ELEVEN
Why Climate Changes
CYCLES AND OSCILLATIONS
Just as the records of past peoples help us understand human society, the records of past climates help us learn how the Earth system works. And just as modern political scientists can test their ideas against the history of humans, Earth system scientists can test their models against past climate changes.
RICHARD ALLEY, The Two-Mile Time Machine
IN THE PAST SEVERAL CHAPTERS, we have highlighted the work of paleoclimatologists and the tools that have allowed them to re-create the features and patterns of ancient climates in the American West. One reason for their success is the variety of tools and climate archives at their disposal—from the analyses of tree rings, lake levels, and oxygen isotopes as indicators of climate wetness and temperature to the interpretation of coarse sediment layers in floodplains, wetlands, and coastal sediments as indicators of past floods. In this chapter, we explore in more detail the proposed mechanisms that are responsible for past variations in climate and the persistent tendency of climate to follow patterns and oscillations.
Over very long timescales (millions of years), climate change is caused by forces within the earth that drive the very slow movements of tectonic plates—processes that build mountains and spread ocean basins apart. Here, we focus on those processes that occur on relatively short timescales (years, decades, centuries, or millennia). Understanding the causes of climate variations and oscillations over longer time periods gives climate scientists insights into how the climate works, the full range of climates possible, and what we may expect in the future.
FIGURE 29. Paleoclimate researchers drilling Porites coral heads in the Red Sea using a large hydraulic rig. Coral cores are used to reconstruct past seawater conditions. (Photo courtesy of Konrad Hughen, Woods Hole Oceanographic Institute.)
CORAL AND ANCIENT EL NIÑO
We owe much of our understanding of past climate change on Earth to the tiny polyps that create coral reefs. Each polyp, no bigger than the head of a pin, secretes a calcium carbonate skeleton to protect itself. Collectively, over the millennia, millions of generations of polyps have built the magnificent coral reefs found in shallow tropical seas. Coral ecosystems, among the richest communities of species found anywhere, have persisted for over 200 million years, a span of the earth’s history that has seen monumental climate changes. Each generation’s fossilized skeletons, stacked upon the last, bear testament to many of those changes.
The most recent changes—those occurring over the past several thousand years of the Holocene epoch—provide valuable information for understanding our current climate. Paleoclimate scientists have developed methods of coring through the calcium carbonate coral skeletons, whose annual layers have recorded information on water temperature, salinity, and nutrients (see figure 29). For instance, scientists have probed coral reefs across the tropical Pacific Ocean for evidence of the past behavior of the ocean-atmosphere phenomenon known as the El Niño–Southern Oscillation (ENSO). During an El Niño event, the trade winds fail, causing the warm waters in the western Pacific Ocean to shift eastward, warming surface temperatures and shallow coral reefs across the tropical Pacific.
Palmyra Atoll, one of the most spectacular and diverse coral reefs in the world, is located just north of the equator in the central Pacific Ocean, making it ideal for studying past changes in the ENSO. Geochemist Kim Cobb of the Georgia Institute of Technology has been studying coral reefs at Palmyra Atoll for over a decade. She uses scuba gear to dive down to and collect cores from the massive heads of the coral genus Porites, which grow up to several meters (6–10 feet) high. Cobb also collects fossilized corals on the island in order to extend the record to older time periods. Once all of these coral cores are taken to her laboratory, they are X-rayed to delineate density changes in their calcium carbonate skeleton. Denser layers are formed during periods of slower growth in the cooler winter months; less dense layers are formed during the warmer summer months, when the coral grows more rapidly. These annual layers, like tree rings, provide a chronology of the coral. Each sample is analyzed for geochemical signals of climate events that took place when the coral was growing.
Cobb and her team have found that the Palmyra corals have faithfully recorded the El Niño events of the twentieth century as reflected in the ratio of oxygen-18 to oxygen-16 in the coral skeletons. Armed with this knowledge, Cobb has explored the frequency and intensity of El Niño over past periods of climatic change. She found that, during El Niño events, the waters surrounding Palmyra become warmer and rainfall increases—both leading to a reduced amount of the heavier isotope (oxygen-18) relative to the lighter isotope (oxygen-16) in the coral skeleton. Conversely, La Niña brings cooler waters and reduced rainfall to the site, leading to a greater proportion of oxygen-18 relative to oxygen-16.
