Climate of the Western 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—for example, San Francisco—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 of course it can also refer to millions of years.
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. Due to its great variety of environments, from the boreal areas of Alaska to the subtropics of Hawai’i, residents of the West generally have a diverse wardrobe. The most variable climates, however, are in the interior areas and mountains of the western continental US. These areas can vary from frigid in the winter to scorching in the summer. By contrast, coastal climates have only moderate seasonal variation, while Alaska is always cool and Hawai’i is always warm. The West’s climate is also extremely variable with respect to moisture, from the arid deserts of Nevada to the rainforests of western Washington.
Climate, like other parts of the Earth system, is not static but changes over time, on 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. What is now the West has been tropical or temperate through most of its history, but it has also ranged from very wet to very dry.
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 that of the West. 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.
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, 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 will never be repeated. 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 the 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.
Much of the light from the sun passes unimpeded through the atmosphere and hits the Earth. Approximately 70% of that light is absorbed and retransmitted from the surface as heat. The transmitted heat, which has a longer wavelength than light, is trapped by gases in the atmosphere including water vapor, carbon dioxide, and methane. The similarity between this process and that which warms a greenhouse earned these “greenhouse gases” their moniker.
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 rearranging 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 the cool periods, there is usually persistent ice at the poles; during the 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 much hotter for much of its history (Figure 9.2). Through the shifting global climate and the movement of the tectonic plates, parts of what is now the West have at times been at the bottom of a shallow sea, a collection of islands scattered across a tropical ocean, a coastal plain with swamps and rivers, covered with ice, and wracked by great floods.
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.
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 towards a series of cooling feedbacks and caused ice to spread from pole to pole.
See Chapter 6: Glaciers to find out more about glaciations.
An ice-covered planet would remain that way because almost all of the sun’s energy would be reflected back into space, but 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. These gases are usually removed from the atmosphere by organisms and the weathering of rocks, but this was not possible through miles of ice! After millions of years, the concentrations of methane and carbon dioxide 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 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 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 continental crust that was to become North America, including some of the West, drifted around the surface of the Earth. 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 that are 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 North American portion of Rodinia was near the equator, and there were two more Snowball Earths during this time. The fact that North America was at such a low latitude, yet had glaciers, is strong evidence that the Earth really did freeze over completely. There is no direct evidence for these events in the West, although some evidence can be found in the rocks of Idaho.
Life and Climate
See Chapter 3: Fossils to learn more about fossils of the West’s ancient seas.
By 635 million years ago, the Earth had warmed again, and the North American continent moved towards the equator. Throughout much of the Paleozoic, North America’s terrestrial margin ran through Idaho, Arizona, and easternmost Utah, with the continental shelf extending out through California, Oregon, and Washington (Figure 9.4). The West had a warm climate, and fossils such as trilobites, brachiopods, and archaeocyathids—extinct reef formers—found in eastern California and Nevada provide evidence of warm, shallow seas. Sea level rose in the Ordovician, and both deep- and shallow-water marine deposits are known from that time. Conodonts and graptolites are found in deep-water deposits from northeastern Washington and shallow-water marine rocks throughout Nevada and southeastern California, revealing that the climate remained warm during the Ordovician. At the end of the Ordovician, from 460 to 430 million years ago, the Earth fell into another ice age, but corals found in California indicate it remained warm enough for tropical seas to exist there. Silurian-age rocks tell us much the same story.
Figure 9.4: The Western United States at 600 million years ago. The entire continent is located in the southern hemisphere, near the equator.
Alaska is almost entirely composed of ancient terranes and volcanic island arcs that drifted together at different times during Earth’s history and had not assembled into anything resembling modern Alaska until the Jurassic. Most of these terranes appear to have experienced warm, wet climates, and geologists have concluded that they all originated in tropical areas of what is now the Pacific Ocean. Alaska’s Cambrian- through Silurian-age rocks all formed on these tropical microcontinents. This is also true for much of central and western Washington, all of Oregon, and most of northwestern California, which formed in a similar manner as terranes and island arcs accreted onto the edge of the continent at its subduction zone.
