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The fossilised remains of snails are helping scientists to understand how a fall in carbon dioxide levels signalled the start of a far colder and quite different climate.
LONDON, 22 April – British and American scientists have used a new technique to pinpoint an epoch-making moment of climate change.
They have used isotopes from land snails in what is now Hampshire and the Isle of Wight in England to reconstruct a fateful fall in carbon dioxide levels in the atmosphere, when average air temperatures fell by up to 6°C, summer freshwater temperatures plummeted 10°C and great sheets of ice began to form.
Almost 34 million years ago, the late Eocene epoch gave way to the Oligocene. In a much warmer world, carbon dioxide levels in the atmosphere had reached 1,000 parts per million (ppm), and then started to fall precipitately.
Within about 400,000 years huge glaciers dominated the polar regions, sea levels fell, faunas were extinguished and the world had changed forever.
Palaeontologists, climate scientists and geophysicists have repeatedly tried to reconstruct the sequence of events that turned a hot, marshy world into a freezing one, but any physical evidence of ancient planetary catastrophe has been buried, or eroded, or washed away.
Marine sediments have preserved the temperature conditions as they changed. Changes in the fossil sequence tell a story of extinctions in Europe and the migration of new genera from Asia. But like most of the great extinctions from the past – and this was a relatively minor period of mass extinction – the cause remains a mystery.
“By understanding past climate transitions, we can better understand the present and predict impacts for the future”
But isotope evidence from fossil shells of the snail Viviparius lentus seems to have settled one point: the dramatic shift is firmly linked to changes in carbon dioxide levels in the atmosphere.
Michael Hren of the University of Connecticut and colleagues report in the Proceedings of the National Academy of Sciences that heavy isotopes of carbon and oxygen in snail fossils could be interpreted to serve as records of temperatures at the time the shells formed.
The past, too, has lessons for the future. Carbon dioxide levels in the atmosphere, thanks to the burning of fossil fuels, are rising again: nearly 400 ppm now and in danger of increasing once again to 1,000 ppm in the next 100 years.
If a sharp drop in temperature fundamentally altered life on Earth 34 million years ago, another, even swifter rise in atmospheric and fresh water temperatures during the next century would do the same.
“One of the key principles of geology is that the past is the key to the present: records of past climate inform us of how the Earth system functions”, said Dr Hren. “By understanding past climate transitions, we can better understand the present and predict impacts for the future.” – Climate News Network