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- Herausgeber: John Wiley & Sons
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
Theories and opinions about climate change abound - from those claiming human-induced climate change is already beyond control to those who express scepticism about the real extent of these changes. How should we weigh up the scientific evidence, and what role does climate change play in the history of the Earth? In this comprehensive history of the climate and climate change, Antonello Provenzale explains how the planetary climate system works and how the climate has evolved over millions of years. Starting from the catastrophic events that marked the early history of the Earth, including seas of magma, global glaciations and mass extinctions, he demonstrates how the climate has fluctuated between hot and cold periods, with the Earth hot and lush with forests at certain times and almost entirely covered by a thick layer of ice at others. The mechanisms that determine the modifications of the climate are multiple and complex and include external factors, such as solar luminosity and variations in the Earth's orbit, as well as internal processes connecting the atmosphere, the oceans, the crust, the mantle and the biosphere, composed of living organisms. While the climate has fluctuated a great deal over the Earth's long history, there are two features of our current situation that are a source of real concern. First, the rise in temperature of the last fifty years has been extremely fast, making it difficult for the environment to adapt to the new conditions. Second, the human population is much greater than it was in the past, and this population needs water, food, energy and shelter to survive and flourish. If temperatures continue to rise as they have in recent decades, ours will not be an easy world in which to live. To appreciate what is at stake, we need to understand how the climate works and how human activity is affecting it - not in order to save the planet, which will do just fine on its own and probably better without us, but to save ourselves.
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Table of Contents
Cover
Title Page
Copyright Page
Introduction
1. From the Ocean of Magma to the Great Oxygenation
The Beginnings
The Atmosphere in the Archean
Anoxic Oceans
The Kingdom of Oxygen and Sulphur
Magma from the Depths
The Ozone Layer
The Faint Young Sun and the End of the Archean Era
2. A World of Fire and Ice
Ice Everywhere (or Just About)
Is It Always Oxygen’s Fault?
The Volcanoes Set the Planet Back in Motion
Emerging Rocks
How the Planet Keeps the Greenhouse Effect at Bay
Rocks that Sink
Ice in Tropical Seas
Volcanoes and the Start of a New World
3. Light Reflected, Light Re-radiated
Children of the Sun
Shining with Reflected Light
Energy Absorbed and Re-radiated
The Earth’s Blanket of Gas
How Is This Blanket Made?
Albedo Again
Multiple Balances
Sorcerer’s Apprentices
4. The Explosion that Changed the World
From Bacteria to Living Sacks
Australian Enigmas
Avalon
The Dawn of a New World
Between Avalon and Burgess
Punctuated Equilibria
5. Between Catastrophes and Opportunities
The Climate during the Phanerozoic
Measuring Geological Time
Stable Isotopes
Land Ho!
Mosses, Forests and Ice
Things Do Not Always Go to Plan
6. The Living Planet
A Grand Organic Whole
Ecosystem Engineers
Sand, Bushes and Monsoons
The Lake beneath the Trees
The Bacterial Kingdom
Gaia
7. Winds Up High and Currents in the Deep
Energy Received, Energy Re-radiated: A Question of Latitude
The Earth’s Fluid Envelope
An Atmosphere in Dynamic Equilibrium
Jet Streams and Storm Tracks
Planet Ocean
Oceanic Currents
A Question of Transport
8. The Big Heat
An Equable Climate
Winds and Hurricanes in the Equable Climate
Is the Answer in the Clouds?
So Just How Much Carbon Dioxide Was There in the Eocene?
A Big, Sudden Heat
9. Rain, Snow and Clouds: The Planetary Water Cycle
The Triple Point
Vapour and Droplets
A Cloudy Planet
Rain and Snow
The Little Boy off the Coasts of South America
Low Clouds, High Clouds
Clausius and Clapeyron Again!
The Water Cycle
10. The Planet Cools
The Climate’s Pendulum
An Antarctic Story
Is It the Himalayas’ Fault?
Old Trees and New Grasses
Where Did All the Water Go?
11. The Breath of the Ice
Discovering the Glaciations
Children of the Ice Age
Greenhouse Gases and Glaciations
Planets in Chaos
Milankovitch Cycles
Glacial Mysteries
The World of the 100,000 Years
12. Agitated Ice
The Glacial Maximum
An Unstable Climate
Global Fluctuations
The Bipolar Seesaw
The Ice Retreats
Dryas!
13. Conquering the Planet
The Climate of the Holocene
The Green Sahara
The Last 2,000 Years
Romanesque Heat and Baroque Cold
Does the Sun Have the Answer?
The Accomplices Within
14. The Age of Humanity
The Keeling Curve
An Explosive Growth
The Carbon Cycle Again!
The Isotopes Return
The Time of the Anthropocene
Not Just Climate Change
15. Global Warming
One Degree in a Century
The Climate’s Hotspots
Raining Cats and Dogs
Ever Drier Summers
Fire, Fire!
Higher and Higher
The Causes of Warming
Who’s Afraid of Global Warming?
16. Arctic Sentinels
Arctic Amplification
Glaciers in Retreat
New Routes in Arctic Seas
Sinking into the Mud
A Question of Mismatch
Adventures in the Tundra
Restless Vortices and Lazy Currents
17. The Mountain Heat
Mountainous Lands
Amplified Warming
Ice that Flows …
… And that Melts
Water Towers
Erratic Monsoons
Winds from the West
Life at Altitude
Ibex and Prairies
Measuring the Change
18. Digital Twins
Thick Simulations, Tiny Similes
Models of the World
Equations on the Grid
The Science and Art of Parameterization
A Regional Zoom
Imperfect Models
Living with Uncertainty
Validating the Models
But the Road Is Not Yet Finished
The Virtual Earth
19. Knowing in Order to Anticipate, Anticipating in Order to Act
Meteorology and Climate
Climate Scenarios
Unexpected Instabilities
Estimating the Impacts of Climate Change
Levels of Risk
Operational Climatology
Reducing the Concentration of Greenhouse Gases
Intervening on the Climate
Who Pays the Bill?
