Frozen Earth: The Once and Future Story of Ice Ages (5 page)

That glaciers are such rich sources of organic material has only recently been realized.
It is one of those discoveries that is obvious only in hindsight—the animal and plant remains and waste that would decompose quickly in unglaciated regions are preserved, only to be released when the ice melts.
This new knowledge should give pause to
anyone wanting to slake his thirst with water that is “pure as the driven snow,” fresh from the melting snows of a glacier.

Dead birds and animals aren’t the only things stored in permanent icefields.
The transformation of snow crystals into the hard ice of glaciers has also left us a remarkable and detailed record of the changing climate of the current ice age, reaching back through several glacialinterglacial cycles.
Especially in Greenland and the Antarctic, where snow has been accumulating continuously for hundreds of thousands of years and the glacial ice is very thick, the history of local and global climate has been preserved, literally frozen into the ice, with high fidelity.
Deep cores have now been drilled there, the precious samples wrested from the inhospitable poles of the Earth, whisked into refrigerators, and distributed to laboratories around the globe.
The cores are layered with bands like tree rings, each band recording an annual cycle of summer warmth and winter snows.
It is possible to count the layers back into history, a thousand years, ten thousand years, a hundred thousand years.
In some of the layers, tiny particles of volcanic ash that have been carried by the winds to their final polar resting place record huge volcanic eruptions halfway around the world.
Throughout the cores, the ice crystals themselves have a story to tell—they provide a continuous meteorological record because their chemical properties depend on the local temperature when they formed.
And as the snow of thousands of years ago was gradually buried and recrystallized into ice, tiny bubbles of the ambient air were trapped.
With care, these miniature time capsules can be retrieved from the ice cores and analyzed.
It’s a bit like opening Tutankhamen’s tomb or finding an ancient petroglyph.
The bubbles provide a glimpse into the past, making it possible to take direct measurements of the atmosphere as it was thousands of years ago—including, among other things, its content of the greenhouse gases thought to be responsible for global warming.
From such analyses has come a detailed, and in many ways very sobering, record of how the Earth’s climate has changed through the past few glacial cycles.
As we shall see in this
book, some of the climate fluctuations documented from ice cores may have directly influenced the course of history, and possibly even the evolution of our species.

Figure 1.
The glaciers of the Pleistocene Ice Age have left their marks indelibly across much of the Northern Hemisphere.
This polished, scratched, and grooved bedrock in southern Ontario, Canada, is a testament to the abrasive power of flowing ice and the rocks, gravel, and “grit” embedded within it.
Photograph courtesy Professor Kenneth Hamblin, Brigham Young University.

The tremendous power of the ice age glaciers is something that is not easy to imagine.
Some inkling of their ability to shape the landscape can be gleaned from the icecaps of Greenland and the Antarctic, but most of us have never ventured to those remote regions and so have no way to gauge their effects directly.
However, at more temperate latitudes, the deep grooves and scratches that the ice age glaciers left in solid bedrock as they crept across the land are often still visible, silent but very graphic testament to their abrasive power.
At the base of a glacier several kilometers thick, similar to those that covered the northern parts of Europe and North America during the last glacial interval, the pressure is tremendous—roughly equivalent to that at a depth of several kilometers
in the ocean.
Should a glacial interval recur in the future, the ice would not just bury northern cities, it would simply scrape them off the surface of the Earth and, eventually and quite unceremoniously, dump the twisted and mangled remains far to the south.
Figures 1 and 2 give some idea, however imperfect, of just how powerful ice age glaciers are.

Figure 2.
Glaciers shape the landscape on both small and large scales.
This air photo shows a large expanse of land in the Northwest Territories, Canada, affected by Pleistocene glaciation.
Long glacier-produced ridges are lined up parallel to the direction of ice flow, which was toward the bottom of the picture.
By permission, Canadian National Air Photo Library.
Copyright Her Majesty the Queen in Right of Canada.

CHAPTER TWO

Fire, Water, and God

Louis Agassiz’s theory of a global ice age overturned the conventional wisdom about the Earth’s past.
It also bruised a few egos, especially those of scientists who had built their reputations on quite different versions of the planet’s history (in this respect, at least, scientific disputes seem not to have changed much over the intervening century and two-thirds).
They did not appreciate this brash young newcomer—a zoologist at that—with his revolutionary ideas.
Agassiz himself had anticipated opposition to his theory, but he was taken aback at the virulence of some of the critics.
He saw the geological evidence for an ice age as overwhelming, and he could not fathom how others could interpret it differently.
But many scientific advances have been similarly unpopular initially.
Often it requires the genius of a conceptual thinker, which Agassiz clearly was, to launch a new way of thinking about natural phenomena.
Just how clear—and, paradoxically, at the same time unnoticed—the evidence for glaciation was, even to naturalists who were accustomed to careful observation, is nicely illustrated by a passage from Charles Darwin’s autobiography.
In 1831, just a few years before the publication of Agassiz’s ice age theory, Darwin was on a field trip in Wales with a prominent British geologist.
Anxious to find fossils, they scoured hill and valley, examining the rocks in great detail.
But they
completely missed the evidence for glaciation that surrounded them.
Years later and by then fully aware that the Earth had experienced extensive glaciation, Darwin wrote about his earlier field excursion: “On this tour I had a striking instance how easy it is to overlook phenomena, however conspicuous,
before they have been observed by anyone
.
.
.
neither of us saw a trace of the wonderful glacial phenomena all around us; we did not notice the plainly scored rocks, the perched boulders, the lateral and terminal moraines.”

