Chapter Content
## Chapter 41: The Age of Frost
*“I had a dream, which was not all a dream.
The bright sun was extinguish'd, and the stars
Did wander darkling in the eternal space…”*
— Byron, “Darkness”
In 1815, upon the island of Sumbawa in what is now Indonesia, a dormant giant awakened. Mount Tambora, a picturesque volcano, erupted with a force unseen in living memory. The cataclysmic explosion, coupled with the ensuing tsunamis, claimed over one hundred thousand lives. Tambora’s eruption dwarfed anything previously witnessed, a staggering display of raw power. It was the most violent volcanic event in ten millennia – fifteen times the size of the Mount St. Helens eruption of 1980, releasing energy equivalent to sixty thousand Hiroshima bombs.
News traveled sluggishly in those days. It took seven months for the London *Times* to publish a brief report – essentially a merchant's letter – about the eruption. Yet, long before the news arrived, its consequences were being felt. Two hundred and forty cubic kilometers of ash choked the atmosphere, dimming the sun and plunging the planet into a chilling embrace. Sunsets were eerily muted, a phenomenon captured with feverish delight by the English painter J.M.W. Turner. But more often than not, the world was shrouded in a suffocating, oppressive gloom. This funereal darkness inspired Lord Byron to pen the haunting verses that begin this chapter.
Spring faltered, and summer offered no warmth. 1816 became known as "The Year Without a Summer." Harvests failed across the globe. In Ireland, famine and typhus ravaged the population, claiming sixty-five thousand souls. In New England, America, it was branded “Eighteen Hundred and Froze to Death.” Frosts persisted well into June, rendering seeds useless in the ground. Livestock perished en masse due to lack of fodder, or were slaughtered prematurely. By any measure, 1816 was a year of profound hardship – arguably the worst agricultural disaster experienced by modern farmers. Yet, the global temperature dipped by less than a single degree Celsius. Scientists would later realize how delicately balanced the Earth’s climate control system truly was.
The 19th century wasn't particularly cold as centuries go. For two hundred years prior, as we now know, Europe and North America had been experiencing a Little Ice Age. The Thames River routinely froze solid, hosting annual Frost Fairs, and skating races were a common sight on Dutch canals - improbable spectacles for present-day observers. In short, it was an era accustomed to the chill. Perhaps this explains why 19th-century geologists were slow to recognize that they inhabited a relatively mild world compared to times past. Around them stretched landscapes sculpted by the relentless power of glaciers and temperatures that would have utterly extinguished any Frost Fair.
They recognized that something extraordinary had occurred long ago. Anomalies abounded across Europe, defying easy explanation: the skeletal remains of Arctic reindeer discovered in the balmy south of France, massive erratics – boulders – perched incongruously in unexpected locations. This often led to bold, if ultimately untenable, hypotheses. One French naturalist, Dé Luc, attempted to explain the presence of massive granite blocks atop the limestone layers of the Jura Mountains by suggesting they had been catapulted there by compressed air within underground caverns, like a cork from a toy gun. This flimsy explanation struggled to account for the stones’ immense size and vast displacement, but in that era, what mattered more was internal consistency than a precise match to the observable reality of rock movement.
The great British geologist, Arthur Hallam, observed that had James Hutton, the father of 18th-century geology, personally explored Switzerland, he would have immediately grasped the significance of the scarred and eroded valleys, the polished rock surfaces, the telltale moraines – ridges of debris left by retreating ice – and the myriad other clues. All of which spoke of glacial passage. Unfortunately, Hutton was no traveler. Still, even from his secondhand accounts, he flatly rejected the notion that massive rocks had been hurled onto hillsides a thousand meters high by floods – "no water on Earth could have floated them," he pointed out – and became an early advocate for large-scale glaciation. Sadly, his views failed to gain widespread traction; for nearly half a century, most naturalists clung to the idea that the markings on rocks were simply the result of heavy carts or even hobnailed boots.
Conversely, local farmers, unburdened by scientific orthodoxy, often knew better. The Swiss naturalist Jean de Charpentier recounts a story of walking a country path in 1834 with a Swiss woodcutter. They casually discussed the ubiquitous boulders littering the landscape. The woodcutter matter-of-factly stated that the rocks originated far away, in the Grimsel region. “When I asked him how they had come to his region,” Charpentier wrote, “he answered without hesitation: ‘The Grimsel Glacier carried them here by way of the valley, because that glacier once extended as far as the town of Bern.’”
