The Atlantic
- Peter Brannen | Photo Illustrations Brendan Pattengale | Maps| La Tigre
Our climate models could be missing something big.
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Brendan Pattengale
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We live on a wild planet, a wobbly, erupting, ocean-sloshed orb that careens
around a giant thermonuclear explosion in the void. Big rocks whiz by overhead,
and here on the Earth’s surface, whole continents crash together, rip apart, and
occasionally turn inside out, killing nearly everything. Our planet is fickle.
When the unseen tug of celestial bodies points Earth toward a new North Star,
for instance, the shift in sunlight can dry up the Sahara, or fill it with
hippopotamuses. Of more immediate interest today, a variation in the composition
of the Earth’s atmosphere of as little as 0.1 percent has meant the difference
between sweltering Arctic rainforests and a half mile of ice atop Boston. That
negligible wisp of the air is carbon dioxide.
Since about the time of the American Civil War, CO
2’s
crucial role in warming the planet has been well understood. And not just based
on mathematical models: The planet has run many experiments with different
levels of atmospheric CO
2. At some points in the Earth’s history,
lots of CO
2 has vented from the crust and leaped from the seas, and
the planet has gotten warm. At others, lots of CO
2 has been hidden
away in the rocks and in the ocean’s depths, and the planet has gotten cold. The
sea level, meanwhile, has tried to keep up—rising and falling over the ages,
with coastlines racing out across the continental shelf, only to be drawn back
in again. During the entire half-billion-year Phanerozoic eon of animal life,
CO
2 has been the
primary driver
of
the Earth’s climate. And sometimes, when the planet has issued a truly titanic slug of CO
2
into the atmosphere, things have gone horribly wrong.
Today, humans are injecting CO
2 into the atmosphere at one of the
fastest rates ever over this entire, near-eternal span. When hucksters tell you
that the climate is always changing, they’re right, but that’s not the good news
they think it is. “The climate system is an angry beast,” the late Columbia
climate scientist Wally Broecker was fond of saying, “and we are poking it with
sticks.”
The beast has only just begun to snarl. All of recorded human history—at only a
few thousand years, a mere eyeblink in geologic time—has played out in perhaps
the most stable climate window of the past 650,000 years. We have been shielded
from the climate’s violence by our short civilizational memory, and our
remarkably good fortune. But humanity’s ongoing chemistry experiment on our
planet could push the climate well beyond those slim historical parameters, into
a state it hasn’t seen in tens of millions of years, a world for which
Homo sapiens did not evolve.
When there’s been as much carbon dioxide in the air as there already is
today—not to mention how much there’s likely to be in 50 or 100 years—the world
has been much, much warmer, with seas 70 feet higher than they are today. Why?
The planet today is not yet in equilibrium with the warped atmosphere that
industrial civilization has so recently created. If CO
2
stays at its current levels, much less steadily increases, it will take
centuries—even millennia—for the planet to fully find its new footing. The
transition will be punishing in the near term and the long term, and when it’s
over, Earth will look far different from the one that nursed humanity. This is
the grim lesson of paleoclimatology: The planet seems to respond far more
aggressively to small provocations than it’s been projected to by many of our
models.
To truly appreciate the coming changes to our planet, we need to plumb the
history of climate change. So let us take a trip back into deep time, a journey
that will begin with the familiar climate of recorded history and end in the
feverish, high-CO
2 greenhouse of the early age of mammals, 50 million
years ago. It is a sobering journey, one that warns of catastrophic surprises
that may be in store.
The first couple of steps back in time won’t take us to a warmer world—but they
will illuminate just what sort of ill-tempered planet we’re dealing with. As we
pull back even slightly from the span of recorded history—our tiny sliver of
geologic time—we’ll notice almost at once that the entire record of human
civilization is perched at the edge of a climate cliff. Below is a punishing ice
age. As it turns out, we live on an ice-age planet, one marked by the swelling
and disintegration of massive polar ice sheets in response to tiny changes in
sunlight and CO
2 levels. Our current warmer period is merely one peak
in a mountain range, with each summit an interglacial springtime like today, and
each valley floor a deep freeze. It takes some doing to escape this cycle, but
with CO
2 as it is now, we won’t be returning to an ice age for the
foreseeable future. And to reach analogues for the kind of warming we’ll likely
see in the coming decades and centuries, we will need to move beyond the past 3
million years of ice ages entirely, and make drastic jumps back into the alien
Earths of tens of millions of years ago. Our future may come to resemble these
strange lost worlds.
Before we move more dramatically backwards in time, let us briefly pause over
the history of civilization, and then some. Ten thousand years ago, the big
mammals had just vanished,
at human hands, in Eurasia and the Americas. Steppes once filled with mammoths and camels and
wetlands stocked with giant beavers were suddenly, stunningly vacant.
The coastlines that civilization presumes to be eternal were still far beyond
today’s horizon. But the seas were rising. The doomed vestiges of mile-thick ice
sheets that had cloaked a third of North American land were retreating to the
far corners of Canada, chased there by tundra and taiga. The roughly 13
quintillion gallons of meltwater these ice sheets would hemorrhage, in a matter
of millennia, raised the sea level hundreds of feet, leaving coral reefs that
had been bathed in sunlight under shallow waves now drowned in the deep.
