18 Miles

by Christopher Dewdney

  The answer had to be volcanoes — volcanoes with a series of eruptions of epic proportions. On average, volcanoes pump 10 billion tons of carbon dioxide into the atmosphere a year. If several supervolcanoes erupted at once, the amount of carbon dioxide would increase a thousandfold. Luckily for us, rain washes volcanic carbon dioxide out of the air. But since there wasn’t any rain, since all the water on the planet was frozen, the carbon dioxide would not be “scrubbed” (as they refer to the carbon dioxide removal process on the space station). Kirschvink did some quick calculations on ratios of carbon dioxide and greenhouse warming and finally realized exactly how Snowball Earth came to an abrupt, and cataclysmic, end.

  These days we don’t need to be told how potent a greenhouse gas carbon dioxide is. Even at current levels, way less than 1 percent of our atmosphere, its warming effects are felt. Kirschvink’s calculations revealed that after 10 million years, carbon dioxide levels had risen to occupy 10 percent of Snowball Earth’s atmosphere. No matter how cold the planet, the greenhouse effect of carbon dioxide at such high levels was irresistible. And that was how the snowball melted. But there was more. The transition from snowball to hothouse was extraordinarily abrupt. The evidence suggested that the planet went from an average temperature of -40°C to an average temperature of 23°C in less than a few hundred years. Now all Kirschvink needed was proof of the great meltdown. It was 1990.

  The Final Hurdle

  In 1992, Harvard geologist Paul Hoffman met Kirschvink and became an instant convert to the theory. Hoffman said he knew the very place to look for evidence: Namibia. Hoffman phoned an old friend, a geochemist by the name of Daniel Schrag, and convinced him to come along on an expedition to the 600-million-year-old calcium carbonate cliffs in Namibia.

  Geologists had never known what to make of these geological formations, but now, armed with the Snowball Earth meltdown hypothesis, Hoffman and Schrag had a good idea. They found the layer of dropstones precisely where it should be, at the 600-million-year-old level, and immediately above it, they found massive calcium carbonate deposits. Schrag’s chemical analysis of these deposits left no doubt: they had been formed by calcium leached out of surface rock by carbonic acid. Here was proof that the vast amount of carbon dioxide in Snowball Earth’s atmosphere had bonded with the first rain to fall for 15 million years and turned it into carbonic acid. The carbon in the acid rain then bonded with the calcium from the dissolved rocks to form calcium carbonate. There had indeed been a meltdown.

  And what a meltdown. Hoffman was awestruck by the obvious speed of the transition; there was no gradation between the dropstone layer and the calcium carbonate layer. The heat wave must have come on so fast that it turned the Earth’s climate upside down. Continental glaciers melted within decades. Some scientists believe that giant hurricanes churned the hot oceans, whipping up storm waves 330 feet high. The torrential rains unleashed by this “mother of all greenhouse effects,” as Hoffman put it, probably lasted without interruption for a century. This scenario was the last apocalyptic piece of the puzzle.

  In 1998, Hoffman and Schrag went on a world tour of major universities to expound their theory of Snowball Earth. It all went well until they hit another seemingly impenetrable brick wall. This time, the critics were biologists. Life could not have survived such a cataclysm, the biologists said. Photosynthesis required sunlight and how could sunlight penetrate ice that was more than 65 feet thick? Life only existed in the oceans at that time, so without open water, life wouldn’t have survived. The Snowball Earth thesis was kicked to the curb again.

  But there was one more white knight, a scientist who didn’t just think out of the box but off the planet. Chris McKay was a planetary exobiologist working for the space science division of NASA. His corner of expertise was the survival of life in hostile environments, such as might be found on other planets. When he heard about the biological objections to the Snowball Earth theory, he already knew the answer. He had been to the one place on Earth with the same conditions: Antarctica.

  NASA had been studying the dry valleys near McMurdo Sound in Antarctica because the terrain in these valleys is similar to Mars — a cold, snowless, rocky desert. Two of the valleys contain lakes that are millions of years old. And there, under ice more than 16 feet thick, is water. If life had survived beneath that ice, then it probably would have survived under the thicker ice of Snowball Earth.

