The Human Footprint

A Global Environmental History

Anthony N. Penna

• Publisher: Wiley-Blackwell
• Year: 2014
• Print ISBN: 9781118912461
• eBook ISBN: 9781118912430
• Edition: 2
• Topic: History
• Subtopic: World History
• Index: History

The Human Footprint: A Global Environmental History, Second Edition, presents a multidisciplinary global history of Earth from its origins to the present day.

• Provides a comprehensive, global, multidisciplinary history of the planet from its earliest origins to the present era
• Draws on the most recent research in geology, climatology, evolutionary biology, archaeology, anthropology, history, demography and the social and physical sciences
• Features the latest research findings on planetary history, human evolution, the green agricultural revolution, climate change, global warming and the nature of world/human history interdependencies
• Offers in-depth analyses of topics relating to human evolution, agriculture, population growth, urbanization, manufacturing, consumption, industrialization, and fossil fuel dependency.

Chapter 1
An Evolving Earth

Introduction

Earth is a living, dynamic, and sometimes violent planet. Able to sustain organic life, subject to violent swings in temperature, the modern climate is the result of Earth-defining changes. These changes range from the collision of Earth's continents to the ability of its oceans to absorb solar energy as heat and reflect it as light, and to influence the intensity and flow of ocean currents. Atmospheric changes, tectonic movements, and global climate are interconnected forces that transformed Earth's history and created an environment suitable for the development of all life forms.

On a much longer timescale, scientists believe that our solar system formed because gaseous clouds of debris from older stars condensed into solid matter about 4.6 billion years ago. The formation of our solar system was 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 many trillions of degrees, a super-heated universe, smaller than the size of an atom, 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 reminder of this event and the continuing expansion of the cosmos.

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

About 300,000 years after the big bang, all the ingredients of creation were present: 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 continued to exist. All that has happened is that for the next 13 billion years these same ingredients have arranged themselves in different patterns, which constantly form and dissipate.¹

By collapsing the timescales of the 13-billion-year cosmos by a factor of one billion, the Big Bang took place 13 years ago. In this scenario, Earth's first living organisms appeared about four years ago, and modern humans evolved in Africa about 50 minutes ago. The invention of agriculture and the building of cities, which you will read about in later chapters, occurred five 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 at in another way, if the 10-billion-year projected life of Earth's energy system were compressed into a single year, all of written human history would be represented in less than a minute. And the twentieth century would be less than 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-sized meteors created a liquid mass of molten rock of 1,800°F (980°C). This was the newly forming Earth, with an atmosphere of mostly hydrogen and helium, the main gases around the sun. During a 600-million-year period, repeated bombardments followed by the sinking of the iron cores of those meteors created the molten center of Earth. Its iron core created the planet's magnetic field, which deflected many high-energy and dangerous particles from Earth. In this very important way, it acquired and to this day possesses a protective shield.³

This extremely hot planet created an equally torrid atmosphere including super-heated hydrogen and helium molecules, moving so fast that they escaped Earth's gravity. The young Earth can be thought of as a massive volcanic field that created its own infant atmosphere, releasing gases: water (H₂O) as steam, carbon dioxide (CO₂), and ammonia (NH₃).⁴ As large meteor strikes slowed over a period of about two billion years, however, the surface and its atmosphere changed significantly. Some scientists attribute this climatic transition to the impact of a mega-meteor, estimated to be the 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 that would have normally warmed the tropics. A decrease in solar radiation cooled the surface of Earth and “dried” the atmosphere, causing more radiation to escape from the surface, and initiated glacial expansion and a colder, snow-covered planet (2.3 billion years ago). The advancing glaciers reflected abnormal amounts of light and heat back into space, allowing for further cooling.

A cooling atmosphere made conditions suitable for ice comets emerging from the deep voids in our galaxy and weighing between 20 and 40 tons to bombard our inner space every two to three seconds. According to this “Snowball Earth” theory, these galactic events saturated Earth's atmosphere with increasing amounts of condensation. “Cosmic rain” cooled the white-hot planet and created the world's earliest oceans. Much of the planet's atmospheric carbon dioxide dissolved in these new oceans, creating the conditions for the development of bacteria that could live on the energy provided by the sun and carbon dioxide in the water. A by-product of this interaction between sunlight and carbon dioxide was oxygen (O₂). As atmospheric oxygen levels rose, carbon dioxide levels dropped. Ammonia molecules in the atmosphere separated by sunlight into one nitrogen atom (N) and three hydrogen atoms (H₃), with the latter escaping Earth's gravity and drifting off into space. Although the cooling did not require oxygen, free “atmospheric oxygen levels probably increased considerably about 2 bya [billion years ago] and again near 800 mya, coincident with major evolutionary changes in Earth's biosphere. Carbon dioxide levels are also believed to have been substantially different during the Precambrian Epoch (4.6 bya–570 mya).”⁵

Figure 1.1 Cutaway views showing the internal structure of the Earth.

Source: Jacquelyne Kious and Robert I. Tilling, This Dynamic Earth: The Story of Plate Tectonics (Washington, DC: US Geological Survey, 1996).

Increases in carbon dioxide levels with an atmospheric concentration of 0.035% after the Precambrian affected Earth's atmosphere.⁶ Changes in the land and sea biospheres not only transformed carbon dioxide into oxygen but also sequestered carbon in various “sinks”—the oceans, mountain ranges, and solid rocks. Accordingly, the planet experienced glaciers that covered the land and water from the northern hemisphere to the tropics about 700 mya. The ice sheets locked up about 25% of Earth's carbon. With oceans covered in ice rather than in liquid form, they could not capture carbon dioxide, released by volcanic eruptions. As noted earlier, under normal conditions, liquid oceans would absorb carbon dioxide. Under these circumstances, however, the atmosphere captured increasing amounts of this soluble gas.

The result of these tumultuous and explosive beginnings was an atmosphere unlike any others in our solar system. It was an atmosphere with just enough oxygen (about 21%) to allow bacteria to evolve into living things and for our species to evolve as well. Today, without accounting for water vapor, about 78% of the atmosphere is nitrogen, argon and other gases make up about 1%, and carbon dioxide represents 0.04% or about 400 parts per million of air and rising.⁷

Incidentally, the Snowball Earth theory provides a plausible explanation for the coming of the ice but not an answer to the vital geophysical problem of how Earth eventually corrected itself to its current 23.5° tilt. To date, the most plausible scientific explanation is that Earth's land mass was clustered together at the South Pole 600 mya. The weight of this clustering tipped Earth into its present inclination. Eventually this land mass broke up to form the 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 the Precambrian, when Earth was covered with ice. However, increasing levels of carbon dioxide, a heat-trapping gas, triggered global warming. When that 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. Water vapor is the largest natural greenhouse gas because it traps ...

Table of contents

• Cover
• Title Page
• Copyright
• Dedication
• List of Figures
• Acknowledgments
• Introduction
• Chapter 1: An Evolving Earth
• Chapter 2: Evolving Humanity
• Chapter 3: Foraging, Cultivating, and Food Production
• Chapter 4: Populating the Earth: Diet, Domestication, and Disease
• Chapter 5: The Making of an Urban World
• Chapter 6: Mining, Making, and Manufacturing
• Chapter 7: Industrial Work
• Chapter 8: Trade and Consumption
• Chapter 9: Fossil Fuels and Wind, Water, Nuclear, and Solar Energy
• Chapter 10: A Warming Climate
• Epilogue
• Index
• End User License Agreement