Conclusion: The Journey Continues
Bibliography
1 From the Ocean of Magma to the Great Oxygenation
2 A World of Fire and Ice
3 Light Reflected, Light Re-radiated
4 The Explosion that Changed the World
5 Between Catastrophes and Opportunities
6 The Living Planet
7 Winds Up High and Currents in the Deep
8 The Big Heat
9 Rain, Snow and Clouds: The Planetary Water Cycle
10 The Planet Cools
11 The Breath of the Ice
12 Agitated Ice
13 Conquering the Planet
14 The Age of Humanity
15 Global Warming
16 Arctic Sentinels
17 The Mountain Heat
18 Digital Twins
19 Knowing in Order to Anticipate, Anticipating in Order to Act
Conclusion: The Journey Continues
Index
End User License Agreement
List of Illustrations
Chapter 1
Figure 1.1 Approximate reconstruction of the concentration of oxygen in the atmosphere, as ...
Chapter 4
Figure 4.1 A reconstruction of the
Opabinia regalis
, discovered among the fossils of...
Chapter 5
Figure 5.1 Schematic illustration of hydrogen isotopes (deuterium,
2
H, and triti...
Figure 5.2 The layers of the Gola del Bottaccione, near Gubbio, where Álvarez father...
Chapter 6
Figure 6.1 Image of the Negev desert close to Sde-Boker in Israel, with its characteristic ...
Chapter 7
Figure 7.1 Power (energy per second in watts, W) by square metre absorbed by the Earth’s...
Figure 7.2 Average temperature profile in the atmosphere as a function of altitude.
Figure 7.3 General circulation of the atmosphere: above, in the case of a single Hadley cel...
Figure 7.4 Vertical profile of temperature, salinity and density in typical ocean condition...
Figure 7.5 A simplified diagram of oceanic thermohaline circulation.
Figure 7.6 Schematic structure of the wind-driven ocean circulation.
Chapter 8
Figure 8.1 The Zachos curve. Time runs from right to left. On the left-hand vertical axis, ...
Chapter 9
Figure 9.1 The normal condition of tropical circulation in the Pacific (above) and that of ...
Figure 9.2 A simplified diagram of the Earth’s water cycle with its various componen...
Chapter 11
Figure 11.1 Oscillations in temperature and ice extension reconstructed through the isotopic...
Figure 11.2 Record of the isotopic ratio of deuterium to hydrogen (bottom curve) that provid...
Chapter 12
Figure 12.1 The temperature record, estimated from the isotopic ratio of oxygen-18 to oxygen...
Chapter 13
Figure 13.1 The continuous line shows the reconstruction of average global temperatures in t...
Figure 13.2 Reconstruction of global temperature fluctuations over the last two millennia, r...
Figure 13.3 Reconstruction of summer temperatures (June–August) in Scandinavia and th...
Figure 13.4 Estimate of soil humidity in the south-western United States for the period from...
Figure 13.5 The number of sunspots (on a monthly average) from Galileo’s observations...
Chapter 14
Figure 14.1 The Keeling Curve, which shows the concentration of carbon dioxide in the atmosp...
Figure 14.2 Concentration of atmospheric carbon dioxide in the last 10,000 years, reconstruc...
Chapter 15
Figure 15.1 Average global temperatures measured by observation networks around the world. H...
Figure 15.2 Average difference in temperature for the months December–January–...
Figure 15.3 The average percentage difference in total precipitation in the months December–...
Figure 15.4 The variation of drought conditions throughout the world, from 1950 to today. In...
Figure 15.5 Variations by percentage of the area burned by summer fires in the period 1985–...
Figure 15.6 The estimate of the causes of climate change over recent decades. On the left, t...
Chapter 16
Figure 16.1 The basin of the Bayelva river on the island of Spitzbergen. In the summer, the ...
Figure 16.2 Average September Arctic sea ice extent from 1979 to 2021, using data provided b...
Figure 16.3 Increase in the average temperature of the Arctic permafrost in regions where it...
Figure 16.4 The Amundsen–Nobile Climate Change Tower (CCT) installed by the Italian C...
Figure 16.5 Schematics of the tropospheric circumpolar vortex and the stratospheric polar vo...
Chapter 17
Figure 17.1 The average progress of mass balances in eleven mountainous regions all over the...
Figure 17.2 Variation in length (in metres) of a number of large Alpine glaciers, compared t...
Figure 17.3 An image of the Miage glacier covered in debris, in 2014. The icy tongue descend...
Figure 17.4 An adult male alpine ibex (
Capra ibex
) at the Gran Paradiso National Park...
Chapter 18
Figure 18.1 Schematic illustrating the hierarchy of climate models. This ranges from the con...
Figure 18.2 Global temperature obtained from four archives of observational data interpolate...
Chapter 19
Figure 19.1 Expected growth in average global temperature (top) and decrease in Arctic marin...
Figure 19.2 Projection of the average retreat of the fronts (snouts) of fourteen glaciers in...
Figure 19.3 Map of the percentage rise in the area burned by summer fires compared to averag...
Figure 19.4 Expected impacts of a particular rise in global temperature (indicated on the le...
Guide
Cover
Table of Contents
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HISTORY OF CLIMATE CHANGE
From the Earth’s Origins to the Anthropocene
Antonello Provenzale
Translated by Alice Kilgarriff
polity
Copyright Page
Originally published in Italian as Coccodrilli al Polo Nord e Ghiacci all’Equatore: Storia del clima della Terra dalle origini ai giorni nostri © First published in Italy by Rizzoli, 2021
This edition published by arrangement with Grandi & Associati
This English edition © Polity Press, 2023
The translation of this work has been funded by SEPS
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All rights reserved. Except for the quotation of short passages for the purpose of criticism and review, no part of this publication may be reproduced, stored in a retrieval system or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior permission of the publisher.