  Divers had been exploring the icy waters of both Antarctic lakes since the early 1970s and had discovered the same ancient form of cyanobacteria that predated Snowball Earth. Lots of it. As well, they found mats of algae as big as sheets. Another surprise was the amount of light penetrating the thick ice. McKay, an expert on polar ice, knew that when ice forms slowly, under extremely cold conditions, it is much more transparent than ordinary sea ice. So he didn’t doubt that enough sunlight had penetrated the equatorial ice on Snowball Earth to sustain photosynthesis. Harland’s theory had finally passed its last test. Snowball Earth had made it into the official geological record of our planet.

  Life, hardly given the warmest of welcomes, survived. Bacteria, algae and anaerobic organisms living near geothermal deep-sea vents and in hot springs, continued to exist for millions of years until, in the great thaw, the icy clutch of winter was broken. The cosmic spring that followed witnessed life’s resurrection in a renaissance that would change the face of the Earth.

  Of course, when such a counterintuitive theory got the official nod, then any scientist with a hankering for posterity (and there are a lot) began looking for other, older glacial ages. And they found them. In the four-billion-year history of our planet, ice ages have struck at least six times, perhaps seven. Harland, Kirschvink and Hoffman’s Snowball Earth now has the official name of the Marinoan glaciation. (I don’t know why, after all their work, it didn’t end up being called the Hoffmanian.) It was preceded by the Sturtian glaciation, which lasted much longer (720 to 660 million years ago), although it was not as global as the Marinoan or the even earlier Huronian glaciation, which gripped the planet 2.3 billion years ago, after the first oxygen hit the atmosphere.

  The causes of some of these glaciations are still unknown. We know much more about the Pliocene-Quaternary glaciation — the one that we are in the middle of and the one that almost wiped us off the planet 200,000 years ago.

  11

  Climate Change Past and Present

  The Arctic Eocene

  An ice age is not normal. During the 4.5 billion years of our planet’s existence, only 517 million years have been spent in ice ages. That’s less than a 10th. Earth’s default climate is subtropical. Normally, palm trees grow as easily in the Arctic as they do at the equator. Up to 2.6 million years ago, when our current glacial age began, we had had a pretty long run of hot weather: 257 million years worth. Indeed, at the height of that extended summer, a time called the Eocene Optimum, which peaked about 45 million years ago, the planet was particularly warm. Carbon dioxide and methane levels rose precipitously at the beginning of the Eocene Optimum and then stabilized at those levels for millions more years. Antarctica was fringed with subtropical rainforest, and palm trees grew in Alaska. Meanwhile, the Canadian high Arctic was home to vast forests of dawn redwoods and palmettos inhabited by an extraordinary menagerie of creatures. Fossils of tapirs, boa constrictors, rhinoceroses, pygmy hippopotamuses, giant land tortoises, eight-foot-long monitor lizards, alligators and the six-foot flightless “terror bird” called diatryma have been found on Ellesmere Island in the Canadian Arctic.

  Due to their northerly latitude (polar landmasses were less than five degrees from their present positions), the Arctic forests experienced months of midnight sun and months of Arctic night. But even during the protracted darkness, the temperature never dipped below freezing. Some evolutionary biologists think that this might have been where some nocturnal mammals first evolved, prowling the warm, polar nights. Fossils of gliding lemurs have been discovered on Ellesm
ere, and perhaps the first bats found their wings under similar conditions.

  So when the first snow fell, it must have come as rude awakening. Indeed, here is reason to believe the transition from the endless global summer to our current ice age was quite abrupt. The fossils from the forests of Ellesmere Island aren’t even fossilized, despite being millions of years old. The mummified wood burns easily and the leaf litter from the floor of the ancient forest is still spongy. It is as if the whole forest had been freeze-dried in situ.

