Cold (11 page)

This Ice Age at its worst, when what would become New York City was under ice, and woolly mammoths strolled over what would
become great cities, was not as horribly cold as it might have been. For truly cold weather, one has to go back seven hundred
million years, back to the time of Snowball Earth.

It is September twenty-third. Back in Anchorage, leaves on the birch trees have turned yellow. It has been raining while I
was away. It is raining now. Everything is soaking wet at the lower elevations. Everyone is speculating about a very snowy
winter. “A few degrees colder,” people say, “and this would be snow.” In the mountains, at elevations above the road system,
snow already covers the ground. A certain class of women, not quite able to wait for the really cold weather, already wears
fashionable full-length coats. People are putting on their spiked snow tires. The spikes click and clack as they contact wet
pavement.

My pet caterpillars, Fram and Bedford, stayed in the refrigerator while I was in Britain. They seem a bit subdued but otherwise
fine. I give them fresh willow leaves to eat. They crawl on the leaves, but they have no appetite. For them, it is close to
winter. It is nearly time to freeze up.

Friends come to dinner. One of them brings me a copy of the
Anchorage Daily News,
dated September ninth. I had been driving on the ninth, crossing Scotland’s glacier-formed landscape. “Lodge Owner Trapped
by Lawnmower Dies,” the headline says. Andrew Piekarski had been cutting the grass at his lodge thirty miles from Anchorage.
He was on a riding mower. Nighttime temperatures in his area were dropping into the thirties. On a small hill, the mower toppled
and pinned him to the ground. He lay under the mower all night, unhurt but trapped and slowly freezing to death. A state trooper
was interviewed. “He couldn’t get it off his legs,” the trooper reported, “and he couldn’t get out from under it and he died
from exposure, from hypothermia.” As Piekarski slowly froze, there would have been time for existential thinking. Before the
hypothermia slowed his thinking, before the stupor, before the hallucinations of warmth, he must have considered the absurdity
of his situation, the odd reality of deadly hypothermia before the end of summer.

I show off Fram and Bedford to my dinner guests. The two caterpillars remain listless. It is clear that they are ready to
sleep. I have two jewelry boxes, and I line each with willow leaves. I put Fram in one box and Bedford in the other. After
a brief ceremony, I put them in the freezer, behind the frozen peas and under a slab of salmon, condemning them to a snowball
cocoon for winter.

Reconstruction of past climates is not a simple business. Even today, three centuries after Daniel Fahrenheit developed the
mercury thermometer, understanding recent global climate change leads to controversy. Which thermometers are reliable? Do
thermometers near cities reflect local warming or real patterns in global temperature change? Do the number of measurements
taken in the Northern Hemisphere outweigh and overwhelm those taken in the Southern Hemisphere? Is the average temperature
the important number? Or the hottest day? Or the coldest day?

There are dozens of ways to reconstruct past climates. In western Europe, the timing of the grape harvest has been recorded
for hundreds of years. Late harvests reflect cold, wet summers. Emmanuel Le Roy Ladurie, author of
Times of Feast, Times of Famine: A History of Climate Since the Year
1000, wrote, “Bacchus is an ample provider of climatic information. We owe him a libation.” Where records exist, grape harvest
data can be backed up by grain harvest data. Another climate historian visited forty-one art museums to look at more than
six thousand paintings dating from the beginning of the Little Ice Age. In the paintings, he saw a slow increase in cloudiness
from the late 1400s until about 1750. Low clouds settled in after 1550 but cleared away around 1850.

Much can be said without written history or paintings. Tree rings can be measured, often stretching back a century or more.
Corals, too, leave growth rings. In both cases, wide rings indicate good growing conditions, while narrow rings indicate cold.
The chemical composition of ice cores says something about the climates when the ice formed. Cores pulled from Antarctic ice
can span four hundred thousand years. Past sea levels say something. When it is cold, water is locked in ice, and sea levels
drop. River channels extending out onto today’s continental shelves speak of lower sea levels during past cold snaps and glacial
periods. Beach terraces on hillsides speak of higher sea levels and past warm spells and interglacials.

And there is geology. There are erratics and moraines. There is bedrock scored by the action of ice. There are fossils of
animals and plants with known temperature tolerances. There are sand wedges, originally formed as long-ago ice wedges, identical
to those of today’s Arctic. There are dropstones — stones carried by ice over a sea or a lake and dropped to the bottom when
the ice melts, there deforming soft mud, leaving an unmistakable sign of ice and cold. There is topographical graffiti left
by massive flooding that followed warm spells and broken ice dams. The Channeled Scablands of eastern Washington State were
scrawled across the landscape when the ice dam that formed Lake Missoula melted thousands of years ago. Water from the lake
ripped across what is now the northwestern United States. For a few hours, water flowed at a rate something like sixty-five
times that at which today’s Amazon River flows. During these few hours, soil disappeared and boulders were suspended like
particles of clay. Gouges hundreds of feet thick were cut into the earth.