ISBN-13: 978-1-5095-5393-8 – hardback
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Library of Congress Control Number: 2022948541
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Introduction
The climate of our planet is continually changing. At certain periods in its history, the Earth was hot and lush with forests, and at others, it was almost entirely covered by a thick layer of ice. At times, there have been crocodiles at the North Pole, and at others, ice at the equator. Over the course of millions of years, sudden rises in temperature caused by massive volcanic eruptions have been followed by slow cooling periods associated with the dance of the continents, moved by convective motions in the Earth’s mantle. And all of this has been punctuated, every so often, by monstrous catastrophes that have swept away most of that epoch’s living beings.
Over the last 3 million years, the climate has been dominated by the almost regular alternation between prolonged cold and dry glacial periods and shorter, hot and rainy interglacials, in response to slow variations in the Earth’s orbit. The time in which we are currently living, the Holocene, began around 12,000 years ago when the ice retreated after the last glacial peak. Unlike many of the periods that preceded it, the Holocene has been characterized up until now by a stable climate with modest variations in temperature, something that has facilitated the birth of agriculture and the shift from nomadic hunter-gatherer populations to sedentary human societies.
The mechanisms that determine the continuous modifications in our climate are multiple and depend on both external forces – such as variations in solar luminosity, Earth’s orbital characteristics and volcanic activity – and, most of all, on the many internal processes connecting the atmosphere, the oceans, the ice, the biosphere, the crust and the mantle. These are extremely complex phenomena in which the response is often disproportionate to the stimulus, and it can activate a myriad of feedback processes. They are associated to changes across all scales of time and space, with the amplification or reduction of the effects of external forcing factors. On top of these, the bond between organisms and environment is what makes the Earth a living planet whose evolution is inextricably linked to the action of the biosphere.
Among the many players involved, two play a central role. One is the composition of the atmosphere, in particular the concentration of greenhouse gases such as carbon dioxide, methane and water vapour. The other is the albedo of terrestrial surfaces and the clouds, or, rather, the quantity of solar energy that is reflected into space without entering into the planet’s thermal machine. Alongside these principal actors, we also have the transport of heat and matter in the atmosphere and the ocean; the water cycle with its transformation of water into liquid, solid and vapour; and the biogeochemical cycles that remix and redistribute crucial elements such as carbon, phosphorus, nitrogen, iron and many others among the various components of the Earth System.
Over the last two centuries, a new factor has been added to these mechanisms of the climate’s natural variability: human activity. This has reached planetary dimensions, emitting enormous quantities of carbon dioxide into the atmosphere and profoundly modifying the land, while eliminating an incredibly large number of natural ecosystems, deforesting the tropics, dramatically reducing terrestrial and marine biodiversity, and moving unimaginable quantities of sediment. All of these changes, in particular the growth in the concentration of carbon dioxide in the atmosphere, have led to a rapid increase in the average global temperature, which has risen by more than 1 degree centigrade over the last century. In some areas, such as the Arctic, the warming has been double that. And these changes in temperature have an impact on all of the climate’s components – leading, for example, to an intensification of drought conditions in the Mediterranean basin and higher rainfall in Northern Europe.
Of course, an increase in temperature of almost 1 degree is negligible when compared to the much more conspicuous variations of the past. Some 50 million years ago, the average global temperature was probably 10 degrees higher than today. So, what are we worrying about?
Essentially, two aspects. First, there is the fact that the temperature increase of the last fifty years has been extremely fast, probably faster than has ever occurred in the past, making it more difficult for the environment to adapt to the new conditions. And there is the not-insignificant fact that, as of 2022, almost 8 billion people inhabit the Earth within fixed infrastructures and megalopolises that grow ever larger, often along densely populated coastlines that are the site of intense production activity. This is a population that needs water, food, energy. We are a highly technological society, but one that is very vulnerable to environmental changes. If the mercury continues to climb, there will be more droughts and consequent famines but also, paradoxically, increasingly violent floods, more extensive fires, a rise in sea level and more intense coastal erosion events. Some ecosystems could even collapse, and there will be difficulties in agricultural production. And, presumably, there will also be mass migration, social and economic instability and an even greater risk of war. If, by the end of the century, the temperature has increased by 4 °C compared to 100 years ago, as is expected if we continue to behave in the way we have done so far, the world will not be an easy place to live.
In order to understand fully what is happening, to put it in perspective and attempt to resolve the problems we ourselves have created, we must not deny the factual evidence – global warming is caused by human activity – and nor should we scream that this is the end of the world. Instead, it is necessary to understand how the climate functions, how it has changed and why, what is happening now, what the risks are, and what the past, even the remote past, can teach us. And how we can attempt to predict the future, and what strategies we can use to tackle the climate crisis we are living in. In short, we need to apply our intelligence and capacity for analysis. This book aims to make a small contribution in this regard, discussing what we have understood about the fascinating and complex system that is the Earth’s climate, and what remains unclear. It does so by following a path that begins in the darkness of time, in the era when our planet was formed, through conditions so very different from those we see today that they bring to mind alien worlds – all the while, trying to understand why things have happened in a particular way.
We will journey through the history of the climate of the Earth, investigating the mechanisms by which it functions, exploring the most extreme conditions of the past and the great instabilities that have changed the world, finally arriving at the last century, at the cumbersome presence of humanity and the effects caused by the rise in the concentration of carbon dioxide in our atmosphere and oceans. We will do this in order to compare what is happening today with what we have learned from the Earth’s past, and to use our knowledge of previous disasters to avoid (where possible) new ones.
The knowledge and capacity to forecast that come from this cannot, however, remain an end in themselves. These things must inform those taking decisions and be used to resolve real problems. At this moment, one of the issues we must face up to is how to combat anthropogenic climate change, as well as the need to stop the loss of biodiversity and fight poverty the world over. These three aspects are strongly connected to one another and involve the health of human beings and ecosystems alike, as well as the availability of water of sufficient quantity and quality for both human beings and the natural environment. In the final chapters of this book, we will consider some of these aspects with the knowledge that the solution is not a return to a golden age that never actually existed, but by taking as a starting point those technologies and that development which created the problem and using them to resolve it. We need to imagine and implement innovative approaches, even more advanced technologies, societies that are less delusional and more sustainable, working with nature and not against it. Preferring utopia to dystopia and trying to create it in reality, at least as far as possible. Not so much to save the planet, which will do just fine on its own and probably better without us, but to save ourselves.
