Today on Far Future Horizons we present a BBC Horizon
documentary concerning the Snowball Earth Hypothesis.
The Snowball Earth hypothesis puts forth the incredible
proposition that the Earth's surface became entirely or nearly entirely frozen
at least once, some time earlier than 650 million years ago. The Snowball
Earth scenario envisions the Paleo-Earth’s temperatures plummeting as a result of runaway glaciation.
Proponents of
the hypothesis argue that it best explains sedimentary deposits generally
regarded as of glacial origin at tropical paleolatitudes, and other otherwise
enigmatic features in the geological record. Opponents of the hypothesis
contest the implications of the geological evidence for global glaciation, the
geophysical feasibility of an ice- or slush-covered ocean,[2][3] and the
difficulty of escaping an all-frozen condition. There are a number of
unanswered questions, including whether the Earth was a full snowball, or a
"slushball" with a thin equatorial band of open (or seasonally open)
water.
For over fifty
years a group of scientists has been trying to prove this incredible period of
Earth history. Struggling against scepticism and disbelief, now finally the
many mysteries have been solved and the scientific community is slowly coming
around to the extraordinary idea not just of the dramatic freeze, but of an
equally dramatic thaw. Scientists across the world are starting to believe that
in the past the Earth froze over completely for ten million years... then
warmed up rapidly about 600 million years ago. Almost all life was wiped out.
But out of the freeze emerged the first complex creatures on Earth. Scientists
now believe that the so-called Snowball Earth theory could hold the key to the
evolution of complex life on this planet.
The discovery of
this theory is a classic scientific detective story. For decades there had been
a growing 'X-File' of geological anomalies haunting the scientific community.
Telltale signs of past glaciation have been found in places that should have
been much too hot - very near the equator. Even during the most severe ice age,
scientists believed that the ice only reached as far down as Northern Europe
and the middle of the USA.
So what could these tropical deposits mean?
Back in the
1960s one of the first climate modellers, Mikhail Budyko, stumbled on an
ingenious answer. Through some simple mathematical formulae, he calculated that
if the polar ice caps had spread past a crucial point, a runaway freezing
process would have followed, eventually freezing over the whole of the planet.
The idea fascinated scientists, but no one thought his runaway glaciation was
anything more than a theoretical result. Surely it had never actually happened
on planet Earth?
The idea
foundered because according to the model, once the Earth was frozen there was
no way out - the Earth would remain frozen forever. The big freeze would wipe
out all life; we would not exist today. It seemed patently absurd. But then
came a series of insights and inspirations from a geologist in California, Joe
Kirschvink, who came up with a brilliant solution - that volcanoes, protruding
above the frozen landscape, would have carried on pumping out carbon dioxide,
the greenhouse gas, even though the world had entered the deep freeze. On
Snowball Earth there was no rain to wash this carbon dioxide out of the
atmosphere. Instead it would have built up to higher and higher concentrations
- until eventually it sparked off not just global warming but global meltdown.
From the baking
landscape of Africa to ice-covered Antarctica,
Horizon follows the tale of a theory which, if true, would have huge
implications. Because scientists now believe this cycle of freezing and frying
may have created the unique conditions needed for the evolution of complex
life, including our own.
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