Über die Autorin bzw. den Autor

Charles H. Langmuir is the Higgins Professor of Geochemistry at Harvard University. Wally Broecker is the Newberry Professor of Earth and Environmental Sciences at Columbia University and the author of Fixing Climate and The Great Ocean Conveyor (Princeton), among other books. Both are members of the National Academy of Sciences.

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"In this comprehensive and engaging tour of environmental science, world-leading authorities Charles Langmuir and Wally Broecker provide the residents of the only habitable planet we know with the essential knowledge of how we got here and where we might be going."--Richard Alley, Pennsylvania State University

"As NASA continues to assess the habitability of our planetary neighbor, Mars, this insightful and approachable book is a timely reminder of how important it is to understand the habitability of our own Earth. Comprehensive and up-to-date, it exposes how ideas, imperfect understanding, and controversies drive scientific knowledge forward."--Roger Everett Summons, Massachusetts Institute of Technology

"This is a magnificent book, a successful and very worthwhile revision of its legendary and coveted first edition. The new edition offers more than a minor dusting off of the material. There are some completely new chapters and the authors have also done a good job of introducing newer discoveries. This book is more timely than ever, and I greet this revision with uncontained enthusiasm."--Raymond T. Pierrehumbert, University of Chicago

"This book is exceptionally well written and easy to read. The authors have taken a huge and complex topic and simplified it, removed the jargon, used analogies common to everyday experience, and as a result made a book that should be accessible and enjoyable to readers with little background in science."--Becky Alexander, University of Washington

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"In this comprehensive and engaging tour of environmental science, world-leading authorities Charles Langmuir and Wally Broecker provide the residents of the only habitable planet we know with the essential knowledge of how we got here and where we might be going."--Richard Alley, Pennsylvania State University

"As NASA continues to assess the habitability of our planetary neighbor, Mars, this insightful and approachable book is a timely reminder of how important it is to understand the habitability of our own Earth. Comprehensive and up-to-date, it exposes how ideas, imperfect understanding, and controversies drive scientific knowledge forward."--Roger Everett Summons, Massachusetts Institute of Technology

"This is a magnificent book, a successful and very worthwhile revision of its legendary and coveted first edition. The new edition offers more than a minor dusting off of the material. There are some completely new chapters and the authors have also done a good job of introducing newer discoveries. This book is more timely than ever, and I greet this revision with uncontained enthusiasm."--Raymond T. Pierrehumbert, University of Chicago

"This book is exceptionally well written and easy to read. The authors have taken a huge and complex topic and simplified it, removed the jargon, used analogies common to everyday experience, and as a result made a book that should be accessible and enjoyable to readers with little background in science."--Becky Alexander, University of Washington

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HOW TO BUILD A HABITABLE PLANET

PRINCETON UNIVERSITY PRESS

Contents

PREFACE...............................................................................................................................xvChapter 1. Introduction: Earth and Life as Natural Systems............................................................................1Chapter 2. The Setting: The Big Bang and Galaxy Formation.............................................................................27Chapter 3. The Raw Material: Synthesis of Elements in Stars...........................................................................51Chapter 4. Preliminary Fabrication: Formation of Organic and Inorganic Molecules......................................................83Chapter 5. The Heavy Construction: The Formation of Planets and Moons from a Solar Nebula.............................................113Chapter 6. The Schedule: Quantifying the Timescale with Radionuclides.................................................................141Chapter 7. Interior Modifications: Segregation into Core, Mantle, Crust, Ocean, and Atmosphere........................................171Chapter 8. Contending with the Neighbors: Moons, Asteroids, Comets, and Impacts.......................................................209Chapter 9. Making It Comfortable: Running Water, Temperature Control, and Sun Protection..............................................249Chapter 10. Establishing the Circulation: Plate Tectonics.............................................................................285Chapter 11. Internal Circulation: Mantle Convection and Its Relationship to the Surface...............................................315Chapter 12. Linking the Layers: Solid Earth, Liquid Ocean, and Gaseous Atmosphere.....................................................349Chapter 13. Colonizing the Surface: The Origin of Life as a Planetary Process.........................................................383Chapter 14. Dealing with the Competition: The Roles of Evolution and Extinction in Creating the Diversity of Life.....................427Chapter 15. Energizing the Surface: Coevolution of Life and Planet to Create a Planetary Fuel Cell....................................453Chapter 16. Exterior Modifications: The Record of Oxidation of the Planetary Surface..................................................475Chapter 17. Planetary Evolution: The Importance of Catastrophes and the Question of Directionality....................................509Chapter 18. Coping with the Weather: Causes and Consequences of Naturally Induced Climate Change......................................539Chapter 19. The Rise of Homo Sapiens: Access to Earth's Treasure Chest Permits a Planetary Takeover...................................567Chapter 20. Mankind at the Helm: Human Civilization in a Planetary Context............................................................597Chapter 21. Are We Alone? The Question of Habitability in the Universe................................................................649GLOSSARY..............................................................................................................................669INDEX.................................................................................................................................687