By 9,000 years ago, humans in the Fertile Crescent, China, Mexico, and the Andes
had independently developed agriculture and—after 200,000 years of wandering—had
begun to stay put. Sedentary settlements blossomed. Humans, with a surfeit of
calories, began to divide their labor, and artisans plied new arts. The Earth’s
oldest cities, such as Jericho, were bustling.
By 5,000 years ago, sunlight had waned in the Northern summer, and
rains drifted south toward the equator again. The green Sahara began to
die, as it had many times before.
It’s easy to forget that the Earth—cozy, pastoral, familiar—is nevertheless a
celestial body, and astronomy still has a vote in earthly affairs. Every 20,000
years or so the planet swivels about its axis, and 10,000 years ago, at
civilization’s first light, the Earth’s top half was aimed toward the sun during
the closest part of its orbit—an arrangement today enjoyed by the Southern
Hemisphere. The resulting Northern-summer warmth turned the Sahara green. Lakes,
hosting hippos, crocodiles, turtles, and buffalo, speckled North Africa, Arabia,
and everywhere in between. Lake Chad, which today finds itself overtaxed and
shrinking toward oblivion, was “Mega-Chad,” a 115,000-square-mile freshwater sea
that sprawled across the continent. Beneath the Mediterranean today, hundreds of
dark mud layers alternate with whiter muck, a barcode that marks the Sahara’s
rhythmic switching from lush green to continent-spanning desert.
Imprinted on top of this cycle were the last gasps of an ice age that had
gripped the planet for the previous 100,000 years. The Earth was still thawing,
and amid the final approach of the rising tides, enormous plains and forests
like Doggerland—a lowland that had joined mainland Europe to the British
Isles—were abandoned by nomadic humans and offered to the surging seas. Vast
islands like Georges Bank, 75 miles off Massachusetts—which once held mastodons
and giant ground sloths—saw their menagerie overtaken. Scallop draggers still
pull up their tusks and teeth today, far offshore.
By 5,000 years ago, as humanity was emerging from its unlettered millennia, the
ice had stopped melting and oceans that had been surging for 15,000 years
finally settled on modern shorelines. Sunlight had waned in the Northern summer,
and rains drifted south toward the equator again. The green Sahara began to die,
as it had many times before. Hunter-fisher-gatherers who for thousands of years
had littered the verdant interior of North Africa
with fishhooks and harpoon points
abandoned the now-arid wastelands, and gathered along the Nile. The age of
pharaohs began.
By geologic standards, the climate has been
remarkably stable ever since, until the sudden warming of the past few decades. That’s unsettling, because
history tells us that even local, trivial climate misadventures during this
otherwise peaceful span can help bring societies to ruin. In fact, 3,200 years
ago, an entire network of civilizations—a veritable globalized economy—fell
apart when minor climate chaos struck.
“There is famine in [our] house; we will all die of hunger. If you do not
quickly arrive here, we ourselves will die of hunger. You will not see a living
soul from your land.” This letter was sent between associates at a commercial
firm in Syria with outposts spread across the region, as cities from the Levant
to the Euphrates fell. Across the Mediterranean and Mesopotamia, dynasties that
had ruled for centuries were all collapsing. The mortuary-temple walls of Ramses
III—the last great pharaoh of Egypt’s New Kingdom period—speak of waves of mass
migration, over land and sea, and warfare with mysterious invaders from afar.
Within decades the entire Bronze Age world had collapsed.
Historians have advanced many culprits for the breakdown, including earthquakes
and rebellions. But like our own teetering world—one strained by souring trade
relations, with fractious populaces led by unsteady, unscrupulous leaders and
now stricken by plague—the eastern Mediterranean and the Aegean were
ill-prepared to accommodate the deteriorating climate. While one must resist
environmental determinism, it is nevertheless telling that when the region
mildly cooled and
a centuries-long drought
struck around 1200 B.C., this network of ancient civilizations fell to pieces.
Even Megiddo, the biblical site of Armageddon, was destroyed.
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The Jökulsárlón glacier lagoon in Iceland
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This same story is told elsewhere, over and over, throughout the extremely mild
stretch of time that is written history. The Roman empire’s imperial power was
vouchsafed by centuries of warm weather, but its end saw a return to an arid
cold—perhaps conjured by distant pressure systems over Iceland and the Azores.
In A.D. 536, known as
the worst year to be alive, one of Iceland’s volcanoes exploded, and darkness descended over the Northern
Hemisphere, bringing summer snow to China and starvation to Ireland. In Central
America several centuries later, when the reliable band of tropical rainfall
that rings the Earth
left the Mayan lowlands and headed south, the megalithic civilization above it withered. In North America, a
megadrought about 800 years ago made ancestral Puebloans abandon cliffside
villages like Mesa Verde, as Nebraska was
swept by giant sand dunes and California burned. In the 15th century, a 30-year drought bookended by equally unhelpful deluges
brought the Khmer at Angkor low. The “hydraulic empire” had been fed and
maintained by an elaborate irrigation system of canals and reservoirs. But when
these canals ran dry for decades, then
clogged with rains, invaders easily toppled the empire in 1431, and the Khmer forfeited their
temples to the jungle.
