An Evolving Earth...Penna C01.tex V2 - 09/05/2014 10:07 A.M. Page 17 AN EVOLVING EARTH 17 cooled the...

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Chapter 1

An Evolving Earth

Introduction

Earth is a living, dynamic, and sometimes violent planet. Able to sustainorganic life, subject to violent swings in temperature, the modern climateis the result of Earth-defining changes. These changes range from the colli-sion of Earth’s continents to the ability of its oceans to absorb solar energyas heat and reflect it as light, and to influence the intensity and flow of oceancurrents. Atmospheric changes, tectonic movements, and global climate areinterconnected forces that transformed Earth’s history and created an envi-ronment suitable for the development of all life forms.

On a much longer timescale, scientists believe that our solar systemformed because gaseous clouds of debris from older stars condensed intosolid matter about 4.6 billion years ago. The formation of our solar systemwas but one of many significant cosmic events in the history of the universe,which most scientists agree took place about 13 billion years ago. At manytrillions of degrees, a super-heated universe, smaller than the size of anatom, began expanding faster than the speed of light in its first few seconds.This was the Big Bang. The universe’s background radiation is a reminderof this event and the continuing expansion of the cosmos.

For the next 300,000 years, the universe remained a super-heated entitymuch like the interior of the sun in our solar system. As the universeexpanded and cooled, a phenomenon that continues to this day, energyand matter separated. As described by the historian David Christian:

About 300,000 years after the big bang, all the ingredients of creation werepresent: time, space, energy, and the basic particles of the material universe,now mostly organized into atoms of hydrogen and helium. Since that time,nothing has really changed. The same energy and the same matter have con-tinued to exist. All that has happened is that for the next 13 billion years these

The Human Footprint: A Global Environmental History, Second Edition. Anthony N. Penna.© 2015 John Wiley & Sons, Inc. Published 2015 by John Wiley & Sons, Inc.

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16 AN EVOLVING EARTH

same ingredients have arranged themselves in different patterns, which con-stantly form and dissipate.1

By collapsing the timescales of the 13-billion-year cosmos by a factor ofone billion, the Big Bang took place 13 years ago. In this scenario, Earth’sfirst living organisms appeared about four years ago, and modern humansevolved in Africa about 50 minutes ago. The invention of agriculture andthe building of cities, which you will read about in later chapters, occurredfive minutes and three minutes ago, respectively.2 Thought about in this way,the life of humans is a relatively recent addition to Earth’s history. Looked atin another way, if the 10-billion-year projected life of Earth’s energy systemwere compressed into a single year, all of written human history would berepresented in less than a minute. And the twentieth century would be lessthan a third of a second long.

The Origins of Earth and Its Unique Atmosphere:From Hot to Cold Planet

More than 4.6 billion years ago the explosive atomic energy of mega-sizedmeteors created a liquid mass of molten rock of 1,800∘F (980∘C). Thiswas the newly forming Earth, with an atmosphere of mostly hydrogenand helium, the main gases around the sun. During a 600-million-yearperiod, repeated bombardments followed by the sinking of the iron cores ofthose meteors created the molten center of Earth. Its iron core created theplanet’s magnetic field, which deflected many high-energy and dangerousparticles from Earth. In this very important way, it acquired and to this daypossesses a protective shield.3

This extremely hot planet created an equally torrid atmosphere includingsuper-heated hydrogen and helium molecules, moving so fast that theyescaped Earth’s gravity. The young Earth can be thought of as a massivevolcanic field that created its own infant atmosphere, releasing gases: water(H2O) as steam, carbon dioxide (CO2), and ammonia (NH3).4 As largemeteor strikes slowed over a period of about two billion years, however, thesurface and its atmosphere changed significantly. Some scientists attributethis climatic transition to the impact of a mega-meteor, estimated to bethe size of the planet Mars, that struck Earth about two billion years ago.The impact knocked it on its side, deflecting much of the sunlight thatwould have normally warmed the tropics. A decrease in solar radiation


AN EVOLVING EARTH 17

cooled the surface of Earth and “dried” the atmosphere, causing moreradiation to escape from the surface, and initiated glacial expansion and acolder, snow-covered planet (2.3 billion years ago). The advancing glaciersreflected abnormal amounts of light and heat back into space, allowing forfurther cooling.

A cooling atmosphere made conditions suitable for ice comets emergingfrom the deep voids in our galaxy and weighing between 20 and 40 tonsto bombard our inner space every two to three seconds. According tothis “Snowball Earth” theory, these galactic events saturated Earth’s atmo-sphere with increasing amounts of condensation. “Cosmic rain” cooledthe white-hot planet and created the world’s earliest oceans. Much of theplanet’s atmospheric carbon dioxide dissolved in these new oceans, creatingthe conditions for the development of bacteria that could live on the energyprovided by the sun and carbon dioxide in the water. A by-product ofthis interaction between sunlight and carbon dioxide was oxygen (O2). Asatmospheric oxygen levels rose, carbon dioxide levels dropped. Ammoniamolecules in the atmosphere separated by sunlight into one nitrogen atom(N) and three hydrogen atoms (H3), with the latter escaping Earth’s gravityand drifting off into space. Although the cooling did not require oxygen,free “atmospheric oxygen levels probably increased considerably about

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Figure 1.1 Cutaway views showing the internal structure of the Earth.Source: Jacquelyne Kious and Robert I. Tilling, This Dynamic Earth: The Story ofPlate Tectonics (Washington, DC: US Geological Survey, 1996).



