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greasy slick. The surface thickened and froze into a thin transparent layer. As this layer thickened, it
grew grey and then opaque from salt and air bubbles that filled its inner voids. In some places the
greasy ice congealed into large round pancakes, with raised edges like giant lily pads, where they
bumped and smashed against each other. And, a nice touch this, the fresh young sea ice grew a coating
of frost flowers, each one the size and shape of an edelweiss.
Sea ice bends. Unlike freshwater ice, which can shatter like glass, the ice that forms first on the
surface of the sea is elastic. When you try to walk on it, your legs unexpectedly buckle. But as it thick-
ens it becomes reassuringly firm, like solid rock.
Though sea ice is grey when it first forms, it whitens year by year as its brine drains back into the
sea. Even grey young ice is often dusted with white snow. But a frozen ocean is far from monochrome.
Gashes of open seawater, created as the pack ice is ripped apart by wind and weather, expose the deep
turquoise roots of the floating sea ice. And the dark ocean reflects in the clouds, streaking them the
colour of a bruise. “Water sky” this is called, and polar sailors have long used it as a clue for where to
point their ship next as they navigate perilously through the pack.
Where waterways have frozen over, the ice is smooth and level. Where the edges of an old water
wound have been cauterized together again, untidy piles of ice blocks are an astonishing bright blue.
Ice cracks suddenly like a whip. Sometimes pack ice groans and creaks as the wind crams floes to-
gether or prepares to break them open. But for the most part, the frozen polar oceans are shrouded in
silence—eerie and absolute. There is no scene more alien on Earth.
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For perhaps a few thousand years, the white menace stole unheeded towards the equator. Earth's
primitive life-forms had neither the eyes to see the encroaching ice nor the wit to fear it. Most of them
lived their dull lives in a band around the Earth's waist, and as the ice advanced steadily from the far
north and south, they bathed unconcerned in the warmth of their shallow, equatorial seas.
An occasional storm might have whipped up waves near the shore. Perhaps the surf tore at the rub-
bery microbial mats that coated the seafloor and sprayed nearby rocks with scraps like soggy chicken
skin. Stromatolites built up their stone reefs, layer by microscopic layer. Geysers blew. Rain fell. The
sun shone again. There was no hint of the devastation to come.
But when the ice reached the tropics, its slow creep became a sprint. In a matter of decades, it en-
gulfed the tropical oceans and headed for the equator.
Ice spread out from shallow bays and grew first a skin, then a carapace over the oceans. It clung
to the beaches and scraped the mats on the seafloor. In some places this shell was still thin enough to
crack and seal again. In others it was thousands of feet deep.
For a few hundred, perhaps even a few thousand years after the oceans were capped with ice, the
land remained bare. But ice began to accumulate, gradually, in the thin air of mountain ranges, creating
great frozen rivers that flowed down to fill the surrounding valleys. In the end, the whiteout was com-
plete. Earth's surface looked like the frigid wasteland of Mars, or one of Jupiter's ice-covered moons.
Sunlight bounced off the bright surface and was dazzled back into space. The mercury hit a staggering
minus 40 degrees C. (Or it would have, except that at those temperatures mercury itself would have
frozen.) There was little wind or weather of any kind. Clouds, by and large, disappeared, save perhaps
for tiny ice crystals high in the atmosphere, which scattered sunsets into strange, lurid colours, blue
and green, rimmed with vibrant pink. No rain fell and little snow. Every day brought silent, unremit-
ting cold.
The Snowball wasn't just another humdrum old “ice age” like those from more recent eras. The
events we call ice ages were merely brief cold blips in an otherwise fairly comfortable world. There