Chapter One

Introduction

The origin and evolution of our inhabited world is both a single subject and a topic of enormous diversity. It is "natural science" in the sense the term was used hundreds of years ago—the understanding of nature—but with a vastly greater panorama of scientific fields and data. The fields include the fundamental sciences of physics, chemistry, and biology, and also the integrative and historical sciences of astronomy and earth science. Most subjects broached in this book could each occupy an entire term of study, so the task is daunting for all of us.

Our aim is to explore Earth's history in detail as an example of a habitable planet, and from that history try to deduce the likelihood of similar histories occurring elsewhere. Within this story there are large numbers of exciting scientific developments and outstanding questions. And the story is the grandest that can be told, the scientific creation story of the universe whose evolution has led to us, human beings who are able to question and investigate the origin of our existence and the universal laws that led to us and surround us.

One of the challenges we face is that the range of scales we will need to encompass is almost unfathomable, from the minute atoms of which we and our planet are made, to the grander scales of solar system and universe of which we are a minuscule part. The smallest scales pertain to how atoms are formed and how molecules combine. The smallest scale of concern to us is the size of the hydrogen nucleus—a starting point for all atoms—which is 0.000000000000001 meters (m) in size. Dealing with very small (or very large) numbers is obviously cumbersome, so we will use exponential notation and abbreviations (Table 1-1). The hydrogen nucleus has a diameter of 10^-15 m. At vastly larger scales, stellar distances are measured in light years—the distance light travels in a year. Since the speed of light is 3 × 10⁸ m/sec multiplied by the ~3 × 10⁷ seconds in a year, a light year is 9 × 10¹⁵ meters. The nearest star is 3 light years away, our galaxy the Milky Way is 100,000 light years across, and the universe is estimated to be billions of light years in diameter, or ~10²⁶ m. Hence, our task encompasses 10²⁶m /10^-15m, or 41 orders of magnitude in terms of distance!

Similar large magnitudes exist for time. As we will discover in Chapter 2, the age of the universe is roughly 14 billion years (14 Ga), or 4.2 × 10¹⁷ seconds. The time of the atomic reactions that are involved in the creation of matter can be nanoseconds (10^-9 seconds). Our range of time encompasses 26 orders of magnitude.

The challenge of working with these huge ranges of time and space is that our experience as human beings is so limited. The same size of figure on a page (Fig. 1-1) can be used to portray vastly different scales. We might tell the story of the evolution of the universe taking place over billions of years with the flavor of a story about our summer vacation—without recognizing the difference in scale between the story of everything and that of our small life. The journey through the story of the universe will make much more sense if we remain mindful at each moment of the scale of the phenomena we are investigating.

