Frozen Earth: The Once and Future Story of Ice Ages (30 page)

From that first core in Peru, Thompson never looked back.
In the intervening years, he and his colleagues have brought back high-altitude ice cores from China, Bolivia, Tibet, Alaska, and Mt.
Kilimanjaro itself.
But there was a price to pay in physical hardship—in the tropics and subtropics, permanent glaciers can only exist at extreme altitudes, and the logistics of living and drilling at such heights are equally extreme.

Thompson’s record was in Tibet, where, in 1997, he and his crew drilled into a glacier at 7,200 meters above sea level—an amazing feat.
Even in the cold of that high elevation, there is the question of whether there might be gaps in the record—warm years when the winter snowfall melted away in the summer sun.
In the small mountain glaciers, annual layers in the ice are not nearly as pronounced as they are in the polar ice caps, and dating is difficult.
Often a few marker layers—dust from a volcanic eruption that has been dated elsewhere, or a change in some chemical parameter that is recognized in other cores—are the only reliable sources of age information.
Still, it is clear that some of the mountain glaciers contain ice that dates back well into the last glacial period.
A core from Bolivia penetrated ice that appears to be at least 25,000 years old, and, although there is controversy about the exact ages, Thompson believes that some of the Himalayan glaciers contain ice that is hundreds of thousands of years old.

Perhaps the greatest value of the ice cores from mountain glaciers is that they provide an almost global coverage.
Early workers on glacial climates assumed that the tropics probably enjoyed a fairly stable climate even while high latitudes suffered through the extremes of the Pleistocene glacial-interglacial cycles.
Proxies for temperature in
deep-sea cores from the tropics seemed to be in agreement with this conclusion, and in any case, it had been well known since Milankovitch made his calculations that variations in the incident solar energy in the tropics during orbital cycles are much smaller than they are nearer the poles.
The ice-core data from Greenland and the Antarctic provided accurate information about the large temperature shifts at high latitudes.
But until the work of Thompson—now joined by several other groups drilling mountain glaciers—there were no such data for regions nearer the equator.
Studies of the low-latitude glaciers, combined with new approaches to studying ocean sediment cores from the tropics and large-scale computer simulations of the Earth’s climate, have now shown that equatorial regions play a more important role in climate change than was believed earlier.
This seems to be true of changes that range all the way from the glacial-interglacial cycles of the ice age to the four-or five-year timescales that characterize recurrent El Niño events.

The ice cores from low-latitude glaciers provide a record of changing local climate that stretches far back into the past, but the work has also focused attention on the present-day fate of these ice caps.
Most of them are melting—fast.
Lonnie Thompson and his colleagues, after completing a drilling project on Mt.
Kilimanjaro that recovered ice cores spanning 11,700 years of climate history for tropical Africa, made headlines around the globe when they predicted that the famous glacier would be gone within a few decades if the current climate persists.
They noted that during the twentieth century, the extent of Kilimanjaro’s ice cap had decreased by 80 percent (some of this decrease may be due to local conditions rather than global warming).
Large-scale shrinkage has been documented for other low-latitude glaciers as well.
This melting back is not just a regrettable natural phenomenon, or loss of another tourist attraction for curious sightseers.
Nearly three-quarters of the world’s population lives in the tropics.
Some depend on glacial meltwater for at least part of the year; it may not be a reliable source for very much longer.

CHAPTER TEN

Ice Ages, Climate, and Evolution

Mrs. Antrobus: What about the cold weather?

Telegraph boy: Of course I don’t know anything . . . but they say there’s a wall of ice moving down from the North, that’s what they say. We can’t get Boston by telegraph, and they’re burning pianos in Hartford.

. . . it moves everything in front of it, churches and post offices and city halls.