2 bya [billion years ago] and again near 800 mya, coincident with majorevolutionary changes in Earth’s biosphere. Carbon dioxide levels are alsobelieved to have been substantially different during the Precambrian Epoch(4.6 bya–570 mya).”5

Increases in carbon dioxide levels with an atmospheric concentrationof 0.035% after the Precambrian affected Earth’s atmosphere.6 Changes inthe land and sea biospheres not only transformed carbon dioxide into oxy-gen but also sequestered carbon in various “sinks” – the oceans, mountainranges, and solid rocks. Accordingly, the planet experienced glaciers thatcovered the land and water from the northern hemisphere to the tropicsabout 700 mya. The ice sheets locked up about 25% of Earth’s carbon. Withoceans covered in ice rather than in liquid form, they could not capture car-bon dioxide, released by volcanic eruptions. As noted earlier, under normalconditions, liquid oceans would absorb carbon dioxide. Under these cir-cumstances, however, the atmosphere captured increasing amounts of thissoluble gas.

The result of these tumultuous and explosive beginnings was an atmo-sphere unlike any others in our solar system. It was an atmosphere with justenough oxygen (about 21%) to allow bacteria to evolve into living thingsand for our species to evolve as well. Today, without accounting for watervapor, about 78% of the atmosphere is nitrogen, argon and other gases makeup about 1%, and carbon dioxide represents 0.04% or about 400 parts permillion of air and rising.7

Incidentally, the Snowball Earth theory provides a plausible explanationfor the coming of the ice but not an answer to the vital geophysical problemof how Earth eventually corrected itself to its current 23.5∘ tilt. To date, themost plausible scientific explanation is that Earth’s land mass was clusteredtogether at the South Pole 600 mya. The weight of this clustering tippedEarth into its present inclination. Eventually this land mass broke up to formthe continents, a topic described in greater detail later in this chapter.

Icehouse Planet and Greenhouse Planet

These geological events took place over millions of years during thePrecambrian, when Earth was covered with ice. However, increasing levelsof carbon dioxide, a heat-trapping gas, triggered global warming. Whenthat happened, millions of years of accumulated global ice melted away.As atmospheric temperatures rose, evaporation from oceans and surface



water increased, and Earth’s climate progressively became warmer. Watervapor is the largest natural greenhouse gas because it traps solar radiationemitted from Earth’s surface.

In the beginning, these processes created the near-perfect environmentfor early life forms, including oxygen-producing blue-green algae. Risinglevels of oxygen in the atmosphere “made possible the more complex chem-istry used by multi-celled animals.”8 The original greenhouse effect trig-gered the “Cambrian explosion” (570–530 mya), the burst of life that sawthe origin of many species.

Current debates about the potentially harmful effects of global warm-ing on the climate system continue, but billions of years ago, global warm-ing complemented the production of oxygen-producing blue-green algae,triggered the ice melt, and promoted the reaction of oxygen and iron inwater. The presence of oxygen caused iron to separate from water and formdeposits at the bottom of the oceans. Additionally, extremely high surfacetemperatures caused considerable evaporation and consequently consider-able rainfall. “Rainfall would have washed the carbon dioxide out of theair, turning it into carbonic acid. The acids then weathered continents, andthrough chemical reactions, those sediments turned into limestone whenthey were washed into the oceans.”9 Iron and limestone deposits collected inocean-bottom sediments that in geological time became deposits in futureglacial rocks and were very important to future human manufacturing andindustrial activities described in later chapters.

About 550 mya, changes in our planet’s global carbon budget resultedin significant changes in the global climate. During this entire period, theamount of atmospheric carbon dioxide dropped to about two or threetimes modern levels. These declines occurred as carbon became locked insolid rock formations of all kinds, as the ocean floors spread out, and asland-based ecosystems with their diverse and expanding flora evolved.

While this warming continued to green the planet, decreasing amountsof carbon dioxide caused a slow and gradual cooling of Earth over millionsof years. These fluctuations in carbon dioxide levels also provide a plausi-ble explanation for the cyclical history of “ice ages” experienced during thepast 150 million years. A seemingly regular pattern of ice ages has appearedmore recently during the past 740,000 years. Every 100,000 years an ice ageof about 95,000 years with melting extending over thousands of the finalyears is followed by an interglacial period of approximately 10,000 years.Given this pattern, human civilization has blossomed during the currentinterglacial period.



Plate Tectonics, Super-continents,and Climate Change

In addition to the volatile changes in the global climate system, major alter-ations in the topography of Earth and in the location of the continents accel-erated widespread cooling over millions of years. The movement of majorplates, including the breakup of major landmasses into the continents thatwe know today, changed the climate in ways that altered the ecology ofthe planet.

As Earth’s surface cooled during its four-billion-year history, the60-mi-thick (96 km) crust called the lithosphere was created. In time,volcanic activity and earthquakes caused the lithosphere to break into hugetectonic plates. Repeated activity at the boundaries of these plates causedthem to move and drift. Lava flows continued to create and reconfigurethe fabric of Earth’s surface. Giant cracks in the plates permitted magma toflow from Earth’s epicenter to its surface. Old rock formations fractured asmagma and molten lava flows created new surfaces and a new sea floor. Theexplosive capacity and intensity of these disruptions caused some oceans toexpand and others to contract. Mountain ranges pressed upward as giantearthen plates crashed against each other.

The process that explains the movement of these plates is called con-tinental drift and its origin emerged from the hypothesis that in the dis-tant past a super-continent existed. Eventually, it broke apart into severalpieces that drifted apart over millions of years of Earth’s history. Stitchingthe present-day continents together, as you would a puzzle, along their con-tinental shelves and not along their shorelines, demonstrates the originalconfiguration of this super-continent. That drift occurs seemed plausiblebut scientifically explaining the source of the energy that powered the move-ment remained elusive until the second half of the twentieth century.