Hopscotching through these human disasters to the present day, we pass perhaps
the most familiar historical climate event of all: the Little Ice Age. Lasting
roughly from 1500 to 1850, the chill made ice rinks of Dutch canals, and swelled
up Swiss mountain glaciers. Tent cities sprung up on a frozen Thames, and George
Washington endured his winter of cold and privation at Valley Forge in 1777
(which wasn’t even particularly harsh for the times). The Little Ice Age might
have been a regional event, perhaps the product of an exceptional run of
sunlight-dimming volcanism. In 1816, its annus horribilis, the so-called year
without a summer—which brought snows to New England in August—global
temperatures dropped perhaps a mere half a degree Celsius. While it is
perennially plumbed by historians for insights into future climate change, it is
not even remotely on the same scale of disruption as that which might lie in our
future.
As Europe emerged from its chill, coal from 300-million-year-old jungles was
being fed into English furnaces. Although the Earth was now in the same
configuration that, in the previous few million years, had invited a return to
deep, unthinkable ice ages, for some reason
the next ice age never took. Instead the planet embarked on an almost unprecedented global chemistry
experiment. Halfway through the 20th century, the climate began behaving very
strangely.
So this is the climate of written history, a seemingly eventful stretch that has
really been the random noise and variability of a climate essentially at peace.
Indeed, if you were to find yourself in an industrial civilization somewhere
else in the universe, you would almost certainly notice such similarly strange
and improbably pleasant millennia behind you. This kind of climate stability
seems to be a prerequisite for organized society. It is, in other words, as good
as it gets.
As we jump back 20,000 years—to yesterday, geologically—the world ceases being
recognizable. Whereas all of recorded history played out in a climate hovering
well within a band of 1 degree Celsius, we now see what a difference 5 to 6
degrees can make—a scale of change similar to the one that humans may engineer
in only the next century or so, though in this case, the world is 5 to 6 degrees
colder, not warmer.
An Antarctica’s worth of ice now rests atop North America. Similar sheets
smother northern Europe, and as a result, the sea level is now 400 feet lower.
The midwestern United States is carpeted in stands of stunted spruce of the sort
that would today look at home in northern Quebec. The Rockies are carved up, not
by wildflower-dappled mountain valleys, but by overflowing rivers of ice and
rock. California is a land of dire wolves. Where the Pacific Northwest edges up
against the American Antarctica, it is a harsh and treeless place. Nevada and
Utah fill up with cold rains.
During World War II, at Topaz, the desolate Japanese American internment camp in
Utah, prisoners combed the flats of the Sevier Desert for unlikely seashells,
fashioning
miraculous little brooches
from tiny mussel and snail shells to while away their exile. The desert
seashells were roughly 20,000 years old, from the vanished depths of the giant
Pleistocene-era Lake Bonneville—the product of a jet stream diverted south by
the ice sheet. This was once a Utahan Lake Superior, more than 1,000 feet deep
in places. It was joined by endless other verdant lakes scattered across today’s
bleak Basin and Range region.
Elsewhere, the retreat of the seas made most of Indonesia a peninsula of
mainland Asia. Vast savannas and swamps linked Australia and New Guinea, and of
course Russia shared a tundra handshake with Alaska. There were reindeer in
Spain, and
glaciers in Morocco. And everywhere loess, loess, and more loess. This was the age of dust.
Ice is a rock that flows. Send it in massive sterilizing slabs across the
continents, and it will quarry mountainsides, pulverize bedrock, and obliterate
everything in its path. At the height of the last ice age, along the crumbling
margins of the continental ice sheets, the rocky, dusty spoils of all this
destruction spilled out onto the tundra. Dry winds carried this silt around the
world in enormous dust storms, piling it up in seas of loess that buried the
central U.S., China, and Eastern Europe under featureless drifts. In Austria,
not far from the site of the voluptuous Venus of Willendorf figurine, carved
some 30,000 years ago, are the remains of a campground of the same age—tents,
hearths, burnt garbage pits, hoards of ivory jewelry—all abandoned in the face
of these violent, smothering haboobs. Ice cores from both Antarctica and
Greenland record a local environment that was 10 times dustier than today. All
of this dust
seeded the seas with iron, a vital nutrient for carbon-hogging plankton, which bloomed around Antarctica
and pulled gigatons of CO
2 out of the air and deep into the ocean,
freezing the planet further.
This parched Pleistocene world would have appeared duller from space, hosting as
it did a quarter less plant life. CO
2 in the atmosphere registered
only a paltry 180 ppm, less than half of what it is today. In fact, CO
2
was so low, it might have been unable to drop any further. Photosynthesis starts
to shut down at such trifling levels, a negative-feedback effect that might have
left more CO
2—unused by plants—in the air above, acting as a brake on
the deep freeze.
This was the strange world of the Ice Age, one that, geologically speaking, is
still remarkably recent. It’s so recent, in fact, that today, most of Canada and
Scandinavia is still bouncing back up from the now-vanished ice sheets that had
weighed those lands down.
The floods carried 30-foot boulders on biblical waves, through what were
suddenly the world’s wildest rapids.