Cobb was able to extend the record back 1,100 years by splicing, drilling, and dating dozens of exposed fossilized coral heads on the beaches of Palmyra. These studies reveal that, during the Medieval Climate Anomaly between the tenth and fourteenth centuries, sea surface temperatures were cooler in the central Pacific than today—conditions associated with La Niña. We know from studying modern climate patterns that cooler waters in the central and eastern Pacific are associated with drier conditions over the American West and northern Mexico. Cobb’s results suggest that the Medieval droughts (described in chapter 9), which coincided with the collapse and migration of the Ancestral Pueblo and other cultures across the Southwest and California, may well have been caused by extended La Niña conditions in the Pacific Ocean.
Another intriguing result of the Palmyra coral record reveals that El Niño events became more frequent and intense during the Little Ice Age—particularly in the seventeenth century—than we have experienced for most of the twentieth century. Today, we know that El Niño events are associated with increased precipitation in the Southwest. As discussed in chapter 10, the cooler climate during the Little Ice Age coincided with wetter conditions and increased flooding in California and the American West.
THE ENSO OVER THE AGES
Studies of the past behavior of the ENSO have led to new understandings elsewhere as well. For example, southern Ecuador is a region adjacent to the tropical Pacific with a climate closely tied to El Niño. Two geologists, Donald Rodbell and the late Geoffrey Seltzer, selected a study site there, Lake Pallcacocha, to assess the effects of the ENSO over the entire Holocene.
El Niño winters typically involve periods of heavy rainfall and flooding in this region, leading to rapid sediment erosion from the surrounding mountains. This erosion results in thick layers of light gray sediment being deposited on the lake bottom. Rodbell, Seltzer, and their team analyzed these layers in long sediment cores taken from Lake Pallcacocha to calculate the frequency and intensity of past El Niño events. They found that, prior to 5,000 years ago, during the early to mid-Holocene, flood layers occurred on average every fifteen years, whereas during the late Holocene, flood layers occurred every two to ten years. This result suggests that El Niño events have been occurring more frequently in recent times.
VARIABILITY IN THE PACIFIC DECADAL OSCILLATION
Researchers have also looked for clues to past climate changes in the strength of the Pacific Decadal Oscillation (PDO)—another pattern of ocean-atmosphere interaction that influences precipitation in the American West (as described in chapter 4). When the PDO is in a “negative,” or cool, phase, sea surface temperatures are typically cooler than average; conversely, when the PDO is in a “positive,” or warm, phase, sea surface temperatures are warmer than average. Researchers who use paleoclimate data to reconstruct past sea surface temperatures are able to see patterns of change in the phase of the PDO and even in the strength of the PDO. Such studies suggest, for example, that the PDO was generally in the negative (cool) phase during the middle part of the Holocene, about 7,000 to 4,000 years ago, so sea surface temperatures along the Pacific coast were cooler. Conditions during this period were wetter in the Pacific Northwest and drier in the Southwest and interior western North America (as described in chapter 7). The wetter conditions of the Neoglacial that followed (as described in chapter 8) correspond to a period when the PDO was in a positive (warm) phase.
The PDO returned to the negative (cool) phase again in the late Holocene for the four centuries from AD 900 to 1300. As we described in chapter 9, this was a period marked by extreme and prolonged droughts. The connection between the PDO phase and this megadrought period has been inferred from tree-ring records from Southern California and western Canada—which are at the extreme ends of the North American region affected by the PDO. Tree rings from these regions reveal that the Southwest was dry and the Northwest was wet at that time. A study using moisture-sensitive tree-ring chronologies from Southern California and northern Baja California has shown that the PDO was unusually weak during the Little Ice Age compared to that of the centuries immediately before and after it.