Then, the development of sonar technology allowed scientists to map theocean floor. The motion of the plates moves the floor of the ocean into deeptrenches, where the plates and sediments are carried into Earth’s intenselyhot interior. There, they melt and return as lava to the surface of the seafloor. Called subduction, this cyclical process of submerging old sedimentreduced the size of the sea floor and simultaneously replaced it with moltenlava. In the process, it spread the size of the ocean floor. “In other words, itis the heat of Earth’s interior that provides the power needed to move great



plates. That heat is generated largely by radioactive materials within Earth,which had been formed in the supernova explosion that occurred just beforethe creation of our solar system.”10

Although rigid, Earth’s eight large tectonic plates and seven small onesare not static but rather part of a dynamic process of development andchange in the planet’s material composition. Over millions of years, theworld experienced a complex series of massive disruptive volcanic activ-ities and earthquakes followed by periods of tranquility. These collisionsand uplifts had widespread effects. Throughout geological history, thesetectonic plates fused into super-continents, and their eventual breakup andcontinental drift created the present continents. Their constant movementalso had a powerful effect on global climate change that depended on thepositioning of major landmasses over the North and South Poles.

About 280 mya, the continents in the southern hemisphere, namelyAfrica, Antarctica, Australia, South America, and the landmass that wenow know as India, were one large continent known as Gondwana. Theexistence of major glaciers may have coincided with low carbon dioxideconcentrations. North America, Europe, and Asia were separate floatingcontinents and there was also glacial ice on the world’s oceans. Over 10 mil-lion years, Gondwana and the separate floating continents drifted togetherto form the super-continent Pangaea (meaning “all Earth”). “Whether wedescribe it as a landlocked planet with an immense saltwater lake, or anocean planet with an immense island is only a matter of definition. It mighthave seemed a friendly world. At least, you could walk anywhere; therewere no distant lands across the sea.”11

The Warming

Pangaea (270–180 mya) existed during a time of dramatic climatic changeson Earth. After millions of years of major glaciations, global climates beganwarming again. The dew point rose quickly and the ice planet meltedand became covered with great swamps. Before its breakup, Pangaea wascomposed of large connected landmasses that we now “see” as separateentities. Greenland and the British Isles were a part of Europe. Indonesia,Malaysia, and Japan were connected to Asia. Siberia and Alaska were one,and, although now extinct, large, shallow inland seas covered much oftoday’s landscape.


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Within 15 million years, however, shocking changes again visited theplanet as Pangaea began to fragment. A major volcanic eruption of super-plume magnitude ejected molten lava from Earth’s core, through its crustto the surface. “Texas, Florida, and England were then at the equator. Northand South China, in separate pieces, Indochina and Malaya together, andfragments of what would later be Siberia were all large islands. Ice agesflickered on and off every 2.5 million years, and the level of the seas cor-respondingly fell and rose.”12

Earth’s surface had been entirely transformed by the end of this15-million-year period. Lava flows buried entire regions. They reworkedSiberia’s landscape. What remained of Pangaea drifted northward, movingSiberia to its present location, closer to the North Pole.

Mega-monsoons, torrential seasonal rains on a much larger scale thanhumans have ever witnessed, drenched and flooded the land. South Chinaslowly crumpled into Asia. Many volcanoes blew their tops together,belching sulfuric acid into the stratosphere. The biological consequenceswere profound – a worldwide orgy of dying, on the land and at sea, the likesof which has never been seen before or since.13

Increased tectonic fractures, volcanic eruptions, and the weathering ofcarbon-rich rocks and sediments in geological time elevated carbon dioxidelevels and pushed the climate system into the extended warmth of theMesozoic Era (230–65 mya).14

As the breakup of Pangaea continued, about 180 mya a narrow oceanstrait separated the two large pieces of the super-continent’s puzzle, nowknown as South America and Africa. They would slowly continue tomove apart as the sea floor’s spreading along the enormous mid-oceanrift expanded the Atlantic Ocean’s size. Simultaneously, older ocean floordisappeared into deep ocean trenches or forced itself under continentalland masses, the subduction zones described earlier. This drifting andcolliding caused considerable tectonic instability. Today, the Atlantic Oceangrows about one and a half inches a year or at about the same rate as ourfingernails grow.15 Throughout this lengthy geological history, Earth wasmostly ice free, suggesting the existence not only of a warm climate butalso of elevated carbon dioxide levels.16


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Figure 1.3 The breakup of Pangaea. Source: Jacquelyne Kious and Robert I.Tilling, This Dynamic Earth: The Story of Plate Tectonics (Washington, DC: USGeological Survey, 1996).



The Cooling

During the further fragmentation of Pangaea, its southernmost part brokeinto two super-continents. Gondwana had collided with Pangaea millions ofyears before it broke away in two parts, one named Gondwana and a secondnamed Laurasia, sometime between 200 and 180 mya (the mid-MesozoicEra). Their separation in the southern hemisphere triggered the transitionfrom a global greenhouse to a cooler planet. The release of trapped frigidwaters at the South Pole, continental drift and collision, and the creationof the Tibetan Plateau, as well as many other significant geological events,accelerated this transition. India separated from Antarctica 130 mya andbegan its slow drift toward the Asian subcontinent. About 60 mya, what isnow the island of Mauritius and the continent of Australia began to separatefrom Antarctica. By 33 mya, the final underwater land connection betweenAntarctica and Australia and stretching south of Tasmania was broken, cre-ating la grande coupure, the great cut.17

As Australia drifted northward, an ocean with deep, cold bottom waterspassed through the great cut, creating the thermal isolation of Antarctica.During the earlier greenhouse phase, Antarctica had supported abundanttemperate-climate vegetation. Today, 8,155 ft3 (2,486 m3) of frigid oceanwater passes between Antarctica and Australia every second. This volumerepresents 1,000 times the flow of the mighty Amazon River.18 Calledthe Antarctic Circumpolar Current by scientists, it has been identified asthe condition that ended Earth’s lengthy greenhouse period and initiatedglobal cooling and its cycle of ice ages. By keeping cold waters circulatingaround Antarctica, it created the southern polar ice cap and maintained thecold ocean bottoms around Earth. According to Australian ecologist TimFlannery, “the establishment of the Antarctic Circumpolar Current was theswitch that turned on the modern world … at the time of the great cutmean global temperatures dropped by an extraordinary five to six degreesto just 5 degrees Celsius (41∘F).”19

In the southern hemisphere, icebergs from Antarctica began appearingmore than 20 mya, about 35 million years after the separation of Australiafrom Antarctica. Caused once again by ocean floor spreading, the sepa-ration “made possible the free flow of oceanic currents around the SouthPolar continent[;] the effect, for a time, was greatly increased snowfall on theSouth Pole area, producing the heavy flow of ice that occurred five millionyears ago.”20 The ice spread far beyond the continental boundaries, creat-ing ice cliffs almost 1,000 ft (305 m) high. As they grew in size and weight,



they descended into the ocean depths as much as 1,900 ft (580 m), scouringthe ocean floor and creating deep cold-water ocean bottoms. At no time inhuman history has this region been free of ice.

