Our Fragile Moment

How Lessons from Earth's Past Can Help Us Survive the Climate Crisis


By Michael E. Mann

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In this sweeping work of science and history, the renowned climate scientist and author of The New Climate War shows us the conditions on Earth that allowed humans not only to exist but thrive, and how they are imperiled if we veer off course.

For the vast majority of its 4.54 billion years, Earth has proven it can manage just fine without human beings. Then came the first proto-humans, who emerged just a little more than 2 million years ago—a fleeting moment in geological time. What is it that made this benevolent moment of ours possible? Ironically, it’s the very same thing that now threatens us—climate change.

The drying of the tropics during the Pleistocene period created a niche for early hominids, who could hunt prey as forests gave way to savannahs in the African tropics. The sudden cooling episode known as the “Younger Dryas” 13,000 years ago, which occurred just as Earth was thawing out of the last Ice Age, spurred the development of agriculture in the fertile crescent. The “Little Ice Age” cooling of the 16th-19th centuries led to famines and pestilence for much of Europe, yet it was a boon for the Dutch, who were able to take advantage of stronger winds to shorten their ocean voyages.

The conditions that allowed humans to live on this earth are fragile, incredibly so. Climate variability has at times created new niches that humans or their ancestors could potentially exploit, and challenges that at times have spurred innovation. But there’s a relatively narrow envelope of climate variability within which human civilization remains viable. And our survival depends on conditions remaining within that range.
In this book, renowned climate scientist Michael Mann will arm readers with the knowledge necessary to appreciate the gravity of the unfolding climate crisis, while emboldening them—and others–to act before it truly does become too late.




We live on a Goldilocks planet. It has water, an oxygen-rich atmosphere, and an ozone layer that protects life from damaging ultraviolet rays. It is neither too cold nor too hot, seemingly just right for life. Despite our ongoing search—which, with the recent advent of the James Webb telescope, now extends out nearly fourteen billion light years—we have thus far found no other planet in the universe with such benevolent conditions. It’s almost as if this planet, Earth, was custom made for us. And yet it wasn’t.

For the vast majority of its 4.54 billion years, Earth has proven it can manage just fine without human beings. The first hominids—proto-humans—emerged a little more than two million years ago. Only during the past 200,000 years have modern humans walked the Earth. And human civilizations have existed for only about 6000 or so years, 0.0001 percent of Earth’s history—a fleeting moment in geological time.

What is it that made this fragile yet benevolent moment of ours possible? Ironically, it’s the very same thing that now threatens us: climate change. The asteroid impact sixty-five million years ago that generated a global dust storm chilled the planet, killing off the dinosaurs and paving the way for our ancestors, tiny shrew-sized proto-mammals that scurried about, hiding from their saurian predators. With the dinosaurs no longer around, these critters could now come out from the shadows, fill new niches, and gradually branch out to produce primates, apes, and eventually us. Though such an event would prove devastating for modern human civilization if it happened today, our real and urgent threat is from fossil fuel burning and carbon pollution, and it is warming, not cooling.

Figure 1. Estimated global temperature changes over the past 24,000 years. The fragile moment is defined by the period from around 6000 years ago to the mid-twentieth century (the “zero” of the time axis).

Climate has shaped and guided us from the start. The drying of the tropics as the planet cooled during the Pleistocene epoch of the past 2.5 million years created a niche for early hominids, who could hunt prey as forests gave way to savannas in the African tropics. Yet drying today threatens drought and wildfire in many regions. The sudden cooling episode in the North Atlantic Ocean 13,000 years ago known as the Younger Dryas, which occurred just as Earth was thawing out of the last ice age, challenged hunter-gatherers, spurring the development of agriculture in the Fertile Crescent. A similar North Atlantic cooling event looms today as Greenland ice melts, freshens the waters of the North Atlantic, and disrupts the northward ocean conveyor current system. It could threaten fish populations and impair our ability to feed a hungry planet. The Little Ice Age of the sixteenth to nineteenth centuries led to famines and pestilence for much of Europe, and contributed to the collapse of the Norse colonies. Yet it was a boon for some, such as the Dutch, who were able to take advantage of stronger winds to shorten their ocean voyages. The Dutch West and East India Companies became the dominant maritime trading companies, holding a near monopoly on European shipping routes to South and North America, Africa, Australia, and New Zealand. They seemingly ruled the world. For a while. Just like the dinosaurs did. For a while.