The italics above are mine.
They emphasize the common experience that is implicit in the phrase “point out the obvious.”
Some things are invisible until someone shows them to you; then they pop up everywhere.
Darwin marveled that he and his geological colleague could have overlooked these glacial features that he later found so conspicuous that “a house burned down by fire did not tell its story more plainly than did this valley.”
But it is also true that it would have required Darwin and his colleague to make a huge leap of understanding to interpret the glacial features correctly.
Agassiz argued for a global-scale ice age by extrapolation from observations he made studying glaciers in the Alps.
The true extent of the two great continental ice sheets that still existed on the Earth, in Greenland and the Antarctic, was still unknown.
It was to be more than a decade before explorers established that a single massive icecap covered Greenland.
With no well-known natural analog, it was difficult for most people to imagine the magnitude of the phenomenon that Agassiz proposed, and even harder to envision glaciers in places like Wales, with no high mountains and more than a thousand kilometers from even the small Alpine glaciers of Switzerland.

The ice age theory came to prominence in the 1830s.
Historians of science often refer to the final decades of the eighteenth and the early decades of the nineteenth centuries as the “heroic period” of geology.
Thousands of people—obviously very few of them actual practitioners of the subject—regularly turned out to hear lectures by the geological superstars of the early 1800s.
The Geological Society of London,
founded in 1807, had concerns that too many people would apply to join.
Considering that the middle class, although expanding rapidly, was still small, and the traditional educated elite smaller still, these facts speak impressively of a pervasive interest in the Earth and its history.
Geology was, in its early days, an activity in which almost anyone with time and the means to travel a bit could participate, and even contribute to.
Not only that, it offered an opportunity to get out into the countryside, away from the rapidly industrializing and heavily polluted cities.

This was also a time when European (and North American) philosophy, science, and society were undergoing rapid change.
The changes had actually begun a century or more earlier, but the pace was accelerating.
It was a time when science, based on mathematics, experimentation, and observation using new instruments such as microscopes and telescopes, began to challenge the status quo.
During the seventeenth and eighteenth centuries, momentous advances were made in understanding the “universe,” a term that at the time encompassed virtually all of natural science.
But it is worth remembering that the new discoveries were being made, not through a scientific enterprise anything like that of today, but largely by individual geniuses, some of them in universities, some in government, some independent inventors.
They were “natural philosophers,” not scientists in the present-day sense of the word.
Their philosophy, their view of the world, was a radical departure from earlier philosophies, which tended to be based in religion and founded on decree rather than reason.
Galileo was interrogated by the Inquisition for his heretical proposal, based on observation, that the Earth moved in an orbit around the sun and was not at the center of the universe.
But even giants such as Newton, while seeking truth through observations and mathematics, were at pains to point out that their work was consistent with, not antithetical to, a theological or scriptural foundation for the world, and especially for man.

The natural philosophers of the seventeenth and eighteenth centuries corresponded regularly with one another and met to present their work at gatherings such as those of the Royal Society in London.
Through networks of people with similar interests, their ideas gradually spread and gained prominence.
There was an understanding, at least in intellectual circles and among the educated segments of society, that one should be skeptical of dogma, and that logic and reason could solve many problems in all realms of life.
However, by the beginning of the nineteenth century, the century that would see the idea of ice ages proposed and eventually accepted, there was a considerable backlash against many of the tenets of the Age of Reason.
The idea that nature, man, and indeed the universe were governed by a set of physical laws was a concept that was simply too stark and inhuman for many people to accept.
The thought that there might not be any mystery, that there might not be a Divine Providence backstage pulling the strings, was contrary to everything that most Europeans believed.
They were also concerned that commercial and practical concerns, rather than spiritual ones, were becoming the driving forces for their governments, and that church was being separated from state—formally, in the case of the newly independent United States.
On top of that, Europe and North America were in the throes of an Industrial Revolution that was sending shock waves through society and changing the way thousands, if not millions, of people worked.
There was also political turbulence: first the American and then the French Revolution had overturned the old order.
One of the outcomes of all this turmoil was an upswing in religious conversions and the founding of a rash of new religious sects.

How did the young science of geology, which, more than most sciences, intersected with long-held religious beliefs, fit into this turbulent mix of cultural change?
And how were the developing ideas about ice ages affected?
This is not the place for a detailed history of the field, but some background is necessary to put the work of Agassiz and his forerunners in context.
As the title for this chapter hints, there were a few prominent themes that guided ideas about the Earth in the early days of natural science and geology.
One was spiritual, the idea that there was a creator who had constructed the Earth and everything on it.
In Europe and the Americas, this evolved from a strict adherence to the six
days of Creation described in the Bible to much looser interpretations as more and more geologic evidence accumulated that was contradictory to the biblical story.
Then there was a long-standing controversy about the origin of the rocks in the Earth’s crust: had they all been laid down in some primordial ocean, the land gradually emerging as the seas withdrew (this scenario too had religious overtones, because it was sometimes linked to the biblical Flood) or was there internal heat and fire that fused minerals and made the materials that we today call igneous rocks?