SNOWBRAWLS - Snowball Earth - page 86

At first, Paul wouldn't hear of this. The oceans were fully covered with ice, and that was final. But
once again he was forced to change his mind. And the impetus for this came from a rival ice world, a
pretender to the Snowball crown, which began to tug at everyone's attention. This was a newer, gentler
snowball, with a moniker of its own: “Slushball Earth”. Climate modellers created it. They use com-
puter programs to do what geologists can't— rerun the Earth's experiment and see what happens. As
soon as they heard about the Snowball, they fired up their machines and tried to make one.
They couldn't.
However much the modellers wanted to generate an ice-covered world, their computers wouldn't
oblige. Modelling had developed into a much more sophisticated affair since Mikhail Budyko's prim-
itive attempts first turned up the “ice catastrophe” back in the 1960s. And the modern models stuck
at a sort of halfway house, where ice advanced to somewhere near the tropics, but no further. A few
models could generate ice on land near the equator—which would explain the ice rocks from Aus-
tralia. But the equatorial oceans remained stubbornly ice-free. 10
So the modelers began to talk of an alternative to Paul's “hard” Snowball, a new, softer variant.
Nick Christie-Blick thought this Slushball was a wonderfully moderate solution to the Snowball
conundrum; neither one extreme nor the other, it was a comfortable answer. It could also explain some
of the evidence that had consistently bothered Nick. For instance, in many parts of the world the ice
rocks are hundreds of feet thick. To make those deposits, icebergs had to be free to wander offshore
and melt and drop their loads on the seafloor, and they certainly couldn't do this if the oceans were
fully frozen. Paul argued that the ice rocks formed at the beginning and the end of the Snowball, when
there was still a little open water. But Nick felt that to make such thick deposits, the process would
have to continue throughout the Snowball. Open oceans at the equator, he felt, provided the perfect an-
swer. For the biologists, too, the Slushball was just right. This, they felt, was exactly what life needed.
Paul, however, hated the Slushball. He called it “Loophole Earth”, and said the models needed a
few reality checks of their own. The Earth's climate is insanely complicated, and nobody claims that
its every nuance can be encapsulated inside a computer. Modellers are good at reproducing today's
climate mainly because they can compare their model output with records of temperature, wind and
weather. But the only Precambrian weather reports are the ones written in rocks. And according to
Paul, the Slushball came nowhere near explaining this geological evidence. It couldn't account for the
ironstones, the cap carbonates, or the strange chemical signatures in the rocks. Most important of all,
it couldn't explain the extremely long duration of the ice.
Paul pointed in particular to the findings of Nick Christie-Blick's graduate student, Linda Sohl.
Her magnetic work had shown that the glaciations must have lasted at least hundreds of thousands, if
not millions, of years. The Slushball, said Paul, simply couldn't last that long. It was precarious, like
a pencil balanced on its tip. Nudge the model world one way or another, and you would quickly force
it to choose: Snowball or no-ball.
If you cooled the Slushball a little, Paul said, ice would quickly take over. White ice reflects sun-
light, which cools the Earth, which breeds more ice in a runaway cycle, which, Paul said, would freeze
over the tropical ocean. Warm the Slushball a little, on the other hand, and its ice would soon vanish.
Warming melts ice, which opens up dark patches of ocean, which absorb more sunlight until all the
remaining ice races back to the poles.
What, then, was Paul's explanation for how life survived the ice? Well, living things are ex-
traordinarily resilient, especially simple ones. Bacteria survive—somehow—at the South Pole. Other
bacteria have shown up beneath glaciers, and even inside solid rock. Unknown to the authorities, a
small colony of Streptococcus mitis hitched a ride to the Moon in 1967 inside an Apollo TV camera,