Charpentier was elated, as he had independently arrived at a similar conclusion. However, when he presented his ideas at a scientific gathering, they were met with dismissive silence. Charpentier’s close friend, a Swiss naturalist named Louis Agassiz, initially skeptical, slowly came around to embracing and then championing the theory.
Agassiz, a former student of Cuvier in Paris, held a professorship in natural history at the Academy of Neuchâtel, Switzerland. Another friend, Karl Schimper, a botanist, coined the term “Ice Age” (German: *Eizeit*) in 1837. He believed that evidence suggested ice sheets had not only blanketed the Swiss Alps but had also covered vast swaths of Europe, Asia, and North America. This was a radical proposition. He shared his notes with Agassiz – a decision he would later regret as Agassiz increasingly appropriated his ideas, while Schimper felt, with considerable justification, that it was his theory. Charpentier also became a bitter rival of his old friend. Alexander von Humboldt, another acquaintance of Agassiz, once remarked upon the three stages of scientific discovery: first, people deny that it is true; then, they deny that it is important; finally, they credit the wrong person. Humboldt had Agassiz in mind, at least in part, when he made that observation.
Regardless, Agassiz dedicated himself to the cause. To understand glacial forces, he went wherever they led – plunging into the depths of perilous crevasses, scaling the treacherous peaks of the Alps, often, it seemed, unaware that he and his team were climbing mountains no human had ever set foot upon. Yet, almost everywhere Agassiz went, his theory was met with disbelief. Humboldt urged him to abandon his near-obsessive focus on glaciers and return to his expertise: the study of fossil fish. But Agassiz was a man driven by an idea.
In Britain, support for Agassiz’s theory was even scarcer, as most naturalists had never seen a glacier and struggled to grasp its immense power. "Are the striated and polished surfaces of rock to be attributed solely to glacial action?" Roderick Murchison asked sarcastically at a meeting, apparently imagining the surfaces were merely covered in a light frost. He remained deeply distrustful of "glacial-mad" geologists until his death. William Hopkins, a Cambridge professor and leader of the Geological Society, echoed this sentiment, declaring that the notion of glaciers moving rocks "is mechanically absurd" and unworthy of the Society’s attention.
Undeterred, Agassiz tirelessly traveled and lectured, proselytizing his theory. In 1840, he presented a paper at a meeting of the British Association for the Advancement of Science in Glasgow, only to be publicly criticized by the esteemed Charles Lyell. The following year, the Geological Society of Edinburgh passed a resolution acknowledging that Agassiz's theory *might* have some merit, but certainly did not apply to Scotland.
Lyell eventually changed his mind. His epiphany came when he suddenly recognized that a pile of moraine – a long ridge of rocks – near his Scottish home, which he had passed hundreds of times, could only be explained by the action of glaciers. However, even with this private conversion, Lyell lacked the courage to publicly endorse the glacial theory. It was a difficult period for Agassiz. His marriage disintegrated, Schimper publicly denounced him for stealing his work, Charpentier stopped speaking to him, and Lyell, the world’s foremost geologist, only offered lukewarm, equivocating support.
In 1846, Agassiz traveled to the United States to deliver a series of lectures, and there, he finally achieved the recognition he craved. Harvard University offered him a professorship and established a world-class Museum of Comparative Zoology in his name. His settling in New England certainly played a role, as its long winters fostered a certain sympathy for theories of prolonged cold periods. Six years later, Agassiz undertook his first scientific expedition to Greenland. That helped too. They found that nearly the entire island was covered by an ice sheet, much like the ancient ice sheets that Agassiz theorized. His ideas were beginning to gain genuine support. One crucial piece of the puzzle, however, remained: Agassiz could not explain what caused the Ice Ages. Help would come from an unexpected source.
In the 1860s, British newspapers and scholarly journals began receiving articles on topics ranging from hydrostatics and electricity to other scientific disciplines, all penned by a James Croll of Anderson’s University in Glasgow. One particular piece, arguing that variations in the Earth’s orbit might be responsible for triggering glacial periods, was published in *Philosophical Magazine* in 1864 and was immediately hailed as a work of the highest scholarship. But surprise and perhaps a little embarrassment rippled through the academic community when it became known that Croll was not a researcher at the university, but simply its janitor.
