Climate of the Northwest Central US

Climate is a description of the average temperature, range of temperatures, humidity, precipitation, and other atmospheric/hydrospheric conditions a region experiences over a period of many years. These factors interact with and are influenced by other parts of the Earth system, including geology, geography, insolation, currents, and living things.

Because it is founded on statistics, climate can be a difficult concept to grasp, yet concrete examples can be illuminating. Terms like “desert,” “rain forest,” and “tundra” describe climates, and we have gained a general understanding of their meaning. Climate can also encompass the cyclical variations a region experiences; a region with a small temperature variation between winter and summer—San Francisco, for example—has a different climate from one that has a large variation, such as Buffalo. Scientists have settled on 30 years as the shortest amount of time over which climate can be defined, but it can of course also define time periods millions of years in length.

You cannot go outside and observe climate. Weather, on the other hand, can be observed instantly—it is 57 degrees and raining right now. Weather varies with the time of day, the season, multi-year cycles, etc., while climate encompasses those variations. Our choice of clothing in the morning is based on the weather, while the wardrobe in our closet is a reflection of climate. Residents of the Northwest Central have a diverse wardrobe, especially in low-lying areas that experience seasonal extremes of hot and cold. The entire area experiences great seasonal variation, although hot summer temperatures are moderated at higher elevations.

Past Climates

Climate, like other parts of the Earth system, is not static but changes over time, on both human and geologic time scales. Latitude, for example, has a very direct effect on climate, so as the continents shift over geologic time, the climates on them also shift. Furthermore, the conditions on Earth as a whole have varied through time, altering what kinds of climates are possible. Throughout its long history, parts of the Northwest Central US have been covered in ice, filled with subtropical swamps and forests, and submerged in warm, shallow seas.

Ancient climates are reconstructed through many methods. Written records and tree rings go back hundreds of years, glacial ice cores hundreds of thousands of years, and fossils and rocks that indicate different climates go back hundreds of millions of years. These clues, coupled with modeling and a knowledge of physics and chemistry, help climatologists put together an increasingly detailed history of the Earth’s climate, and of that of the Northwest Central. Unfortunately, we do not have as clear an understanding of climate for the earliest part of Earth history as we do for the later parts, because the oldest rocks are much more difficult to find. However, we can still say something about the climate of the ancient Earth, in large part due to our knowledge of atmospheric chemistry.

Ancient Atmosphere

Not long after the Earth first formed, more than 4.5 billion years ago, its atmosphere was composed mostly of hydrogen and helium. Volcanic activity and collisions with meteorites and comets added water vapor, carbon dioxide (CO2), and nitrogen to the atmosphere. As the Earth cooled enough for liquid water to form, the vapor formed clouds from which the rain poured forth in such a deluge as the planet will never experience again. These torrential rains were constant for millions of years, absorbing salt and other minerals from the earth as the rainwater coursed to the lowest areas, forming Earth’s oceans and seas.

At this time, the sun produced significantly less energy than it does today, so one might expect that once the oceans formed, they would continue to cool and eventually freeze. Yet temperatures stabilized, perhaps because there was a greater concentration of potent greenhouse gases in the atmosphere and less land surface to reflect light, so temperatures remained high enough for liquid water to exist. Indirectly, the ocean was responsible for the final ingredient of the modern atmosphere because it was home to the first life on Earth. Photosynthetic bacteria appeared perhaps as early as 3.5 billion years ago, but abundant iron and organic matter quickly absorbed the oxygen they produced. After hundreds of millions of years, these sinks were exhausted, and free oxygen could finally build up in the atmosphere. With this addition, the modern atmosphere was complete, though the relative amounts of the gases composing it would, and still continue to, shift. The composition of the atmosphere and the huge volume of water on Earth are two of the most important factors affecting climate.

While the atmosphere was forming about 3.7 billion years ago, the surface of the Earth was cooling to form a solid crust of rock (although there are indications that this process may have started as early as 4.4 billion years ago). Regardless of precisely when this took place, it represented the beginning of tectonic processes that have continued ever since. Molten rock from the mantle constantly wells up from deep fissures and solidifies into relatively dense rock, while more buoyant rock floats higher on the magma and is pushed around on the slow conveyor belts of mantle-formed rock (Figure 9.1). Denser rock forms oceanic plates that are lower and covered in water, and lighter rock forms continental plates, though part or all of a continental plate may be submerged under a shallow sea. The motion of these plates, the rearrangement of the continents, and the amount and types of minerals exposed to the atmosphere play a huge role in the climate. Not only do the continents and oceans move through different climate zones, but the continents also affect climate based on their size, and the weathering of rock on the continents plays a large role in the composition of the atmosphere. For example, rock that is enriched in organic matter will release abundant amounts of carbon dioxide as it weathers, while rock rich in feldspar and mica will take up carbon dioxide.

Figure 9.1: The layers of the Earth include the rigid crust of the lithosphere, which is constantly moving over the plastically flowing asthenosphere.

Nearly one billion years ago, the Earth began fluctuating between warm and cool periods lasting roughly 150 million years each. During cool periods, there is usually persistent ice at the poles, while during warm periods there is little or no glaciation anywhere on Earth. Today, we are still in a cool period—although the world has been cooler than it is at present, it has been far hotter for much of its history (Figure 9.2). Through the shifting global climate and the movement of the continents, what is now the Northwest Central has at times been submerged beneath a shallow sea, a plain filled with swamps, rivers, and grasslands, and even buried under thick ice.

