus, Eocene, Florissant, Colorado. Panel 1: Photo of a jaw from above on a black background. Panel 2: Illustration of a group of four Mesohippus in an Eocene environment. Most of the habitat is open, although there are trees in the background. The horses are medium brown in color with elongated white spots or white stripes.

An ancient horse (Mesohippus), Eocene Florissant Fossil Beds, Teller County, Colorado. Mesohippus measured up to 70 centimeters (2 feet) at shoulder height. Left: Jaw with teeth. Right: Reconstruction of living animals. Photo and reconstruction by National Park Service/NPS (public domain).


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 showing the Isthmus of Panama region before the Isthmus closed. Ocean water circulated between the Atlantic and Pacific Oceans.

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 CommonsCreative Commons Attribution-ShareAlike 4.0 International license, image cropped and modified).


2-Panel image of the closing of stages in the closing of the Isthmus of Panama at 20 million years ago and 15 million years ago.

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).

The large ice sheets in the Northern Hemisphere did not extend into the Southwest, even at their maximum area. Large glaciers were found at higher elevations, and temperatures were cool. Fossil mammals adapted to colder temperatures are found in the Pleistocene of Colorado. In southern New Mexico, Pleistocene fossil mammals are found that now live at higher elevations in the mountains of northern New Mexico, indicating cooler temperatures and more available moisture in the area during the late Pleistocene.

Large lakes formed in low areas, and the Southwest’s most striking ice age feature was Lake Bonneville, a massive pluvial lake that covered much of Utah. Its remnant exists today as the Great Salt Lake. The last glacial advance of the modern ice age peaked some 18,000 years ago. Today nearly all the glaciers in the Southwest are gone, and the climate is in an arid state.


2-panel image of mammoth prints at White Sands, New Mexico. Panel 1: Photo of mammoth prints. The prints look like shallow circular depressions in flat ground. A broad-brimmed hat and human boot prints indicate the scale of the mammoth prints (they are larger). Panel 2: Close of of a single footprint. The print is circular with a raised edge.

Columbian mammoth (Mammuthus columbi) tracks, Pleistocene, White Sands National Park, New Mexico. Left photo and right photo from NPS, courtesy David Bustos (public domain).


2-Panel image of ancient Lake Bonneville. Left: Map of Lake Bonneville at its greatest surface area in the Pleistocene. The lake extends almost the entire length of western Utah and is widest in the north, narrowing toward the south. Panel 2: Graph showing fluctuating lake levels of ancient Lake Bonneville. The lake reached its peak a little less than 15,000 years ago at 5250 feet. Approaching 10,000 years ago, lake levels dropped precipitously and have stayed low since. The lake is called the Great Salt Lake beginning about 10,000 years ago.

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.


2-panel image showing Andrews Glacier in August 1916 (left) and July 2013 (right). The glacier occurs in a saddle between two slopes and flows downhill, ending at the shore of a lake. In 1916, the glacier appears slightly more massive than in 2013.

Andrews Glacier in Rocky Mountain National Park, Colorado, at two points in time. Glaciers in the Colorado Rockies are sustained largely by avalanches and wind-blown snow. In general, it is expected that high alpine glaciers in the Colorado Rockies will disappear as the climate continues to warm. Photo credits: 1916 photo from USGS (public domain), 2013 photo by daveynin (flickrCreative Commons Attribution 2.0 Generic license, image resized).

Future climate of the southwestern U.S.

The Southwest contributes significantly to climate change. The population of any industrialized and particularly wealthy country produces pollution; the majority of these emissions come from the use of petroleum. The more than 16 million residents of the Southwest use carbon-rich fossil fuels to provide electricity for lighting, cooling, and appliances, to fuel their transportation and industry, and to make the products they use. Burning those fossil fuels releases carbon into the atmosphere, which warms the Earth.

Of the southwestern states, Arizona emits the most greenhouse gases, releasing 94 million metric tons of carbon dioxide (CO2) per year. Although this pales in comparison to emissions from the nation’s highest CO2 producer—Texas, which releases nearly 656 million metric tons of CO2 per year—Arizona’s greenhouse gas emissions are rising rapidly compared with the nation as a whole. In the last decade, the United States has decreased the total amount of energy-related carbon dioxide emissions by almost 10%, yet Arizona’s emissions have increased by 9% thanks to a growing population that relies heavily on oil and natural gas for energy. Emissions from Colorado and Utah have also increased over the past decade, growing 7% and 2%, respectively.

On the other hand, southwestern states are making changes to reduce human impact on the climate. New Mexico has reduced its CO2 emissions by more than four metric tons in the last decade. The cities of Aspen and Lafayette, Colorado, as well as the state of New Mexico, were early adopters of the 2030 Challenge, an effort to reduce fossil fuel use in buildings so that both new and renovated buildings would qualify as carbon neutral by the year 2030. Additionally, states are beginning to step up their use and production of renewable energy. As of 2015, Arizona ranks 30th in the nation for renewable energy production, much of which it produces from hydroelectricity and biomass.


Photograph of Phoenix, Arizona, from a high point near the city. The photo shows development (roads, buildings) on a flat plain with mountains rising in the distance. Between the mountains and the nearest buildings is a cluster of tall buildings in downtown Phoenix.

Sprawling development of Phoenix, Arizona, a city in the Sonoran Desert, 2009. Photo by Bigmikebmw (Wikimedia Commons, Creative Commons Attribution-ShareAlike 3.0 Unported license, image cropped).


Photo of the Agua Caliente solar farm, Arizona. Photo shows arrays of solar panels arranged in multiple grids on a flat, barren landscape. Mountains can be seen on the horizon.

Agua Caliente solar farm, Maricopa County, Arizona. Photo by Center for Land Use Interpretation (Creative Commons Attribution-Noncommercial-ShareAlike 3.0 license, image resized).


Trends and predictions

Studies show that the southwestern states' climate is changing right now and that change has accelerated in the latter part of the 20th century. These changes include the following:

  • The number of days with temperatures above 35°C (95°F) and nights above 24°C (75°F) has been steadily increasing since 1970, and the warming is projected to continue.
  • The onset of stream flows from melting snow in Colorado has shifted two weeks earlier due to warming spring temperatures. Flows in late summer are correspondingly reduced, leading to extra pressure on the state’s water supplies.
  • Streamflow totals for the last decade in the Great Basin, Rio Grande, and Colorado River were between 5% and 37% lower than their 20th century averages.
  • Since 1980, tree mortality in forests and woodlands across the Southwest has been higher and more extensive than at any time during the previous 90 years. This mortality is attributed to higher temperatures, drought, and the eruption of bark beetles that are able to survive through warmer winter weather.
  • Increased heat in the Pacific Ocean has altered the weather patterns of Pacific storms, decreasing snowfall in the mountains of western Utah and Arizona.
  • In the last decade, the Southwest's frost-free season has increased by approximately 7% compared to the average season length for the 20th century.
  • The seasonality and transmission frequency of insect-borne diseases and other infectious diseases prevalent in the Southwest, including plague, valley fever, and Hanta, are influenced by warming trends.


Photograph of a light brown dear mouse with a white belly sitting on a log.

A deer mouse (Peromyscus maniculatus) in Portal, Arizona, 2004. Deer mice are the most important rodent carriers of hantavirus in the Southwest. Photo by Gregory Smith (flickr, Creative Commons Attribution-ShareAlike 2.0 Generic license, image cropped and resized).


Future temperature

Recent warming within the Southwest has been among the most rapid in the United States, and models predict that the area's climate will continue to warm. The average annual temperature in most of the Southwest is predicted to rise 2.2° to 5.5°C (4° to 10°F) by 2100. Summer heat waves will become hotter and longer, while winter cold snaps will occur less often. These increased temperatures lead to a whole host of other effects, including a decrease in snowpack, declines in river flow, drier soils from more evaporation, and the increased likelihood of drought and fires.

In winter, rising temperatures have increased the number of frost-free days. Today, most of the Southwest experiences about 17 fewer freezing days than it did over the last century. By 2070, one can expect up to 38 more days of freeze-free weather each year. These warmer temperatures and increased precipitation have helped bring on longer growing seasons. While changes in the growing season can have a positive effect on some crops (such as melons and sweet potatoes), altered flowering patterns due to more frost-free days can lead to early bud bursts, damaging perennial crops such as nuts and stone fruits.