I would like to thank everyone who has helped me to write this book. I am grateful to Giò and Maria for their support during the months of writing. Thank you to Marco Ferrari, Silvia Giamberini, Elisa Palazzi and Maddalena Pennisi, who had the patience to read everything I wrote and have always given me encouragement, advice and relevant suggestions. Thank you to Chiara Boschi, Andrea Dini, Gianfranco Di Vincenzo and Eleonora Regattieri, who provided me with precious guidance on those chapters closest to their own scientific expertise. A special thank you goes to Laura Grandi, who pushed me to take on this task and followed it throughout each of its phases, putting up with my delays and providing me with important advice. Thank you also to the Festival della Mente in Sarzana, Italy, where the idea for this book was born. Finally, sincere thanks to Elise Heslinga of Polity Press, who gently helped me throughout the preparation of the English version of this book and firmly reminded me of deadlines; to Alice Kilgarriff, who translated the book from Italian and with whom I worked closely and pleasantly; and to Leigh Mueller for the copy-editing. Obviously, it goes without saying that all of the errors are, however, exclusively my own.
1From the Ocean of Magma to the Great Oxygenation
The Beginnings
Below is an ocean of molten magma the colour of red fire. Above, an incessant rain of meteorites and small asteroids, bolides that crash onto the burning liquid surface of the planet, releasing even more heat and scattering carbon compounds that will later be incorporated into the molecules of life. In the first 200 million years of its existence, the Earth boiled incessantly, resembling the most pulp-like descriptions of a Dantean Hell.
During this first geological eon, appropriately named the Hadean, inside the planet a dense nucleus that was rich in metals was differentiating itself from a lighter, rocky mantle made up of siliceous and partially molten materials. Over this time period that lasted around 500 million years and ended 4 billion years ago, it is believed that the orbit of a planetoid known as Theia, which was almost as large as Mars, most likely crossed into the orbit of our own planet, colliding with the Earth in a crash of colossal proportions. The two celestial bodies fused together and the violence of the impact broke off a gigantic incandescent fragment that solidified over time, forming our now ice-cold satellite: the Moon.
In the millions of years that followed, the surface of the Earth slowly cooled and became solid, covered first in basalt and later in granite, while asteroid impacts decreased, the continents began to form and widescale volcanic eruptions dominated the planetary stage. The Earth’s crust was born. The volcanoes freed enormous quantities of thermal energy still present in the bowels of the earth, releasing gas and water that went on to form the primordial atmosphere. Other energy was produced by the decay of radioactive elements that had been incorporated into the mantle during the formation of the Earth. At that time, with the reduction in surface temperature, new oceans of liquid water covered the surface, the atmosphere began its long chemical evolution and the Earth transformed itself into a planet ready to host life. This was the beginning of the Archean Eon, the period that spans from almost 4 to 2.5 billion years ago.
Life on Earth, therefore, appeared relatively early on. Numerous clues suggest the presence of fully developed cellular life more than 3.5 billion years ago – so, shortly after conditions grew less hostile. How life on Earth originated remains one of science’s most fascinating enigmas, bound of course to the question of whether living organisms were also able to develop on other planets. But for now, we are interested in what the climate was like in that distant time, how the ocean, atmosphere and planet’s surface interacted with one another and what the global climate conditions were like.
The Atmosphere in the Archean
During the Archean, the atmosphere was different from the one we know today. For a long time, scientists believed that the Earth’s primitive atmosphere was strongly ‘reducing’, meaning it was poor in oxygen and rich in molecules that contained hydrogen, such as methane, hydrogen sulphide and ammonia, substances that are, in truth, not very palatable for us humans. In 1952 and 1953, two American researchers, Stanley Miller, at the time a young assistant, and Harold Urey, his professor, carried out an experiment that continues to be hailed as a milestone. Following the intuitions of Alexander Oparin, a Russian biochemist who thirty years earlier had proposed a theory on the biogeochemical origin of life, Miller and Urey inserted a mixture of gas – considered at the time to be similar to the Archean atmosphere – into a sealed container. They added water and caused various sparks inside the container to simulate the effect of lightning.
The result – a surprising one – was that many complex organic molecules were produced, including different kinds of amino acids that are present in living organisms today. The ensuing enthusiasm and emotion were momentous, and it seemed that we were close to understanding how life on Earth was born.
However, the journey from Miller and Urey’s organic compounds to the function of a living cell is a long one. In the decades that followed, other possibilities were explored. For example, the suggestion that life was born around submarine hydrothermal vents, or by the catalytic action of certain kinds of clay on the formation of protocellular membranes. Or that it began in the depths of the Earth’s crust, many kilometres below the surface, with organisms able to use methane as an energy source. There are also those who, like Fred Hoyle, a famous British astrophysicist, supported the hypothesis of panspermia, which suggested life was brought to Earth by comets and asteroids hailing from other planets (though this doesn’t resolve the problem of how life was formed on those other planets). Today, we know that complex organic molecules can be observed in interstellar clouds and found on those meteorites that fall to Earth. In 2019, for example, a complex sugar, ribose, was identified in two different meteorites. But for now, at least, we are still not able to reconstruct fully the chain of events that led these complex molecules to become the first functioning cell.
Even the Archean atmosphere was not, in reality, as reducing as we had believed up until a few years ago. In 2011, Dustin Trail and two of his colleagues at the Astrobiology centre at the Rensselaer Polytechnic Institute in the state of New York published a scientific article in which they analysed the composition of the oldest minerals present on Earth. The results of the study suggest that the primordial atmosphere was primarily composed of carbon dioxide, sulphur dioxide and water, all of which are very stable molecules, meaning this atmosphere was much less reducing and decidedly different from that hypothesized in the experiment by Miller and Urey. The search for the origins of life on Earth, therefore, continue today and will likely occupy researchers for many years to come.