Although ocean conditions of the past seem to explain the broader patterns of precipitation in the West, other hypotheses have been put forward to explain some of the more frequently recurring climate cycles that we see in the past. The more intriguing of these hypotheses are ones that involve extraterrestrial causes of climate change.
EXTRATERRESTRIAL CAUSES OF CLIMATE CHANGE: SUNSPOT CYCLES
Whereas the PDO and the ENSO are climate oscillations produced internally within the earth’s ocean-atmosphere system, our planet’s climate is also influenced by extraterrestrial events, including changes in the output of energy from the sun. The study of solar variations has a long history, extending back to the early seventeenth century when Galileo Galilei systematically studied the sun at dawn and dusk with his rudimentary telescope. He observed dark patches on the sun’s surface, carefully recording their locations and numbers, and noticed that the number and positions of the dark patches shifted from year to year. Galileo and other astronomers recorded a ten- to twenty-fold decrease in these dark spots beginning in AD 1645. For the next seventy years, between AD 1645 and 1715, the sun’s surface was almost devoid of sunspots.
In the late nineteenth century, the sunspot records were reexamined by astronomer Edward Walter Maunder. This period of low sunspot numbers from AD 1645 to 1715 is now known as the “Maunder Minimum.” Maunder noted that this period coincided with the peak of the Little Ice Age, the cool interval between the fifteenth and nineteenth centuries, and he proposed a relationship between sunspots and climate.
Two other periods of low sunspot activity occurred during the past millennium: the “Wolf Minimum,” peaking at AD 1300, and the “Sporer Minimum,” peaking at AD 1500. These, and the Maunder Minimum, are spaced about 200 years apart, remarkably similar to the recurrence intervals of extreme climate events—floods and droughts—in the American West and elsewhere across the globe. In the West, these extreme events were recorded in the stratigraphic records of megafloods in the Santa Barbara Basin, changing lake levels of Mono Lake, and fluctuating salinity in the San Francisco Bay, to name just a few.
The pattern recurs beyond the western United States. For example, severe floods and droughts have been found to recur every two centuries in Central America. The Mayan empire of the Yucatan, which had flourished for thousands of years, was buffeted by periodic droughts and ultimately collapsed at the peak of its power nearly 1,000 years ago. The Mayans had long kept records of astronomical phenomena, and the droughts coincided with a pronounced 208-year cycle of increased solar intensity, a fact they may have noted with grave concern.
The Sunspot-Climate Connection
Although the connection between sunspots and climate is intriguing, the causal mechanism remains somewhat mysterious. The change in solar energy received by the earth between solar sunspot maxima and minima is a mere one-tenth of 1 percent. How could such a small change cause such relatively large climate shifts on the earth?
Sunspots appear when loops of magnetism form deep within the sun and rise to the surface, leading to a drop in temperature that appears as a dark spot when viewed from the earth. During periods with more sunspots, solar output is greater, with more solar energy being received at the top of the earth’s atmosphere. Satellite observations have shown that the solar constant—the total amount of solar energy flux received by the earth—is not actually “constant.” Rather, it varies on timescales of days to years, apparently related to the number of sunspots. Climatologists (and economists) have noted an eleven-year sunspot cycle that has been correlated with such climate-related phenomena as agricultural production in the United States. During periods of high sunspot activity, the value of the solar constant increases, triggering a chain reaction in the atmosphere and ocean that ultimately leads to changes in precipitation and temperature patterns in different regions of the earth.
Scientists are now exploring the details of changing solar output and associated effects on the earth’s climate system. They use climate models to show the effects of small energy changes received in the upper atmosphere. These changes can be amplified through positive feedbacks in the planet’s climate system. Climate modeler Gerald Meehl and his colleagues at the National Center for Atmospheric Research in Boulder, Colorado, have proposed two mechanisms for this amplification. In the “top-down” mechanism, increased sunspot activity leads to an increase in the amount of ultraviolet light hitting the stratosphere—enhancing the formation of ozone, absorbing more ultraviolet light, and thereby heating the stratosphere. The amount of heating in the stratosphere is not uniform across the globe but varies with latitude. The temperature differences lead to an increase in winds and tropical precipitation.
