DID the polar regions once bask in summer heat waves, while the tropics froze
under ice? It鈥檚 possible, claim geologists in Australia and the US, who say they
have shown how the direction of the Earth鈥檚 spin axis could have changed at
breakneck speed.
There is evidence that ice sheets covered continents within just 10掳 of
the equator between about 800 and 600 million years ago. Why such tropical
regions froze has puzzled geologists. One explanation, known as the 鈥渟nowball
Earth鈥� theory, is that icecaps growing out from polar regions caused runaway
glaciation, covering the entire planet in ice a kilometre thick
(This Week, 5 September, p 11).
But Darren Williams of Pennsylvania State University in Erie is unconvinced.
He thinks many organisms that survived this period would have died off if the
entire Earth froze. 鈥淎ll photosynthetic life forms should have been wiped out,鈥�
he says.
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Instead, Williams backs an explanation for tropical glaciation suggested in
the 1980s. Suppose the Earth鈥檚 polar axis once tilted down towards the Sun at an
angle of 54掳 or more. Polar regions would then have received more solar
energy than the equator, and glaciers could have grown in the tropics, while the
poles stayed ice-free.
The trouble with this idea was that no one could explain why the tilt changed
to its present-day value of about 23掳 by around 430 million years ago, as
geological evidence suggests. How could the Earth鈥檚 axis shift through tens of
degrees in less than 170 million years? In this week鈥檚 Nature (vol 396,
p 453), Williams and his colleagues suggest how this could have happened.
If the polar axis was tilted towards the Sun 600 million years ago, as
Williams believes, ice would have built up around the equator. But the tilt of
the Earth鈥檚 axis has a small natural 鈥渨obble鈥�, and any small decrease in its
tilt would have warmed the equator and cooled the poles鈥攅specially as many
continents were clustered around the poles at that time, encouraging ice to
build up there.
This process would have slightly altered the shape of the Earth. It naturally
bulges out at the equator, but the polar ice sheets would have made it more
spherical. Using equations describing the Earth鈥檚 spin, Williams鈥檚 team
calculates that this could cause the Earth鈥檚 tilt to decrease even more. This
would create a feedback effect that could have moved the Earth鈥檚 axis to its
current position in the allotted time.
Paul Hoffman of Harvard University, a proponent of the snowball Earth idea,
is sceptical. A highly tilted Earth would have had trouble growing glaciers at
the poles, he says, because although the winters would be very cold, summers
would be hot. 鈥淭hat is not favourable for glaciation.鈥�
Bruce Bills of the NASA Goddard Space Flight Center in Greenbelt, Maryland,
calls the idea 鈥渁 very interesting and provocative hypothesis鈥�. However, he
points out that Williams鈥檚 theory depends on very specific climate conditions:
鈥淲hether the climate system actually behaved in that way is the biggest unknown
right now.鈥�
