London: Earth could get much warmer than expected in future, say scientists who have reconstructed past global warming and carbon cycle on earth 56 million years ago.
Global warming from greenhouse gas emissions depends not only on the size of the emissions, but also on the warming effect that the extra amount of gas has on the atmosphere.
This effect, called climate sensitivity, is usually defined as the warming caused by the doubling of the amount of carbon dioxide (CO2) in the atmosphere.
Climate sensitivity depends on a number of properties of the earth's climate system, such as the composition of clouds and cloud cover.
"The research shows that climate sensitivity was higher during the past global, warm climate than in the current climate, said Gary Shaffer, professor at the University of Magallanes in Chile.
"This is bad news for humanity as greater climate sensitivity from warming will further amplify the warming," said Shaffer, also a professor at the Niels Bohr Institute of the University of Copenhagen in Denmark.
The study was based on reconstructions and climate modelling of a period of global warming 56 million years ago.
The period known as the Palaeocene-Eocene Thermal Maximum (PETM) was triggered by massive releases of carbon into the atmosphere and researchers have long known it as a time that could in some ways be analogous to today's global warming.
Reconstructions of past temperatures show that even before the PETM the Earth was about 10 degrees warmer than today and then warmed an additional 5 degrees during the PETM.
Researchers estimated that where it was about 4.5 degrees Celsius before the PETM, the temperature rose to about 5.1 degrees Celsius during the PETM. Climate sensitivity is currently around 3 degrees.
"Our results show that the amount of carbon that drove the PETM warming was about the same amount as the current 'easily accessible' fossil fuel reserves of about 4,000 billion tonnes," said Shaffer.
"But the warming that would result from adding such large amounts of carbon to the climate system would be much greater today than during the PETM and could reach up to 10 degrees," he said.
This is partly due to the current atmosphere containing much less CO2 - about 400 parts per million (ppm) - compared to before the PETM, where the concentration was about 1,000 ppm and partly because we emit carbon into the atmosphere at a much faster rate than during the PETM.
"If we then also take into account the fact that climate sensitivity increases with the temperature, it means that it is all the more urgent to limit global warming as soon as possible by reducing the human-made emissions of greenhouse gases," said Shaffer.
The results are published in the scientific journal, Geophysical Research Letters.
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