How Earth’s Climate Has Guided Human History

Misunderstanding climate variability can stall climate action, but navigating nuance is the path to positive change.

The following is an excerpt from Our Fragile Moment: How Lessons from Earth’s Past Can Help Us Survive the Climate Crisis by Michael Mann. 

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Our Fragile Moment: How Lessons from Earth's Past Can Help Us Survive the Climate Crisis

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Our Fragile Moment: How Lessons from Earth's Past Can Help Us Survive the Climate Crisis


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 6,000 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.

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.

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.

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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?!”

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.

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 doom-ism, 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.

Excerpted from Our Fragile Moment: How Lessons from Earth’s Past Can Help Us Survive the Climate Crisis by Michael Mann. Copyright © 2023. Available from PublicAffairs, an imprint of Hachette Book Group, Inc. 

Meet the Writer

About Michael Mann

Dr. Michael Mann is the author of Our Fragile Moment: How Lessons from Earth’s Past Can Help Us Survive the Climate Crisis, a professor of Earth & Environmental Science, and the Director of the Penn Center for Science, Sustainability and the Media at the University of Pennsylvania in Philadelphia, PA.

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