In 2021, we find ourselves in an unusual situation: We live on a world with
massive ice sheets, one of which covers one of the seven continents and is more
than a mile deep. For most of the planet’s past, it has had
virtually no ice
whatsoever. The periods of extreme cold—like the ultra-ancient, phantasmagoric
nightmares of Snowball Earth, when the oceans might have been smothered by ice
sheets all the way to the tropics—are outliers. There were a few other
surprising pulses of frost here and there, but they merely punctuate the balmy
stretches of the fossil record. For almost all of the Earth’s history, the
planet was a much warmer place than it is today, with much higher CO
2
levels. This is not a climate-denying talking point; it’s a physical fact, and
acknowledging it does nothing to take away from the potential catastrophe of
future warming. After all, we humans, along with everything else alive today,
evolved to live in our familiar low-CO
2 world—a process that took a
long time.
How long, exactly? Fifty million years ago, as our tiny mammalian ancestors were
still sweating through the jungly, high-CO
2 greenhouse climate they
had inherited from the dinosaurs, India was nearing the end of an extended
journey. Long estranged from Africa and the august, bygone supercontinent of
Gondwana, the subcontinent raced northeast across the proto–Indian Ocean and
smashed into Asia in slow motion. The collision not only
quieted CO2-spewing volcanoes
along Asian subduction zones; it also thrust the Himalayas and the Tibetan
Plateau toward the stars, to be continually
weathered and eroded away.
As it turns out, weathering rocks—that is, breaking them down with
CO
2-rich rainwater—is one of the planet’s most effective long-term
mechanisms for removing carbon dioxide from the atmosphere, one that modern
geoengineers are frantically trying to reproduce in a lab, for obvious reasons.
Adding to this colossal Himalayan CO
2 sink, the more recent buckling,
tectonic mess that lifted Indonesia and its neighbors from the sea over the past
20 million years or so also exhumed vast tracts of highly weatherable ocean
crust, exposing it all to the withering assault of tropical rainstorms. Today
this corroding rock accounts for roughly
10 percent of the planet’s carbon sink. Over tens of millions of years, then, the stately march of plate
tectonics—the balance of volcanic CO
2 and rock weathering—seems to
have driven long-term climate change, in our case toward a colder, lower-CO
2
world. As we’ll see, humans now threaten to undo this entire epic,
geologic-scale climate evolution of the Cenozoic era—and in only a few decades.
When Earth’s blanket of CO
2 was
finally thin enough, the planet’s regular wobbles were at long last sufficient to trigger deep
glaciations. The ice ages began. But the climate was not stable during this
period. The ice advanced and retreated, and while the descent into the wild
episodes of the Pleistocene epoch could be leisurely—the depths of planetary
winter taking tens of thousands of years to arrive—the leap out of the cold
tended to be sudden and violent. This is where positive feedback loops come in:
When the last ice age ended, it ended fast.
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Glacial ice near the Torfajökull volcano, in Iceland
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Coral reefs marking the ancient sea level—but today lying deep off the coasts of
Tahiti and Indonesia—reveal that about 14,500 years ago, the seas suddenly
jumped 50 feet or so in only a few centuries, as meltwater from the late, great
North American ice sheet raged down the Mississippi. When a 300-foot-deep lake
of glacial meltwater spanning at least 80,000 square miles of central Canada
catastrophically drained into the ocean, it shut down the churn of the North
Atlantic and arrested the seaborne flow of heat northward. As a result, tundra
advanced to retake much of Europe for 1,000 years. But when ocean circulation
kicked back into gear, and the dense, salty seawater began to sink again, the
system rebooted, and currents carried the equator’s heat toward the Arctic once
more. Temperatures in Greenland suddenly leaped 10 degrees Celsius in perhaps a
decade, fires spread, and revanchist forests reclaimed Europe for good.
In Idaho, ice dams that had held back giant lakes of glacial meltwater about six
times the volume of Lake Erie collapsed as the world warmed, and each released
10 times the flow of all the rivers on Earth into eastern Washington. The floods
carried 30-foot boulders on biblical waves, through what were suddenly the
world’s wildest rapids. They left behind a labyrinth of bedrock-scoured canyons
that still covers the entire southeastern corner of the state like a scar. When
the Earth’s climate changes, this is what it can look like on the ground.
As the ice sheets of the Northern Hemisphere finally lost their grip, darker
land around the melting margins became exposed to the sun for the first time in
100,000 years, accelerating the ice’s retreat. Permafrost melted, and methane
bubbled up from thawing bogs. Colder, more CO
2-soluble oceans warmed,
and gave up the carbon they’d stolen in the Ice Age, warming the Earth even
more. Relieved of their glacial burden,
volcanoes in Iceland, Europe, and California awoke, adding even more CO
2 to the atmosphere.
Soon the Sahara would green again, Jericho would be born, and humans would start
writing things down. They would do so with the assumption that the world they
saw was the way it had always been. “We were born only yesterday and know
nothing,” one of them would write. “And our days on earth are but a shadow.”
As we leap back in time again, we emerge before the final Pleistocene
glaciation. We’ve gone tremendously far back, 129,000 years, though in some ways
we’ve only returned to our own world. This was the most recent interglacial
period, the last of many breaks between the ice ages, and the last time the
planet was roughly as warm as it is today. Once more, the seas have risen
hundreds of feet, but something is awry.