Thornton Wilder,
The Skin of Our Teeth

Weather is always a topic of conversation, and even today’s city dwellers, quite insulated from the natural world, tune in to the Weather Channel to learn the latest about weather in their region or across the globe.
Climate is simply average weather on a long timescale.
Thornton Wilder’s play
The Skin of Our Teeth
deliberately mixes geological periods—modern Americans, dinosaurs, and an ice age exist simultaneously—and weather, as often in literature, is a metaphor for crisis and conflict.
Some anthropologists and biologists think that in real life, it is much more than a metaphor: that climate—especially the climate of the Pleistocene Ice Age—has had a direct effect on human evolution.
The arguments are circumstantial and have to do mainly with timing.
But more complete fossil records for other species show strong links
between evolution, extinction, and the glacial-interglacial climate swings of the Pleistocene Ice Age.
And much farther back in the Earth’s history, ice ages may also have had an important influence on the way life evolved on our planet.

The evolution of modern human beings can be traced back through a number of species, collectively known as the hominids, to a common ancestor with the chimpanzees, our closest living relatives among the primates, some five or six million years ago.
As far as we can tell, chimpanzees and other members of the family of “great apes” have not changed radically from that distant ancestor.
We, on the other hand, have changed a lot.
The interesting questions are, Why?
and, How?
Clues to these questions are hard to come by, and we may never know the answers with complete certainty.
But the timing at least suggests an interesting connection with climate and the Pleistocene Ice Age.
For several million years after our common ancestor, hominids evolved slowly.
They developed the ability to move around on the ground with an upright posture, although their body structure suggests that they were still expert tree climbers.
Then, right around the time when the Earth’s average temperature plunged downward at the beginning of the Pleistocene Ice Age—about three million years ago—the rate of change accelerated drastically.
Hominids quickly evolved away from their apelike ancestors, developing increasingly sophisticated tools and weapons, hunting, planning, complex language, and eventually agriculture, writing, airplanes, and computers.
During that time there was an increase in brain size by more than a factor of three in less than three million years, a breathtakingly rapid change compared to the normal course of evolution.
That change took place entirely within the Pleistocene Ice Age.
Is there a cause-and-effect relationship?
The detailed climate records from deep-sea sediments and polar ice cores discussed in chapter 9 have provided evidence that is, at the very least, suggestive of a link.
This raises the question, Would we be here at all were it not for the Pleistocene Ice Age?
Exploring that question can be a bit unsettling.
Based on the evidence that is currently available, it
seems possible that we are here, not because we won out over other species in some survival-of-the-fittest battle, or because of an inevitable march of evolution toward higher intelligence, but because of the fickle ice age climate of Africa.
If this turns out to be true, the evolution of our species really was a roll of the dice, a chance outcome that could not easily have been predicted.

To investigate this premise requires a closer look at several aspects of the problem: What is it that really makes us human, and when in our history did these traits evolve?
And, what was the ice age climate really like as these characteristics arose, and can a logical connection be made?
Quite a few biologists, paleontologists, and anthropologists have examined these questions, especially in the recent past.
Their main impetus has been the unequivocal coincidence in timing between the start of the Pleistocene Ice Age and the beginning of rapid change toward modern humans among the hominids.
But this single observation could indeed be coincidence.
After all, most of the fossil evidence for early hominid evolution comes from tropical Africa, a region far removed from the large-scale continental ice sheets of the polar regions.
The climate cooled in tropical Africa as the ice sheets expanded, but temperatures remained moderate.
And at higher latitudes, where temperature changes were much more extreme, many species dealt with the ice age climate swings, not through rapid evolutionary change, but simply by migration to regions of more equable climate.
One would not, a priori, expect annual temperature changes of a few degrees Celsius to have affected early hominids very severely.

But let’s look first at those characteristics that are generally agreed to distinguish us from all other creatures on this Earth.
First is an upright posture, the ability to walk on two feet rather than four.
The second is more important: intelligence that is much more advanced than that of even our closest relatives among the primates.
It is that developing intelligence, linked to the rapid increase in brain size observed among the fossil hominids, that led to highly coordinated body movement, sophisticated toolmaking abilities, and the emergence of complex
language as a means of communication.
Yet another characteristic of humans is that we mature very slowly compared to most animals.
A human infant is completely helpless at birth, wholly dependent on its parents for survival, incapable even of locomotion for a year or more.