THE POWER AND LIMITATIONS OF SCIENTIFIC REDUCTIONISM

Our approach in this book is to try to relate the smallest of parts to the largest of systems. This may appear to contrast with the traditional scientific approach of so-called reductionism. Much scientific understanding has come about by discovering governing mathematical equations, or laws, that account for diverse phenomena. In this approach, understanding comes from our ability to "reduce" the whole to the fundamental laws of physics from which all phenomena arise. Then phenomena calculated at the most fundamental level can, at least in theory, explain and predict the whole.

The great scientific revolution of the seventeenth century exemplifies the power of the reductionist approach. Newton's mathematical expression of gravity was able to account for Kepler's laws describing the motion of the planets around the sun and the careful measurements of falling objects carried out by Galileo. The big idea stemming from Newton's successes was that fundamental physical laws, described by mathematical equations, can explain everything we see. Today we are probably only partially able to conceive of the wonder of these results when they first appeared—everything we observe, from the tiny scale of a rolling marble to the movements of celestial objects, governed by mathematical law discernible by the mind of man. As Alexander Pope wrote of Newton's discoveries:

Nature and Nature's laws lay hid in night: God said, Let Newton be! and all was light.

Emerging from these startling successes came the idea of the "clockwork universe," where everything is explained by physical law rather than divine intervention. Once the laws are understood, everything can be accurately described and predicted through calculation. This is often understood as the fundamental scientific approach.

An aspect of this approach is the belief that understanding complex phenomena comes from breaking them down to their simplest parts. If we wish to accurately describe a crystal or a gas, the behavior of individual atoms provides the ultimate answer, and if we wish to understand the behavior of individual atoms, we must understand subatomic particles and quantum theory, and ultimately the strings of string theory. Understanding then comes from isolation of variables, improvement of the resolution and precision of observation, and discoveries of fundamental laws from which calculations are theoretically possible from first principles.

In this way, phenomena that appear to be miraculous become explained. Any human being prior to the scientific revolution, if they were to hear an amplified sound system or view images on a television screen, might believe they were in the presence of a miracle (or more likely the presence of the devil). But once the machine is taken apart and all the components are understood, the actions of physical laws appear. Understanding the operation of some of the electronic components would require reduction and observation to microscopic levels, ultimately down to the fundamental particles that make up the atoms involved. The same approach applies to the processes of life. The "miracle" of medications derives from understanding the processes of the body and the action of the drug on the molecular scale. The apparent miracle of evolution can be reduced to individual mutations of DNA molecules. Many of the topics of this book reflect the efficacy of this approach. Understanding how laws that operate at small scales manifest on much larger scales is one of the great triumphs of the scientific method.

Despite its obvious successes, however, reductionism falls short when we try to calculate or understand many natural phenomena. From the practical point of view, very few natural phenomena can actually be calculated from first principles. Let's take a simple example of calculating the atmospheric pressure at some point on the surface of Earth, say at the top of your head as you read this book. This is a straightforward, one-dimensional problem that involves simply summing the weight of the atmospheric column directly above you. We can measure pressure very precisely indeed—what would it take to calculate it as well as we can measure it?

To calculate it, we would need to know the density of the atmosphere at each point along the column. Thermodynamics helps with the general pressure-temperature-volume relations, but quantitative thermodynamic calculations apply best to closed systems, and the air over your head is in movement. The density of air also depends on the concentration of water vapor, which can vary both laterally and vertically. Winds are a response to pressure gradients, so pressure changes continually owing to movements and forces exterior to your personal atmospheric column. We could take an average temperature profile for this time of year, and assume a clear sky with constant relative humidity and no wind, but that gives an approximate pressure, nowhere near as precise as what we can measure. We could send a probe up through the atmosphere and measure the temperature and water vapor, but that is a bit like simply measuring the pressure, and by the time we got the data and processed it, the atmosphere could have undergone small changes from time of day and weather, leading to small errors.

This simple example illustrates the fundamental point that natural systems at any moment can never be specified completely. They are open systems without clear boundaries. Energy and material continually flow in and out; physical and chemical properties are not constant and homogeneous. For the gases of the atmosphere, the water of the ocean, the rocks of Earth's mantle, the liquid metal of Earth's outer core, or the plasma of the sun's interior, pressure and temperature change in space and time, mass and energy are constantly flowing in and out of the system, and we cannot make enough measurements to define accurately the state of the system on any but approximate scales, or as a long-term or broadly spaced average.

