by Lee Billings
The rise of vascular land plants caused so much carbon sequestration during the late Devonian and early Carboniferous that atmospheric CO2 levels plummeted. The diminished greenhouse effect dropped global temperatures by only a few degrees, but that seemingly slight change was enough to tip the world into a long-term ice age. Ice caps formed and grew at the poles as cooler summers failed to melt the accumulated snows of previous winters. The bright white spreading glaciers reflected more sunlight into space than the darker lands and seas, driving temperatures lower still. On average, every few tens of thousands of years or so, glaciers advanced from the poles into lower latitudes, locking water in their icy clutches to reduce global sea levels and turn climates more arid. Each time, terrestrial species in polar and temperate latitudes were forced down to the tropics ahead of advancing walls of ice four kilometers (two and a half miles) high. Each time, falling sea levels exposed the life-packed continental shelves to open air, disrupting marine ecosystems. Inevitably the glacial advance would wane, the walls of ice would retreat to the poles, and marine and terrestrial life would once again thrive in an interglacial period.
For a hundred million years, throughout the Carboniferous and most of the following geological period, the Permian, Earth’s ice caps endured, occasionally sending glaciers down from the poles. Those ice caps finally melted away around 260 million years ago, when increased volcanic activity and decreased oceanic absorption of carbon rapidly pumped atmospheric CO2 back to mid-Devonian levels. Abundant polar ice would not return to our planet until around 35 million years ago. Those polar ice sheets expanded just over two and a half million years ago, when the outpourings of undersea volcanoes formed the Isthmus of Panama and sutured together North and South America, creating new oceanic and atmospheric circulation patterns that further lowered global temperatures. This occurred at the dawn of the Quaternary Period, a span of time that, at its tail end, would give rise to anatomically modern humans. Since the Quaternary’s beginning, and even today, with Antarctica and Greenland still locked in ice, the Earth has technically been in an ice age. That this is a rather remarkable state of affairs has only very recently come to be appreciated. Polar ice caps, despite their presence for the entirety of human history, are surprisingly infrequent occurrences in the history of Earth. As far as geologists can discern, over the course of its 4.5-billion-year existence, ice caps have graced our planet’s poles for only a sum total of about 600 million years—about an eighth of the Earth’s life thus far.
In our present ice age, glacial walls of ice repeatedly pulsed from the Arctic to cover the sites of modern-day Toronto, New York City, and Chicago, as well as much of northern Pennsylvania. They carved out Hudson Bay and the Great Lakes, and at their edges spat out glacial moraines—chunks of broken land that became places such as Long Island and Cape Cod. The glaciers last retreated some 12,000 years ago, at the beginning of an interglacial epoch we call the Holocene. The rise of agriculture, cities, commerce, industry, science, and technology that we recognize as human civilization and chronicle as human history has all occurred within the abnormally mild and stable Holocene interglacial, the climatic equivalent of a twelve-thousand-year summer.
The signs of glacial advance and retreat can be tracked in sedimentary rocks and isotopic analysis of seawater, but some of the most high-fidelity evidence of climate oscillations comes from within the glaciers themselves, in bubbles of trapped, ancient air. Found in ice cores extracted from today’s melting glaciers, each bubble is a snapshot of the atmosphere on a day in the distant past, when a minuscule puff of air was trapped in fresh-fallen snow that became part of the ice. The oldest bubbles are of impressive vintage—detailed analysis revealed they formed some 800,000 years ago. In aggregate, the gas in the bubbles tracked the changing composition of Earth’s atmosphere for the majority of the past million years, long before modern humans ambled onto the planetary scene. The bubbles display a clear pattern that connects greenhouse-gas levels to glaciation: when glaciers were advancing, out of every million molecules of air trapped in their ice approximately 200 were CO2. When glaciers were retreating, the amount of CO2 in ice-locked air rose to about 300 parts per million (ppm).