That distant world was very different from the one we know today. There was no life on the land, only single-celled life (predominantly organisms similar to bacteria and Archaea) in marine waters, and an atmosphere that was different from the one we breathe in today. If one of us were to be transported back to that environment by a time machine, we would suffocate instantly. There was no molecular oxygen in the atmosphere, no ozone layer to protect from the sun’s ultraviolet rays. In short, it was an alien planet with an ecosystem based predominantly on anaerobic metabolism, sustained by chemical reactions that can only happen in the absence of oxygen. But this is the environment in which, after a long chain of successive evolutions, life developed, leading to us and the world we know.
Anoxic Oceans
The oceans of the Archean were also different from those we know today. Ocean salinity was probably not all that different, but as there was no free oxygen in the atmosphere, there was none dissolved in the marine waters. The oceans were anoxic. Thanks to this total absence of oxygen, the hot waters of the Archean contained high quantities of dissolved soluble iron (known as ‘ferrous iron’, an iron atom with two positive charges) that came from rocks and which would be immediately oxidized in today’s oceans and rendered insoluble, causing them to sink into the sediment. The single-celled organisms present in those seas lived in anaerobic conditions, meaning their metabolism required an almost complete absence of free oxygen and a lifecycle based on chemical reactions between atoms of dissolved iron and carbon dioxide. Other kinds of metabolism used sulphur, which was widely available in the Archean oceans. For example, the process of photosynthesis developed by the so-called purple sulphur bacteria was based on an anaerobic cycle that used sunlight and hydrogen sulphide.
In that world based on the chemistry of iron and sulphur, however, other single-celled organisms were also present, capable of a different kind of photosynthesis that was much more efficient. Cyanobacteria, also known as blue-green algae, were able to use carbon dioxide, water and sunlight from the atmosphere in a new photosynthetic process to generate energy, producing molecular oxygen as a waste material. We know that cyanobacteria were almost certainly already present 2,700 million years ago, perhaps even earlier. In that world, oxygen was a toxic gas, lethal for anaerobic organisms, and these new arrivals would come to expand from the ecological niches in which they had developed and overrun the anoxic world of the Archean.
The presence of dissolved iron and the production of oxygen by cyanobacteria in the oceanic waters of the late Archean Eon gave rise to the spectacular sedimentary rocks known as banded iron formations. These highly colourful structures, central to the mineral extraction of iron, are visible today in the oldest rocks of Australia, Brazil, Canada, South Africa, India, Russia and the United States. The thickness of each band varies from less than a millimetre to a metre, and is composed of hematite or magnetite, both iron oxides. Between these bands we find siliceous sedimentary rocks composed of chert and shale.
But how are these bands formed? We can presume that oxygen production by the cyanobacteria played an essential role. We have seen that, during photosynthesis, oxygen is produced as a waste material. The oxygen produced by the cyanobacteria in the shallow waters of the Archean continental platforms was therefore rapidly eliminated because it combined with the dissolved iron, generating insoluble iron oxides that sank to the bottom and created the rich iron deposits we see today in rocks the world over.
Why, though, was the precipitation process of the iron oxides not continuous, creating alternating layers of sediments, some rich in iron and others from siliceous rocks that instead contain barely any? At the moment, there is no definitive answer. One possibility is that the ecosystem of the cyanobacteria underwent a series of oscillations in which their efficiency varied, meaning their ability to produce oxygen through photosynthesis varied also. Equally, there could have been fluctuations in the chemical characteristics of the environment that rendered it less or more suited to the iron’s precipitation. Recently, a group of Australian, German and Canadian researchers proposed a different explanation. According to their hypothesis, supported by an analysis of the microscopic structure of the rocks with banded iron formations, the sedimentation of the iron was roughly continuous. Only after deposition, in the process of diagenesis and metamorphism (which modify sediments and sedimentary rocks when the geological processes lead them to be buried deeper down and exposed to temperatures and pressure much higher than those on the surface), is there a ‘migration’ of the iron oxides that separate from their siliceous sediment to form alternate bands with a high concentration of iron.
Independently of whether banded iron formations originated from oscillations in the production of oxygen or from the processes that took place after sedimentation, these banded structures tell us two important things. The first is that, 2.5 billion years ago, single-celled organisms capable of photosynthesis, and which produced oxygen as a waste material, already existed. The second is that the oxygen was present in relatively low concentrations, otherwise the iron would not have been able to stay dissolved and would instead have oxidized as soon as it came into contact with the sea water, as happens in modern oceans. It is no coincidence that there are no banded iron formations younger than 1,800 million years old, if we exclude a significant episode around 600 million years ago, to which we will return in the next chapter.
The Kingdom of Oxygen and Sulphur
Around 3 billion years ago, molecular oxygen was almost entirely absent from the Earth’s atmosphere and the oceans. Today, the atmospheric concentration of oxygen is around 21 per cent and the oceans are oxygenated to great depths. Something must therefore have happened between that distant past and more recent times that caused the quantity of free oxygen in the atmosphere to grow significantly and drastically modify our planet’s environment.
In the relatively shallow waters that covered the continental platforms of the Archean seas, cyanobacteria initially produced small quantities of oxygen that combined with the dissolved iron to produce oxides that were quickly deposited among the sediments on the seabed. The more the cyanobacteria population grew, the more oxygen was produced. Not all of the oxygen was reused by the biological cycle (cyanobacteria, like plants, produce oxygen in the reactions of photosynthesis and reabsorb it during nocturnal respiration). In fact, part of the organic material produced through photosynthesis was buried in the sediment, leaving a surplus of oxygen in the water. As a consequence, the available iron was oxidized and oxygen began to accumulate in the marine water, passing from there into the atmosphere. The anaerobic organisms were confined to marginal areas, beneath the mud on the seabed or at great depths, whilst on the surface the world grew increasingly richer in oxygen.