As we can see, the story of human life on Earth is a complicated one. Climate variability has at times created new niches that humans or their ancestors could potentially exploit, and challenges that caused devastation, then spurred innovation. But the conditions that allowed humans to live on this Earth are incredibly fragile, and there’s a relatively narrow envelope of climate variability within which human civilization remains viable. Today, our massive societal infrastructure supports more than eight billion people, an order of magnitude beyond the natural “carrying capacity” of our planet (the resource limit of what our planet can provide in the absence of human technology). The resilience of this infrastructure depends on conditions remaining the same as those that prevailed during its development.

The concentration of carbon dioxide (CO2) in the atmosphere today is the highest since early hominids first hunted on the African savannas. It is now already outside the range during which our civilization arose. If we continue to burn fossil fuels, it is likely that the planet will warm beyond the limit of our collective adaptive capacity. How close are we to the edge? In the pages that follow, I set out to answer that question.

We’ll look at how we have arrived here, and the incredible gift of a stable climate that the planet gave us along the way so that we, humans, could not just exist but thrive. And we’ll learn how our civilization will be imperiled if we continue on our current path. We’ll delve into the field known as paleoclimatology—the study of prehistoric climates—which offers crucial lessons as we contend with the greatest challenge we’ve faced as a species. You already, no doubt, know that we face a climate crisis. In the following pages, I’ll arm you with the knowledge necessary to fully appreciate the extent of the unfolding threat, while emboldening you to act before it truly does become too late. Only by understanding the climate changes of the past and what they tell us about the circumstances that allowed us to thrive, can we appreciate two seemingly contradictory realities. On the one hand, there is the absolute fragility of this moment in time—driven home on a daily basis by each devastating wildfire and every “once in a century” hurricane or 110°F day, collective signs that we seem to be slipping into the chasm of an unlivable planet. On the other hand, however, the study of Earth’s history betrays some degree of climate resilience. Climate change is a crisis, but a solvable crisis.1

An important point we’ll come back to often throughout this book is this: we must embrace scientific uncertainty. The scientific process builds on itself. New data come to light that help us refine our understanding. Sometimes it changes our previous understanding. Contrarians insist that this uncertainty is a reason for climate inaction, the implication being that we can’t trust it, or we might somehow overreact in a way that, for example, could hurt the economy. But just the opposite is true. Many key climate impacts—the increase in deadly and devastating extreme weather events, the loss of glacial ice, and the resulting inundation of our coasts—have already exceeded the earlier scientific projections. Uncertainty isn’t our friend. It is, however, a very good reason for even greater precaution and more concerted action.

A consequence of this uncertainty, as we’ll see, is that the answers aren’t always cut and dry—this is particularly true as we go back in time and the data become both sparser and fuzzier. Our instinct is to try to come up with simple analogs and definitive conclusions. But science doesn’t work that way, and a complex system like Earth’s climate certainly doesn’t work like that. So we must embrace nuance, too—and indeed it is one of our greatest tools as we seek answers to the key questions about our climate past and our climate future.

Different scientific studies often come to at least modestly different conclusions. It is only by assessing the collective evidence across numerous scientific studies that we reach more firm conclusions and begin to establish scientific consensus. I’ve always loved this story told by Ira Flatow, the amiable host of NPR’s Science Friday, about a fact-finding congressional inquiry into the potential threat posed by supersonic air travel during the early 1970s:

Senator Edmund Muskie (D-ME) was the chairman of the committee assigned to find the answers to these questions. He, in turn, appointed an august committee from the National Academy of Sciences (NAS) to study the issue. Six months later they were to report to the congressional committee. All the newspapers were there and the cameras were rolling.

