Scientific studies show that a massive release of carbon into the atmosphere caused the ancient global warming event known as the Paleocene-Eocene Thermal Maximum (PETM) that began about 55 million years ago.

    The resulting greenhouse effect heated the earth as a whole by about 9 Fahrenheit (5 Celsius) in less than 10,000 years, geologic records show.

    The increase in temperatures lasted about 170,000 years, altering the world rainfall patterns, making the oceans acidic, affecting plant and animal life and spawning the rise of our modern primate ancestors, according to the study by Mark Pagani, associate professor of geology and geophysics at Yale University.

    "The PETM is a stunning example of carbon dioxide-induced global warming and stands in contrast to critics who argue that the Earth temperature is insensitive to increases in carbon dioxide," said Pagani.

    "Not only did the Earth warm by at least 9 Fahrenheit (5 Celsius), but it did so during a time when Earth average temperature was already 9 Fahrenheit (5 Celsius) warmer than today."

    However, what has not been clear is how much carbon was responsible for the temperature increase and where it came from.

    Scientists have speculated that it might have come from massive fires from burning coal and other ancient plant material, or from an increase of methane from the continental shelves that rapidly turned into atmospheric carbon dioxide.

    "According to this work, if the PETM was caused by the burning of plant material then climate sensitivity to carbon dioxide is more than 4.5 times per carbon dioxide doubling. And if methane was the culprit, then Earth climate must be extremely sensitive to carbon dioxide increasing, over 10 Fahrenheit (5.56 Celsius) per carbon dioxide doubling," noted Pagani.

    This finding contradicts the position held by many climate-change skeptics that the Earth climate is resilient to such carbon dioxide emissions.

    "The last time carbon was emitted to the atmosphere on the scale of what we are doing today, there were winners and losers," said Ken Caldeira, a co-author from the Carnegie Institution's Department of Global Ecology.

    "There was ecological devastation, but new species rose from the ashes. Our work provides even more incentive to develop the clean energy sources that can provide for economic growth and development without risking the natural world that is our endowment," he said.