Basing global warming forecasts on atmospheric carbon alone may deliver predictions that fall well short of the potential for temperatures to rise, a new study of a 55 million year old global warming event suggests.
When a trio of United States scientists examined planetary warming during the Palaeocene–Eocene Thermal Maximum (PETM), they found that the globe warmed by 5-9°C over several thousand years, but that carbon dioxide in the atmosphere could only explain between 1°C-3.5°C of the warming.
Writing in Nature GeoScience Online on Monday, the authors concluded that in addition to CO2's effect on temperature, other feedback or forcing mechanisms must have contributed the greater portion of the warming effect.
Forecasts of future temperature caused by human-induced global warming only account for greenhouse gas emissions into the atmosphere.
The authors say their PETM study highlights the need to consider other factors which may escalate temperatures well beyond the effect modelled under greenhouse gas scenarios.
Richard Zeebe of the University of Hawaii, James Zachos of the University of California and Gerald Dickens of Rice University, Texas, used carbon isotopes, deep-sea carbonate records and computer modelling to consider carbon’s role in the PETM.
The onset of the event 55 million years ago was marked by a dramatic global increase in temperatures within a few thousand years, and a release of terrestial carbon large enough to acidify the oceans.
It is unclear where the carbon came from, nor could the authors say whether the carbon release forced warming that was then amplified by feedback mechanisms, or whether carbon was released in response to warming and itself amplified the effect.
The paper also made it clear that the world was a different place during the PETM: continents were in different configurations and atmospheric and ocean circulations bore no resemblance to today's.
However, the paper suggests that the release of carbon during the onset of the PETM, and human-caused carbon releases over the past 50 years, may be in a similar order of magnitude.
"The PETM may therefore serve as a case study for the consequences of the carbon dioxide released at present by human activities," the authors said.
"…our results imply a fundamental gap in our understanding of the amplitude of global warming associated with large and abrupt climate perturbations.
"This gap needs to be filled to confidently predict future climate change."
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