In the second, “bottom-up,” mechanism, greater sunspot numbers lead to increased absorption of solar energy by the surface ocean in the subtropics and so increased evaporation of seawater. The trade winds become stronger, and the evaporated moisture carried by the trade winds increases precipitation in the western Pacific and subtropical zone. These stronger winds blowing from east to west across the Pacific increase the upwelling of cooler water in the eastern equatorial Pacific, lowering sea surface temperatures, similar to conditions during a La Niña event.
Although the mechanisms linking sunspots and climate are still being investigated, these climate models are important steps in understanding why precipitation patterns in the American West and elsewhere appear to be closely tied to sunspot cycles.
Radiocarbon and Past Solar Activity
In an attempt to understand past changes in solar output in deeper geological time, Earth scientists have devised ingenious proxy methods for solar activity. One of these methods is the amount of radiocarbon produced in the earth’s atmosphere, which is closely related to the output of energetic particles from the sun (the solar wind). Periods of low solar activity correspond to high radiocarbon production in the earth’s atmosphere and vice versa. Radiocarbon (carbon-14) combines with oxygen to form carbon dioxide and is taken up by plants and trees during photosynthesis. Years with higher levels of radiocarbon production will thereby be recorded in the growth rings of the trees. Geochemists measure the minute amounts of carbon-14 in tree rings and compare this radiocarbon age to the actual (calendar year) age of the tree rings, obtained by counting the annual rings.
The tree rings used in these studies are those of the long-lived bristlecone pines in the White Mountains of eastern California, with living trees that are 4,000 years old and fossilized trees extending the record back to 11,000 years before the present. These studies have shown that, in addition to a 200-year cycle in solar activity, there are solar cycles with periods of 55, 90, 400, 1,000, and 2,500 years. One key finding is that these periodicities are seen in a number of climate records in the West, including extreme floods and droughts, suggesting that solar activity somehow affects when and where precipitation falls by influencing atmospheric circulation.
Another cycle that climate scientists have detected has a 1,500-year periodicity. For decades, geologists have noted that glaciers throughout the Holocene advanced and retreated every 1,500 years or so. More recently, this 1,500-year cycle has also been documented in oceanic sediment cores in the form of discrete layers of debris transported to the North Atlantic by icebergs (known as “ice-rafted debris”). In the eastern Pacific, changes in sea surface temperatures from the Santa Barbara Basin similarly exhibit a 1,500-year cycle. Thus, this cycle appears to correlate with solar activity, but details of how it influences climate are still being investigated.
TERRESTRIAL CAUSES OF CLIMATE CHANGE: VOLCANIC ERUPTIONS
Volcanic eruptions—violent natural events in their own right—have been linked with past climate variations. Although eruptions occur relatively frequently, they are not cyclical. Over timescales of millions of years, changes in the number of volcanic events on land and undersea (where volcanism occurs at mid-ocean ridges and during the formation of oceanic plateaus) eventually alter the flux of carbon dioxide in the atmosphere, modifying climate. For instance, the Mesozoic Era, when dinosaurs roamed the earth, had a much balmier climate primarily because of increased undersea volcanism associated with faster seafloor spreading and oceanic plateau formation. This volcanism increased the amount of carbon dioxide from the interior of the earth to the atmosphere, resulting in atmospheric levels higher than 2,000 parts per million and high global temperatures.
Over shorter timescales of months to years, volcanic eruptions have had the opposite effect on temperatures. Following an eruption, particles of dust and ash are ejected into the atmosphere, where they are suspended, blocking sunlight and reducing solar radiation for a year or more. Volcanoes also eject sulfur gases, which combine with water vapor in the stratosphere to form tiny droplets of sulfuric acid. These droplets absorb solar radiation and scatter it back into space, causing global temperatures to fall.