As the Earth’s wobble and orbit conspired to melt more ice than the poles have
shed so far today, the planet absorbed more sunlight. As a result, global
temperatures were little more than 1 degree warmer than today’s Anthropocene
chart-toppers—or
maybe even the same. But sea level was 20 to 30 feet higher than it is now. (A full third of
Florida was sunk beneath the waves.) This is “sobering,” as one paper put it.
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The Dallol sulfur springs in the Danakil Depression, Ethiopia, one
of the hottest places on Earth
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Modelers have tried and mostly failed to square how a world about as warm as
today’s could produce seas so strangely high. Provisional, if nightmarish,
explanations like the runaway, catastrophic collapse of monstrous ice cliffs
more than 300 feet tall in Antarctica, which may or may not be set into motion
in our own time,
are fiercely debated
in conference halls and geoscience departments.
Very soon, we may well have
warmed the planet enough to trigger
similarly dramatic sea-level rise, even if it takes centuries to play out. This
is what the Exxon scientist James Black meant in 1977 when he warned higher-ups
of the coming “super-interglacial” that would be brought about—as a matter of
simple atmospheric physics—from burning fossil fuels. But our trajectory as a
civilization is headed well beyond the warmth of the last interglacial, or any
other interglacial period of the Pleistocene, for that matter. So it’s time to
keep moving. We must take our first truly heroic leap into geologic time,
millions of years into the past.
We’re more than 3 million years in the past now, and carbon dioxide in the
atmosphere is at 400 parts per million, a level the planet will not again see
until September 2016. This world is 3 to 4 degrees Celsius warmer than ours, and
the sea level is up to 80 feet higher. Stunted beech trees and bogs line the
foothills of the Transantarctic Mountains not far from the South Pole—the last
members of a venerable line of once-majestic forests that had existed since long
before the age of the dinosaurs.
What we’ve glossed over in our journey back to this ancient present: the entire
evolutionary history of
Homo sapiens, three Yellowstone super-eruptions,
thousands of megafloods, the last of the giant terror birds, a mass extinction
of whales, and the glacial creation and destruction of innumerable islands and
moraines. As we make our way backwards in time to the Pliocene, the glaciations
get briefer, and the ice sheets themselves become thinner and more
temperamental. About 2.6 million years ago they all but disappear in North
America, as
CO2 levels continue their slow climb.
When we arrive in the middle of the Pliocene, just over 3 million years ago,
CO
2 levels are high enough that we’ve escaped the cycle of ice ages
and warm interglacials altogether. Lucy the
Australopithecus roams a
heavily forested East Africa. We are now outside the evolutionary envelope of
our modern world, sculpted as it was by the temperamental northern ice sheets
and deep freezes of the Pleistocene. But as to atmospheric carbon dioxide, 3
million years is how far back we have to go to arrive at an analogue for 2021.
Despite the similarities between our world and that of the Pliocene, the
differences are notable. In the Canadian High Arctic—where today tundra spreads
to the horizon—evergreen forests come right to the edge of an ice-free Arctic
Ocean. Though the world as a whole is only a few degrees warmer, the Arctic, as
always, gets the brunt of the extra heat. This is called “polar amplification,”
and it’s why maps of modern warming are crowned by a disturbing fog of maroon.
Models struggle to reproduce the extreme level of warming in the Pliocene
Arctic. It’s a full 10 to 15 degrees Celsius warmer in the long twilight of
northern Canada, and the pine and birch woodlands of these Arctic shores are
filled with
gigantic forest-dwelling camels. Occasionally this boreal world erupts in wildfire, a phenomenon echoed by the
blazes that today sweep ever farther north. Elsewhere, West Antarctica’s ice
sheet may have disappeared entirely, and Greenland’s, if it exists at all, is
shriveled and pathetic.
A common projection for our own warming world is that, while the wet places will
get wetter, the dry places will get drier. But the Pliocene seems to defy this
saw for reasons not yet fully understood. It’s
a strangely wet world, especially the subtropics, where—in the Sahara, the Outback, the Atacama, the
American Southwest, and Namibia—lakes, savannas, and woodlands replace deserts.
This ancient wetness might come down to
inadequacies in how we model clouds, which are under no obligation to behave in physical reality as they do in
simplified lines of computer code. Hurricanes were almost certainly more
consistently punishing 3 million years ago, just as our storms of the future
will be. And a more sluggish circulation of the atmosphere might have lulled the
trade winds, turning El Niño into “El Padre.” Perhaps this is
what brought rains—and lakes—to the Mojave
at this time.
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Angeles National Forest, California
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Our modern coastlines would have been so far underwater that you’d have to take
great pains to avoid getting the bends if you tried scuba diving down to them.