There is this inescapable fact: the farther back in time one goes, the more speculative climate reconstruction becomes. It
is easy enough to find signs of the most recent glaciations of the late Pleistocene Ice Age, to see hints of its many glacial
and interglacial swings, but dip farther into prehistory, and records are obliterated. Rocks are worked and reworked, clues
are muffled, and fossils are scarce or even absent. The earth has been around for something like four and a half billion years.
It took the first billion years for life to invent itself. This was simple life, more like a living slime than something biblical.
It took another three billion years for more complex life to form, the kind of life that leaves abundant fossils, the kind
of life with hard shells and bones and exoskeletons. Throughout this time, continents drifted. Tectonic plates rode about
like loose barges, occasionally colliding, one forced downward and one upward. And there was erosion from wind and water and
ice. The rocks enshrining the planet’s history were recycled, cast away like dusty books remaindered from a publisher’s warehouse.

Exactly how, then, does one reconstruct the climate of seven hundred million years ago or a billion years ago or two billion
years ago? From work emerging over the past fifty years or so, but really coalescing over the past ten years and largely through
the efforts and personality of one man, the idea of Snowball Earth has taken hold. It is science at its ugliest, when evidence
is scarce and inconsistent, when speculation and ego and charisma mix with observations, when data are insufficient to unambiguously
confirm or refute ideas. And for now, the speculation and ego and charisma of Paul Hoffman — backed up by a certain amount
of hard-won data from remote locations in the Canadian Arctic, the Namib Desert, and Australia’s Flinders Ranges — has won
the day. The earth of seven hundred million years ago, Hoffman believes, was frozen from pole to pole, one more or less continuous,
frigid, blood-thickening, numbing ball of ice: Snowball Earth.

His evidence is in the geology of Precambrian rocks. It is the same sort of evidence left by the Pleistocene Ice Age, but
the rocks are much older and occur at all latitudes. The tropics, seven hundred million years ago, were not so tropical. The
oceans were frozen over. One could have ice-skated between tropical islands. Across much of the earth’s surface, a mercury
thermometer would be of little use because the mercury would be frozen solid. Mean global temperature could have been something
like minus sixty degrees. At the equator, temperatures would have hovered around negative ten, what Apsley Cherry-Garrard
would have called forty-two degrees of frost. The planet would have been only slightly more hospitable than, say, Mars.

As early as the 1870s, people were finding scattered evidence of long-past ice ages, ice ages much older than Agassiz’s original
Ice Age. There was, for example, a paper by H. Reusch published in 1891 with the title “Skuringmærker og Morængrus Eftervist
i Finn-marken fra en Periode meget Aeldre end ‘Istiden’ ” — in English, “Glacial Striae and Boulder-Clay in Norwegian Lapponie
from a Period Much Older Than the Last Ice Age.” In 1948, the Antarctic explorer Sir Douglas Mawson spotted signs of ancient
ice ages in Australia. “Verily,” he told the Royal Geological Society of Australia, “glaciations of Precambrian time were
probably the most severe of all in earth history; in fact, the world must have experienced its greatest ice age.”

Others slowly built on the idea of an ancient ice age, but it took the ambition, ego, and entrepreneurial maneuverings of
Paul Hoffman to bring the idea home. He has been quoted as saying, “Everyone’s entitled to my opinion.” During a scientific
debate, he once challenged a Snowball Earth naysayer to “try me out in the Boston Marathon some year.” And his own self-assessment,
according to a biographer: “Gosh, I’m awful. I don’t know how I’d react to me.” But the history of science shows that new
ideas take hold because they are pushed by strong personalities. In 1998, Hoffman, with a couple of coauthors, published “A
Neoproterozoic Snowball Earth” in the prestigious academic journal
Science
. The idea and the people behind it were immediately attacked, both verbally and in print. The “Snowball” jargon had charisma,
but it was a charisma that worked both ways. Opponents wrote papers with titles such as “The Snowball Earth Trip: A Neoproterozoic
Snow Job?” and “Has Snowball Earth a Snowball’s Chance?” Some talked of a “Slushball Earth,” not quite as cold as Hoffman’s
Snowball and with oceans that were not entirely frozen over. Others rejected the idea entirely. Still others have gone in
the other direction, delving farther back in time, suggesting that an even colder Snowball Earth circled the sun some two
billion years earlier than Hoffman’s did.

Hoffman has been awarded the Logan Medal by the Geological Association of Canada, the Miller Medal for outstanding research
in earth science by the Royal Society of Canada, and the Alfred Wegener Medal by the European Union of Geosciences. He is
a member of the National Academy of Sciences. He is a professor at Harvard. But he also has been accused of founding the Church
of the Latter-day Snowballers. He stirs up bitter debate. It has been said that he sometimes alienates people who were once
friends. He inspires passions, both positive and negative. He is, in short, a successful scientist. He has, if nothing else,
made the world of science consider the possibility of a chilled earth, an icicle planet, one big ball-shaped skating rink
in bad need of a tropical vacation.