The world will be what we make it--perhaps what you make it. The timelines are indeed that short.

Consider the speed of change. The earth has experienced five mass extinctions before the one we are living through now, each so complete a wiping of the fossil record that it functioned as an evolutionary reset, the planet’s phylogenetic tree first expanding, then collapsing, at intervals, like a lung: 86 percent of all species dead 450 million years ago; 70 million years later, 75 percent; 125 million years later, 96 percent; 50 million years later, 80 percent; 135 million years after that, 75 percent again. All but one of these involved climate change produced by greenhouse gas. The most notorious was 250 million years ago; it began when carbon dioxide warmed the planet by five degrees Celsius, accelerated when that warming triggered the release of methane, another greenhouse gas, and ended with all but a sliver of life on Earth dead. We are currently adding carbon to the atmosphere at a considerably faster rate--by most estimates, at least ten times faster. The rate is one hundred times faster than at any point in human history before the beginning of industrialization. And there is already, right now, fully a third more carbon in the atmosphere than at any point in the last 800,000 years--perhaps in as long as 15 million years. There were no humans then. The oceans were more than a hundred feet higher.

Many perceive global warming as a sort of moral and economic debt, accumulated since the beginning of the Industrial Revolution and now come due after several centuries. In fact, more than half the carbon exhaled into the atmosphere by the burning of fossil fuels has been emitted in just the past three decades. The United Nations established its climate change framework in 1992, building a political consensus out of a scientific consensus and advertising it unmistakably to the world; this means we have now done as much damage to the environment knowingly as we ever managed in ignorance. Global warming may seem like a distended morality tale playing out over several centuries and inflicting a kind of Old Testament retribution on the great-great-grandchildren of those responsible, since it was carbon burning in eighteenth-century England that lit the fuse of everything that has followed. But that is a fable about historical villainy that acquits those of us alive today--unfairly. The majority of the burning has come since the 1994 premiere of Friends. A quarter of the damage has been done since Barack Obama was elected president, and Joe Biden vice president, in 2008. Since the end of World War II, the figure is about 90 percent. The story of the industrial world’s kamikaze mission is the story of a single lifetime--the planet brought from seeming stability to the brink of catastrophe in the years between a baptism or bar mitzvah and a funeral.

It is the lifetime of many of the scientists who first raised public alarm about climate change, some of whom, incredibly, remain working--that is how rapidly we have arrived at this promontory, staring down the likelihood of three degrees Celsius of warming by the year 2100. Four degrees is possible as well--perhaps more. According to some estimates, that would mean that whole regions of Africa and Australia and the United States, parts of South America north of Patagonia, and Asia south of Siberia would be rendered brutally uncomfortable by direct heat, desertification, and flooding. Certainly, it would make them inhospitable, and many more regions besides. Which means that, if the planet was brought to the brink of climate catastrophe within the lifetime of a single generation, the responsibility to avoid it belongs with a single generation, too. We all also know that second lifetime. It is ours.

I am not an environmentalist and don’t even think of myself as a nature person. I’ve lived my whole life in cities, enjoying gadgets built by industrial supply chains I hardly think twice about. I’ve never gone camping, not willingly anyway, and while I always thought it was basically a good idea to keep streams clean and air clear, I also always accepted the proposition that there was a trade-off between economic growth and its cost to nature--and figured, well, in most cases I’d probably go for growth. I’m not about to personally slaughter a cow to eat a hamburger, but I’m also not about to go vegan. In these ways--many of them at least--I am like every other American who has spent their life fatally complacent, and willfully deluded, about climate change, which is not just the biggest threat human life on the planet has ever faced but a threat of an entirely different category and scale. That is, the scale of human life itself.