In North America, mean temperatures dropped more than 18∘F (10∘C),resulting in massive extinctions of marine life, flora, and fauna and theonset of seasonal variability with tropical summers and frigid winters.Before this cooling, rainfall covered the world more evenly throughoutthe year than today. Without seasonal dry periods, grasslands, variedvegetation, and deserts were uncommon. Earth was mostly covered byoceans and shallow seas, and global temperatures stabilized worldwide.Arctic seas, frozen regions, and glaciers were either nonexistent or severelylimited. With global cooling, mixed evergreen forests gave way to mixedhardwood forests in North America. Changes occurred throughout theworld as the modern glaciated world came into existence.

As the cool climate replaced the warm, wet one, the dry, shallow coastaland inland seas that covered much of North America retreated, uncover-ing larger bodies of land. With cooling, temperatures worldwide becamemore variable. As a consequence of this cooling, a reduction in precipita-tion allowed open habitats of savanna and grassland to replace dense forests.Although the long trend was clearly in the direction of a cooling of theEarth and the eventual formation of glacial ice, climatic fluctuations fromwarmer to colder and vice versa continued. These climatic fluctuations withthe newer phases of cooling and drying led to the development of summerand winter. “Seasonality requires plants to adapt once or twice yearly repro-ductive phases, with seeds that can survive bad weather while lying in adormant state. The grasses were particularly successful at doing this, result-ing in open savanna grasslands.”21 Happening slowly, this transformationproved to be a significant change, leading to the development and evolutionof mammal life. These geological events exercised a controlling influence onfaunal evolution and were essential to the eventual evolution of hominids,the ancestors of anatomically modern humans.

The Elevation of the Tibetan Plateau and Its Effecton the Global Climate

From 66.4 to 57.8 mya, Earth experienced increasing continental uplift.India collided with Asia, and “the compression stresses that built up afterthe two continents collided somehow forced Tibet upward, although the



exact mechanism responsible for the uplift is still being debated.”22 At thepoint of collision between India and Asia, rock from both was pushed,compressed, crushed, and pulverized. Some was pushed upward to createthe Himalayas, the youngest mountain range in the world. Some waspushed downward into the mantle of Earth to create the base of the moun-tains, which prevented them from collapsing under the weight of the uplift.The density of the mantle thickened. The mountains themselves representthis thickening of Earth’s crust brought about by upward compression.

As Asia and India collided, the ocean plate and part of the landmass ofIndia pushed under the edge of Asia. Some of India’s land was not as denseas Earth’s thick mantle, however, so, rather than being pushed downward, itmoved horizontally, creating a wedge between Asia and India. The impact ofthis horizontal movement uplifted the Tibetan Plateau as one solid piece ofrock. “The rock (called the continental crust) beneath Tibet is an astonishing46 miles (74 kilometers) thick, about twice the average crustal thickness [ofEarth].”23

Called the “roof of the world,” the Tibetan Plateau is almost a third aslarge as the continental United States. At 15,000 ft (4,570 m) in elevation,or 3 mi (4.8 km) above sea level, it exceeds the height of most of the RockyMountains. The Himalayas, 7 mi (11.2 km) above sea level, form a 1,800mi (2,900 km)-long rampart between India and Central Asia. For geologistsand climatologists, how this uplift changed global climates and created theconditions for the evolution of humans remains an important question.

As the formation of the Tibetan Plateau and the Himalayas coincidedwith the continuing transition from warm worldwide greenhouse climatesto more temperate climates about 24 mya, the evolutionary pattern forwarm-blooded species became sustainable.

Seasonality occurred because the building of the Himalayas and theTibetan Plateau changed the climate of Asia. In Asia, seasonality, with a dryclimate during the winter and rain in the summer, created the monsoon.“This occurs because the vast, high Himalayan region is quickly warmed bythe summer sun, heating the air and causing it to rise. This draws in moistair from the sea and the moisture falls as rain.”24 Before these climaticchanges began, Tibet possessed tropical and subtropical forests. As recentlyas 10 mya, the region contained deciduous forests very much like thosefound in the temperate regions of the world. Today, the vegetation differsmarkedly from that of the past. It is primarily grass and scrub vegetation,“adapted to the harsh steppe climate, which involves severe winters andseasonal drying.”25


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Figure 1.4 The Himalayas: two continents collide. Source: Jacquelyne Kious andRobert I. Tilling, This Dynamic Earth: The Story of Plate Tectonics (Washington, DC:US Geological Survey, 1996).