This state of affairs influences our ability to calculate and predict. Every calculation requires specification of initial conditions. Calculation of the weather tomorrow starts from our knowledge of the weather today. Initial conditions for real systems can never be measured simultaneously everywhere. Predictions become difficult for the immediate future, and progressively more uncertain for more distant times. This is particularly true for systems that contain "feedback," where movement in one direction causes a countervailing movement. This common characteristic of the real world often even leads to chaos.

CHAOS

Nowhere is the uncertainty of prediction more evident than chaotic systems. Chaos occurs in common equations where the outcome is so sensitive to the tiniest of changes in initial conditions or constants in the equations that long-term prediction is impossible. A simple illustration of this point comes from a time series generated by a "feedback" equation where the initial value of x is permitted to vary between 0 and 1. Subsequent values of x are then calculated with the following equation:

F(x_n) = Ax_n(1 - x_n) (1-1)

A is a constant. To construct the time series, the equation is used repeatedly with the output from one step used as the input for the next step, i.e., x_n+1 = F(x_n). Whenever x is large, the Ax term gets larger, but the (1 - x) term gets smaller, and vice versa. This is a negative feedback, where an increase in one term causes a decrease in the other. Negative feedbacks are very important in many natural processes. If A = 3 and we start with a value of x = 0.5, then F(x) = 0.75. For the next step, x = 0.75 and F(x) = 0.5625 and so on. This can easily be set up on a spreadsheet or a simple computer program to calculate what happens after large numbers of steps—a recommended exercise for the reader.

Equation (1-1) is an inverted parabola (see Fig. 1-2), and we can track the evolution of the time series as a path from one point on the parabola to the next. Figure 1-3a illustrates what happens to a time series for moderate values of A. When A = 2, the system proceeds rapidly to a steady-state value of 0.5, while for A = 2.8 the steady-state value is near 0.64. The steady state values for a particular value of A are independent of the initial value of x. The time series begins to exhibit more interesting behavior once A exceeds values of 3.0. For values of 3.2, for example (Fig. 1-3b) the time series proceeds to an oscillation between two states, also independent of the initial value of x. For A = 3.9, no such regularity appears in the result—no matter how many the number of steps.

To have a more comprehensive view of the behavior of this simple function, Figure 1-4 plots on the vertical axis the range of values of x obtained after a large number of steps, for different values of A plotted along the horizontal axis. For values of A less than 3, the time series arrives at a steady state value, independent of the starting value of x. Up to values of A less than (~3.45) the time series oscillates between two values. At values of A slightly above this number there are four stable oscillating states, and the number of stable states increases up to values of A of about 3.57, above which chaos begins. Then values vary over large ranges in almost random fashion between an upward and lower bound. Steady-state values reappear for A = 3.83, and then chaos reappears up to values of 4. For the chaotic states, the most minuscule change in the value of A leads to a completely different series of results, such that the state of the system at future time cannot be predicted.

In the chaotic regime, the result after a fixed number of steps also varies with minuscule changes in initial conditions—i.e., the initial value of x—and these changes are nonintuitive. Table 1-2 illustrates the result after 100 steps of the time series for A = 3.9. No matter how precisely the initial value is specified, while the first step is the same, the final value after a sufficient number of steps can vary across the entire range. "Extreme sensitivity to initial conditions" is the hallmark of chaotic systems and has been described by the so-called butterfly effect, where the beating of a butterfly's wings in China could lead eventually to a hurricane in the Atlantic.

(Continues...)

Excerpted from HOW TO BUILD A HABITABLE PLANET by Charles H. Langmuir Wally Broecker Copyright © 2012 by Princeton University Press. Excerpted by permission of PRINCETON UNIVERSITY PRESS. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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