Volcanologists have compared the climate effects of recent eruptions and observed that the greatest cooling is caused by sulfur-rich eruptions. The 1982 eruption of El Chichon in Mexico, for example, emitted a smaller volume of ash than the 1980 eruption of Mount St. Helens in Washington but forty times the volume of sulfur gases, and El Chichon resulted in five times the amount of atmospheric cooling. Although these effects are relatively short-term, they can be significant enough to have bigger climatic impacts. Two major volcanic eruptions are thought to have contributed to global cooling during the Little Ice Age. In Iceland, the Laki eruption in 1783 was the largest basaltic eruption in recorded history, with 3.4 cubic miles of lava erupting over a period of eight months. The eruption caused extremely severe winters in Europe and North America. At the time, Benjamin Franklin observed a constant fog over much of North America and Europe, and he wondered whether the unusually low winter temperatures experienced that year in the eastern United States, where temperatures were 8°F lower than they had been the previous 225 years, were related to the Laki eruption.
Three decades later, an even larger eruption (the largest of the past 10,000 years) occurred when Mt. Tambora erupted in Indonesia in 1815. Some 36.5 cubic miles of ash were sent up into the atmosphere, resulting in what became known as the “year without a summer,” featuring unusually low temperatures, frost, and snow even during summer months in Europe and New England. The amount of ash spewed into the air by Mt. Tambora was about a hundred times that of Mount St. Helens in 1980.
ICE AGE CYCLES
The relatively rapid climate fluctuations of the past 11,000 years are superimposed on more gradual changes in the orbit of the earth around the sun. The earth’s climate has fluctuated in and out of ice ages over the past two and a half million years, as described in chapters 5 and 6. What caused these recurring ice ages, how were they discovered, and how have they affected climate during the Holocene?
The theory of the nature and causes of these longer-term variations in climate has evolved over the past two centuries with the help of a distinguished group of astronomers, geologists, mathematicians, climatologists, and physicists. One of the earliest and most unlikely, and thus remarkable, contributors to our understanding of these cycles was James Croll. He grew up in Scotland in the early nineteenth century and was forced to drop out of school at the age of thirteen to work on the family farm. After his long and arduous days in the fields, Croll returned home to study physical science late into the night. He had a passion for understanding the forces governing the natural world. His family could not afford a formal university education, so, when Croll reached his late teens, he was forced to embark on a series of jobs in which he had little interest or aptitude, including millwright, carpenter, salesman, shopkeeper, hotel keeper, and life insurance salesman. Only in his spare time could he study his true passion—earth science.
A major turning point for Croll came in middle age, when he accepted a job as a janitor at the Andersonian College and Museum during the 1860s. His salary was meager, but he had access to an extensive scientific library where he spent evenings studying physics and geology. He was drawn to a hotly debated topic of the day: the features and causes of past ice ages. He was particularly interested in the hypothesis that past ice ages were related to changes in the earth’s orbit around the sun.
The shape of the orbit itself changes on long timescales, pulsing from near-circular to slightly more elliptical. The gradual distortion of the earth’s orbit is caused by the gravitational pull of other nearby planets. When the orbit is elliptical, the earth receives more sunlight than at other times because of the slight change in its distance from the sun. Today, the earth’s orbit is very close to circular, and there is only a small difference in the amount of radiation received during its yearlong revolution.
Croll calculated that, for each revolution of the earth around the sun, the amount of solar radiation received on the earth’s surface is slightly different than it was the year before. The difference is infinitesimal but cumulative. He speculated that these small changes in the earth’s orbit could alter the amount of solar radiation reaching the earth’s surface just enough to influence the earth’s climate. His laborious calculations of the earth’s orbit over the past three million years showed cyclical variations between more circular and more elliptical shapes, with periods lasting 100,000 years.
Another aspect of the earth-sun relationship also interested Croll. He hypothesized that maximum ice growth, leading to an ice age, would occur when the orientation of the earth’s axis of rotation was in a certain position. The earth’s axis of rotation also changes cyclically; it “wobbles” like a spinning top. Today, the earth’s axis at the North Pole points toward the North Star. Over time, however, the orientation of the earth’s axis slowly changes, tracing a circle in the sky that takes 26,000 years to complete. Croll reasoned that the most likely time for the growth of a large ice sheet in the northern hemisphere (where most of the global landmass is located) is when the earth’s orbit is most elongated and when the axis of rotation is pointing away from the sun during the winter.
