Will
Earth freeze to death?
A
new continental drift could result in the end of human life, says
Gabrielle Walker.
Picture the world encased in ice more than a mile thick; temperatures
100 degrees colder than today; a blue planet made white. Many
researchers believe that the Earth experienced this super ice
age, dubbed "Snowball Earth", about 600 million years
ago.
If they are right, our home planet
can experience sudden lurches in climate more violent and deadly
than anyone had imagined. And, according to David Evans, a geologist
from Yale University in Connecticut, we could be heading for another.
In
a so-called Snowball Earth, the entire planet, including
the oceans, would be covered in ice. Human life would
perish; only simple life forms would survive
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The scale of this biggest of chills would be catastrophic, dwarfing
recent ice ages of woolly mammoth fame.
During these, ice reached only as far as New York. In a Snowball
Earth, ice stretches all the way from the poles to the equator,
the oceans freezing over.
Norse mythology has a word for it: after Fimbulwinter, the "winter
of winters" in which the Earth was gripped with ice, comes
Ragnarok, the end of the world.
This romantic mythology can't have been inspired by cold, hard
experience. When the last Snowball Earth occurred, there were
no sentient beings to witness it, since the planet was populated
by single-celled, primordial slime.
That was lucky for life, because such simple creatures are hardy,
and at least some of them made it through. The next one, though,
would be another matter.
It will not happen in our lifetimes. A good few hundred million
years will pass before the right conditions come around. But a
new Snowball would still be disastrous for the complex, multi-celled,
specialised animals that now populate the Earth - humans included.
Though the details of the deep freeze are sketchy, it's clear
that something catastrophic happened to the climate 600-700 million
years ago. Rocks dating from that time show that glaciers once
scraped over every continent; when they thawed, melting icebergs
dropped pebbles and boulders into the soft mud of the ancient
seafloors.
Geologists have known about this for decades, but because continents
drift around the Earth's surface, most people assumed that they
bore signs of ice because they must all have been clustered around
the icy poles.
But evidence has emerged that at least some of the continents
spent the freeze-over near the equator. Since this is the hottest
part of the Earth, a frozen equator means that everything else
must have been frozen, too.
That prompted many researchers to try to figure out why the entire
Earth iced over. The problem they face is not creating a deep
freeze - that turns out to be unexpectedly easy - but explaining
why it happens so rarely.
A Snowball would be easy in principle because white ice reflects
sunlight back to space, which cools the planet a little and causes
more freezing. If there is any ice at all on the planet - near
the poles, say - it should be able to grow and cool and grow some
more until there is a complete white-out.
Last year, Dan Schrag, of Harvard University, realised what usually
protects us from this fate. He suggested that high-latitude continents
act as a brake on any encroaching ice.
The normal arrangement of the world's continents is something
like the one we have today - with at least some land masses at
high latitudes stretching towards the poles. If the polar caps
ever started to grow, says Schrag, these nearby continents would
act as a barrier, to stop the ice in its tracks.
Continental rocks behave like a sophisticated planetary thermostat
preventing the world from overheating, soaking up greenhouse gases,
such as carbon dioxide, and stopping the Earth from getting too
warm.
But if high-latitude continents were covered with ice, this process
would stop, the Earth would warm up a little, and the ice would
be forced to shrink again.
Thinking about this, Schrag wondered what would happen if all
of the continents happened to lie in a band around the equator.
In that case, he realised, ice could reach out with impunity from
both the north and south poles. It could grow and cool and grow
unchecked until it became unstoppable.
That, he suggests, is what happened 600 million years ago. What's
more, such a specific arrangement of the continents would be very
unusual since they drift around the Earth's surface at random,
which would also explain why Snowballs are so rare.
If Schrag is right, this should be encouraging news; with Canada,
Russia and northern Europe we have plenty of high-latitude land
masses to protect us.
However, David Evans believes that that peculiar continental
arrangement 600 million years ago was no accident. He thinks that
an extraordinary event sent every single continent flying towards
the equator, and provided the ultimate trigger for the Snowball.
That event, says Evans, may have been the building of a supercontinent.
Every so often, thanks to the random jitterbug of continental
drift, all the landmasses collide into one giant lump. One of
these - called Rodinia - occurred just before the last Snowball
Earth, and Evans believes it made the planet dramatically unstable.
Rotating objects always prefer to have most of their weight around
their middles, which is why it's easier to knock over a tall,
thin spinning top than a short fat one. If the instability is
too much to handle, the object will try to readjust until the
excess weight is spinning around its waist, and the system is
safely back in balance.
Evans believes that the same applies to our spinning planet.
He thinks the imbalance caused by Rodinia tipped the Earth's crust
over and sent all the land masses racing for the equator at what
for them was the break-neck speed of several feet a year. (Normally
they travel at only an inch or so a year - the same rate that
fingernails grow.)
This idea is alarming because we're building another supercontinent
right now. Africa is shoving its way into Europe, which is already
joined at the hip to Asia. India is crashing into southern Asia
and Australia is heading north to join the fray.
And, according to one attempt at constructing the future, over
the next few hundred millions years, the Atlantic will begin to
close again bringing North and South America back into the fold,
and Antarctica will head north to join India.
If Evans is right, the new supercontinent would lurch to the
equator, break apart, and another Snowball would be on its way.
The frozen polar oceans would begin to reach out with tentative
feelers of ice.
Finding nothing to halt them, they would spread like a disease
until they became unstoppable. Global temperatures would plummet;
rain would stop; clouds no longer form. The white-out would be
complete.
For complex creatures, the result would be disastrous. But this
wouldn't be the end of all life on Earth any more than the last
one was. Even after Ragnarok, say the myths, a new world will
arise filled with good things and devoid of wickedness.
The Earth is a born survivor; it's we humans who are the fragile
ones.
Gabrielle Walker's book Snowball
Earth is published on April 7 by Bloomsbury and is available for
£14.99 plus £1.99 p&p. To order call Telegraph
Books Direct on 0870 155 7222.
She will lecture on Snowball Earth at the Royal Institution in
London on May 7 at 7.30pm and at the Cheltenham Festival of Science
on June 7.
To request a brochure for the Cheltenham Festival of Science,
June 4-8, telephone 01242 237377 or email festival [email protected].
Bookings can be made on 01242 227979.
4 april
2003

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