Warmer temperatures also make it easier for insect pests to overwinter and produce more generations. Bark beetles, which normally die in cold weather, have been able to survive through the winter and reproduce, increasing tree mortality. For example, high winter temperatures between 2000 and 2003 correlated to bark beetle outbreaks that devastated pinyon pine throughout the Southwest, leading to nearly 90% mortality at some sites in Colorado and Arizona. As of 2010, bark beetles in Arizona and New Mexico have affected more than twice the forest area burned by wildfires in those states.


Photo of a hillside covered with conifers. Live green tress are mixed with many dead trees that have orange needles or no needles.

Future precipitation and water supply

Water supply is an important issue in the Southwest, and communities will need to adapt to changes in precipitation, snowmelt, and runoff as the climate changes. Agriculture accounts for more than half of the Southwest’s water use, so any major reduction in the availability of water resources will create a serious strain on ecosystems and populations. Drier days and higher temperatures will amplify evaporation, increasing the desertification of already arid areas and affecting natural ecosystems as well as increasing pressure on the water supply for agriculture and cities. An increased frost-free season length also leads to increased water demands for agriculture and heat stress on plants.

Cattle ranches throughout the southwestern states rely on rain-fed grazing forage, making them extremely susceptible to climate change and drought. In addition, temperature increases and recent drought have resulted in earlier spring snowmelt and decreased snow cover on the lower slopes of high mountains, bringing about more rapid runoff and increased flooding. These changes to rain and snow-pack are already stressing water sources and affecting agriculture.

Precipitation has become more variable from year to year, and heavy downpours across the U.S. have increased in the last 20 years. Because higher temperatures mean greater evaporation and warmer air can hold more water, precipitation will occur in greater amounts at a time, but less frequently. Although there has so far been little regional change in the Southwest’s annual precipitation, the area’s average precipitation is expected to decrease in the south and remain stable or increase in the north. Most models predict a decrease in winter and spring precipitation by the middle of the century, and more frequent precipitation extremes during the last half of the century.


Satellite photographs showing Lake Mead on the Arizona-Nevada border at two points in time: August 7, 2000, and August 6, 2021. In the photo from 2000, the lake covers more area; in 2021, the water level is noticeably lower, as indicated by the arms of the lake being thinner and some parts of the lake appearing to be nearly completely dry.

Lake Mead, the lake created by the Hoover Dam, at two points in time about 21 years apart. Images by Lauren Dauphin, NASA Earth Observatory (used following NASA's image use policy).


Satellite photographs showing Lake Powell in Arizona at two points in time: September 1, 2017, and August 27, 2021. In the photo from 2017, the lake covers more area; in 2021, the water level is noticeably lower, as indicated by the arms of the lake being thinner.

Lake Powell, the lake created by Glen Canyon Dam, at two points in time about four years apart. Photos by Lauren Dauphin, NASA Earth Observatory (used following NASA's image use policy).


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. However, 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.


Close-up photograph of hailstones, rounded balls of ice.

Hailstones from a storm in Limon, Colorado, 2010. According to the photographer, the largest stones were 1.5 to 2.5 centimeters (0.6 to 1 inch) size. Photo by James St. John (flickr, Creative Commons Attribution 2.0 Generic license, image cropped and resized).


Photograph of a car windshield showing impact marks with radiating cracks where hailstones hit the glass.

A car with a windshield damaged by hailstones, Limon, Colorado, 2010. Photo by James St. John (flickr, Creative Commons Attribution 2.0 Generic license, image resized).

Resources

Resources from the Paleontological Research Institution

Books

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.

Websites

Digital Encyclopedia of Earth Science: Why talk about climate change?: https://earthathome.org/de/talk-about-climate/

Digital Encyclopedia of Earth Science: What is climate?: https://earthathome.org/de/what-is-climate/

Digital Encyclopedia of Earth Science: Evidence for and causes of recent climate change: https://earthathome.org/de/recent-climate-change/

[email protected]: Quick guides & FAQ: Climate and Energy: https://earthathome.org/quick-faqs/#climate

[email protected]: Here on Earth: Introduction to Climate: https://earthathome.org/hoe/climate/


Go to the full list of resources about the climate of the southwestern U.S.

Go to the full list of general resources about climate