Although Croll’s calculations showed that the earth’s orbital changes caused only slight variations in the amount of solar energy reaching its surface, he surmised that ice sheets, once they begin to grow, reflect sunlight back to space, causing even more cooling and resulting in more ice—a so-called “positive feedback” loop. Croll can be credited with being the first to recognize this important process. He also proposed that growing ice sheets near the poles would increase the temperature differences between the equator and poles, leading to more vigorous winds and surface currents, ultimately bringing more moisture toward the poles. These processes would enhance ice sheet growth and are still recognized as important today.
Despite his humble beginnings, his lack of formal education, and his need to work long days in a variety of jobs ill-suited to him, Croll became a prominent scientist, publishing a book entitled Climate and Time in Their Geological Relations: A Theory of Secular Changes of the Earth’s Climate. He finally landed a position at the Scottish Geological Survey, and ultimately his impressive achievements were honored by his election to the Royal Society of London.
By the late nineteenth century, however, Croll’s ice age theory was questioned and finally rejected. His proposed date for the end of the last ice age, some 80,000 years ago, was shown to be off by tens of thousands of years. A much later date, 10,000 years, was estimated by geologists who studied the erosion of young glacial deposits by the Niagara River. This disparity led the geologists at the time to reject Croll’s entire theory, and, sadly, he died an unappreciated scientist.
MILANKOVITCH CYCLES
Fortunately, Croll’s ice age theory was revived two decades later by a young Serbian mathematician, Mulatin Milankovitch, who had read Croll’s book and was so inspired that he made it his life’s goal to understand the earth’s past climate. He began with calculations of the amount of solar radiation received by the earth’s surface at different latitudes and seasons over the past 130,000 years, based on the orbital cycles. Milankovitch calculated that variations in the tilt of the earth’s rotational axis seemed to produce the largest changes in solar radiation (and temperature) on the earth’s surface. The tilt of the spin axis with respect to the earth-sun plane, currently 23.5°, fluctuates between 22° and 24.5°. The importance of this becomes more evident when we consider that the earth’s tilt controls our seasons: when the Northern Hemisphere tilts away from the sun’s rays, it experiences winter, and when the Northern Hemisphere tilts toward the sun, it experiences summer. Each day, the earth completes a rotation around this axis that is slightly different than the day before. The difference again is tiny, but it forms a tick in a long cycle that takes 41,000 years to complete, during which our seasons become more or less pronounced.
Contrary to Croll’s findings, Milankovitch calculated that the most likely time for the growth of large ice sheets is not during the Northern Hemisphere winter but during its summer. When summer temperatures in the mid- to high latitudes of the Northern Hemisphere are cooler, the snow that has accumulated over the winter persists during the summer, rather than melting. Year by year, snow continues to build up, finally turning to ice under the accumulated weight and pressure, eventually growing into a large ice sheet.
EVIDENCE FROM THE DEEP SEA
The final link in the great ice age puzzle was discovered in the 1950s, when geologists began drilling cores in the sediments of the deep seabed. These sediments include tiny carbonate microfossils (the foraminifera) that continuously rain down through the water column. The microscopic fossils allowed geologists to derive the pattern of the ice ages for the past two million years.
Pioneering research measuring the oxygen isotopes in tiny fossil shells was conducted by Cesare Emiliani in the laboratory of his mentor, Harold Urey, at the University of Chicago. Emiliani developed the use of oxygen isotopes in paleoclimate research—a technique whose value, as we have already seen, continues to this day. It is based on the more rapid evaporation of the lighter isotope of oxygen (oxygen-16) from the ocean than the heavy isotope (oxygen-18). This evaporated water vapor is carried to higher latitudes and continents, some precipitating as snow on land, building great ice sheets during glacial periods. As the oxygen-16-rich water is removed from the oceans and stored as ice on land, sea level is lowered, and the ocean water becomes more enriched in the heavy isotope of oxygen (oxygen-18). When marine organisms (such as the microscopic foraminifera) build their shells of calcium carbonate, they incorporate this oxygen, recording changes in sea level and glacier growth on land.