For every change in elevation of approximately 1.5 mi (2.4 km), the tem-perature drops about 32∘F (18∘C). The changes in vegetation and the evi-dence from sedimentary formations suggest that the Tibetan plateau hasrisen about 3 mi (4.8 km) above sea level in the past 25 million years, withone-half of the uplift occurring during the past 10 million years. The adja-cent regions of southern China and Southeast Asia, unaffected by the uplift,grow vegetation suited for a warm climate, one that the fossil and pollenremains in Tibet today suggest was present there 40 million years beforethe uplift. “Even more definitively, uplift accounts for the presence of oceansediment … approximately 70 million years old in Tibet.”26

What was the effect of the Tibetan Plateau uplift on climate change inthe Eurasian regions affected by the tectonic collisions of India into Asia?First, the uplifted region blocked the natural west-to-east wind currents atthe surface and in the upper atmosphere through the middle latitudes ofEarth. With the westerly winds blocked, the east winds approaching theplateau were diverted northward around the plateau. One additional resultwas a “meander” or large southward return flow. These changes in circula-tion represent long-term climatic patterns rather than short-term weatherreporting.27

Second, a high plateau affected winter and summer air circulation. With-out the uplift, precipitation stretched out over a larger geographical areaand fell with less intensity. The sun rapidly heated the thin atmosphere ofthe uplifted areas. Because of its low density, warm air rose rapidly. Risingair cooled and lost its capacity to hold water vapor. Since moisture from theIndian Ocean was blocked by the lower atmospheric pressure of the plateau,more intense rainfall in the form of summer monsoons hit the southeastregion.28 India and Southeast Asia became warmer and wetter as a resultof the Tibetan Plateau. In their wake, monsoons today bring flooding, soilerosion, the destruction of crops, and loss of life to the low-lying areas ofthe Indian subcontinent.

Third, the effects of seasonal heating and cooling were global.

In the summer the rising motion of air over the plateau leads to a compen-sating sinking of air over surrounding regions, including the high-pressureregions that lie over the oceans (which are cool compared with land temper-atures) at subtropical latitudes. Heating over the huge Tibetan Plateau alsoinduces air to sink over adjacent areas in the Mediterranean and central Asia.The sinking air is dry because it originates from high elevations far fromoceanic moisture sources. Compressional heating, which takes place as theair sinks, also lowers the relative humidity.29



The reverse occurred during the winter months. Air over the plateau fellas air over the low-pressure oceans rose. Warmer ocean temperatures pre-vailed relative to the cold air masses of the elevated plateau. Like the sum-mer, the effect of falling air pressure on the plateau in the winter had globalclimate effects that altered the climates of Asia, Africa, and Europe.

The summer winds, which contained more moisture, switched from thewest to a drier northeast. The drier air of the elevated region blocked mois-ture coming from the Indian Ocean. These climatic conditions explain theexistence of cooler summers and colder winters in northern Asia. Duringthe millions of years of continental uplift, the flora altered to reflect the grad-ual changes in climate. Over time the sub-Arctic forests and tundra replacedthe warm deciduous forests of 20 mya. The gradual transformation is shownby the evolution “from forest to steppe and even to desert vegetation, asshown in the fossil record of the past 20 million years.”30

Because of the uplift, Europe’s atmospheric circulation wind currentsshifted in response to the location and size of the Tibetan Plateau. Sincewind arrived from the northeast and flowed in a north-to-south direction,the continent experienced colder and drier winters and cooler summers.Because of these wind currents, the Mediterranean, the Arabian Peninsula,and northwest Africa were drier. The evidence to support these and otherobservations was found in sediment. Deep-sea sediments contain millionsof years of dust and debris blown across the continents into the oceans of theworld. During periods of continental aridity, larger amounts of wind-blownsediments appear in the oceans, serving as indicators of atmospheric windintensity and direction.

For example, the sediments, calcium carbonate, wind-blown quartz andclay, and siliceous microfossils found in the Arabian Sea, blown from thearid African continent, serve as useful measures of the extent of the arid-ity and the strength of the summer Indian Ocean monsoon. Also, sedi-ments found in the north Pacific Ocean east of Japan are the remains ofwind-blown soil from central China 2,400 mi (3,900 km) away depositedduring five separate 95,800-year glacial cycles.31

So, the collision of India and Asia and the continuing pressure ofplate movement had profound ecological and climatic effects in theregion and beyond. The uplifting of the land into broad plateaus and thebuilding of mountain chains changed the Earth’s climate. Continentalmountain-building resulted in cooler temperatures in the areas of theuplift, and because of the altered atmospheric circulation patterns hadglobal cooling and drying effects.



For example, early climate records of the Bighorn basin of Utah and theBadlands regions of South Dakota in the American West support the con-clusion that the onset of aridity was triggered by continental uplift. Bothexperienced an average annual rainfall of 40 inches (101 cm) before theuplift and less than 16 inches (40.5 cm) thereafter.32 What geologists havebeen claiming for more than a century, namely that mountain-building cancause climate change, has essentially been proven by climatologists. “Onepotentially important factor was the uplift of the Tibetan and North Amer-ican plateaus, which have led to elevated continental weathering rates andthe drawdown of atmospheric CO2.”33

High mountains and plateaus contrasted strongly with the prevailinglow continental landscapes during the earlier greenhouse phase, millionsof years before. “The evolution of certain plant types about seven millionyears ago that were capable of a distinct photosynthetic pathway forcarbon (called C4 plants) may have resulted from lower atmospheric CO2levels.”34 Since both the Tibetan and North American plateaus have risensubstantially during the past 40 million years, they provide a plausibleexplanation for global cooling.

An abrupt warming, by several degrees, punctuated Earth’s tectonicactivity. It occurred approximately 55 mya and lasted for about 100,000years. Its environmental impact was felt in the northern and south-ern hemispheres and in the deep oceans. Because this approximately100,000-year anomaly occurred at the Paleocene and Eocene boundaries,it is often referred to as the Paleocene–Eocene Thermal Maximum (55.8mya). Shifts in global rainfall patterns and in vegetation are present invarious fossil records. Because this warming anomaly took place duringa major reshaping of Earth’s landscape and an overall cooling phase, therelationship of these events remains a subject for scientific inquiry.