A few years ago, I began collecting stories of climate change, many of them terrifying, gripping, uncanny narratives, with even the most small-scale sagas playing like fables: a group of Arctic scientists trapped when melting ice isolated their research center, on an island populated also by a group of polar bears; a Russian boy killed by anthrax released from a thawing reindeer carcass, which had been trapped in permafrost for many decades. My file of stories grew daily, but very few of the clips, even those drawn from new research published in the most pedigreed scientific journals, seemed to appear in the coverage about climate change the country watched on television and read in its newspapers. In those places, climate change was reported, of course, and even with some tinge of alarm. But the discussion of possible effects was misleadingly narrow, limited almost invariably to the matter of sea-level rise. Just as worrisome, the coverage was sanguine, all things considered. As recently as the 1997 signing of the landmark Kyoto Protocol, two degrees Celsius of global warming was considered the threshold of catastrophe: flooded cities, crippling droughts and heat waves, a planet battered daily by hurricanes and monsoons we used to call “natural disasters” but will soon normalize as simply “bad weather.” More recently, the foreign minister of the Marshall Islands offered another name for that level of warming: “genocide.”

This is not a book about the science of warming; it is about what warming means to the way we live on this planet. But what does that science say? It is complicated research, because it is built on two layers of uncertainty: what humans will do, mostly in emitting greenhouse gases, but also in how we adapt to the environment we have transformed and how the climate will respond, both through straightforward heating and a variety of more complicated and sometimes contradictory feedback loops. But even shaded by those uncertainty bars, it is also very clear research, in fact terrifyingly clear. The United Nations’ Intergovernmental Panel on Climate Change (IPCC) offers the gold-standard assessments of the state of the planet and the likely trajectory for climate change. In its latest report, the IPCC suggested the world was on track for about 3 degrees of warming, bringing the unthinkable collapse of the planet’s ice sheets not just into the realm of the real but into the present.

Because these numbers are so small, we tend to trivialize the differences between them--one, two, four, five. Human experience and memory offer no good analogy for how we should think of those thresholds, but, as with world wars or recurrences of cancer, you don’t want to see even one.

At two degrees of warming, the ice sheets will likely begin their collapse, 400 million more people could suffer from water scarcity, and major cities in the equatorial band of the planet will become lethally hot in summer. There would be thirty-two times more extreme heat waves in India, and each would last five times as long, exposing ninety-three times more people. This is our best-case scenario.

At three degrees, southern Europe would be in permanent drought, and the average drought in Central America would last nineteen months longer and in the Caribbean twenty-one months longer. In northern Africa, the figure is sixty months longer--five years. The areas burned each year by wildfires would double in the Mediterranean and sextuple, or more, in the United States.

At four degrees, damages from river flooding could grow thirtyfold in Bangladesh, twentyfold in India, and as much as sixtyfold in the United Kingdom. In certain places, six climate-driven natural disasters could strike simultaneously. Conflict and warfare could double.

Even if we pull the planet up short of two degrees by 2100, we will be left with an atmosphere that contains 500 parts per million of carbon--perhaps more. The last time this was the case, sixteen million years ago, the planet was not two degrees warmer; it was somewhere between five and eight, giving the planet about 130 feet of sea-level rise, enough to draw a new American coastline as far west as I-95. Some of these processes take thousands of years to unfold, but they are also irreversible and therefore effectively permanent. You might hope to simply reverse climate change; you can’t. It will outrun all of us.

This is part of what makes climate change what the theorist Timothy Morton calls a “hyperobject”--a conceptual fact so large and complex that, like the internet, it can never be properly comprehended. There are many features of climate change--its size, its scope, its brutality--that alone satisfy this definition; together, they might elevate it into a higher and more incomprehensible conceptual category yet. But time is perhaps the most mind-bending feature, the worst outcomes arriving so long from now that we reflexively discount their reality.

All the paths projected from the present--to two degrees warming, to three, even to four or five--will be carved overwhelmingly by what we choose to do now. There is nothing stopping us from three degrees other than our own will to change course, which we have yet to display. Because the planet is as big as it is, and as ecologically diverse; because humans have proven themselves an adaptable species and will likely continue to adapt to outmaneuver a lethal threat; and because the devastating effects of warming will soon become too extreme to ignore or deny, if they haven’t already; because of all that, it is unlikely that climate change will render the planet truly uninhabitable. But if we do nothing about carbon emissions, if the next thirty years of industrial activity trace the same arc upward as the last thirty years have, whole regions will become unlivable by any standard we have today, as soon as the end of this century.

