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Ancient Climate-Change Event Puzzles Scientists
07-19-2009, 02:58 AM,
Ancient Climate-Change Event Puzzles Scientists
Ancient Climate-Change Event Puzzles Scientists

By Phil Berardelli
ScienceNOW Daily News
14 July 2009
Carbon dioxide (CO2) gets a bad rep for contributing to global warming, and deservedly so. But scientists say they can't entirely blame the greenhouse gas for a curious spike in Earth's temperature 55 million years ago. New research reveals that something else also seems to have warmed the planet during that time, though no one's quite sure what it was.

Over the past couple of decades, researchers have been gathering data about a mysterious event known as the Paleocene-Eocene Thermal Maximum (PETM). The data, derived from drill cores brought up from the deep seabed in the Atlantic and Pacific Oceans, show that the surface temperature of the planet rose by as much as 9°C within 10,000 years during the PETM, which itself started out warmer than our current world. Temperatures stayed at this elevated level for nearly 100,000 years.

On the surface, the culprit appeared to be CO2. For reasons unknown, atmospheric concentrations of the gas rose by about 700 parts per million, from 1000 ppm to 1700 ppm--more than four times higher than today's level of 385 ppm--during the PETM. That much of an infusion of the well-established greenhouse gas should have been plenty to spike temperatures.

But a new analysis doesn't fully support this scenario. Oceanographer Richard Zeebe of the University of Hawaii, Manoa, and colleagues ran carbon-cycle simulations of the oceans and atmosphere based on the data yielded by the sediment cores. They even simulated what would happen to global temperatures when they increased the atmosphere's sensitivity to doubling CO2 levels--to 2000 ppm--during the PETM. The most they could achieve was a warming of 3.5°C, they report online this week in Nature Geoscience. That means some other phenomenon must have pushed up temperatures by as much as 5.5°C, the team says. So at present, the unexplained warming represents a gap in understanding about what causes significant and rapid climate change.

"It's possible that other greenhouse gases such as methane could have contributed to the [PETM] warming," Zeebe says. It's also possible that the models are underestimating the climate response to CO2 increases. If that's the case, it "would mean our understanding of the climate system is incomplete," he says.

Zeebe's team is now looking at smaller warming events that occurred within several million years after the PETM. "We're currently trying to find out whether or not [they] were caused by the same mechanism," he says. The idea is to determine whether the PETM warming was unique "or a universal feature."

Geochemist Gabriel Bowen of Purdue University in West Lafayette, Indiana, applauds the work. "We've long had a hunch that something was fishy about the climate response during the PETM," he says. "This study puts the nails in the coffin of the idea that climate during the PETM responded to CO2 alone." Says Bowen, "The urgent challenge now facing us is to find out what was amplifying [temperatures] during this event and understand what it means for Earth's future."
07-19-2009, 03:04 AM,
Ancient Climate-Change Event Puzzles Scientists
Letter abstract

Nature Geoscience
Published online: 13 July 2009 | doi:10.1038/ngeo578

Carbon dioxide forcing alone insufficient to explain Palaeocene–Eocene Thermal Maximum warming

Richard E. Zeebe1, James C. Zachos2 & Gerald R. Dickens3

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The Palaeocene–Eocene Thermal Maximum (about 55 Myr ago) represents a possible analogue for the future and thus may provide insight into climate system sensitivity and feedbacks1, 2. The key feature of this event is the release of a large mass of 13C-depleted carbon into the carbon reservoirs at the Earth's surface, although the source remains an open issue3, 4. Concurrently, global surface temperatures rose by 5–9 °C within a few thousand years5, 6, 7, 8, 9. Here we use published palaeorecords of deep-sea carbonate dissolution10, 11, 12, 13, 14 and stable carbon isotope composition10, 15, 16, 17 along with a carbon cycle model to constrain the initial carbon pulse to a magnitude of 3,000 Pg C or less, with an isotopic composition lighter than -50permil. As a result, atmospheric carbon dioxide concentrations increased during the main event by less than about 70% compared with pre-event levels. At accepted values for the climate sensitivity to a doubling of the atmospheric CO2 concentration1, this rise in CO2 can explain only between 1 and 3.5 °C of the warming inferred from proxy records. We conclude that in addition to direct CO2 forcing, other processes and/or feedbacks that are hitherto unknown must have caused a substantial portion of the warming during the Palaeocene–Eocene Thermal Maximum. Once these processes have been identified, their potential effect on future climate change needs to be taken into account.
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1. School of Ocean and Earth Science and Technology, Department of Oceanography, University of Hawaii at Manoa, 1000 Pope Road, MSB 504, Honolulu, Hawaii 96822, USA
2. Earth and Planetary Sciences Department, University of California, Santa Cruz, California 95064, USA
3. Department of Earth Sciences, Rice University, Houston, Texas 77005, USA

Correspondence to: Richard E. Zeebe1 e-mail:

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