Emiliani published a landmark paper in 1957 that described oscillations every 100,000 years between two opposing climate states: cold glacial periods, lasting up to 90,000 years, and warmer interglacial periods, lasting only about 10,000 years. The temperature difference between the two states is approximately 6°C (11°F) or more. The shift from the warmer to the cooler state into an ice age is relatively slow, as ice builds up on the Northern Hemisphere continents over the course of tens of thousands of years. In contrast, the shift from the cooler to the warmer state (from glacial to interglacial) takes only thousands of years. This pattern of climate change over the past million years has been described as “sawtooth” because of its sharp up-and-down shape when temperature or sea level is plotted against time.
EVIDENCE FROM ICE CORES
Ice sheets on Greenland and Antarctica contain in their layers of ice detailed information about past global temperatures, atmospheric composition, windiness, and more. Over the past two decades, cores extracted from these enormous ice sheets have been used to reconstruct past climate. In one project, researchers extracted atmospheric gases from bubbles trapped in the ice to measure past levels of carbon dioxide. Their results show that carbon dioxide levels in the atmosphere varied with temperature during the ice ages over the past two million years. Carbon dioxide dropped to concentrations as low as 180 parts per million (ppm) during the peak of the glacials and rose to about 280 ppm during the interglacials.
Carbon dioxide causes a climate feedback by amplifying the small changes in solar radiation received by changes in the earth’s orbit. During glacial periods, carbon dioxide is absorbed by the oceans, decreasing its concentration in the atmosphere, leading to further cooling, in a positive feedback loop. During the transitions from glacials to interglacials, carbon dioxide is released back to the atmosphere, further warming the earth, in yet another positive feedback loop. Carbon dioxide levels reached their “interglacial” value of 280 ppm about 6,000 years ago but have climbed to the unprecedented level (over the past two million years) of 390 ppm over the past century. Many climate scientists warn that a continued rise in atmospheric carbon dioxide will warm the earth’s climate for the foreseeable future.
The ice cores also show that, during glacial periods, the climate changed quite abruptly—warming rapidly and then cooling—every few thousand years. Although we have not experienced such drastic climate swings during the Holocene, climate scientists are actively studying these past events in order to understand what caused them and to determine if they might recur in a future warmer world.
THE LAST BIG THAW
At the peak of the last glacial period, some 20,000 years ago, ice sheets reached a maximum size (and sea level its lowest level) during a period called the Last Glacial Maximum. Solar radiation in the Northern Hemisphere summer dropped to a minimum at that time, and the global climate was as cold as it had been in 100,000 years. Gigantic ice sheets blanketed North America, Europe, Asia, and Scandinavia. These ice sheets were between one and two miles thick; their weight was so great that they depressed portions of the continents by as much as 1,200 feet. The water locked up in these enormous ice sheets originated from the oceans, causing global sea levels to drop by about 360 feet.
Gradual shifts in aspects of the earth’s orbit explain the thawing and melting of the ice ages beginning 20,000 years ago as well as the broad climate patterns that have occurred since then. Summer solar radiation in the Northern Hemisphere began increasing from a minimum 20,000 years ago to peak values about 10,000 years ago, when the earth’s tilt reached its maximum of 24.5°. However, these astronomical conditions were not enough to account for the 18°F difference between glacial temperatures and Holocene interglacial temperatures.
Climate feedbacks are required to accentuate these nudges in global conditions. As the glaciers retreated, the white cover of snow and ice, which reflects most of the sunlight, was removed from the continents, replaced with a darker earth surface. That darker surface absorbed more of the sunlight, transferring the heat to the surrounding area rather than reflecting it back into space. During the early Holocene, once the southern edges of the ice sheets began to retreat, the earth’s reflectivity (or albedo) decreased, with less solar radiation reflected back into space and more of it retained by the earth’s surface, where it warmed the land and melted more ice. Many such feedback loops across the globe have worked together in leading the earth into the ice ages and back out again.