Fossilized ocean and continental records reveal a release of massiveamounts of carbon from any of these likely sources: volcanic activity duringtectonic uplift, the discharge of methane (CH4) from the decompositionof clathrates on the ocean floor, or the oxidation of organic matter.35 Thisevent has become one that is studied closely by climatologists because therapid release of carbon into the atmosphere by natural causes is similar tothe release today by humans. The effects of the former not only changedatmospheric conditions, raising temperatures 9–14.5∘F (5–8∘C), but alsochanged deep ocean circulation patterns in the span of a few thousandyears. That it took an additional 100,000 years for the climate systemto recover from the Thermal Maximum has not been lost on scientists



attempting to predict the length of time that it would take for the world torecover from the warming since the beginning of industrialization.36 “Theevent is a striking example of massive carbon release and related extremeclimatic warming.”37

The Birth, Death, and Rebirth of the MediterraneanSea and Its Hemispheric Environmental Effects

With the breakup of Pangaea, a new era of continental collisions began.Africa struck Europe, creating the Swiss Alps. The tectonic pressure ofAfrica moving northward against Arabia and the Eurasian land masssqueezed the ancient Tethys Sea, which once separated Africa from Arabia.Once it had collided with Eurasia, only a small eastern section of theTethys Sea remained as a remnant of the once great warm-water belt thatdominated the region from the Atlantic to the Indian Ocean and swampedlarge areas of Europe, northern Africa, and the middle of Southwest Asia.For much of its 300-million-year existence, the Tethys Sea was inhabitedby an extensive variety of warm-water invertebrates living in average watertemperatures of 77∘F (25∘C) in areas that today are in the cold temperatezone. Twenty-five mya, sand dollars lived everywhere and coral reefs dom-inated large areas of this sea.38

The pressure of folding plates and uplift of the deep Tethys Sea sedimentscreated the mountain ranges from the Alps to the Caucasus in Ukraine andthe Zagros in Iran. When the two continents separated ever so slightly, theMediterranean Sea was all that remained of this ancient sea. Today, the pres-sure exerted by the movement of the African tectonic plate is responsible forthe volcanic and earthquake activity that extends from Portugal across thenorthern coast of the Mediterranean Sea to Turkey and Iran.39 Collidingcontinents caused continental compression and led to wider ocean basinsand lower sea levels, thereby creating more land surface. A larger surfacepermitted easier movement of flora and fauna. More land usually meantmore plants to reflect much more solar heat, possibly aiding in the coolingof the planet.

With colliding continents, exchanges of plants and animals from Africaand Eurasia became commonplace. Horses and bovines spread to Africawhile elephants and primates dispersed from Africa to Eurasia. The move-ments of human species (hominids) from Africa represent an importantchapter in this global exchange. “This topographical configuration, and the



population movements it permitted, was the topological foundation for theemergence and spread of successive species of Homo.”40 This migration ofhominids will be discussed more fully in Chapter 2.

Ecological changes and their impact on human evolution are reflected inthe 20-million-year history of the Mediterranean Sea. Before the closing ofthe Tethys Sea, cold ocean water flowed freely from what was to become theAtlantic and Indian Oceans. It sustained a diverse marine life from the coldsedimentary bottom to the warmer subsurface levels. The climate of thesurrounding landmass was also moderated by the open passages from oneocean to another. About 15 mya, the connection to the Indian Ocean closed.Tectonic pressure caused by converging plates and mountain-buildingclosed Gibraltar sometime between eight and five mya. The MediterraneanSea became isolated from the Atlantic Ocean, and in the process began toevaporate into a dry desert.

Given the following calculations, today, it would take about 1,000 yearsfor the Mediterranean Sea to evaporate completely. Taking into account theclimate in the region today, about 2,400 mi3 (3,860 km3) of seawater evap-orate each year. Fewer than 300 mi3 (482 km3) of water from rain and freshwater from rivers emptying into the sea replace it each year. Through thenarrow Straits of Gibraltar, the sea receives about 2,100 m3 (3,380 km3) ofseawater each year, which together with the rain and fresh water maintainsit at a near-constant level.41 Despite these calculations, however, it seemsthat the Mediterranean did not evaporate that quickly eight mya becausefreshwater from rivers continued to empty into it.

Eastern Europe … was covered at that time by a vast body of brackish water,called Lac Mer. This extended all the way from Vienna to the Ural Mountainsand the Aral Sea. The present Caspian Sea and Black Sea are its last remnants… The bottom of the Mediterranean basin [was] partly covered by a series oflarge, brackish lakes for some time between 8 and 7 million years ago … Theydisappeared about 7 million years ago, when the converging plates elevatedthe Carpathian Mountains, sufficiently to change the drainage pattern, andthe waters of Lac Mer then escaped to the north … and the lakes evaporatedcompletely.42

Without water, continents with exposed shelves surrounded the Mediter-ranean. High plateaus with steep slopes descended to the floor of the formerTethys Sea. The exposed floor revealed peaks and plateaus of submarinevolcanoes. They dotted the former seascape as sunken valleys and basinsabout 2 mi (3.2 km) below sea level. It was a barren, desert landscape with



temperatures reaching 150∘F (65.5∘C).43 The transformation of theMediterranean into a great desert basin helps to explain the dramaticchange in the climate of central Europe from 20 million to 5 mya. “TheVienna woods were changed into steppes and palms grew in Switzerland.”44To reach the desolate and desiccated Mediterranean, the great prehistoricrivers, the Nile and the Rhone, cut deep canyons, at least 900 ft (275 m)below the present sea level. These canyons represent the remains of ancientriver systems created when the Mediterranean became a desert.

About 5.5 mya, the movement of the tectonic plates that had closed thearea around Gibraltar’s straits 2.5 million years before caused it to open.The new opening revealed a very high scarp that exposed a steep drop tothe Mediterranean Sea floor 2 mi (3.2 km) below. Over time, erosion of therock barriers that dammed the straits turned a trickle into a cascade, andfinally into a great waterfall. The flow of water from the Atlantic would needto exceed the loss from evaporation by a factor of 10 in order to fill theMediterranean.45

The relationship between land and the flow of water played a major rolein the changing climate and environmental conditions of the Mediterraneanregion. A dry, arid, dusty, and inhospitable region would one day become acenter for civilization. The role of the rivers that emptied into the Sea and therole of the port cities that bound the region together were inextricably linkedto the life-sustaining qualities of the Sea. This relationship also proved to beimportant in other parts of the world as seafarers from a more recent timewould depart from these port cities and begin anew the binding of the worldtogether as they traveled and traded in China, India, and the Americas.

