Vast sheets of ice that threatened to
freeze much of the Earth may have been turned back by tiny changes in the level
of sunlight, scientists have found. The finding could be a vital breakthrough in
solving one of science's great mysteries.
Researchers have long known
the planet has gone through about 20 ice ages in the past 2.5m years. What they
have been unable to work out is why each of those ice ages ended.
The
mystery was solved when a boy playing football in Nanjing, China, suddenly fell
through a hole in the ground. He found himself in a cave that contained
stalagmites dating back tens of thousands of years.
In theory, when
much of the planet is covered in ice, it should reflect so much sunlight back
into space that it might never warm up again. In reality, however, ice ages have
culminated in a rapid global thaw that has left Europe and America basking in
warmth, until the start of the next one.
"What's striking from the
geological record is just how fast the ice melted each time the thaw started,"
said Larry Edwards, professor of earth sciences at Minnesota University. "It
suggests that when they reach a certain size, the ice sheets became vulnerable
to relatively small environmental changes that can prompt a rapid thaw. But we
haven't known why that happened."
To answer this question, Edwards
needed to know exactly when each of the thaws had occurred to see if they
correlated with other geological events. His problem was the difficulty in
pinpointing the exact dates of events that took place so long ago. The
breakthrough came after the boy's fall revealed ancient
stalagmites.
For geologists, the value of stalagmites is that the
composition of the minerals undergoes tiny variations according to the
prevailing climate. It means the layers of a stalagmite provide a powerful proxy
record of past climate change, allowing scientists to work out exactly when ice
ages came and went.
Using such methods, Edwards found that the end of
at least the past four ice ages had correlated closely with variations in the
Earth's orbit that made it slightly warmer. Such variations have long been
suspected of involvement in thawing prehistoric ice sheets, but the lack of
evidence about when ice ages ended made it impossible to be
sure.
Edwards said the surge in sunshine seemed to set in motion a
series of events that collectively restored the planet to a warmer state. One of
the key elements was that the initial surge of cold fresh water released by
melting ice dramatically slowed the circulation of the oceans.
This
meant the tropical oceans became warmer and warmer, forcing them to release
their dissolved carbon dioxide into the atmosphere, where it accelerated the
warming caused by the extra sunshine.
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