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 most of its long history, the Southwest has been tropical or temperate, 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 of that of the Southwest. 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.

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.

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 filled, 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 8.1). 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 8.1: 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 8.2). Through the shifting global climate and the movement of the continents, what is now the Southwest has at times been at the bottom of a shallow sea, a coastal plain with swamps and rivers, and even inundated by monsoonal rains or clouded by intense dust storms.

Figure 8.2: 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 8.3). 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, tipping the Earth toward a series of cooling feedbacks and causing ice to spread from pole to pole.

An ice-covered planet would remain frozen 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. 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 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.

Figure 8.3: Snowball Earth periods during the Proterozoic.

For the next 1.5 billion years, the Southwest, free of ice, drifted around the surface of the Earth. A new supercontinent—Rodinia—formed, and the part that is now North America was stable, creating 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. Since the Southwest was part of that craton, it was probably underwater for most of this time. About 850 million years ago, during the Cryogenian, the Earth entered a 200-million-year ice age, during which there were two more Snowball Earth cycles. Although the part of Rodinia that would eventually become North America was located near the equator, 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. Tillites—rocks composed of ancient glacial sediment—found in Utah provide direct evidence for Proterozoic glaciation.

Life and Climate

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 US was located near the Tropic of Capricorn (Figure 8.4). 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. Although there is a rich marine fossil record from the areas between these islands, we have no record of what kinds of plants colonized the land after land plants evolved in the late Ordovician and Silurian.

Figure 8.4: The location of the continents during the A) early and B) late Cambrian. Note the position of North America relative to the equator.

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, which was associated with the first of five major mass extinctions that have occurred over the last half-billion years. Although global sea level dropped during this event, 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.

In the Silurian and Devonian (430 to 300 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 the same time—while the Southwest was still submerged—the oceans between Gondwana and North America began to close (Figure 8.5). In the early Carboniferous, 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.

Figure 8.5: By the late Devonian (375 million years ago), the oceans between Gondwana and Euramerica had begun to close.

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. In the late Carboniferous, 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.

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—from waterloving ferns and horsetails 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. 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-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. 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.

Pangaea began to break up during the Jurassic, rifting apart into continents that would drift toward their modern-day positions (Figure 8.6). The supercontinent was split by spreading along the mid-Atlantic ridge, initiating the formation of the Atlantic Ocean. 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 (Figure 8.7); the water was rich with mosasaurs, giant clams, and other marine life. In the late Cretaceous, however, sea level dropped 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—though warmer than today—was cooler than at the start of the era.

Figure 8.6: The breakup of Pangaea began approximately 220 million years ago.

Figure 8.7: The Western Interior Seaway.

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.

After this event, the climate may have cooled briefly, but it soon rebounded to a warmer state, and the world reached one of its warmest episodes during the Eocene. Right at the boundary between the Paleocene and Eocene epochs (approximately 56 million years ago), temperatures spiked upward in what geologists call the Paleocene-Eocene Thermal Maximum. During this event, which lasted perhaps only approximately 10,000 years, the atmosphere and ocean warmed by as much as 8°C (14°F) in as little as 4000 years, and deep oceans became acidic, with low levels of dissolved oxygen. The causes of this event remain unclear, but may have involved the sudden release of methane from sediments on the seafloor. The resulting greenhouse effect persisted for 100,000 years. The abrupt climatic change was associated with major migrations, the extinction of plants and animals on land, and a mass extinction in the deep sea. The Southwest’s climate was warm and wet, with strong volcanic activity, and large mammals roamed the forested landscape. Large lakes covered parts of northern Utah and Colorado.

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. This allowed for the formation of the Antarctic Circumpolar Current, which insulated Antarctica from warm ocean water coming from lower latitudes and led to the formation of the continent’s glaciers. The continents approached their modern configuration, and India began to collide with Asia to form the Himalayas. Global temperatures fell further in the late Miocene thanks to the formation of the Himalayas—this event had a significant impact on global climate, as weathering of the newly exposed rock began to serve as a sink to take up atmospheric CO₂. With the reduction of this greenhouse gas, temperatures cooled worldwide, and this cooling has continued more-or-less to the present day. 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.

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. Surprisingly, a major contributing factor to this event was a geological change that occurred half a world away. 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. These changes in ocean circulation throughout the Caribbean and Gulf of Mexico also affected nutrient supplies in the coastal ocean, which may have contributed to an increase in the extinction of marine animals (including everything from mollusks and corals to whales and dugongs) during the late Pliocene.

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 a mere 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 (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 largest. However, 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 (Figure 8.8). Its remnant exists today as the Great Salt Lake. The last glacial advance of the modern ice age peaked some 18,000 years ago, and today nearly all the glaciers in the Southwest are gone, while the climate is now in an arid state.