Since the last glacial retreat at the beginning of the Holocene, circa 10,000 B.C., the average amount of CO2 in the atmosphere had held steady at around 275 ppm. Glacial ice cores show that value beginning to climb at the turn of the nineteenth century, just as the world’s population of humans reached one billion. Not coincidentally, this was when societies were embroiled in the Industrial Revolution, powering newly commercialized steam engines by burning ancient stores of coal. In all the time since, as populations surged and technology spread, CO2 levels continued to sharply rise. By 1950, there were 2.5 billion people on the planet, and atmospheric CO2 levels had surpassed 300 ppm—a level now considered the approximate threshold beyond which ice-age conditions cannot prevail. By 1980, the world held 4.5 billion people, and atmospheric CO2 was 340 ppm.
By 2000, with the world host to more than 6 billion people, and with CO2 at 370 ppm and rising some 2 ppm per year, a Nobel Prize–winning atmospheric chemist, Paul Crutzen, could no longer convince himself that he still lived in the Holocene. He had received his Nobel in 1995, based on his work clarifying how trace emissions of exotic gases—man-made refrigerants called chlorofluorocarbons—had in the latter half of the twentieth century eaten gaping holes in the planet’s protective atmospheric layer of ozone. The ozone hole was only one small part of a much larger trend, Crutzen believed. Aided by engines and turbines, amplified by fossil carbon and petrochemical fertilizers, humans had commandeered huge swaths of the planet’s flows of energy and nutrients, channeling them to new purposes and altering global geochemistry. The resulting growth was exponential, and—at least if confined to a single planet—unavoidably transitory. Geological history’s darkest passages began to echo through his mind.
Evidence of the world’s remaking was everywhere before his eyes. He could see it in the dissipating stratospheric ozone, the disappearing polar ice, and the thawing tundra permafrost. He glimpsed human hands in the shifting seasonal patterns of migratory animals and flowering plants, in the “once-in-a-century” storms, droughts, and heat waves that now came every few years, in the corals that lay bleached and dying in the too-warm waters of shallow seas, in the clear-cut forests, dammed rivers, and runoff-clogged streams. Crutzen felt compelled to coauthor an influential paper with the aquatic ecologist Eugene Stoermer arguing that our newfound planetary powers placed us in an entirely new geological period. In this “recent age of man,” the “Anthropocene,” human dominance altered the skies and seas, and even changed the very rocks that through the eons would endure as mute testament to our era.
For many millions of years to come, the Holocene-Anthropocene transition will be clearly visible to the naked eye wherever a proper rock face is exposed. In marine basins where white carbonate sediment once settled to form limestone and chalk, CO2-saturated ocean water will have become more acidic, depositing instead dark carbonate-depleted clays and muds. If atmospheric CO2 continues to climb unabated, carbon-rich black shales could dramatically re-enter the geological record, as rising temperatures and sea levels once again lead to widespread deepwater anoxia. We cannot say, however, whether whatever may later find the Anthropocene’s new shales will, as we did, extract and burn the accompanying deposits of oil and gas to build a global technological civilization. Fossils abutting the Holocene-Anthropocene transition will record a planetary mass extinction event, the sixth in Earth’s history, one in which species-rich ecosystems developed over tens of millions of years suddenly, irretrievably vanished and were replaced with agrarian homogeneity. Upper Holocene fossil beds will reward any future paleontologists with finds such as lithified coral reefs and carbonized amphibians; lower Anthropocene beds will more likely offer corn cobs, cow bones, petrified oil palms, and perhaps even some remains of their human masters, to whose needs they were bent and bred. Very rarely
, a warped, weird band of rusted metal-laced rock may be found—the remnants of some major coastal city long sunken and buried beneath sediments from a great ancient river. If you hope to appear as a fossil in some fractured far-future cliff face, you could do worse than a burial on the Mississippi Delta of slowly sinking New Orleans.