Born into poverty in 1821, Croll's formal education ended at the age of thirteen. He drifted through a series of jobs – carpenter, insurance salesman, temperance hotel keeper – before landing his position at Anderson’s University (now the University of Strathclyde). He persuaded his brother to assist him with many of his duties, allowing him to spend quiet evenings in the university library, immersing himself in physics, mathematics, astronomy, hydrostatics, and other emerging fields of science. Gradually, he began to write a series of papers, focusing primarily on the movement of the Earth and its effect on climate.
Croll was the first to propose that the cyclical changes in the Earth’s orbit, from elliptical (that is, somewhat egg-shaped) to nearly circular and back again, could be the driving force behind glacial advances and retreats. Before him, no one had explained changes in Earth’s weather through the mechanics of astronomy. Thanks to Croll’s compelling arguments, the notion that parts of the Earth had been under glacial control at some point in the past began to find acceptance in England. Croll's genius was universally recognized, securing him a position with the Scottish Geological Survey and earning him a cascade of honors: fellowship in the Royal Society of London and the New York Academy of Sciences, an honorary degree from the University of St. Andrews, and more.
Sadly, just as Agassiz’s theory was finally gaining acceptance in Europe, he was relentlessly exploring untouched areas of the Americas. Everywhere he went, even near the equator, he found evidence of glacial action. Eventually, he became convinced that ice had once covered the entire Earth, obliterating all life that God had created. None of the evidence Agassiz cited supported such an idea. Nonetheless, his status in his adopted country steadily grew until he became something of a saint, second only to God, so much so that upon his death in 1873, Harvard University deemed it necessary to hire three professors to replace him.
Agassiz’s theory, however, soon fell out of favor. This does sometimes happen. Less than a decade after his death, his successor as chairman of Harvard's Geology department wrote: "The so-called Ice Age…which a few years ago was riding the geological high horse, is now by geologists quietly set aside."
In part, the problem was Croll's calculations suggested the last ice age ended 8,000 years ago, but the geological record increasingly pointed to something severe having occurred much more recently, within the last 3,000 years. Without a credible explanation for the *cause* of an Ice Age, the entire theory remained vulnerable. This would remain the case until a Serbian scholar named Milutin Milankovitch entered the scene. Milankovitch had no background in celestial mechanics – he was a trained mechanical engineer – but in the early 20th century, he became captivated by the problem. He realized that the trouble with Croll's theory was not that it was wrong, but that it was too simple.
As the Earth moves through space, not only does its orbital path shift in length and shape, but its angle relative to the sun – its obliquity, precession, and eccentricity – also changes in regular patterns, all of which affect the length and intensity of sunlight falling on any point on the planet. In particular, the Earth experiences three variations in its position over vast stretches of time, known as axial tilt, precession, and eccentricity. Milankovitch suspected that these complex, cyclical shifts were somehow linked to the onset and retreat of glacial periods. The difficulty was that the timescales of these variations differed dramatically – some around 20,000 years, others 40,000 years, yet others 100,000 years, with almost every cycle spanning millennia. This meant that pinpointing their intersection over long periods of time required an almost endless series of intricate calculations. Essentially, Milankovitch had to calculate, for the past million years, the angle and duration of sunlight falling on every latitude of the Earth, in every season, as all three factors constantly shifted.
Fortunately, such a monumental task was perfectly suited to Milankovitch’s temperament. Over the next two decades, armed with only a pencil and slide rule, Milankovitch tirelessly computed his tables of cycles, even while on vacation – a task that could now be accomplished in a day or two with a computer. The calculations were done in his spare time. However, in 1914, Milankovitch suddenly had much spare time. World War I broke out, and he was arrested as a reservist in the Serbian army. For much of the next four years, he was confined to Budapest, Hungary under house arrest, leniently supervised with only weekly reporting to the police, allowing him to spend his days toiling in the library of the Hungarian Academy of Sciences. He was, perhaps, history’s happiest prisoner of war.
The culmination of his efforts was the publication, in 1930, of *Mathematical Climatology and the Astronomical Theory of Climate Change*. Milankovitch was right: glacial periods were indeed linked to perturbations in the Earth’s motion. Although, like most people, he thought that intensely cold winters caused long cold periods. It was Russian-born German meteorologist Wladimir Köppen -- father-in-law to tectonic geologist Alfred Wegener -- who discovered the process was more complex, and more sinister, than that.