The committee’s chief scientist said, “Senator, we’re ready to testify,” and Muskie responded, “Okay, tell me what the answer is. Is this going to be a danger?” The scientist then slapped down his giant sheaf of papers on the desk and said, “I’ve got these papers here that definitely tell us this is going to be a danger.” Muskie was ready to conclude right there, but then the NAS scientist interjected, “On the other hand, I have another set of papers over here that says these papers aren’t good enough to know the answer.” In exhaustion, the senator looked up and yelled, “Will somebody find me a one-handed scientist?!”2

It’s a cute story, but with a serious lesson. Everybody wants a “one-handed scientist,” but that’s not how science works.

Complicating matters further is that press releases and media coverage tend to emphasize “blockbuster” studies: ostensibly shocking new discoveries that garner clicks and pageviews. So we get the so-called whiplash effect, where we’re told one week about a study, for example, that shows that eating chocolate or drinking coffee or wine (basically all the good stuff life offers) is healthy, only to read a headline the following week about a new study insisting it’s bad for you.3

As a result, we get a skewed view of scientific understanding as more polarized and more mercurial than it actually is. The phenomenon is readily seen in the climate discourse, where we’re told one week, for example, that the Greenland Ice Sheet—and all the sea level rise that comes with it—may be on the verge of collapse, while a study the following week suggests it’s more stable than we thought. We’re frequently bombarded with dire headlines about “doomsday glaciers” and “methane bombs” that belie the still dire but more nuanced and, importantly, far less hopeless picture that emerges from an objective assessment of the underlying scientific evidence.

Keeping uncertainty and all its implications in mind, we’ll look at the big question on everybody’s mind: Are we doomed? The answer, as we’ll learn, is that it is entirely up to us. The collective evidence from the paleoclimate record—the record of Earth’s past climatic changes—actually provides a blueprint for what we need to do to preserve our fragile moment. The greatest threat to meaningful climate action today is no longer denial, but despair and doomism, premised on the flawed notion that it is too late to do anything. Our review of the paleoclimatic record will tell us otherwise.

There is a duality that governs the human species and the climate it enjoys. Human actions, particularly the burning of fossil fuels and the generation of carbon pollution, have impacted the trajectory that our climate has taken over the past two centuries, but the longer-term trajectory of our climate has also impacted us. It’s what got us here. By looking back at that trajectory, we can gain insights into what futures are possible. In my previous book The New Climate War, I examined the lobbying efforts over the past half century by fossil fuel companies and their enablers that have prevented us from thus far taking the actions necessary to avert catastrophic climate change. Thanks to the efforts of those corporations, we’re now coming up against the boundary of habitable life for us humans.

In this book, I’m inverting this perspective. We’re going to look at the influence that Earth’s climate history has had on us and what we can learn from it. But keep in mind that paleoclimate is only one line of evidence. It will not and cannot address all of the questions we might have about human-caused climate change, if for no other reason than there is no perfect analog in our past for what we potentially face in the future. But together with insights from the modern climate record and guidance from state-of-the-art models of Earth’s climate system, it informs our assessment of just how tenuous this moment is, underscoring both the urgency of actions to mitigate, and adapt to, the heightening climate crisis we face and the agency that we still possess in averting disaster.

Figure 2. The geological timescale.


Our Moment Begins

We are at a crossroads in human history. Never before has there been a moment so simultaneously perilous and promising. We are the first species to have taken evolution into our own hands.

—CARL SAGAN, Broca’s Brain

Imperiled today by planetary warming of our own making, we find no small amount of irony in our current plight. For we, in fact, owe our very existence to climate change, albeit the natural sort. A violent asteroid impact here, a sudden warming spike there, with a collision of tectonic plates and a collapsing ocean conveyor thrown into the mix. We wouldn’t have arrived at this moment without a remarkable series of climate episodes and accidents.

We Emerge

Climate change has shaped us from the beginning. It’s a simple statement, but the story is not. Not all evolutionary or societal trends are driven by environmental changes, let alone climate change. Some simply reflect the slow but steady progress of natural selection acting on environmental conditions, chance discoveries, and innovations. But there is no doubt that some of the key developments that made us what we are today were driven by climate events. Let’s start at the beginning.