The Impact of the Isthmus of Panama on GlobalClimate Change

About 10 mya, the Isthmus and presumably all of Central America beganrising from the ocean floor. This natural damming and separating of theequatorial Atlantic and Pacific oceans also altered global climate conditions.On the Caribbean side of the Isthmus, volcanic activity, prevalent from fiveto seven mya, created new land formations from the lava flows. The spread-ing of the Pacific sea floor along the Galápagos Islands rift zone to the southof the Isthmus provides clues to the latter’s rising. The spreading of the seafloor created south-to-north fractures in the entrance of the North Ameri-can Trench along the west coasts of Costa Rica and Panama.



As new ocean floor emerged along the Galápagos Ridge, the northwardmovement of old ocean floor descended into the Middle America Trench.By pushing and burrowing its way under the lithosphere, it caused the upliftof Central America, including the Isthmus of Panama. By four mya, the clos-ing of the “gate” separating the Pacific and Atlantic Oceans was complete,marking the end of the worldwide equatorial warm-water circular system.With separate circular systems flowing in the two oceans, the Gulf Streambecame part of the new Atlantic Thermoline. This change altered the pathof storms and delivered warmer, humid air to central Canada and northernEurope. In winter, when heavy snow accumulations exceeded summer meltover a long period, ice-age conditions prevailed.46

The opening and closing of isthmuses contribute to the cooling and dry-ing of Earth. They separate one large body of water from another and changethe ocean currents, altering climate conditions. The closing of the Isthmus ofPanama prevented warm Atlantic water from circulating around the equa-tor, accelerating the further cooling of the global climate. It also connectedtwo large continents, which prompted the movement of fauna, flora, and,much later, humans across this passage. Humans may have made their waysouth in boats as well as on foot 30,000 to 15,000 bp, and they may havelanded on the Isthmus as they traveled south.

The Mid-Pliocene, Glacial and Interglacial Cycles,and “Modern” Times

The Mid-Pliocene (3.3 to 3 mya) was a time when mean global temperatureswere higher by 3.5 to 5.5∘F (2 to 3∘C) than those found during the entirepreindustrial period spanning thousands of years, but not higher than thosefound before the emergence of hominids, the earliest human ancestors, inAfrica. By the Mid-Pliocene, the location of the continents and oceans wassimilar to their current configuration, making global temperature compar-isons with the modern era possible. Atmospheric carbon dioxide concen-trations were estimated to be between 360 and 400 parts per million (ppm)during the Mid-Pliocene, higher than preindustrial levels but equal to cur-rent levels in 2014. At these carbon dioxide concentrations, geological andice-core evidence prove that sea levels were at least 49 to 82 ft (15 to 25 m)above modern levels.

An atmospheric-ocean experiment simulating Mid-Pliocene sea surfaceand air conditions conducted by scientists in 2010 arrived at the following



conclusions. Carbon dioxide concentrations of 400 ppm produced a warm-ing of 5.5 to 9∘F (3 to 5∘C) in the North Atlantic and 1.8 to 5.4∘F (1 to3∘C) in the tropics, suggesting that the northern latitudes may be moresensitive to elevated concentrations of carbon dioxide. Among the possi-ble causes were an increased movement of heat from the tropics due to aquicker deep ocean circulation, or faster surface winds increasing the flowof surface ocean currents.

These possible causes differ from models of twenty-first-century warm-ing that postulate a slowing of the North Atlantic Deep Ocean circulationcaused by glacial melt and freshwater discharge into the oceans. Chapter10 provides a fuller explanation of the relationship between global climatechange and deep-water circulations. Clearly, understanding the dynamicsof climate-warming during the Mid-Pliocene may help in predicting theeffects of elevated carbon dioxide concentrations and the role played byocean circulations in today’s globally warmer world.47 Documentation fromice cores points out that it took 100,000 years for carbon dioxide concentra-tions to stabilize and return to preagricultural levels of 180 ppm.

These same paleo-climate records document the sequence of glacialand interglacial cycles for the past 740,000 years, with evidence from deepocean sediments indicating other cycles for several million years. The bestdocumentation comes from the past 740,000 years, with glacial–interglacialcycles lasting approximately 95,000 years and the warm interglacial modefor these cycles lasting about 10,000 years. Within this full cycle, additionalcycles occur, responding to changes in Earth’s spin or tilt as it orbits aroundthe sun. At 41,000 years, one full cycle is determined by the tilt in Earth’saxis, which controls the amount of radiation from the sun in the highlatitudes. At either 23,000 or 19,000 years a “wobble” in Earth’s rotationcauses a much weaker warm–cold cycle with greater effect over the lowlatitudes near the equator.

During every 95,800 years over the past 740,000 years, major climatechanges resulted from these interglacial–glacial cycles. Forests grew andretreated. Water levels in lakes, rivers, and streams rose and fell and the con-tours of continental shelves appeared and then were inundated by chang-ing sea levels. In the high latitudes and in high mountain ranges, glaciersadvanced. In front of the advancing ice appeared an expanding and broadband of peri-glacial and permafrost landscapes. Climate changes and theirimpact on the environment were frequent and predicable.48



2100 higher emissions scenario

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Figure 1.5 Analysis of air bubbles trapped in an Antarctic ice core extendingback 800,000 years. Source: National Oceanic and Atmospheric Administration/Department of Commerce .

The following brief description of Earth’s “recent” climate history coversmore than a million years. At 2.5 mya, moderately sized ice sheets existed inthe northern hemisphere but these were replaced by more extensive glaciers900,000 years ago. During the Pleistocene Epoch (1.8 mya to 11,600 bp),large ice sheets covered much of the northern hemisphere; this was fol-lowed by periods of warming with retreating glaciers. A mostly dry andarid climate existed, with low levels of atmospheric carbon dioxide suitablefor hunting and gathering but inhospitable for plant cultivation or farm-ing. The most recent epoch, the Holocene, covers the past 11,600 years andis characterized by a warming global climate, retreating large glaciers, andgrowing human populations. During recent decades, scientists have notedthe important role of trace gases in abruptly changing Earth’s global envi-ronment during the transition 11,600 years ago.