In the classical view of evolution as a gradual process that winnows out undesirable characteristics and preserves useful ones, none of the physical traits just mentioned—bipedalism, a larger brain, slow maturation—would seem to be especially advantageous.
Walking or running on two feet, especially with the waddling gait that must have characterized early hominids with hip structures that had not yet fully adapted to upright posture, would not be a very effective way either to escape fleet-footed predators or to chase fleeing prey.
Bigger and bigger heads at birth could mean death for both mother and child—a very effective roadblock to proliferation of any species, especially one with a small number of offspring.
Helpless infants are not an efficient way to propagate a species either—they are unable to escape predators on their own.
And yet our species evolved with those characteristics.
Several scientists, notably the paleontologist Steven Stanley and the biologist-neurophysiologist William Calvin, have developed plausible scenarios for the evolution of
Homo sapiens
with these traits, scenarios that explicitly make a connection with ice age climate.
A good deal of what follows is drawn from their writings.

Darwin was the first to recognize that humans had evolved from primates much like the present-day apes of Africa, which he thus viewed as the probable cradle of our species.
His ideas about our evolution from apelike predecessors, now known from fossil evidence to be wrong in detail, are often portrayed in popular culture by the frequently reproduced cartoon of an ape gradually achieving upright posture and ending with a modern human in a suit carrying a briefcase.
In reality, even our distant ancestors, the australopithecines, had upright posture.
This is evident from their fossilized skeletons, and was dramatically confirmed by the discovery in Tanzania of a set of fossil footprints preserved in volcanic ash.
A pair of barefooted, bipedal
australopithecines strolled across a bed of soft volcanic ash that was wet from recent rain.
The ash, like wet sand on a beach, made perfect casts of their feet.
Their footprints—and those of other, smaller animals that passed the same way—were perfectly preserved when another layer of ash was deposited over them.
We are fortunate that the eastern African home of these early hominids is an area of active volcanism, because volcanic ash layers are ideal for dating.
They are often spread over a wide region essentially instantaneously, and they contain freshly crystallized minerals that can be separated and dated.
The Tanzanian ash has been dated to 3.2 million years ago, and the footprints it contains gives us a tiny, random glimpse into the lives of our ancestors.
That something as ephemeral as a footprint should be preserved over such a great span of time might seem surprising.
But in fact fossil footprints, although not common, are found throughout the geological record.
Dinosaurs, especially, are well represented in this way—and some of their preserved footprints are more than 100 million years old.

Australopithecines like the ones that left their tracks in Tanzania had small brains, similar to those of modern-day apes.
As far as can be gleaned from the fairly sparse fossil record that remains,
their
ancestors had also had similar-sized brains for many millions of years.
Things were going pretty well for the australopithecines.
They were largely vegetarian foragers; the African forests provided both food and shelter from terrestrial predators such as the large cats.
Although they could walk upright, australopithecines were also expert climbers.
They had long arms that they could wrap around a tree trunk to help them shinny upwards like a telephone repairman.
They were, in evolutionary terms, in a period of stasis.
Small differences evolved among them, but they were on the whole in balance with their environment, with little pressure for radical change.
But looming ahead was a crisis that would very quickly put an end to this comfortable situation.

In the 1950s, the eminent biologist Ernst Mayr developed the idea that new species often evolve when some small subset of an existing population becomes isolated from the rest of their species.
In such
circumstances, change can occur rapidly, especially if some trait or physical characteristic is favored reproductively—in other words, if individuals with those characteristics are more successful breeders.
Eventually, the isolated population evolves so far from its ancestors that it can no longer interbreed with the parent population and a new species has been born.
Sometimes the new species dies out, but often the newcomers eclipse their parents.