The atmospheric concentration of oxygen has, nevertheless, grown very slowly since around 2.4 billion years ago. For hundreds of millions of years, oxygen remained well below 10–15 per cent of its current concentration. Figure 1.1, from the work of Donald E. Canfield, a biogeochemist working in Denmark, shows an approximate reconstruction of the level of oxygen in the atmosphere.
Figure 1.1 Approximate reconstruction of the concentration of oxygen in the atmosphere, as a percentage of the current level, taken from the work of Donald E. Canfield (2015).
There is, however, a ‘but’. If the cyanobacteria were the cause of oxygenation and they were already present more than 2.7 billion years ago, why didn’t the concentration of oxygen in the atmosphere start to increase earlier, as soon as they evolved? Why did it instead remain low for so long?
It is difficult to answer this question with any certainty, because the relevant information we have is still scarce. In any case, there are two possible factors that could have limited the concentration of oxygen: scant production by a still scarcely abundant cyanobacteria population, or a rapid elimination of the oxygen in the atmosphere. Or a combination of both of these things. And, more generally, the chemistry of the ocean and the atmosphere necessarily played an essential role in determining the composition of that world.
One possibility is that in the Archean seas there was little phosphorus, an essential nutrient for cyanobacteria and algae, both then and now. In fact, phosphorus was easily absorbed by the iron oxides that sank, creating the banded iron formations (BIF) we spoke of earlier. The phosphorus dissolved in the marine waters would have therefore been far less than the levels found today, significantly limiting the productivity of the cyanobacteria’s ecosystem and, consequently, the production of oxygen through photosynthesis. A cycle of cause and effect would therefore have set in, by which the cyanobacteria produced oxygen that combined with the dissolved iron, causing it to sink to the seabed as iron oxides. The latter, however, absorbed phosphorus, removing it from the water and thus limiting the nutrients and growth of the cyanobacteria and, therefore, the production of oxygen. Such a cycle could have also given rise to temporary oscillations in the entire process, generating the ferrous bands of sediments.
Around 2.4 billion years ago, however, a new event may have interrupted the oxygen–iron–phosphorus cycle. Analysis of sediments and rocks indicates that, at this time, the ocean grew rich in sulphur, which combined with the iron dissolved in the water to form pyrite, with its chemical formula FeS2 (1 iron atom and 2 sulphur atoms). The pyrite sank to the seabed, removing the iron from the water. At that point, the formation of iron oxides and BIF was significantly reduced, phosphorus was no longer eliminated from the water, nutrients became abundant and the cyanobacteria could expand and dominate the ecosystem, leading to a gradual rise in the concentration of oxygen in the water and the atmosphere.
Magma from the Depths
In order to get a full picture, we must not neglect another fundamental piece of the puzzle: the direct removal of oxygen from the water and the atmosphere.
The oxygen is removed from the Earth’s surface in two main ways: through the oxidation of the rocks and organic matter on the planet’s surface, and through chemical reactions with reducing gases emitted by volcanoes, which come from deep in the Earth’s crust and mantle. Both of these processes were active in the Archean (as they still are today), but at a certain point, when oxygen began to accumulate in the atmosphere, they evidently became less efficient.
In particular, it is probable that the Earth’s upper mantle was initially in a chemically reducing state capable of reacting with other compounds and ‘capturing’ oxygen atoms. The gases produced by volcanic activity, which came from deep magma, were also reducing and rich in hydrogen compounds able to react with and absorb the oxygen from the atmosphere. One possible scenario is that, with the passing of time, the superficial rocks were increasingly oxidized and then transported to the mantle through the process of subduction, which we will discuss in greater detail in the next chapter. During subduction, the rocks in the oceanic crust descend to great depths, melting at least partially and melding with the mantle. In this way, the mantle was slowly enriched with oxygen until it went from being reducing to a more neutral situation. After this process, which was hypothetically concluded around 2.4 million years ago, the gases emitted by the volcanoes were less reducing, and the oxygen was finally free to accumulate in the atmosphere.
We do not actually know whether this interpretation is correct or not, and it is almost certain that the entire process of oxygenation would have been much more complicated, influenced by many other factors and difficult to reconstruct. What we do know is that, over a period of many hundreds of millions of years, the concentration of oxygen in the atmosphere rose from the negligible levels of the Archean to levels that are comparable (though inferior) to what they are currently. The world had changed forever. Or, at least, until plant life began to thrive on Earth.
The Ozone Layer
Together with the growth in atmospheric oxygen, the concentration of ozone in the stratosphere also increased. Ozone is, in fact, produced through the ‘photodissociation’ of oxygen molecules (O2, 2 oxygen atoms) by sunlight. Atoms of free oxygen then combine with other substances, but some combine instead with another molecule of diatomic oxygen, forming a new molecule with 3 oxygen atoms (O3) and known as ozone.
The ozone that accumulates in the stratosphere (the atmospheric layer between around 15 and 50 kilometres from the ground) plays a crucial role for life on Earth. It is able to absorb the Sun’s ultraviolet rays, thus screening the planet’s surface from potentially damaging radiation. The majority of organisms are, indeed, vulnerable to ultraviolet rays (just think of the UV lamps used to sterilize medical and surgical instruments) and would not otherwise be able to live on the planet’s surface.
It was probably at the same time as this rise in the concentration of oxygen, around 2.4 billion years ago, that the Earth acquired its protective layer of ozone. Since then, it has remained throughout the stratosphere, protecting plants, animals and human beings from the effects of ultraviolet radiation. Ozone, however, is easily destroyed by the chemical reactions caused by compounds such as chlorofluorocarbons. These are the infamous CFCs and HCFCs whose emission into the atmosphere as a result of human activity caused a ‘hole’ in the ozone layer some decades ago. This thinning of the layer of stratospheric ozone was particularly evident above the Antarctic. Through a chain of photochemical reactions, any chlorine atom contained in a CFC molecule can destroy many thousands of ozone molecules, making these compounds extremely harmful even at low concentrations. The reduction in the use of CFCs and HCFCs, later regulated through more rigorous legislation, finally led to a substantial reconstitution of the ozone layer.