Köppen argued that it was *cool summers*, not harsh winters, that triggered glacial periods. If a region experienced an unusually cool summer, incoming sunlight would be reflected off the land, amplifying the cold, leading to more snowfall and, ultimately, a permanent snow cover. As snow turned to ice, the area would become even colder, accumulating more ice. As glaciologist Gwen Schultz put it: “It is not how much snow falls that determines an ice sheet, but how much does not melt -- however little.” An ice age begins with an unusually cool summer where snow doesn't melt, reflects heat, and exacerbates cold, which McPhee says "is a self-magnifying process, and once an ice sheet forms, it starts to move." And with moving ice, you have an ice age.
By the 1950s, hampered by inadequate dating techniques, scientists could not align the available data on glacial periods with Milankovitch’s precise calculations, and Milankovitch and his calculations became increasingly unfashionable. He died in 1958, never seeing his cycles proven correct. By this time, in the words of one history of the period, "you would have been pressed to find a geologist or meteorologist who regarded [his calculations] as anything but an antique." It wasn't until the 1970s, with the refinement of potassium-argon dating techniques for ancient seafloor sediments, that Milankovitch’s theory was finally vindicated.
The Milankovitch cycles alone, however, are not enough to explain the periodicity of ice ages. Many other factors must be considered – most significantly the distribution of continents, particularly the presence of landmasses at the poles – and our understanding of these remains incomplete. However, there is a saying that if you move North America, Eurasia and Greenland north by 500 kilometers, we would be in a perpetual ice age. It seems we were extremely lucky to have been blessed with good weather. Our understanding of the cycles of relatively warm periods inside an ice age, called interglacials, is particularly poor. Perhaps discouragingly, the whole of human civilisation – the development of agriculture, the building of cities, mathematics, literature, science and everything else – has occurred in a freak era of clement conditions. The last few interglacials have lasted for just 8,000 years, and we're now 10,000 years into this one.
The fact is that we are still in an ice age, but a greatly reduced one – although not as reduced as many people imagine. At the peak of the last glaciation, some 20,000 years ago, about 30 percent of the Earth’s land surface was buried beneath ice. Even today, 10 percent remains covered in ice. (Another 14 percent is permafrost.) Three-quarters of the planet's fresh water is frozen, and there are ice sheets at both poles, which is highly unusual in the history of the Earth. Many regions experience snowfall in winter and temperate zones like New Zealand are overlaid with permanent ice fields, which is usual for us but almost unheard of in the history of the Earth.
Until fairly recently, for the majority of the Earth’s existence, temperatures were higher and there were no permanent glaciers anywhere. The current ice age – or, more accurately, the ice age cycle – began about 40 million years ago, passing from extremely harsh to less harsh. We live in one of the less harsh periods. New ice ages always erase the records of the old ones, so the further back in time you go, the less complete the picture becomes. But for the last 2.5 million years or so, we seem to have experienced at least 17 severe glacial episodes - the period during which *Homo erectus* in Africa and, later, modern humans evolved. Two suspects that caused the current ice age cycle are the uplift of the Himalayas and the formation of the Isthmus of Panama. The former disrupted air currents, the latter disrupted ocean currents. Over the last 45 million years, India, once an island, had drifted 2,000 kilometers and collided with Asia. The result not only raised the Himalayas, but created the vast Tibetan Plateau behind them. The altitude increase of the plateau is thought not only to have chilled the climate, but to have altered wind patterns, directing them northward, toward North America, making it more vulnerable to long periods of intense cold. Then, beginning about 5 million years ago, the isthmus of Panama rose from the sea, connecting North and South America, impacting warm water flow from the Pacific to the Atlantic and at least half of the planet’s precipitation patterns. One result of this was the drying out of Africa, which drove apes from the trees to discover new lifestyles on the emerging savannah.
Regardless, with the oceans and continents in their current configuration, it seems that we will be in a long ice age cycle. According to John McPhee, we have around 50 ice ages yet to come, each lasting around 100,000 years, before we can hope for an extremely long thaw.
Fifty million years ago, there were no recurring ice ages on Earth, but when they did appear on Earth, they were staggering in their scale and duration. The first major ice age occurred about 2.2 billion years ago, followed by a warm period that lasted around a billion years. The ice age that followed was larger than the first – so much so that today, some scientists call this era the Cryogenian Period, or a "super ice age." More colloquially, it is called "Snowball Earth."