From 430 to 300 million years ago, North America moved north across the equator, and the cycle of warming and cooling was repeated yet again. In the Devonian, sea level was higher than it had been earlier, but despite this, Devonian rocks are relatively scarce in the West, with small areas widely scattered across Nevada. The most extensive outcrops of Devonian rocks are located on the tropical accreted terranes, and include some very large reefs. Devonian rocks are also common in northern Alaska. These rocks include carbonate platforms, which are consistent with a low-latitude position and warm climate.
By the Early Carboniferous, ice capped the South Pole and began to expand northward. Although the Earth’s temperature fell during this time and the frozen water far to the south caused sea levels to drop, the West still remained relatively warm because of North America’s low-latitude position. Mountain building in Nevada raised the sea bottom, dividing the marine environment into shallow water to the east and deep water to the west. Nevada’s shallow- water deposits contain reefs, indicating that the climate there was still warm. Later, in the Permian, these shallow areas became beaches and lagoons as sea level dropped. Carboniferous and Permian rocks in Oregon, northern California, western Washington, and Alaska originated on tropical terranes— some contain lush, tropical floras that are completely dissimilar to those on the main part of the continent, a testament to their continued isolation in the ocean far from the West (which had now become part of the supercontinent Pangaea). Carboniferous rocks in northern Alaska contain extensive carbonate platforms, which are indicative of a warm climate. Alaska’s Permian rocks are also indicative of a marine environment.
See Chapter 7: Energy for more information about the production and use of oil in Alaska.
Around 220 million years ago, the West moved north from the equator. Pangaea, a supercontinent composed of nearly all the landmass on Earth, began breaking up into continents that would drift toward their modern-day positions (Figure 9.5). The Earth remained warm until worldwide temperatures began to dip again, around 150 million years ago. At this time, the West was still largely underwater, but the Sierra Nevada Mountains had begun to form as a volcanic island arc close to the edge of the continent. Nevada’s Triassic rocks contain both deep-water marine and terrestrial deposits—its Triassic seas were rich with ichthyosaurs and other marine reptiles, while its terrestrial rocks reflect the aridity and seasonality of the climate farther inland. As mountain building continued into the Jurassic, the seas became shallower, while terrestrial deposits expanded. By this time, terranes were beginning to collide with the continent— some of these collisions included volcanic islands ringed with corals, indicating that the climate remained warm. Similar island arcs formed close to what is now southern Alaska, where a very productive sea flooded the Triassic continental margin. The organic-rich rocks that formed in this sea are some of the most prolific source rocks for oil on Alaska’s North Slope.
Jurassic rocks are widely scattered through Oregon and Washington, where they contain coral-ringed volcanic arcs with associated deeper marine sediments. Jurassic rocks in California reflect both shallow-water marine and terrestrial environments. A large area of shallow-water marine deposits was laid down just off of the rising Sierra Nevada, in what is now the eastern Central Valley. The terrestrial rocks of southeastern California contain ginkgos and cycads that indicate a warm, moderately wet climate. Terrestrial Jurassic rocks in southern Nevada, however, show that the climate on land was still arid. Meanwhile, much of southern Alaska was assembled during the Jurassic as a series of island arcs and terranes collided with northern Alaska, which had now drifted into its current position and existed as a broad, flat shelf of shallow marine rocks.
The Earth warmed near the beginning of the Cretaceous, and sea level rose. Mountain building continued throughout the West with the formation of both the Sierra Nevada and the Rocky Mountains. Erosion predominated, and Nevada and Oregon have a very sparse record of Cretaceous sedimentation and climate; the few outcrops from this time period show that Nevada was terrestrial and Oregon was still largely marine. Washington has a slightly better record, showing that it too was still principally marine. Global climate was warm, but reefs did not form, probably due to the intense mountain building and erosion that shed a great amount of sediment into the interior embayments and the Pacific Ocean. Even though Alaska was closer to the North Pole than it is at present, fossil vegetation indicates that its climate was very similar to that of western Oregon today. Lush swamps and forests occupied lowland areas, and some swamps had become rich coal beds.