Today, traveling east through Virginia, or North or South Carolina, or Georgia,
midway through your drive you’ll pass over a gentle 100-foot drop. This is the
Orangeburg Scarp, a bluff—hundreds of miles long—that divides the broad, flat
coastal plain of the American Southeast. It comprises the eroded and
smoothed-out rumors of once-magnificent sea cliffs. Here, waves of the Pliocene
high seas chewed away at the middle of the Carolinas—an East Coast Big Sur. This
ancient shoreline is visible from space by the change in soil color that divides
the states, and is visible on much closer inspection as well: To the east of
this strange drop-off, giant megalodon-shark teeth and whale bones litter the
Carolina Low Country. Though warped over the ages by the secret workings of the
mantle far below, these subtle banks 90 miles inland nevertheless mark the
highest shoreline of the Pliocene, when the seas were dozens of feet higher than
they are today. But even within this warm Pliocene period, the sea level leaped
and fell by as much as 60 feet every 20,000 years, to the rhythm of the Earth’s
sway in space. This is because, under this higher-CO
2 regime, the
unstable ice sheet in Antarctica
took on the volatile temperament
that, 1 million years later, would come to characterize North America’s ice
sheet, toying with the ancient coastline as if it were a marionette.
So this is the Pliocene, the world of the distant present. While today’s
projections of future warming tend to end in 2100, the Pliocene illuminates just
what sort of long-term changes might inevitably be set in motion by the
atmosphere we’ve already engineered. As the great ice sheets melt, the
permafrost awakens, and darker forested land encroaches on the world’s tundra,
positive feedbacks may eventually launch our planet into a different state
altogether, one that might resemble this bygone world. Nevertheless, human
civilization is unlikely to keep atmospheric CO
2 at a Pliocene
level—so more ancient and extreme analogues must be retrieved.
We’re now deeper in the past, and the planet appears truly exotic. The Amazon is
running backwards, and gathers in great pools at the foot of the Andes. A seaway
stretches from Western Europe to Kazakhstan and spills into the Indian Ocean.
California’s Central Valley is open ocean.
What today is the northwestern U.S. is especially unrecognizable. Today the
airy, columnated canyons of the Columbia River in Oregon swarm with tiny
kiteboarders zipping through gorges of basalt. But 16 million years ago, this
was a black, unbreathable place, flowing with rivers of incandescent rock. The
Columbia River basalts—old lava flows that spread across Washington, Oregon, and
Idaho, in some places more than two miles thick—were the creation of a class of
extremely rare and world-changing volcanic eruptions known as large igneous
provinces, or LIPs.
Some LIPs in Earth’s history span millions of square miles, erupt for millions
of years, inject tens of thousands of gigatons of CO
2
into the air, and are responsible for
most of the worst mass extinctions in the history of the planet. They live up to their name—they are large. But these mid-Miocene eruptions
were still rather small as far as LIPs go, and so the planet was spared mass
death. Nevertheless,
the billowing volcanoes
raised atmospheric CO
2 up to about 500 ppm, a level that today
represents something close to the most ambitious and optimistic scenario
possible for limiting our future carbon emissions.
In the Miocene, this volcanic CO
2 warmed up the world to at least 4
degrees Celsius and perhaps as much as 8 degrees above modern temperatures. As a
result, there were turtles and parrots in Siberia. Canada’s Devon Island, in the
high Arctic, is today a desolate wasteland, the largest uninhabited island in
the world—and one used by NASA to simulate life on Mars. In the Miocene, its
flora resembled Lower Michigan’s.
The sweeping grasslands distinctive to our cooler, drier, low-CO
2
world had yet to take over the planet, and so forests were everywhere—in the
middle of Australia and Central Asia and Patagonia. All of this vegetation was
one of the reasons it was so warm. Forests and shrubs made this planet darker
than our own world—one still painted pallid hues in many places by bare land and
ice—and allowed it to absorb more heat. This change in the planet’s color is
just one of the many long-term feedback loops awaiting us after the ice melts.
Long after our initial pulse of CO
2, they will make our future world
warmer and more alien still.
As for fauna, we’re now so distantly marooned in time from our own world that
most of the creatures that inhabited this leafy planet range from the flatly
unfamiliar to the uncannily so. There were big cats that weren’t cats, and
rhino-size “hell pigs” that weren’t pigs. There were sloths that lived in the
ocean and walruses that weren’t related to today’s walruses. Earth’s
largest-ever meat-eating land mammals, African juggernauts like Megistotherium
and Simbakubwa, not closely related to any living mammals today, tore early
elephants apart with bladed mouths.
And with CO
2 at 500
ppm, the sea level was about 150 feet
higher than today. Approaching Antarctica in the middle Miocene by sea, the
waters would be warmer than today, and virtually unvisited by ice. To get to the
ice sheet, you’d have to hike far past lakes and forests of conifers that lined
the coast. Trudging past the trees and finally over endless tundra, you would
come at last to the edge of a much smaller ice sheet whose best days were still
ahead of it. An axiom about this land-based Antarctic ice sheet in
paleoclimatology is that it’s incredibly stubborn. That is, once you have an ice
sheet atop the heart of Antarctica, feedback loops kick in to make it
exceedingly hard to get rid of. Barring true climatic madness, a land-based
Antarctic ice sheet is essentially there to stay.