What this means is that we have not at all arrived at a new equilibrium. It is more like we’ve taken one step out on the plank of a pirate ship. Perhaps because of the exhausting false debate about whether climate change is “real,” too many of us have developed a misleading impression of its effects. Global warming is not “yes” or “no,” nor is it “today’s weather forever” or “doomsday tomorrow.” It is a function that gets worse over time as long as we continue to produce greenhouse gas. And so the experience of life in a climate transformed by human activity is not just a matter of stepping from one stable ecosystem into another, somewhat worse one, no matter how degraded or destructive the transformed climate is. The effects will grow and build as the planet continues to warm: from 1 degree to 1.5 to almost certainly 2 degrees and beyond. The last few years of climate disasters may look like about as much as the planet can take. In fact, we are only just entering our brave new world, one that collapses below us as soon as we set foot on it. For climate change isn’t something happening here or there but everywhere, and all at once. And unless we choose to halt it, it will never stop.

Just how completely the world below our feet will become unknown to us is not yet clear, and how we register its transformation remains an open question. One legacy of the environmentalist creed that long prized the natural world as an otherworldly retreat is that we see its degradation as a sequestered story, unfolding separately from our own modern lives.

In just the last forty years, according to the World Wildlife Fund, more than half the world’s vertebrate animals have died; in just the last twenty-five years, one study of German nature preserves found, the flying insect population declined by three-quarters. Species individuated over millions of years of evolution, but now forced together by climate change, they have begun to mate with one another for the first time, producing a whole new class of hybrid species: the pizzly bear, the coywolf. The zoos are already natural history museums, the children’s books already out-of-date. Little of this book, however, is about “nature” per se, and none of it concerns the tragic fate of the planet’s animals, which has been written about so elegantly and poetically by others that, like our sea-level myopia, it threatens to occlude our picture of what global warming means for us, the human animal. Until now, it seems to have been easier for us to empathize with the climate plight of other species than with our own, perhaps because we have such a hard time acknowledging or understanding our own responsibility and complicity in the changes now unfolding, and such an easier time evaluating the morally simpler calculus of pure victimhood.

That we know global warming is our doing should be a comfort, not a cause for despair, however incomprehensively large and complicated we find the processes that have brought it into being. That we know we are, ourselves, responsible for all its punishing effects should be empowering, and not just perversely so. Global warming is, after all, a human invention. And the flip side of our real-time guilt is that we remain in command. No matter how out of control the climate system seems--with its roiling typhoons, unprecedented famines and heat waves, refugee crises and climate conflicts--we are all its authors.

Some, like our oil companies and their political patrons, are more prolific authors than others. Each of us imposes some suffering on our future selves every time we flip on a light switch, buy a plane ticket, or fail to vote. Now we all share the responsibility to write the next act. We found a way to engineer devastation, and we can find a way to engineer our way out of it--or, rather, engineer our way toward a degraded muddle, but one that nevertheless extends the promise of new generations finding their own way forward, perhaps toward some brighter environmental future.

Since I first began writing about warming, I’ve often been asked whether I see any reason for optimism. The thing is, I am optimistic. Given the prospect that humans could engineer a climate that is 4 degrees warmer over the course of the next several centuries--a degraded muddle counts, for me, as an encouraging future. Yes, warming of 2 or 2.5 degrees would probably unleash enormous amounts of needless suffering, distributed cruelly to those least able to protect themselves, beyond anything that humans have ever experienced through many millennia of strain and strife and all-out war. But also quite a bit better than where we are currently headed. Given the basic mystery of the future, better than that is still possible too--bringing the planet closer to a state we would today regard as merely grim rather than apocalyptic.