  The First Winter

  During the Eocene, and through the following Miocene and Pliocene epochs, the continents continued to drift as the inexorable subsurface vortices of magma pushed the majority of tectonic plates northward. Antarctica was the wayward plate, drifting in the opposite direction, toward the South Pole. About 40 million years ago, five million years after the Eocene Optimum, Australia separated from Antarctica, and cool ocean currents set up between the two continents. For the first time snowflakes fell in the Antarctic highlands during the winter, although the rest of the continent still enjoyed a subtropical climate. Then, roughly 23 million years ago, during the Miocene epoch, the narrow isthmus of land separating South America from Antarctica was breached by the Drake Passage. Now Antarctica was completely isolated in an increasingly cooler southern ocean. The glaciers that had formed on the Antarctic mountain peaks began to spread, crushing the austral forests before them until, 15 million years ago, the glaciers were well on their way to covering the entire continent.

  The rest of the world still enjoyed the glow of the Miocene epoch, almost as warm as the Eocene, but it was like being in a house where somebody has left an air conditioner on full blast, day and night, in a spare room. It was just a matter of time before something tipped the global climate into an ice age. And sure enough, that triggering set of conditions occurred 2.6 million years ago, causing the climate to cool too quickly for the subtropical forests of the Arctic Pliocene epoch to adapt. Some evidence suggests a catastrophic, fairly rapid transition from subtropical to ice-age temperatures as the first of 11 glacial periods, now known as the Pre-Illinoian glaciations, established ice sheets in the northern hemisphere and initiated the Pliocene-Quaternary glaciation. Global temperatures plummeted and ocean levels dropped as water was locked into vast ice sheets.

  Hominids had just gotten a foothold in Africa at the beginning of the Pliocene-Quaternary glaciation, with two separate hominid lineages: Australopithecus africanus and Homo habilis. It was probably the latter that eventually led to Homo sapiens. Fortunately for us, Africa was spared the more brutal climatic effects of the Pliocene-Quaternary, and the deterioration of world climate held off for the critical gestation period of our species, at least until the onset of the Illinoian glaciation, 191,000 years ago.

  Message in a Bottleneck

  With almost seven billion humans currently alive on our planet, it might seem hard to believe that we were once on the verge of extinction. Yet just after the first anatomically modern humans, Homo sapiens, arose in Africa, the climate took a big turn for the worse. The warm interglacial temperatures that had nurtured our forebears grew much cooler, and food became scarce. The cradle of our species was tipping us out. This cold snap, the full-blown glacial advance of the Illinoian period (referred to by climatologists as Marine Isotope Stage 6 after the ocean bottom sediments laid down at that time) began about 195,000 years ago and lasted, with a few brief interglacial periods, for about 72,000 years.

  According to paleoanthropologist Curtis W. Marean, a professor at Arizona State University, central Africa became virtually uninhabitable, and the only safe haven for our ancient ancestors was the sea coast of South Africa. Ocean levels had dropped more than 330 feet, but here, on the coast, plentiful marine life and edible shore plants tempered the hard, cold millennia. Even then, it was tough going. During a particularly severe period, as the glaciers advanced to their maximum extent, Marean postulates that our species dropped from more than 10,000 individuals to a just few hundred souls. This population choke point left a telltale genetic imprint in our genes. Geneticists discovered it in the early 1990s, like a message in a bottle that had been afloat on the cellular seas of time, a tale from our past selves to our present selves, describing in DNA code a harrowing account of near extinction.

  The only other advanced hominids at the time were Neanderthals. And the paleoanthropological consensus seems to be that the Neanderthals likely wouldn’t have survived. Certainly there is archeological evidence that Neanderthals had fire, clothing and culture, but their physiology wasn’t as efficient as Homo sapiens. If Neanderthals had failed, then the great hominid evolutionary experiment might have failed as well. We would have a world today without cities, without civilization, without the “we.” It would be a planet where mammoths still browsed on young birch trees in Minnesota and Norway, where marsupial wolves still prowled the Australian outback and where dodos strutted through the undergrowth on the island of Mauritius without fear of dying out.