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Clouds in Death Valley, California
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But in the middle Miocene this young Antarctic ice sheet seemed to have a
temper. It might have been “surprisingly dynamic,” as one paper cheerfully puts
it. As CO
2 increased from just below today’s levels up to about 500
ppm, Miocene Antarctica shed what today would amount to 30 to 80 percent
of the modern ice sheet. In the Miocene, Antarctica seemed exquisitely tuned to
small changes in atmospheric CO2, in ways that we don’t fully understand and that we’re not incorporating into
our models of the future. There will undoubtedly be surprises awaiting us in our
high-CO
2 future, just as there were for life that existed in the
Miocene. In fact, the Antarctic ice sheet may be more vulnerable today to rapid
retreat and disintegration than
at any time in its entire 34-million-year history.
In the 16 million years since this mid-Miocene heat, the volcanic hot spot
responsible for the Columbia River basalts has wandered under Yellowstone. Today
it powers a much tamer kind of volcano. It could cover a few states in a few
inches of ash and disrupt global agriculture for years, but it couldn’t launch
the planet into a new climate for hundreds of thousands of years, or kill most
life on the surface. Unfortunately, there is such a supervolcano active on Earth
today: industrial civilization. With CO
2 likely to soar past 500
ppm from future emissions, even the sweat-soaked,
Siberian-parrot-populated world of the middle Miocene might not tell us
everything we need to know about our future climate. It’s time to go back to a
global greenhouse climate that ranks among the warmest climate regimes complex
life has ever endured. In our final leap backwards, CO
2 at last
reaches levels that humans might reproduce in the next 100 years or so. What
follows is something like a worst-case scenario for future carbon emissions. But
these worst-case projections have
continued to prove stubbornly accurate in the 21st century so far, and they remain a possible path for our future.
We’re now about to take our largest leap, by far, into the geologic past. We
hurdle over 40 million years of history, past volcanic eruptions thousands of
times bigger than that of Mount St. Helens, past an asteroid impact that punched
out a gigantic crater where the Chesapeake Bay sits today. The Himalayas slump;
India unhitches from Asia; and the further back we go, the higher the CO
2
level rises and the warmer the Earth gets. The Antarctic ice sheet, in its death
throes, vanishes altogether, and the polar continent instead gives way to monkey
puzzle trees and marsupials. We have arrived, finally at the end of our journey,
in the greenhouse world of the early age of mammals.
Today the last dry land one steps on in Canada before setting out across the
ice-choked seas for the North Pole is Ellesmere Island, at the top of the world.
But once upon a time there was a rainforest here. We know this because tree
stumps still erode out of the barren hillsides, and they’re
more than 50 million years old. They’re all that’s left of an ancient polar jungle now whipped by indifferent
Arctic winds. But once upon a time, this island was a swampy cathedral of
redwoods, whose canopy naves were filled with flying lemurs, giant salamanders,
and hippolike beasts that pierced the waters. At this polar latitude, on some
late-fall evening of the early Eocene, the sun tried and failed to lift itself
from the horizon. A pink twilight reached deep into the jungle, but soon the sun
would set entirely here for more than four months. In this unending Arctic dark,
the stillness would be broken by the orphaned calls of tiny early primates, who
hopped fearlessly over stilled alligators that would start moving again when the
sun returned from beyond the horizon. In this unending night, tapirs hunted for
mushrooms and munched on leaf litter that was left over from sunny days past and
that in the far future would become coal.
Humans now threaten to undo the entire climate evolution of the Cenozoic
era—and in only a few decades.
We have no modern analogue for a swampy rainforest teeming with reptiles that
nevertheless endures months of Arctic twilight and polar night. But for each
degree Celsius the planet warms, the
atmosphere holds about 6 percent more water vapor, and given that global temperatures at the beginning of the age of mammals
were roughly 13 degrees warmer than today, it’s difficult to imagine how
uncomfortable this planet would be for Ice Age creatures like ourselves. In
fact, much of the planet would be rendered off-limits to us, far too hot and
humid for human physiology.
Not only was this a sweltering age, but it was also one cruelly punctuated by
some of the most profound and sudden CO
2-driven global-warming events
in geologic history—on top of this already feverish baseline. Deep under the
North Atlantic, the Eocene epoch kicked off in style 56 million years ago with
massive sheets of magma that spread sideways through the crust, igniting vast,
diffuse deposits of fossil fuels at the bottom of the ocean. This ignition of
the underworld injected something like the carbon equivalent of all currently
known fossil-fuel reserves into the seas and atmosphere in less than 20,000
years, warming the planet by another 5 to 9 degrees Celsius. Evidence abounds of
violent storms and megafloods during this ancient spasm of climate
change—episodic waves of torrential rains unlike any on Earth today. In some
places, such storms would have been routine, separated by merciless droughts and
long, brutal, cloudless heat waves. Seas near the equator may have been almost
as hot as a Jacuzzi—too hot for most complex life. As for the rest of the
planet, all of this excess CO
2 acidified the oceans, and the world’s
coral reefs collapsed. Ocean chemistry took 200,000 years to recover.
The most jarring thing about the early age of mammals, though, isn’t merely the
extreme heat. It’s the testimony of the plants. In higher-CO
2
conditions, plants reduce the number of pores on their leaves, and fossil leaves
from the jungles of the early Eocene have tellingly fewer pores than today’s. By
some estimates, CO
2 50 million years ago was
about 600 ppm. Other proxies point to higher CO
2, just over 1,000 ppm, but even
that amount has long bedeviled our computer models of climate change. For years,
in fact, models have told us that to reproduce this feverish world, we’d need to
ramp up CO
2 to
more than 4,000 ppm.
