by Adam Frank
The work was led by the Danish scientist Willi Dansgaard and the American geophysicist Chester Langway. The mile-thick slab of ice covering Greenland is maintained by yearly layers of snowfall, packed one on top of the other. The strata of ice, built up year by year over the millennia, form a kind of frozen layer cake. Each layer comprises a record of that year’s climate. Within each layer of ice was a chemical marker that served as a proxy thermometer. Using it, scientists built a high-resolution recording of Greenland’s temperatures going back thousands of years.13
After six relentless years of work, Dansgaard, Langway, and their Camp Century team drilled all the way down to bedrock, more than four thousand feet below the top of the ice sheet. Once the “ice core” data retrieved by the drilling was converted into temperatures, Dansgaard and his colleagues could see Earth’s passage out of the last ice age. Moving backward, they first saw a period of roughly constant temperature stretching back eight thousand years. This was the Holocene, the time during which human civilization had been born and grown to thrive. Going farther backward, they could also see the transition from the warmth of our current climate to the frozen glacial age more than ten thousand years ago (the Pleistocene).14
Along with the smooth transition from the last ice age to the current warm interglacial period, the Camp Century data also showed a series of spectacular short-term shifts that would come to haunt our climate future. Around twelve thousand years ago, in a period called the Younger Dryas, the planet appeared to drop from a warming state back into the icebox. It was a stunning discovery. In just a matter of decades, average temperatures around the planet had dropped by five degrees Fahrenheit in some places and as much as twenty-seven degrees in others.15 If comparably dramatic global changes occurred in the modern era, it’s hard to imagine our project of civilization making it through intact.
Later drilling work in Greenland and Antarctica confirmed the Camp Century studies. One American researcher working in Antarctica recalls a moment of truth when just looking at an ice core made the speed of climate change apparent. The ice changing from light to dark across just a few inches in the core was a visceral confirmation of abrupt large-scale swings in global climate.
The history of Greenland temperatures based on ice core records.
The recognition of rapid climate change presented researchers with a warning the importance of which they could not yet understand. At the time, human-driven, or “anthropogenic,” climate change was nothing more than a possibility discussed in the most abstract terms at meetings of scientific experts. Almost no one was ready to conclude that the kind of rapid climate shift seen twelve thousand years ago might be something we could drive through our own actions.
WHICH EARTH?
Whatever quiet preparations were going on in homes across the Earth, William Anders was not part of them. That’s because Anders was on a spaceship. On Christmas Eve 1968, two hundred thousand miles from the planet of his birth, Anders and fellow Apollo 8 astronauts Frank Borman and James Lowell were becoming the first humans to orbit the Moon.
“Oh my God,” Anders said to his crewmates as he marveled at the view outside the small window of his Apollo command module. “Here’s the Earth coming up,” he said, looking out across the moon’s horizon. “Wow, is that pretty.”
Anders asked for a roll of color film while Borman joked, “Hey don’t take that [picture], it’s not scheduled.” Loading up the camera, Anders stopped for a moment to consider the magnitude of the vision before him. Then he snapped an image of the world that would change the world.16
The iconic Earthrise photograph taken by William Anders during the Apollo 8 mission in 1968.
Called Earthrise, Anders’s picture of the blue Earth hanging above the gray moonscape became iconic. Life magazine named it one of the one hundred most influential images in human history.17 Since then, space-based pictures of azure oceans, swirling white clouds, and green-brown continents have become familiar. But that familiarity is undercut by a striking truth that has been emerging since the time of Camp Century: the planet we know today is not the Earth that was. If you had visited our world one hundred million, five hundred million, or three billion years ago, you would have found a planet that looked very different from Anders’s image.
Exhaustive work going back to the 1800s has allowed geologists and paleontologists to construct a timeline of our world’s history. But only in the last half century or so has that timeline been resolved into the details of planetary change. There are four long eons in the Earth’s history, representing the most important transitions in the planet’s climate and life. These eons are subdivided into eras, which are further divided into periods and epochs. The Pleistocene and Holocene, whose transitions were revealed by Camp Century ice cores, are examples of epochs.18
The planet’s story begins with an unnamed cloud of interstellar gas and dust. Almost five billion years ago, that slowly spinning cloud, close to a light-year across, collapsed under its own weight. The Sun formed at the center of the infalling mass, and a rapidly spinning disk surrounding the young star emerged as well. Within this dense disk, particles of dust began colliding frequently enough to form free-floating pebbles. Those pebbles then collided to form rock-sized objects. The rocks then collided to form boulders, and so on, all the way up to asteroid-sized planetesimals. After between ten million and a hundred million years, gravity drew the planetesimals together and assembled the Earth and other rocky planets (Mercury, Venus, and Mars).19
This was the beginning of the Hadean, Earth’s first eon. Lasting from 4.6 billion to 4 billion years ago, its name speaks to the planet’s hellish conditions. Earth during the early Hadean was covered in a globe-spanning sea of molten rock. Eventually, this magma ocean cooled and hardened, forming a solid surface. But asteroids and comets continued to rain down on the planet, ending in a period called the Late Heavy Bombardment, when our solar system cleared itself of planetary construction debris. Each of these apocalyptic impacts shattered the surface, turning some or all of it back into molten rock. Gases released from the bombardment and the magma oceans it regenerated left the Hadean Earth with an atmosphere composed mostly of nitrogen and carbon dioxide.20
Thus, the Earth was once a fire world of molten seas.