One could argue that the possibility of human life began when the first living organism emerged from the primordial ooze somewhere around four billion years ago. But really, it began in earnest several billion years later—sixty-six million years ago to be more precise—when a giant asteroid struck Earth. It was almost eight miles wide, traveling 30,000 miles per hour (more than three times faster than the speed of sound), creating a hundred-mile-wide crater that lies beneath the waters off the coast of the Yucatan Peninsula. This epic collision ejected a massive cloud of debris into the atmosphere, blocking out sunlight and rapidly cooling the planet. Roughly eighty percent of all animal species including the non-avian dinosaurs (that “non-avian” qualifier is required because birds, as a direct lineage of the dinosaurs, are in fact a surviving subclass) disappeared.

As we’ll see throughout this story, tragedy for some often meant opportunity for others. A key consequence of this catastrophic event was that it eliminated the main predators of our extreme distant ancestors, the small rodent-like mammals that had scurried about hiding among the rocks. They could come out of hiding and occupy new niches. The collision marked the end of the Mesozoic era—the age of dinosaurs—and the beginning of the Cenozoic era—the age of mammals.

Roughly fifty-six million years ago, just a brief (in geological time) ten million years after the demise of the dinosaurs, climate change—that is, naturally occurring climate change—generated yet another challenge for life. In this case, it was a sudden warming spike known as the Paleocene-Eocene Thermal Maximum, or PETM, which occurred early in the twenty-million-year-long Eocene epoch. It created evolutionary pressures that opened a niche for a whole new order of mammals: the primates. The first one was a primitive lemur-like creature named Dryomomys. She was indeed our distant relative, though at five inches long and on a fruit diet, she would be awkward to invite to Thanksgiving dinner.1

The warm, humid greenhouse climates of the early and mid-Eocene also favored an increase in plant diversity, which in turn created new environmental niches for primates. Wet tropical and subtropical forests allowed the arboreal early primates to diversify and disperse across North America, Eurasia, and Africa. The climate then slowly cooled over the course of the mid- and late Eocene and into the Oligocene epoch, which began roughly thirty-four million years ago.

What drove this cooling trend? Back in the early 1990s, leading paleoclimate scientists Maureen Raymo and William Ruddiman argued that it was the collision of the Indian and Asian tectonic plates, beginning in the early Eocene. That collision pushed up and created the Tibetan plateau, also building the towering Himalayan mountain range (and the majestic Mt. Everest). The warm, moisture-laden air coming off the Indian ocean collides with the mountain range and rises upward toward the peaks, causing the moisture to condense into rainfall as it rises and cools in the atmosphere. Today we know this system as the South Asian summer monsoon.2

More rainfall means more weathering of rocks; CO2 from the atmosphere dissolves in streams and rivers, where it turns into carbonic acid and dissolves rocks. Silicate rocks known as feldspars, for example, dissolve into clay and calcium and carbonate ions (very small charged molecules). These materials run off into streams and rivers and eventually into the ocean. All this carbon drawn down from the atmosphere and buried in the ocean weakens the greenhouse effect and cools down the planet.

It was during the early Oligocene that the first hints of something we might call an icehouse climate began to emerge. Ice sheets formed, first in Antarctica around thirty-four million years ago and later in North America and Greenland.3

The CO2 drawdown and cooling continued on into the subsequent Miocene epoch that began twenty-four million years ago. The cooler conditions led to the retreat of subtropical forests, which were replaced by woodlands—more open environments that consist of a mix of trees, grasses, shrubs, and other plants, creating a niche for primates that spent more time on the ground and less time swinging from trees. Welcome to the “planet of the apes.” Orangutans appeared during the mid-Miocene. Gorillas appeared a few million years later, and the first chimps a few million years after that. With primates that are now partly upright, use primitive tools, and have a more complex social organization—the great apes, or hominids—we are inching closer and closer to our species, Homo sapiens.

Figure 3. Estimated global temperature changes over the past sixty-five million years.

As forests and woodlands disappeared in Eurasia from six to twelve million years ago, so did the populations of great apes. There was an exodus of hominids to southeast Asia and to Africa, where some eventually evolve into hominins—an even more select group that includes modern humans, extinct human species, and our immediate ancestors. Meanwhile, the carbon drawdown, and cooling, continued. Grasslands expanded. So did ice sheets, which began to form in the Northern Hemisphere, in Greenland and North America.