When I met Mike Arthur in his office in late October of 2011, atmospheric CO2 was 390 ppm—and still rising. According to UN estimates, planet Earth was only days away from a baby’s birth, probably somewhere in southeast Asia, that would bring the world population to seven billion people. Similar estimates charted a course for population to hit ten billion late in the twenty-first century. If the global spread of technology and commerce continued, a very large fraction of the future population would seek to enjoy lifestyles similar to that of most present-day Americans. They would want coal-fired power plants to supply electricity at the flip of a switch, ubiquitous freeways and air travel, a car in every garage, a flat-screen television in every living room, cheap, disposable smartphones and computers, meat at every meal, and fresh produce flown to their dinner tables from halfway round the world. But barring the world’s energy infrastructures somehow turning away from fossil fuels with unthinkable swiftness, those very behaviors would raise CO2 concentrations beyond 500 ppm to reach 1,000 ppm and beyond—with predictably disastrous results. Average global temperatures could rise by 5 to 10 degrees Celsius, the poles would become wholly ice-free, and the rising seas would surge hundreds of kilometers inland around the coasts, to list only the most proximate effects. The Earth would then far more resemble the hothouse world it was during the reigns of the trilobites or the dinosaurs, rather than the cool and sedate planet that had previously nurtured human civilization. In that fevered future, simple survival of individuals and entire societies would become a constant struggle.
Whether all this would happen depended a good bit on when and how the Marcellus and other gas shales were exploited.
“As a scientist, I try to be objective, to base my conclusions on data and tradeoffs,” Arthur said. “The data tell us that greenhouse gases from all the fossil fuels we’re burning are greatly impacting the world. That is indisputable. It’s also indisputable that burning natural gas produces 30 percent less CO2 per unit of energy than oil, 40 percent less than coal. Coal-fired power plants produce more than half of the electricity in the U.S. People talk about converting to completely ‘clean coal,’ but there really is no such thing; mining coal is fundamentally deleterious to the environment. People talk about ethanol from corn or sugarcane. That’s bullshit, a pig in a poke someone sold us. Natural gas is real, it’s cheap, and it’s the cleanest fossil fuel. If we replace a lot of our coal burning with natural gas, we’re not just reducing CO2; we’re reducing emissions of mercury, nitrous oxide, sulfur dioxide, and particulates. We can make cars run on natural gas, too. It’s not hard. That reduces emissions even more. So perhaps this is the lesser evil. But then I look at the sheer amount available, and I worry.”
The Marcellus is singular only in its size. Similar black shales are found throughout the world, on every continent, each an echo and a portent of an Earth warmer and wetter than our own. A deeper, older deposit, the Utica, lies directly beneath the Marcellus in the northeastern United States. There are rich gas shales in Canada, Mexico, and Argentina. There are gas shales in Australia, China, and India. Gas shale is found across Europe—in Germany, in Poland, in the Czech Republic. Gas shale is in Africa, north and south. Gas shale seems, in fact, to be so abundant that it could almost unilaterally transform the fortunes of many developing countries, bringing economic prosperity and soaring levels of consumption and greenhouse emissions. In developed nations already hooked on ancient carbon, the shales could be a lifeline, found and seized just before the Anthropocene’s fossil-fuel boom would otherwise reach its end.
Arthur looked down at his desk, strewn with unsteady piles of paper, charts of geologic sections and maps of stratigraphic thickness—the detritus of a scientist working to unwrap rockbound gifts from the Earth. He briefly closed his eyes and raised his fingers to rub his forehead, as if to stave off the full-steam arrival of a freight-train headache.
Sooner or later, I think we will take most of the shale gas out of the ground and burn it up,” Arthur concluded. “Some people say it will ease our transition into the other alternative energy solutions we need. I worry it might stifle them instead. The view of most conservative politicians in this country today is, hey, why bother investing in solar and wind and other renewables when we’ve got all this gas in the here and now? ‘Drill, baby, drill,’ right? Well, let’s say [the] most optimistic [gas recovery] estimates are right, and let’s say the U.S. for some reason used the Marcellus as its only energy source. At present rates we would burn through all of it in twenty years. Maybe it will last ten times as long, two hundred years. Maybe longer. In the here and now that seems like a good while, but remember, it took about two million years for the entire Marcellus deposition to occur. Geologically, that’s really fast, but it’s still too long for most everyone to comprehend. Humans are now influencing the planet on these larger timescales, but we don’t seem to be very good at planning and accounting for that fact. We ignore the lessons of the past and the prospects of the future at our own peril.”