The planet’s first forms of life may have emerged by the end of the Hadean. The repeated asteroid bombardments would, however, have sterilized the world, forcing biology to potentially start over and over again.21 Either way, by the beginning of the next eon—the Archean—the kind of life we know today was already in place. The Archean lasted from 4 billion to 2.5 billion years ago. It was during this vast span of time that life based on the biochemistry of self-replicating molecules called DNA spread across the world. But in the Archean, all life consisted of simple, single-celled organisms living in the oceans. The reason for this watery fixation was simple: the whole planet was pretty much an ocean.22
While continents now cover about 30 percent of the Earth’s surface, during the Archean they had yet to “grow.” The ground you stand on today is composed of granite that is less dense than the black volcanic basalt making up the ocean floor. Granite is formed deep within the Earth’s mid-layers (called the mantle). Like warm air in a cold room, granite rises slowly upward as it forms, allowing it to become separate from the more dense ocean crust. While there remains controversy about the process, many scientists believe that during the Archean the continent making was still beginning. Rather than planet-spanning continents, the world hosted just one or two proto-continents called cratons. Each craton was smaller than India is today.
Thus, the Earth was once a water world of almost endless ocean.
Life slowly explored new domains of structure and metabolism as the Archean gave way to the Proterozoic eon, lasting from 2.5 billion to half a billion years ago. The earliest cells on Earth had been relatively simple affairs. Called prokaryotes, they include modern-day bacteria. The first prokaryotes lived by breaking down complex molecul
es into simpler structures (basically fermentation). The evolution of early forms of photosynthesis had, however, given some prokaryotes the ability to draw energy directly from sunlight. These were the earliest forms of photosynthesis, whereby cells use sunlight to generate food.23
By the beginning of the Proterozoic eon, life had learned new, more efficient photosynthetic strategies. Some of these came from the development of a wider range of internal machinery, like a cellular nucleus to hold the genetic blueprints of the cell. The emergence of these nucleus-bearing eukaryotic cells changed life’s trajectory on the planet. With the addition of new forms of photosynthesis, more energy became available to cells, allowing them greater flexibility and adaptation. The first multicellular organisms appeared during the Proterozoic, as life began to experiment with the division of labor. Cells specialized into different forms that worked together. Left without the larger organism, however, these specialized cells would die.24
Along with the changes in life, the planet itself was changing. During the billion-year-plus stretch of the Proterozoic, the first cratons grew into full-sized continents. Eventually, the slow movement of the Earth’s crustal plates (plate tectonics) drew them together to form a supercontinent, a vast landmass called Rodinia. Other supercontinents would form and break apart over the course of Earth’s history. Each would change the planet’s climate by altering global ocean circulation and resetting patterns of rock weathering and CO2 cycling.25
Perhaps the most important climate shifts to come during the Proterozoic were the first periods of near-total glaciation. At least four times during this eon, changes in the concentrations of atmospheric greenhouse gases plunged planetary temperatures into the freezer. From the poles all the way to the equator, the entire planet may have become locked in miles-thick layers of ice.26 Seen from space, this snowball world would have appeared as a mottled and cracked Ping-Pong ball with no large expanses of open blue water.
Thus, the Earth was once a snowball world of endless ice.
For all the changes Earth experienced, none was more remarkable or mysterious than life’s sudden burst of creativity just after the Phanerozoic eon began 540 million years ago. Across a remarkably short span of geological time, evolution threw itself a party. What began as still-simple multicellular life rapidly diversified into an orgy of new forms and new species. In just fifty million years, evolution produced all the basic structures that mark life on Earth today. Called the Cambrian Explosion (it occurred during the Cambrian geologic era), it was an evolutionary acceleration on a scale never seen before or after.27
It was only after the Cambrian era that all the “prehistoric” worlds we know from popular fictions arose. There was the Carboniferous era three hundred million years ago, with its vast swamp forests. Those forests eventually became the coal beds we’ve used to power our project of civilization.28 There was also the Jurassic era, dominated by the huge dinosaurs that live on in movies and the dreams of little kids. And finally, there was the more recent cycling of ice ages and interglacial periods, during which we humans appeared and eventually flourished.
The Earth swung back and forth between many versions of itself during the fecund eon of the Phanerozoic. But of particular interest to our own age are the periods when the planetary thermometer rose to fever levels.
Fifty-five million years ago, the supercontinent called Pangaea began splitting apart. The volcanism that accompanies plate tectonics went into overdrive, dumping CO2 into the atmosphere far faster than it could be removed by natural feedbacks. Global average temperatures rose fourteen degrees Fahrenheit above what we experience today. Called the Paleocene-Eocene Thermal Maximum, the result was a planet almost without ice.29 Temperatures in Greenland, where a future Soren Gregersen would endure his subzero glacial summers, stayed at a balmy 70 degrees Fahrenheit.