At five million years ago we had entered into the Pliocene. This was the last time the level of greenhouse gases in the atmosphere was comparable to today, between 380 and 420 parts per million (ppm) in the atmosphere. Yet somehow, the planet was actually 3.5–5°F warmer than today, and the sea level could have been as much as thirty feet higher. What gives? We’ll explore this seeming paradox in a bit.4

Homo sapiens weren’t yet on the scene, but our direct ancestors were. The first upright walking hominins, like Ardipithecus (and, soon, Australopithecus), evolved from earlier hominids on the African continent. Cooling continued, and expansive subtropical savannas and grasslands replaced forests and woodlands. Though these environments were not well suited to the great apes, who retreated to wetter tropical locales (where they remain to this day), they were ideal for the newly evolved hominins, who carved out a niche by walking upright. They were omnivores who supplemented the fruit and nuts favored by their ape ancestors with edible grasses and sedges and the meat they obtained by hunting large game animals in packs on the savannas.

Fast-forward to 2.6 million years ago and we are headed directly into the icehouse—the Pleistocene epoch. Cooling continued and ice sheets began to take hold now in the Northern Hemisphere. Several species of our own genus Homo now roamed the African savannas. Some evolved into faster runners and more effective hunters, taking advantage of the expanded grasslands. Some used rudimentary stone tools. Some developed bigger, better brains and would ultimately evolve into our very own species, Homo sapiens.

By 700,000 years before the present, the climate had cooled yet further, and Earth experienced more extensive glaciation with ice sheets extending well down into North America and Eurasia. These expanded ice sheets displaced the jet stream toward the equator, cooling and drying the subtropics and tropics, including large parts of Africa where the hominins resided. Disrupted climate patterns may have naturally selected for species with higher-powered brains that could develop strategies to contend with the severe challenges created by the changing climate, including the design of more-sophisticated stone tools and development of increasingly more complex social communities, like hunting groups using better-designed spears and sophisticated group hunting strategies to more efficiently hunt game when other food sources were scarce.5

Larger ice sheets fundamentally changed the dynamics of the climate system itself, generating larger, slower swings between cold glacial periods (ice ages) with extensive ice sheets and warmer interglacial periods with greatly diminished ice. These swings are tied to astronomical cycles governing Earth’s orbit relative to the Sun, especially the roughly 100,000-year cycle’s eccentricity (how circular versus elliptical the Earth’s annual orbit around the Sun is). But their magnitude is determined by how cold and icy the planet can get.6

The long-term cooling trend proceeded from around 700,000 years ago, continuing for the next several hundred thousand years. That lead to the intermittent growth of larger and larger ice sheets, causing increasing climate disturbance in the African tropics and subtropics during the ensuing glacial/interglacial cycles. The most recent complete cycle was the largest swing of all, ranging from the extreme warmth of the Eemian period starting 130,000 years ago (which at its peak likely exceeded even today in warmth) to the bitter cold of the Last Glacial Maximum roughly 21,000 years ago when an ice sheet covered what is now New York City. The global temperature change during that swing was about 9°F, and twice that over middle and high latitudes, owing to the amplifying effects of growing or shrinking ice—it’s a so-called positive feedback loop—something that we’ll see is critical in climate change. These huge swings in climate put even greater selective pressure on bigger and better brains that could devise ever more clever coping mechanisms to deal with the challenges presented by climate extremes.

And so, our moment finally arrives. Bones of primitive Homo sapiens first appeared 300,000 years ago in Africa, with skulls suggesting brains of similar size to our own. Anatomically modern Homo sapiens appeared 200,000 years ago, and skulls from 100,000 years ago suggest brains that were indistinguishable from our own in all respects, including both size and shape. Between 100,000 and 200,000 years ago, Homo sapiens were collecting and cooking shellfish and making fishing tools. They were using language. They had become us. But though modern humans had finally arrived, with the help of an asteroid impact, a long-term cooling trend, and the huge seesaw cycles of warming and cooling that marked the late Pleistocene, it would take another series of climate events and accidents to yield the innovation essential for the emergence of human civilization.7

