DOWN UNDER - Snowball Earth - page 73

The Big Tilt itself, after all, faced plenty of problems. For one thing, it couldn't explain nearly as
much as the Snowball purported to. The Tilt accounted only for the equatorial ice and the strange sand
wedges. It said nothing about the cap carbonates, the isotopes or the ironstones.
More seriously, as Paul quickly pointed out, there was no easy mechanism for righting the Earth.
Tilting the planet in the first place would have been quite straightforward if it had happened early
enough. When the solar system was born, there were plenty of planet-sized chunks of rock flying
around the place, crashing into each other like giant pinballs. Most scientists believe that an almost-
planet the size of Mars smashed into the young Earth, creating our moon from the debris. A collision
like this could easily have knocked the Earth over on to its side. But in the relatively sedate time of the
past few hundred million years, what could have righted the Earth to the more gentle tilt that it bears
today? By the end of the Precambrian, any builder's rubble from the early solar system was long gone,
and there's no other easy way to move the Earth back upright. 5
Of course, saying we don't know how the Earth could have righted itself doesn't prove George's
theory to be wrong. Until recently nobody knew what exactly was causing the continents to drift,
though they were still moving for all that. But the Big Tilt had a bigger problem, one that Paul seized
on gleefully. The evidence for glaciation comes in one particular time slice, right at the end of the Pre-
cambrian. So George couldn't rely on the creation of the moon to knock the Earth over. Instead he had
to propose that something else had suddenly tilted the Earth around 700 million years ago, billions of
years after the formation of the moon. And that something else again had caused the Earth to right
itself abruptly again a couple of hundred million years later. That's an even bigger stretch of the mech-
anism problem.
But there still remained the vexing issue of the sand wedges. How did a place that was within per-
haps ten degrees of latitude of the equator experience seasonal changes? Paul did his best. He came up
with a possible explanation involving glaciers that alternately surged forwards and drew back, some-
times insulating the ground they covered from the bitterly cold air and enabling it to thaw a little,
sometimes exposing it for another freeze cycle. Not many people were persuaded by this, though. It
sounded rather too much like special pleading.
Salvation came from a different source, and showed that even Paul had been too conditioned by
how the Earth works today. The key turned out to be how different the Earth's seasons would have
seemed on a planet that was blanketed in ice.
By 2001, veteran climate researcher Jim Walker, from the University of Michigan, had become
intrigued by Paul's Snowball theory, and began tinkering around with a simple climate model to try to
figure out what the weather would have been like. He picked the most extreme of Paul's conditions—a
globally frozen ocean—and set the model on its way. Day to day in Jim's model, the weather was
rather boring. Nothing much changed. There were no travelling storms and no temperamental weather
patterns. At every point on the surface of Jim's model planet, one day was pretty much like the next.
The Snowball must have been a little like Mars, he says. Apart from the occasional dust storm, the
whole Red Planet just settles down into a placid, predictable weather pattern. “The wind there always
blows from the same direction at four o'clock in the afternoon.”
But to Jim's astonishment, the seasons on his model Snowball were an entirely different matter.
They were exaggerated, larger-than-life versions of the seasons we are familiar with. At any one point
on the frozen surface, there was a huge temperature difference between winter and summer, much big-
ger than we see today.
Why? Well, our modern wet and windy Earth has an ingenious built-in mechanism to guard against
extremes. If the climate everywhere simply depended on the direct overhead sunlight, the tropics