Yet, "Snowball" hardly captures the sheer extremity of the period. The theory goes that with a decrease of about 6 percent in the amount of sunlight and a decrease in the ability to generate (or retain) greenhouse gases, the Earth could barely hold onto its heat. The Earth became like Antarctica, a vast ice sheet with temperatures dropping by 45 degrees Celsius. The Earth's surface froze solid, with ocean ice reaching depths of 800 meters near the poles and several tens of meters in the tropics.
A serious problem arises: geologically, it seems the entire Earth, including the equator, was covered in ice. Biologically, however, we can be sure that some open water must have existed somewhere. Cyanobacteria survived, and they photosynthesize. Photosynthesis requires light, but as you look through the ice, the light fades fast, and a few meters down, there's no light at all. One of the two possible explanations for this phenomenon is that a small section of water never froze at all (possibly because there was a very hot spot somewhere), or that some form of translucent ice existed – a phenomenon that exists in nature.
If Earth did freeze, how did it ever get warm again? That is a notoriously difficult question to answer. A planet in a frozen state would stay that way forever, reflecting too much heat away. The force that rescued the situation seems to have come from magma inside the earth. We might have the earth's plate tectonics to thank for this, once again. Volcanoes, we believe, saved us. Volcanic eruptions broke through the icy stranglehold, spewing out heat and gases that melted the ice and allowed the atmosphere to change once again. Intriguingly, this period of extreme cold ended with the Cambrian Explosion – a spring of life development. This spring, of course, wasn't always peaceful, because as the world warmed it experienced the most violent weather, and huge hurricane-size waves crashed everywhere, and there was unbelievable rain.
Polychaete worms, clams, and other life-forms clinging to deep-sea vents undoubtedly persisted throughout this era, as if nothing had happened. For all other life on the planet, however, it was likely a near-extinction event. This time period is very distant, and what we currently understand about it is extremely limited.
Compared to the Cryogenian Period, the more recent ice ages seem relatively small, but they are still extremely enormous by today's standards on Earth. The Wisconsin ice sheet, which covered Europe and North America, was as thick as 3 kilometers in some places, and it progressed at a rate of 120 meters a year. Even at its edges, the ice sheet was as much as 800 meters thick. What a spectacular sight it must have been! Imagine standing at the foot of an ice wall that high, and behind it, millions of square kilometers of nothing but an ice sheet except for a few ice peaks pointing at the sky. Whole regions of land sank under the pressure of the enormous ice sheets, and even after the retreat 12,000 years ago, these regions haven't quite risen to their previous positions. As the ice sheets slowly moved, they shifted enormous rocks and moraines, and they left behind whole regions of land – such as Long Island, Cape Cod, Nantucket, and more. It's no wonder that geologists before Agassiz couldn't understand the enormous power of ice sheets to alter the face of the earth.
If ice sheets were to ever come back, there's no way we could change their direction. In 1964, Prince William Sound in Alaska, North America's largest glacial area, experienced the continent's most violent earthquake on record, reaching 9.2 on the Richter Scale. The earth rose by 6 meters at the location of the fault line, and the earthquake was so enormous that even ponds in Texas splashed over their banks. But what effect did the unprecedented jolt have on Prince William Sound's glaciers? None at all. The glaciers absorbed the quake and continued their advance.
For a long time, we believed that the Earth gradually entered and exited glacial periods, in cycles of hundreds of thousands of years. Now, however, we know that that isn't how things actually work. By measuring ice cores in Greenland, we have a detailed record of the earth's climate changes for over 100,000 years. The results aren't encouraging. The record shows that the Earth has not been the clement place that it was originally believed to have been during the most recent period of its history. Rather, its climate violently swings between warm and cold.
Around 12,000 years ago, the Earth was finishing its most recent glacial period, and the climate started to warm, and warm quickly. Then, abruptly, it reverted to a period of frigid cold that lasted around 1,000 years. In scientific history, that period is called the Younger Dryas. (This term comes from an arctic plant known as Dryas, which was one of the first plants to regrow after a glacier retreated. There is also an older Dryas period, but its characteristics weren't quite as strong.) Near the end of this thousand-year period of cold, the average temperature once again rose suddenly, increasing by 4 degrees Celsius in just 20 years. That might not sound so bad, but it was enough to change Scandinavia's climate into a Mediterranean climate in just 20 years. The changes were even more extreme in some areas. The Greenland ice cores show that the temperature there changed by 8 degrees Celsius in just 10 years. The temperature changes changed the precipitation patterns and also the growth patterns. This was unsettling enough in the sparsely populated past, but today, the consequences would be unimaginable.