Figure 9.2: Changing global climate throughout the last 542 million years. These data were compiled using the ratios of stable oxygen isotopes found in ice cores and the carbonate skeletons of fossil organisms.

Snowball Earth

There is evidence suggesting that the entire surface of the planet has been covered in ice several times, a hypothesis called Snowball Earth (Figure 9.3). Glacial deposits discovered near Lake Huron and elsewhere show that starting about 2.4 billion years ago the entire surface of the Earth may have been covered in ice for as long as 300 million years, an event known in North America as the Huronian glaciation. At that time the continental plates made up less than half as much of the Earth’s surface as they do today and were unified as the continent Arctica. It may have been early life’s production of oxygen that reacted with and lowered the amount of the greenhouse gas methane in the atmosphere, which tipped the Earth toward a series of cooling feedbacks, causing ice to spread from pole to pole.

Figure 9.3: Snowball Earth periods during the Proterozoic.

An ice-covered planet would remain that way because almost all of the sun’s energy would be reflected back into space; however, this did not happen on Earth because of plate tectonics: the Snowball Earth cycle was eventually disrupted by volcanic activity. While the Earth was covered in ice, volcanoes continued to erupt, dumping carbon dioxide and methane into the atmosphere. While these gases are usually removed from the atmosphere by organisms and the weathering of rocks, this was not possible through miles of ice! After millions of years, the concentrations of methane and CO₂ increased to the point that greenhouse warming began to melt the ice sheets. Once the melting started, more of the sun’s energy was absorbed by the surface, and the warming feedbacks began. Because the oceans had been covered, nutrients derived from volcanic gases and chemical changes in the rocks accumulated in the waters. Once they were re-exposed to light, a population explosion of cyanobacteria produced more and more oxygen, which was capable of combining with freshly thawed carbon sources to make more carbon dioxide, further enhancing the warming.

For the next 1.5 billion years, the Northwest Central US, free of ice, drifted around the surface of the Earth. Stromatolites found in Glacier National Park in Montana, as well as in Idaho and Wyoming, indicate periods of warm, shallow seas between 1.7 and 1 billion years ago.

A new supercontinent—Rodinia—formed, and the part that is now North America was stable, forming what is known as a craton, or continental interior relatively free of the folding and faulting that characterizes continental margins subjected to mountain building and other plate tectonic processes. About 850 million years ago, during the Cryogenian, the Earth entered a 200-million-year ice age. The part of Rodinia that would eventually become North America was located near the equator, and there were two more Snowball Earth cycles during this time. Idaho contains deposits from the first of these, called the Sturtian (about 710 million years ago), and the fact that Idaho was at such a low latitude yet still experienced glaciation is strong evidence that the Earth really did freeze over completely. As Rodinia began to break up, another Snowball Earth event occurred during the Marinoan glaciation (about 640 million years ago).

By the late Precambrian, 600 to 550 million years ago, the Earth had warmed again, and the North American continent, including most of the modern Northwest Central US, was again near the equator.

Life and Climate

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. 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 was located just above the equator (Figure 9.4). Cambrian fossils reveal that most of the area was covered by warm, shallow seas during this time.

Figure 9.4: The location of the continents during the A) early and B) late Cambrian. Note the position of North America.

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.

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.

During the late Paleozoic, the sea gradually began to withdraw. The Plains area became terrestrial, but 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.

Around 220 million years ago the Northwest Central 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 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. After reaching its greatest size during the Triassic period, Pangaea began to break apart into continents that would drift toward their modern-day positions (Figure 9.5).

Figure 9.5: The breakup of Pangaea began around 220 million years ago.

Some Triassic rocks now found in the Northwest Central were not, however, part of the 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 US, especially Alaska, 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.

The Jurassic and Cretaceous climates 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 dinosaur localities in North America, including Como Bluff in Wyoming and the Judith River Formation in Montana, are found in the Northwest Central States. 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.

The Earth warmed near the beginning of the Cretaceous, and sea level rose. Throughout the Cretaceous, sea level was an average of 100 meters (330 feet) higher than it is today, largely as a result of water displacement by continental rifting and rapid sea-floor spreading. Shallow seaways spread over many of the continents, and by the start of the late Cretaceous, North America was divided in two by an inland sea known as the Western Interior Seaway (Figure 9.6). Areas in the Northwest Central preserve both the eastern and western shorelines of this sea. Cretaceous fossils from modern-day North Dakota 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.

Figure 9.6: The Western Interior Seaway.

At the close of the Cretaceous, 65 million years ago, global climates (though still much warmer than those of today) were cooler than at the era’s start. At the very end of the Cretaceous, the Gulf Coast experienced an enormous disruption when an asteroid or comet collided with Earth in what is now the northern Yucatán Peninsula in Mexico. 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 US 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. 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. 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.

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. 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.

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 just 800,000 years ago, a type of equilibrium has been reached between warming and cooling, with the ice caps growing and retreating primarily due to the influence of astronomical forces. During the ice sheet’s maximum extent, it reached into Montana, the Dakotas, and Nebraska (Figure 9.7). The portions of the Northwest Central 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 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 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.

Figure 9.7: Extent of glaciation over North America during the last glacial maximum.