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

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