Thus, the Earth was once a jungle world, a sweltering hothouse planet devoid of snow.
Given the scale of the Earth’s changes between one mask and another, the next question we should ask seems clear. What force was powerful enough to drive our world’s dramatic transformations?
THE GREAT OXIDATION EVENT
The engineer asks Donald Canfield if he is claustrophobic. Canfield, a professor of ecology, has just squeezed himself into the cramped confines of Alvin, the world’s most famous deep-sea submersible. It’s a fall day in 1999 on a research ship slowly rolling in the Gulf of California’s blue waters.
“Claustrophobic? No, not at all,” Canfield says, lying enough to make them both feel better.
The engineer flashes him a knowing smile and says, “Good . . . whatever you do, don’t touch the red handle. It’s only for emergencies.”30 The hatch slams closed.
After an hour-long descent, Canfield is skimming along on the floor of the Guaymas Basin in the Gulf of California, more than fifty miles east of the Baja Peninsula and over a mile below the surface. The basin is a “spreading zone” where two of Earth’s continental plates are pulling apart.31 As the plates move away from each other, they carry the Baja Peninsula away from mainland Mexico at a rate of about one inch per year, the same rate as your fingernails grow.32 In between the spreading plates, new seafloor crust is constructed as hot magma upwells from deeper within the planet, cools, and then hardens into solid rock.
From the circular observing port cut into Alvin’s six-foot titanium crew capsule, Canfield gets his first view of the basin’s floor. Far from the well-lit upper ocean, it’s an alien world laid out before him.
“All around us,” Canfield recalls in his book, Oxygen, “We see the effervescence of hot [sulfur]-rich, hydrothermal waters percolating from the accumulating crust.” Boiling water, heated by the Earth’s internal fury, rises in dark columns from the vents. High-temperature geology is, however, only one facet of the otherworldly vision in front of Canfield. Remarkably, life is thriving here in the heat and the darkness. “Great mounds of Riftia tubeworms rise from the shadows swaying gently on expansive hills of gypsum crust,” he writes.33 The enormous tubeworms have no color—none is needed in this world of perpetual darkness.
Everywhere, Canfield makes out what appears to be fallen snow on the gypsum-crusted seafloor. What he sees, however, is not snow, but bacteria. The abundant microscopic creatures draw their energy from the heat and sulfur-based compounds spilling from the hydrothermal vents.34 Their ability to thrive in such an extreme environment is what allows the whole strange ecosystem laid out before Canfield to exist.
Canfield made this trip to the ocean floor to gain insights into the Earth’s past in terms of an alternative biochemistry. What he found at the bottom of the Guaymas Basin were hints pointing to versions of life that need no sunlight. These are, perhaps, vestiges of an early incarnation of the planet before its most significant transformational event: the rise of oxygen in the Great Oxidation Event.
“Try to imagine something so profound, so fundamental, that it changed the whole world,” Canfield writes. “Think of something so revolutionary, that it forever changed the chemistry of the atmosphere, the chemistry of the oceans and the nature of life itself.”35
After posing this question, Canfield surveys the critical moments in human history: the Great Plague, the Renaissance, and World War II. “These were important events,” he writes. “But their influence outside the human realm was small.” He then goes on to consider the extinction event sixty-five million years ago that killed the dinosaurs, and the one 250 million years ago that took down almost 95 percent of all animal species on the planet. Even those events pale in comparison to Canfield’s target. “Each of these major extinctions changed the course of animal evolution, but still, they did not fundamentally alter the fabric of life or surface chemistry of Earth.”36 What, he asks, did so completely transform the Earth? The answer to Canfield’s question turns out to be as simple as drawing a breath.
During Earth’s earliest eras, much of biology may have been powered by chemistry akin to what Canfield saw on his dive. By the mi
ddle of the Archean, however, at least some single-celled organisms had figured out how to tap a new and abundant energy source: sunlight. The first emergence of photosynthetic organisms in the form of what scientists call anoxygenic phototrophs (non-oxygen-producing sunlight eaters) was a major innovation in the history of life. Through the remarkable trial and error of evolution (and lots of time), some bacteria developed molecular light receptors. These were nanoscale machines that absorbed energy from the Sun and used it to power chemical reactions that popped out sugar molecules. Sugar, in whatever form, is the basic chemical battery for all the metabolic shenanigans cells need to stay alive.37
After a billion or so years of non-oxygen-producing photosynthesis, nature got very creative. Sometime in the late Archean, evolution produced a new version of photosynthesis that, for the first time, used water to drive its chemistry. Because water is superabundant on Earth, cells using this new kind of photosynthesis won out over the older forms. But these organisms—called cyanobacteria, or blue-green algae—did more than just multiply. Sucking in water, CO2, and sunlight, they also started spitting out molecules of oxygen as a kind of waste product of their activity.38 In this way, their innovative water-eating, light-powered, oxygen-producing metabolism led them to become the most powerful force in the history of the planet.