ENORMOUS MELTWATER LAKES AND MEGAFLOODS
The melting of enormous ice sheets led to the formation of vast glacial lakes. Rather than draining directly to the sea, the meltwater from the ice sheets sometimes became trapped behind ice dams and formed gargantuan lakes. As these lakes grew, they periodically burst through their dams, forming some of the largest floods in the earth’s history.
Evidence for these enormous glacial floods was first discovered by a young field geologist named J. Harlan Bretz in the early twentieth century. As described in a book by Doug Macdougall, Bretz spent his summers during the 1920s and 1930s documenting and mapping the enigmatic geology of eastern Washington and Idaho. He found gigantic boulders sitting on the landscape 650 feet higher than the present-day Columbia River; intertwining channels and canyons cut into hard basalt bedrock to depths of 1,000 feet; enormous, extinct waterfalls; and giant gravel bars and potholes. These seemingly unrelated features began to tell an almost unbelievable story: this region of ripped-up earth in eastern Washington, the so-called “Channeled Scablands” (see figure 18 in chapter 7), appeared to be the result of a megaflood of unimaginable proportions—possibly the largest ever to sweep across the face of the earth.
Bretz hypothesized that this megaflood originated in northwestern Montana, at the southern boundary of the ice sheet two miles thick—now called the Laurentide ice sheet—that blanketed Canada and the northern United States at the end of the last ice age. As the earth warmed, the Laurentide ice sheet melted. The waters that had been locked in the ice were released to make their way back to their place of origin—the ocean. In North America, these waters took several routes: east through the St. Lawrence Seaway to the North Atlantic; south through the Mississippi River to the Gulf of Mexico; and west through the Columbia River to the Pacific Ocean.
One of the largest of the glacial lakes was named Missoula, and it covered much of northern Montana. Even as Bretz was mapping giant flood features, a glacial geologist named Joseph Thomas Pardee was studying the sediments deposited at this lake’s bottom. As Macdougall’s book also describes, Pardee’s work revealed that Lake Missoula was once the size of Lake Michigan but even deeper: ancient lake shorelines mapped in the hills surrounding Lake Missoula revealed that its depth was, in places, 2,000 feet. Linear hills 45 feet high and separated by over 400 feet were enormous ripple marks that formed just west of Lake Missoula under the huge volume of floodwater draining the lake.
Putting together all the evidence, Bretz hypothesized that an ice dam once held back the lake, and, when that dam melted, the water trapped behind it burst forth in a catastrophic flood. The sound of the bursting dam would have been deafening as it rumbled for hundreds of miles ahead of a wall of water 500 feet high pouring across the Columbia Plateau toward the Pacific Ocean. It would have swept up everything in its path, a brown and churning wave of water, soil, vegetation, and floating icebergs still embedded with boulders. The discharge rate of this massive flood was likely over 200 times that of the largest historical floods on the Mississippi River.
This terrifying flood was probably the largest but not the only one that occurred in the Northwest as the earth thawed from the last ice age. Evidence for multiple flood events comes from the layers of sediment accumulated in the Scablands. The scoured debris and sediments in these floodwaters were eventually carried out to the Pacific Ocean to a final resting place over 600 miles from the Oregon coast, where they were recently cored and recognized as further evidence for the megafloods.
As we have seen, a number of factors influenced climate over different timescales. The earth’s climate is affected by many powerful processes, including changes in its orbit, changes in solar output, volcanic eruptions, and oceanic changes. These factors act simultaneously and involve feedback loops, producing changes in regional and global climate. For regions like the American West, the net result over the past 20,000 years has been a climate that fluctuates broadly between warmer and colder, wetter and drier, and is punctuated by more extreme floods and droughts that recur on a regular basis.
The average climate in the West has been relatively benign over the past century and a half, encouraging human settlement and expansion. A certain faith in the reliability of water needed for the quality of life we enjoy has also fostered a tremendous population growth. In the following chapter, we present an overview of how humans during the past 150 years have adapted to the climate in the West and the ways in which these adaptations, including extensive water development projects, have affected ecosystems in the West, particularly those in the aquatic environment.