• • •
Pennsylvania’s rocks contain a record going just past the beginning of the Cambrian Period, some 542 million years ago, which itself marks the beginning of the Phanerozoic Eon, the half-billion-year stretch of time, up to and including today, wherein geologists can find fossils of complex organisms. “Phanerozoic” is a Greek appellation, and roughly translates to “visible life.” This was when organisms first began building shells and skeletons, which are more easily preserved in rock. The emergence of hard body parts was only one part of a larger surge of biodiversity famously called the “Cambrian explosion.” Within a span of only five or ten million years, organisms larger than a few centimeters in size became commonplace, and physiological innovations as fundamental as spinal cords, jaws, gills, and intestines all emerged for the first time. Nearly every extant animal on Earth today traces its architecture back to this baffling profusion of form. Extending our view a few tens of millions of years deeper into the past, we find the first evidence of creatures like worms and jellyfish, and features such as nerves, muscles, eyes, and radial and bilateral body symmetry. In nearly all Earth’s long history before then, our planet was a world of prokaryotes—single-celled microbes lacking a nucleus.
For decades, scientists have struggled to learn what this vanished, largely alien “Precambrian” world was like, and why it so suddenly changed. Clues can be found in Precambrian rocks, and most of them suggest that, once again, some potent interaction between life and its environment had acted to irreversibly transform the world. The “smoking gun” in this ancient mystery is the very air you breathe—the oxygen in Earth’s atmosphere, which lies at the heart of our planet’s rapid, primeval transition.
Investigating the Precambrian is inevitably challenged by its great distance from us in time—the older any particular rock is, the greater the chance it has at some point in the past been melted or cooked, wiping out nearly all information it might otherwise contain about ancient events and environmental conditions. Three entire eons are contained within Precambrian time—four if you count the formative Chaotian, the eon in which our solar system assembled from its primordial cloud of gas and dust. After the Chaotian, the Hadean Eon began with the great Moon-forming impact some 4.53 billion years ago, and ended nearly 700 million years later. Of the Hadean we know virtually nothing. The Earth must have been quite hot as it cooled from formation, yet a handful of very rare Hadean rocks contain trace evidence of liquid water, suggesting that even then our planet may have had scattered surface seas. The boundary between the Hadean and its successor, the Archean Eon, is not well defined, being set by the smeared-out occurrence of the Late Heavy Bombardment between 4.1 and 3.8 billi
on years ago, when our solar system’s giant planets seem to have hurled huge volumes of asteroids and comets down into the inner solar system. The great impacts that ended the Hadean and commenced the Archean Eon also began our planet’s record of sedimentary rock. Any Hadean sedimentary rocks—and any Hadean life along with it—did not survive the pulverization of the crust and flash-boiling of the planet’s water. It is in the Archean rocks that scientists find the earliest evidence of life, and the beginning of Earth’s metamorphosis into the planet we know today.
Early Archean rocks contain hints of oceans, full-fledged plate tectonics, and the gradual growth of continents, as well as small amounts of organic carbon unquestionably generated by photosynthetic microbes. One thing they do not contain, however, is any significant trace of atmospheric oxygen. The Archean world was a desolate place, dreary beneath a sky clouded with smog-like organic hazes that built up in the anoxic conditions. It was also probably warm: the dominant life forms were likely one broad class of prokaryotes, called methanogens, that gained energy from reacting hydrogen and CO2 together to produce methane, a greenhouse gas that can have even more heat-trapping potency than CO2. The gloomy global reign of the methanogens and other anaerobic microbes seems to have lasted approximately a billion years. It could have lasted far longer if not for the sudden appearance of a new life form: the photosynthetic cyanobacteria.