  The Illinoian glaciation ended with an interglacial period now known as the Sangamonian stage, about 130,000 years ago. And then it was off to the races for Homo sapiens. Earlier, during the hard winter of the Illinoian glaciation, Homo sapiens had acquired the crucial traits of prosocial cooperation and projectile weapons, essential skills for our survival through the dark times. So we were ready when the climate warmed up and megafauna returned to the plains of Central Africa. The Sangamonian summer and autumn lasted some 50,000 years, from about 125,000 to 75,000 years ago, and we used it well. Our species expanded rapidly northward, colonizing the whole of the African continent. Around 70,000 years ago, we spilled out of North Africa, just as one of the bitterest glacial ages ever, the Wisconsin glaciation, was beginning. But that didn’t slow down our global expansion.

  By the time the Wisconsin glaciation entered its most severe period, or last glacial maximum (LGM) about 26,000 years ago, our restless, nomadic species had spread to the far corners of the planet — from northern Europe and throughout Asia to Australia, as well as North and South America. Our colonization of the world took place during the worst ice age since the Andean-Saharan ice age, 460 million years before that. Over the cold, dark millenia of the Wisconsin ice age, we developed complex languages and culture and religion. The Lascaux cave paintings were executed 17,300 years ago at the height of the LGM. Rendered by flickering torchlight on limestone walls, these exquisite paintings of ice-age mammals are snapshots of a lost era when Europe was either covered by Arctic tundra or buried under glaciers. Two-mile-high cliffs of continental glaciers were parked just north of present-day London, England, a mere 435 miles north of Lascaux, and world sea levels were 330 feet lower than today.

  Yet less than 7,000 years later, the Wisconsin glaciation came to a surprisingly sudden end, and our globetrotting forebears had to endure a series of catastrophic climate changes. Greenland ice cores reveal that the end of the Wisconsin was a period marked by intense climatic instability. The mean global temperature flipped several times between temperate conditions and those of an ice age. When the global melt was really underway, about 15,000 years ago, Greenland’s average temperature shot up by 16°C in a period of 50 years, perhaps fewer. And around 12,000 years ago, the definitive end of the Wisconsin, Greenland’s mean temperature skyrocketed by 15°C in one decade. As a result, the reset global temperature, the new “normal,” was probably 6°C warmer. Climatologists now refer to this as “abrupt climate change.” Imagine if this happened today. None of our low-lying nations would be able to build levees and sea walls fast enough to stave off an ocean rising more than three feet a year.

  By the time the oceans peaked, when the Wisconsin glaciers finally retreated to their present positions some 11,000 years ago, humans were thriving on every major continent except Antarctica. The cultural and technological advances we had been nurturing through the long, dark millennia of the Wisconsin glaciation blossomed into grand civilizations. It was
the global spring of our present era: the Holocene interglacial.

  In that 11,000-year period, from the retreat of the continental glaciers to the present, humans have advanced quickly: the first agriculturally based city states arose 6,500 years ago, the domestication of the horse along with the first written language 6,000 years ago, the invention of the wheel 5,400 years ago. Iron was first smelted about 3,200 years ago, and 2,400 years ago Euclid discovered geometry, the basis for modern mathematics. The pace of invention accelerated as technology was increasingly harnessed to advances in science, and today humans stand on the brink of an extraordinary destiny. Maybe Carl Sagan was right when he said, “We are a way for the cosmos to know itself.” Certainly we are coming to understand climate and, in particular, we are making great progress in decoding the mysterious engine that drives climate change. And thanks to Milutin Milankovitch, we are even beginning to understand the cycle that drives the ice ages themselves.

  The Milankovitch Cycle

  Milutin Milankovitch (1879–1958) grew up in the village of Dalj on the banks of the Danube on the eastern edge of the Austro-Hungarian empire. The eldest of seven siblings, Milankovitch was only eight when his father died. His mother, stranded with six children under the age of seven, enlisted Milankovitch’s grandmother and uncle to help with the young family and continued the home schooling Milankovitch’s father had begun. Relatives and friends, some of whom were prominent inventors, philosophers and poets, tutored Milankovitch as well. It was a turbo-charged education, and he was more than prepared when, at age 17, he enrolled in the Vienna Institute of Technology.