See Chapter 3: Fossils for more about plants of the prehistoric West.
The climate cooled again at the end of the Cretaceous, 65 million years ago. This cooling had the greatest impact on northern Alaska, where fossilized forests resemble those found near Anchorage today. But after the end of the Cretaceous, the climate warmed once more—by around 50 million years ago, the West’s climate was actually hot, with palm trees growing in southern Alaska! Alaska’s northern forests once again grew to resemble those of modern forests much farther south. The sea withdrew from most of Oregon and Washington, and plant fossils are abundant throughout these states. One of the best records of the West’s Cretaceous climate is found in Oregon’s John Day fossil beds. Here, plant and animal fossils indicate that from 50 to 35 million years ago this area was home to a subtropical rainforest with banana and citrus trees. Fossil floras in western and north-central Washington tell a similar story, and palm trees were abundant. Between 35 and 20 million years ago, however, the climate became cooler and drier, and prairies and deciduous trees such as oak, maple, and alder flourished. This coincided with the initial uplift of the Cascade Range (37 - 7 million years ago), which began to create a rain shadow (Figure 9.6) to the east. The final uplift of the Cascades and Sierra Nevada created the intense rain shadow that is responsible for the aridity of eastern Washington, eastern Oregon, and Nevada today. Moist Pacific Ocean air moves eastward with the prevailing winds, and it 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 it warms as it moves down the eastern side of the mountain range, promoting evaporation.
Late in the Cenozoic, eruptions in eastern Oregon produced enormous amounts of basalt that flowed north and west, filling the Columbia River basin. These are some of the largest such eruptions in the history of the Earth, and they took place several times over a span of about 11 million years. While evidence that these eruptions influenced global climate is ambiguous, climatic changes are recorded in soils that formed atop some of the lava flows. These soils indicate a decrease in temperature after a period known as the Middle Miocene Climatic Optimum, a brief warming episode that occurred around 16 million years ago.
See Chapter 2: Rocks to learn more about the Columbia Flood Basalts.
Since 800,000 years ago, an equilibrium appears to have been reached between warming and cooling, with Earth’s ice caps growing and retreating primarily due to the influence of astronomical forces. During the last glacial maximum, ice covered the northern part of Washington State (Figure 9.7). The ice sheet did not extend into central or northern Alaska since the local climate was very dry, though an ice cap covered the Brooks Range. During periods of ice advance, the West was colder than it is today, with extensive mountain glaciers occurring throughout the region; in fact, Yosemite Valley was carved out at this time. Microfossil evidence from the Rancho La Brea Tar Pits in Los Angeles tells us that southern California’s climate around 40,000 years ago was similar to San Francisco’s today.
See Chapter 3: Fossils for more on the Rancho La Brea Tar Pits.
Around 12,000 years ago, all of Washington east of the Cascades was inundated and scoured by numerous enormous, violent floods. These occurred when an 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—Glacial Lake Missoula—built up behind the ice dam. When the ice dam later failed, the water was released catastrophically. These floods cut through the dust deposits and basalt that covered much of the region, leaving islands, escarpments, and channels so large that ground-based geologists did not at first recognize their origins.
Figure 9.7: The maximum extent of the Cordilleran and Laurentide ice sheets across western North America and Alaska.
See Chapter 8: Soils to learn more about the soils of Hawai’i.
The Hawaiian Islands began to form 11 million years ago, although most are younger than 7.5 million years. These islands formed from volcanoes erupting from the sea floor over what geologists call a hot spot. Hawai’i has a poorly preserved paleoclimate record, probably because the landscape has been so active, with continuous volcanic eruptions and new lava flows covering the landscape as well as intense erosion occurring on the wetter, windward sides of the islands. Because of the islands’ latitudinal position, geological records found in the nearby deep sea, and the nature and depth of the soils formed on the long-exposed lava, we know that Hawai’i’s climate has always been tropical to subtropical.