The past million or more years and the entire Holocene remain themost intensely studied period in the history of Earth’s climate, and climatechange is a major feature of that period. Glaciers melted after dominating



the landscape and the oceans in the northern hemisphere, having coveredmuch of North America, Europe, and the northern reaches of Asia forhundreds of thousands of years. Ice locked up much of Earth’s conti-nental and ocean water, the resource that most directly influences theglobal climate.

With the melting came warming and an epoch of relative climate stability.Although the long trend was a general warming, Earth was not uniformlywarm. Warm temperatures often exceeded current ones but this interglacialperiod experienced some decades of significant cooling. Historically, lit-tle ice ages, influenced mostly by solar activity, disrupted economic andsocial life during Europe’s first and second millennia. During most of theHolocene, carbon dioxide concentrations remained high, when comparedto the glacial, and stable enough to accelerate the transition to plant cul-tivation and farming. Since 1750, however, burning fossil fuels to powermanufacturing and industrial activity has contributed to elevated levels ofcarbon dioxide and human-induced climate change.

Notes

1 David Christian (2003), Maps of Time: An Introduction to Big History (Berkeley,CA: University of California Press), 26.

2 Ibid., 502–503.3 Ibid., 62.4 Ibid., 63.5 Thomas M. Cronin (1999), Principles of Paleoclimatology (New York: Columbia

University Press), 441.6 Ibid., 442.7 SciJinks (n.d.), “How did Earth’s atmosphere form?” At http://scijinks.jpl.nasa

.gov/atmosphere-formation.8 Cronin, Principles of Paleoclimatology, 442.9 Ibid.

10 Christian, Maps of Time, 71.11 Carl Sagan and Ann Druyan (1992), Shadows of Forgotten Ancestors (New York:

Random House), 29.12 Ibid.13 Ibid., 29–30.14 W. F. Ruddiman (ed.) (1997), Tectonic Uplift and Climate Change (New York:

Plenum Press).15 Christian, Maps of Time, 70.



16 R. M. DeConto and D. Pollard (2003), “Rapid Cenozoic glaciation of Antarcticainduced by declining atmospheric CO2,” Nature, 421(6920), 245–249.

17 Tim Flannery (2001), The Eternal Frontier: An Ecological History of NorthAmerica and Its People (New York: Grove Press), 101.

18 Ibid., 102.19 Ibid.20 Ibid., 172.21 Douglas Palmer (1999), Atlas of the Prehistoric World (New York: Random

House), 139.22 William F. Ruddiman and John E. Kutzbach (1991), “Plateau uplift and climatic

change,” Scientific American, 264(3), 66–75 at 68.23 Palmer, Atlas of the Prehistoric World, 142.24 Ibid.25 Ruddiman and Kutzbach, “Plateau uplift and climatic change,” 68.26 Ibid.27 Ibid., 70.28 Ibid., 71.29 Ibid.30 Ibid., 72.31 Cronin, Principles of Paleoclimatology, 172–173.32 Lisa Cirbus Sloan and Eric J. Barron (1992), “Paleogene climatic evolution:

a climate model investigation of the influence of continental elevation andsea-surface temperature upon continental climate,” in Donald R. Protheroand William A. Berggren (eds), Eocene–Oligocene Climate and Biotic Evolution(Princeton, NJ: Princeton University Press), 202–217 at 207–209.

33 Cronin, Principles of Paleoclimatology, 442.34 Ibid.35 Gabriel J. Bowen, David J. Beerling, Paul L. Koch, et al. (2004), “A humid

climate state during the Palaeocene/Eocene thermal maximum,” Nature, 432(25 November), 495–499.

36 James C. Zachos, Michael W. Wara, Steven Bohaty, et al. (2003), “A transientrise in tropical sea surface temperature during the Paleocene–Eocene thermalmaximum,” Science, 302(5650), 1551–1554.

37 S. Solomon, D. Qin, M. Manning, et al. (eds) (2007), Climate Change 2007: ThePhysical Science Basis. Contribution of Working Group 1 to the Fourth Assess-ment Report of the Intergovernmental Panel On Climate Change (Cambridge:Cambridge University Press), 442.

38 Donald R. Prothero (1994), The Eocene–Oligocene Transition: Paradise Lost(New York: Columbia University Press), 22–23.

39 Walter Sullivan (1991), Continents in Motion: The New Earth Debate(New York: American Institute of Physics, 2nd edn), 164–165.



40 Andrew Sherratt (1996), “Plate tectonics and imaginary prehistories: structureand contingency in agricultural origins,” in David R. Harris (ed.), The Ori-gins and Spread of Agriculture and Pastoralism in Eurasia (Washington, DC:Smithsonian Institution Press), 130–140 at 132.

41 Peter J. Wyllie (1976), The Way the Earth Works: An Introduction to the NewGlobal Geology and Its Revolutionary Development (New York: John Wiley &Sons), 209.

42 Ibid., 210.43 Ibid., 211.44 Sullivan, Continents in Motion, 167.45 Wyllie, The Way the Earth Works, 210–211.46 Sullivan, Continents in Motion, 170–171.47 Thomas M. Cronin, Harry J. Dowsett, Gary S. Dwyer, et al. (2005),

“Mid-Pliocene deep-sea bottom-water temperatures based on ostracodeMg/Ca ratios,” Marine Micropaleontology, 54(3–4), 249–261; A. M. Haywood,B. W. Sellwood, and P. J. Valdes (2000), “Global scale paleoclimate reconstruc-tion of the middle Pliocene climate using the UKMO GCM: initial results,”Global Planetary Change, 25, 239–256.

48 Ruddiman and Kutzbach, “Plateau uplift and climatic change,” 66.

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