The Faint Young Sun and the End of the Archean Era
But what was the average surface temperature during the Archean? This is difficult to say as we have very little to go on. Estimates vary from 10 to 80 °C. More recent results, once more based on the analysis of the composition of the oldest rocks, suggest a less extreme climate, with an average surface temperature lower than 50 °C.
These results, however, conflict with what we know about the evolution of the stars, which suggests that the young Sun emitted less energy than it does today. In the 1950s, astrophysicists such as the German Martin Schwarzschild, who later emigrated to the United States, described how the life of a star is marked by different phases. During the first phase, its luminosity (the energy given off by the star) grows over time, and the Sun is no exception. At the time the Earth was formed, its luminosity was just 70 per cent of what it is currently and it has increased gradually to reach today’s levels.
With less solar energy reaching our planet, the climate of its first 2 billion years should have been extremely cold, with the Earth completely covered in ice. And yet the results provided by the analysis of Archean rocks and zircons more than 4 billion years old tell a different story. The ancient zircons found in the Jack Hills in Australia were formed at depth, crystallizing from magmas that came from the melting of matter found in the Earth’s crust. The isotopic composition of zircons reflects that of the magmas from which they were formed, which in turn preserve at least some of the characteristics of the original crust matter. And yet the composition of zircons reveals that the crust from which the magmas were formed had previously come into contact with liquid water, presumably from an ocean, in a world that could not have been completely frozen. Something doesn’t add up.
This problem, called the paradox of the Faint Young Sun, is first described by Carl Sagan and George Mullen in 1972. Carl Sagan was an excellent astrophysicist and exceptionally gifted scientific educator. He was involved in the golden era of space exploration, also contributing to the Voyager missions, probes that left the solar system carrying information about humanity for other possible inhabitants of the cosmos.
The most obvious way to escape this paradox (as proposed by Sagan and Mullen) is to hypothesize that the primordial Earth had an atmosphere that was extremely rich in carbon dioxide and methane produced by bacteria and capable of trapping infra-red radiation emitted by the Earth, thus generating a greenhouse effect that would have been much more intense than that we see today. This highlights carbon dioxide’s staggering importance to the planet’s climate. These ancient high concentrations of CO2 and methane provided our planet with a thick ‘blanket’ that allowed it to maintain temperatures suited to the presence of life despite weaker solar radiation.
However, towards the end of the Archean, the rise in the concentration of oxygen shocked not only the ecosystems but also the planet’s climate. The oxygen reduced the time methane remained in the atmosphere, while also limiting the distribution of those anaerobic organisms (bacteria and Archaea that live in anoxic conditions) known as methanogens due to their capacity to produce methane through metabolism. In the newly emergent kingdom of oxygen, the atmospheric concentration of methane was therefore reduced. But as the Sun was still faint, the Earth found itself in an unexpected position. For perhaps the first time ever, an epochal change in climate occurred and the planet was covered by a thick layer of ice. Snowball Earth was born.
2A World of Fire and Ice
Ice Everywhere (or Just About)
The probe descended slowly, sending images and data to the mothership orbiting above, beyond the atmosphere of that white planet. On the surface below, ice covered just about everything. Only the peaks of the highest mountains emerged from the dazzling blanket that reflected most of the light received from the star. At the equator, perhaps, the ice opened up leaving a space for the dark ocean waters, which could only have any direct contact with the air at that point and managed to absorb a little light and oxygen, even if we still do not know the actual size of the ice-free area. And yet those same oceans nevertheless hosted a wealth of life – predominantly single-celled organisms, but also more complex, multi-cellular beings.
This is how our planet would have looked to hypothetical space explorers some 600 million years ago. It was Snowball Earth, a climate state characterized by an extensive and thick layer of ice covering almost the entire surface of the Earth. Between the end of the Archean almost 2.5 billion years ago and the beginning of the Cambrian era some 542 million years ago came the 2 billion years of the Proterozoic. During this eon, the Earth went through devastating climate changes, alternating between eras in which ice covered most of the planet and those in which the climate was temperate, or even hot.
The first episode of planetary glaciation probably happened at the beginning of the Proterozoic, shortly after oxygen had begun to accumulate in the atmosphere. This is known as the Huronian glaciation, discovered in 1907 by Arthur Coleman, who was analysing rocks close to the mineral township of Cobalt, in Ontario, Canada. Coleman noticed that enormous and very ancient masses were embedded in the region’s geological strata, and that these masses had been transported a great distance from their area of geological origin. They presented all the signs of glacial action, such as smoothed and striated surfaces, similar to those found on the erratic masses of the last, most recent glacial periods. And so, one of our planet’s oldest glaciations was discovered.
Similar evidence was recently found in the rocks of many other regions of North America, but also in South Africa, India and Australia. At the time of the Huronian glaciation, the continents were arranged in a different way from today, but this geographical extension suggests that ice covered most of the Earth, and not just for a brief time. The duration of the Huronian glaciation was around 300 million years, a period in which many species of living organisms became extinct.
Is It Always Oxygen’s Fault?
The obvious question is, naturally, why a glaciation of this scale happened. Which mechanisms caused the planet to fall into this icy grip? The most likely hypothesis (though not the only one) is once again associated with oxygen and the enormous upheaval this gas brought to the planet.
As we have seen, it is highly likely that methane was a key element in the atmosphere’s composition, responsible for a particularly intense greenhouse effect and capable of countering the scarce solar luminosity of that time, meaning surface temperatures would continue to be mild or even elevated. But the arrival of oxygen threw a spanner in the works. Oxygen reacted easily with methane, eliminating it from the atmosphere and drastically reducing the greenhouse effect, which caused a subsequent crash in temperature. This is the chain of events that probably led to the first great planetary glaciation.
