New Haven, Conn. -- Global warming 55 million years ago suggests a high climate sensitivity to carbon dioxide, according to research led by Mark Pagani, associate professor of geology and geophysics at Yale and published in the December 8 issue of Science.
For some years, scientists have known 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 geologic record shows that the resulting greenhouse effect heated the planet as a whole by about 9° F (5° C), in less than 10,000 years.
That temperature increase lasted about 170,000 years, altered the world's rainfall patterns, made the oceans acidic, affected plant and animal life in the seas and on land, and spawned the rise of our modern primate ancestors.
"The PETM is a stunning example of carbon dioxide-induced global warming and stands in contrast to critics who argue that the Earth's temperature is insensitive to increases in carbon dioxide," said Pagani. "Not only did the Earth warm by at least 9°F (5°C), but it did so during a time when Earth's average temperature was already 9°F 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 'burps' of methane from the continental shelves that rapidly became 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°F (2.5°C) per carbon dioxide doubling. And if methane was the culprit, then Earth's climate must be extremely sensitive to carbon dioxide -- increasing, over 10°F (5.6°C) per carbon dioxide doubling," noted Pagani.
This finding contradicts the position held by many climate-change skeptics that the Earth's climate is resilient to such carbon dioxide emissions and suggests that Earth's temperature will rise substantially with atmospheric carbon dioxide concentrations that are expected to double around mid-century.
"The last time carbon was emitted to the atmosphere on the scale of what we are doing today, there were winners and losers," remarked 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."
Other authors on the paper include David Archer in the Department of Geophysical Sciences, University of Chicago, and James C. Zachos in the Earth Sciences Department, University of California, Santa Cruz.
Citation: Science: (December 8, 2006).