In the Wilderness

For our first hundred thousand years we were out in the wilderness—literally. Early Homo sapiens existed as nomadic, hunter-gatherer tribes. They grew in numbers and expanded into other continents including Europe and Asia, following the migrations of other archaic Homo


  • "Deeply-researched, sprawling in scope and with insights and surprises on every page. This is the sort of historical understanding that leads to wisdom."—Seth Godin, Founding Editor of The Carbon Almanac
  • "This detailed and yet marvelously readable look at our climatic past offers us the information we need to understand our climatic future--and more importantly, to act to shape that future in the here and now."—Bill McKibben, author of The End of Nature
  • “Mann has a tremendous depth of knowledge about the history of our planet’s climate, which is why his words of warning and optimism are so important. This book provides important lessons from humanity’s past to empower readers to help protect our future.”—Former U.S. Vice President Al Gore
  • “Reading Our Fragile Moment is like taking a spectacular hike through billions of years of Earth’s climate history with one of the great scientists of our time. Oh look — there’s the meteor that wiped out the dinosaurs! There’s the great ocean conveyor! There’s the Rossby waves! When you reach the summit of Mann’s wonderful book, you will understand just how rare and beautiful our moment is — and why we need to fight harder to protect it.”—Jeff Goodell, author of The Heat Will Kill You First
  • “Mann shows that over the last few hundreds of millions of years, Earth has been snow-ball cold, tropic hot, rainforest wet, and desert dry. Its atmosphere has been oxygen poor, oxygen rich, or choked with deadly gas. But Earth has never been through anything quite like humankind. Our current comfortable climate is disappearing— because of us. It’s cause for thundering alarm, but is not cause for despair or doomist gloom. It’s time for action. Don’t believe me? Read this book.”—Bill Nye, Science Educator, CEO, The Planetary Society
  • "Mann has masterfully woven the climate story from our past to the future. Drawing upon a wealth of data, research and expertise, he slays the persistent zombie theories that climate scientists ignore historical context."—Dr. Marshall Shepherd, international expert in weather and climate, and Distinguished Professor of Geography and Atmospheric Sciences at the University of Georgia
  • “Written with clarity, brevity and wit, Mann presents a riveting and instructive narrative of Earth’s climate changes to help us navigate this new epoch of human-altered climate. This honest, informed look at planetary history serves as both a defense against doomism and a call to action to forge a livable world that is still well within our grasp.”—David Grinspoon, Astrobiologist and author of Earth in Human Hands
  • “A renowned climatologist and science journalist casts a hard eye on the probability that climate change is irreversible… An evenhanded take on a crucial topic. While our goose may not be cooked, it’s still time to reduce the heat.”—Kirkus
  • “A gripping tale of Earth’s climate history, this book is a must-read for every global citizen. It dispels common climate myths with surgical clarity and provides an essential roadmap to understanding our past and choosing our future.”—Katharine Hayhoe, climate scientist, distinguished professor at Texas Tech University, UN Champion of the Earth, author of Saving Us

On Sale
Sep 26, 2023
Page Count
320 pages

Michael E. Mann

About the Author

Michael E. Mann is the Presidential Distinguished Professor and Director of the Center for Science, Sustainability and the Media at the University of Pennsylvania.
He has received many honors and awards, including NOAA's outstanding publication award in 2002 and selection by Scientific American as one of the fifty leading visionaries in science and technology in 2002. Additionally, he contributed, with other IPCC authors, to the award of the 2007 Nobel Peace Prize.
More recently, he received the Award for Public Engagement with Science from the American Association for the Advancement of Science in 2018 and the Climate Communication Prize from the American Geophysical Union in 2018. In 2019 he received the Tyler Prize for Environmental Achievement. In 2020 he was elected to the U.S. National Academy of Sciences. He is the author of numerous books, including Dire Predictions: Understanding Climate Change, The Hockey Stick and the Climate Wars: Dispatches from the Front Lines, and The Madhouse Effect: How Climate Change Denial is Threatening our PlanetDestroying Our Politics, and Driving Us Crazy. He lives in State College, Pennsylvania.

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