Oxygen’s responsibility for triggering a glaciation was first identified by Canadian geologist S. M. Roscoe, once again studying the rocks around Lake Huron, not far from where Coleman had found proof of the Huronian glaciation. Roscoe noted that, just below the blocks striated by the glaciation, there were strata that contained detritus of pyrite and uraninite, minerals that can only be deposited in sediment in conditions of oxygen scarcity. Just above the strata of glacial rocks, there was instead evidence of oxidized sediment, a sign that oxygen was present in the atmosphere. This observation led to the hypothesis that it was precisely this rise in atmospheric oxygen that reduced the concentration of methane and therefore the greenhouse effect, unleashing the glaciation.
In this case, we also see how the accumulation of oxygen, due to the actions of photosynthetic organisms, caused irreversible changes in the planet’s climate and environment. As the ice started to expand, the planet’s surface became increasingly white and reflective. This meant less sunlight was absorbed, which led to a further drop in temperature. Around 2.4 billion years ago, therefore, our planet first ran the risk of falling into a state unsuited to life. But if we look at the Earth today, it is clear that it managed to find a way out.
The Volcanoes Set the Planet Back in Motion
We might then ask ourselves how our planet managed to escape the ice’s frozen clutches. One possibility is linked to volcanic activity. In the Proterozoic, the volcanoes were extremely active and the eruptions more violent and frequent than those we see today. On the land that emerged, there were no expanses of forest or prairie. The landscapes were barren and rocky, the organic soils produced by the actions of living organisms had yet to form, and the wind and rain swept across continents inhabited solely by bacteria in the areas with the most water, along streams and rivers, in swamps and around hydrothermal areas.
With the glaciation, the emergent lands were also covered (at least partially) by glaciers. The blanket of ice and the low temperatures slowed down the processes of surface rock erosion and weathering (physical and chemical alteration), the main geological mechanism that removes carbon dioxide from the atmosphere, as we shall discuss further in due course. But carbon dioxide was still emitted by volcanoes, which filled the atmosphere with large quantities of the gas. Even without the methane, which had by then been overcome by the oxygen, over the course of millions of years it is probable that the concentration of carbon dioxide continued to rise, increasing the greenhouse effect until it reached a critical threshold that caused temperatures high enough to melt the ice sheets and bring about the end of the Huronian glaciation. The entire melting process may well have been more complicated – aided, for example, by the cloud cover which was probably present even above the ice. In any case, just over 2 billion years ago, the Earth escaped this icy grip and a new era began. For a long period of time, almost 2 billion years, the planet would have been hot, with little or no ice coverage, and with more and more oxygen in the atmosphere.
Emerging Rocks
There were no particularly significant planetary events in the billion years that followed the end of the Huronian glaciation, leading to it being nicknamed the Boring Billion. However, during this period, other aspects of the planet were very active indeed, such as the shifting of the continents due to movement deep in the Earth’s mantle, the birth of a new continental crust, and plate tectonics.
Our planet’s surface is formed by the Earth’s crust, of which there are two kinds: the continental crust, which is mostly made up of granite, and the oceanic crust, primarily composed of basalt, which is denser than granite. The Earth’s mantle is formed from even denser rocks and has a temperature that rises the deeper it goes, to such an extent that the rocks in the mantle are rendered malleable and, in some parts, partially molten. The crust varies in thickness from 5 kilometres at the oceanic crust to 70 kilometres under the highest mountain ranges of the continental crust, whilst the mantle has a thickness of around 2,900 kilometres. If we descend even farther, we come to the realm of the core, composed predominantly of molten metals on the outside and, at its centre, solidified metals rendered extremely dense by the enormous pressure. At the heart of our planet, therefore, is an enormous solid block of metal covered by an ocean of molten metal in constant motion. Truly a scene worthy of Jules Verne.
In the mantle are radioactive elements, trapped there since the Earth’s formation, which decay slowly over time. This decay generates the heat that warms the mantle from the inside, causing the very slow movement and deformation of rocks. This movement is much like the circulation of water in a saucepan when heated from below. The hot liquid rises to the surface where it releases heat and slowly returns to the bottom, creating circulation loops known as ‘convection cells’. The heated air moves in the same way, generating sometimes violent phenomena such as storms.
Convection cells in the mantle obviously move more slowly than those in the air, and are more akin to the moulding of a block of plasticine than to the circulation of a liquid. A full rotation on this merry-go-round, from the depths of the mantle to the surface and back again, takes between 50 and 200 million years. The entirety of the Earth’s surface is subdivided into plates, very large regions that correspond to the enormous convective cells in the mantle, which, together, move the rocks and carry heat generated in the depths up to the surface. These movements are described today by plate tectonics, the modern incarnation of the theory of continental drift first proposed (though unappreciated at the time) by Alfred Wegener, a German geologist and meteorologist who spent a long time studying in the Arctic (today, Germany’s institute for marine and polar research is named after him). Plate tectonics is a crucial process for our planet that was only discovered in the 1970s and is still not fully understood today. It is curious to think that, decades after the theory of relativity, after quantum mechanics and space travel, we still had no idea how the inside of the Earth was structured; nor were we familiar with the most fundamental geological mechanism characterizing our planet.
In the ascending branch of convection cells, the lightest and partially molten components of the rocks in the mantle rise to the surface, accompanied by the formation of volcanic systems and eruptions of magma that, when they solidify, generate basalt, the black rocks of the oceanic crust. In the middle of the Atlantic, roughly parallel to the American and African coasts, a huge underwater mountain range signals the presence of the ascending branch of a convection cell. Similar mid-ocean ridges can be found in the Pacific, the Indian Ocean and in many other areas of our planet. The Mid-Atlantic Ridge runs through Iceland, creating the volcanic landscape found on that island of fire and ice. In Ethiopia, in the largest depression of the Rift Valley, the African plate is breaking up, moving away from the Arabian plate and creating the basis for a new ocean that will see the Red Sea grow much larger.