This ancient planet is far more extreme than anything being predicted for the
end of the century by the United Nations or anyone else. After all, the world
that hosted the rainforests of Ellesmere Island was 13 degrees Celsius warmer
than our own, while the current global ambition, enshrined in the Paris
Agreement, is to limit warming to less than 2, or even 1.5, degrees. Part of
what explains this glaring disparity is that most climate projections end at the
end of the century. Feedbacks that might get you to Eocene- or Miocene-level
warmth play out over much longer timescales than a century. But the other, much
scarier insight that Earth’s history is very starkly telling us is that we have
been missing something crucial in the models we use to predict the future.
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Mount Ruapehu and Mount Ngauruhoe volcanoes, in New Zealand
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Some of the models are starting to catch up. In 2019, one of the most
computationally demanding climate models ever run, by researchers at the
California Institute of Technology, simulated global temperatures suddenly
leaping 12 degrees Celsius by the next century if atmospheric CO
2
reached 1,200
ppm—a very bad, but not impossible, emissions pathway. And
later that year, scientists from the University of Michigan and the University
of Arizona were similarly able to
reproduce the warmth of the Eocene
by using a more sophisticated model of how water behaves at the smallest scales.
The paleoclimatologist Jessica Tierney thinks the key may be the clouds. Today,
the San Francisco fog reliably rolls in, stranding bridge towers high above the
marine layer like birthday candles. These clouds are a mainstay of west coasts
around the world, reflecting sunlight back to space from coastal California and
Peru and Namibia. But under higher-CO
2 conditions and higher
temperatures, water droplets in incipient clouds could get bigger and rain down
faster. In the Eocene, this might have caused these clouds to fall apart and
disappear—inviting more solar energy to reach, and warm, the oceans. That might
be why the Eocene was so outrageously hot.
This sauna of our early mammalian ancestors represents something close to the
worst possible scenario for future warming (although some studies claim that
humans, under truly nihilistic emissions scenarios, could make the planet even
warmer). The good news is the inertia of the Earth’s climate system is such that
we still have time to rapidly reverse course, heading off an encore of this
world, or that of the Miocene, or even the Pliocene, in the coming decades. All
it will require is instantaneously halting the super-eruption of CO
2
disgorged into the atmosphere that began with the Industrial Revolution.
We know how to do this, and we cannot underplay the urgency. The fact is that
none of these ancient periods is actually an apt analogue for the future if
things go wrong. It took millions of years to produce the climates of the
Miocene or the Eocene, and the rate of change right now is almost unprecedented
in the history of animal life.
Humans are currently injecting CO
2 into the air 10 times faster than
even during the most extreme periods within the age of mammals. And you don’t
need the planet to get as hot as it was in the early Eocene to catastrophically
acidify the oceans. Acidification is all about the rate of CO
2
emissions, and we are off the charts. Ocean acidification could reach
the same level it did 56 million years ago by later this century,
and then keep going.
When he coined the term
mass extinction in a 1963 paper, “Crises in the
History of Life,” the American paleontologist Norman Newell posited that this
was what happened when the environment changed faster than evolution could
accommodate. Life has speed limits. And in fact, life today
is still trying to catch up
with the thaw-out of the last ice age, about 12,000 years ago. Meanwhile, our
familiar seasons are growing ever more strange: Flycatchers arrive weeks after
their caterpillar prey hatches; orchids bloom when there are no bees willing to
pollinate them. The early melting of sea ice has driven polar bears ashore,
shifting their diet from seals to goose eggs. And that’s after just 1 degree of
warming.
Subtropical life may have been happy in a warmer Eocene Arctic, but there’s no
reason to think such an intimately adapted ecosystem, evolved on a greenhouse
planet over millions of years, could be reestablished in a few centuries or
millennia. Drown the Florida Everglades, and its crocodilians wouldn’t have an
easy time moving north into their old Miocene stomping grounds in New Jersey,
much less migrating all the way to the unspoiled Arctic bayous if humans
re-create the world of the Eocene. They will run into the levees and
fortifications of drowning Florida exurbs. We are imposing a rate of change on
the planet that has almost never happened before in geologic history, while
largely preventing life on Earth from adjusting to that change.
Taking in the whole sweep of Earth’s history, now we see how unnatural,
nightmarish, and profound our current experiment on the planet really is. A
small population of our particular species of primate has, in only a few
decades, unlocked a massive reservoir of old carbon slumbering in the Earth,
gathering since the dawn of life, and set off on a global immolation of Earth’s
history to power the modern world. As a result, up to half of the tropical coral
reefs on Earth have died, 10 trillion tons of ice have melted, the ocean has
grown 30 percent more acidic, and global temperatures have spiked. If we keep
going down this path for a geologic nanosecond longer, who knows what will
happen? The next few fleeting moments are ours, but they will echo for hundreds
of thousands, even millions, of years. This is one of the most important times
to be alive in the history of life.
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