Most alarmingly, we don't know – not really – what natural phenomenon causes the Earth's temperature to change so quickly. As Elizabeth Kolbert pointed out in *The New Yorker*: "There is no known outside force – not even a hypothetical one – that would cause temperatures on earth to change as drastically and as often as the ice cores show. There seems to be," she continues, "a vast and terrifying feedback loop at work." It probably has something to do with the disruption of normal ocean and ocean currents, but we have a very long way to go before we truly understand it.
One theory says that during the Younger Dryas, a high quantity of melting snow flowing into the ocean lowered the saline concentration (as well as the density) of the water in the northern hemisphere. This directed the Gulf Stream southward, like a driver changing direction to avoid a collision. With the absence of the heat generated by the Gulf Stream, the climate of the higher latitudes of the northern hemisphere reverted to frigid conditions. However, this doesn't explain why, a thousand years later when the earth began to warm once again, the Gulf Stream didn't return to its earlier direction. Rather, we entered an unusually stable period known as the Holocene, the period in which we now live.
There's no reason to believe this period of climate stability will last much longer. In fact, some meteorological authorities believe that our climate is becoming more severe than ever before. Most people naturally think that global warming is impeding the Earth from returning to a glacial state. But as Kolbert points out, when you are faced with unpredictable climate fluctuations, "the very last thing you want to do is proactively mess around with [the climate] on a large scale." Some even believe that rising temperatures could spur the arrival of the next glacial period. This argument might seem unintuitive, but it actually makes sense. A slight increase in temperature would hasten evaporation, which would cause a thickening in the cloud cover, which would in turn cause a constant increase in snow coverage in the higher latitudes. In fact, global warming could cause some regions of North America and northern Europe to become even colder. It makes sense, but it is paradoxical.
Climate is the result of several variables – increases and decreases in the sulfur dioxide content, continental drift, solar activity, changes in the Milankovitch cycle – so understanding the past is just as difficult as predicting the future. There are many things we simply cannot understand. For example, after Antarctica drifted to the Antarctic zone, it went at least 20 million years without any glaciers and instead was covered in vegetation. All of that sounds like fantasy today.
Even more unfathomable are some of the known habitats of late-era dinosaurs. British geologist Stephen Truelly discovered that forests within a range of 10 latitudes around the Arctic were home to large animals like the *Tyrannosaurus rex*. "This is simply incomprehensible," he wrote, "because for three months each year, such high latitudes are in the dark." What's more, there's now evidence that the winters in these high-latitude regions were extremely cold. Oxygen isotope studies show that the climate in Fairbanks, Alaska during the late Cretaceous Period was the same as it is today. So what was the *Tyrannosaurus rex* doing there? It was either migrating long distances according to the season or spending long portions of the year in darkness as it struggled through ice and snow. In Australia – which was farther south than it is now – it would have been impossible to retreat to warmer climates. How did dinosaurs manage to survive in that kind of environment? We can only guess.
It's important to remember that if ice sheets were to start forming again for any reason, there would be much more water available this time around. The Great Lakes, Hudson Bay, and Canada's endless lakes didn't exist back then, and they didn't provide the raw materials for the last ice age. They are products of the last ice age.
On the other hand, the next phase in our history will see massive ice sheet melting rather than massive ice sheet formation. If all the ice sheets were to melt, the ocean level would rise by 60 meters – that's 20 stories – and all coastal cities worldwide would be underwater. More likely, at least in the short term, is that the West Antarctic ice sheet will collapse. In the last 50 years, the temperature of the water around Antarctica has increased by 2.5 degrees Celsius, and the ice sheet collapse has already hastened considerably. The geological composition of that area makes large-scale collapse even more probable. If that happens, the global ocean level will increase by 4.5 to 6 meters on average – and fast.
What's very clear is that we don't know whether the future holds an era of cold or an era of heat. What is clear is that we are living on the edge.
Incidentally, an ice age is not necessarily a bad thing for the earth in the long run. Glaciers grind rock, leaving new and fertile soil. They carve out freshwater lakes, providing plentiful nutrients for hundreds of animal species. They also promote the migration of animals and plants, which fills the earth with vitality. As Tim Flannery said: "To determine the fate of humanity on a landmass, you simply have to ask that landmass this question: 'Have you had a proper ice age?'" With that in mind, we should now consider how one type of ape dealt with these changes.