Study: scientists demonstrate ancient warming occurred in tropical waters

By DAVID WILLIAMSON
UNC News Services

CHAPEL HILL -- Using newer, sophisticated techniques to measure the geochemical properties of single-celled organisms that have lain in the Pacific Ocean seafloor for about 55 million years, scientists have now resolved an important lingering question about global warming.

The scientists, led by Dr. James C. Zachos, a University of California at Santa Cruz paleoceanographer, found that tropical sea surface temperatures climbed significantly during one of the Earth’s early and most important episodes of global warming.

Before the new research findings, just published online by the journal Science, scientists knew that ocean temperatures climbed by about 18 degrees Fahrenheit at the poles during that period, known as the Paleocene-Eocene thermal maximum. But investigators faced conflicting evidence of a comparable rise at tropical latitudes, which models predicted.

"This significance of this work is relatively straightforward," Zachos said. "Based on other lines of evidence, it has been demonstrated through modeling that the climatic warming was driven by a rise in greenhouse gas levels.

"If so, according to climate theory, warming should occur at all latitudes, including the tropics, and the polar warming should be amplified," he said. "This pattern is a fairly robust result in climate model simulations of greenhouse forcing (effects). Our records provide a missing piece of the puzzle that is consistent with the theory."

Amanda Brill, a geological sciences graduate student in the University of North Carolina at Chapel Hill’s College of Arts and Sciences, is one of Zachos’ co-authors. She said the extreme global warming of 55 million years ago lasted about 100,000 years.

Environmental effects included mass extinctions of species of bottom-dwelling organisms and proliferation of planktonic organisms, major changes in precipitation and migration and dispersal of land mammals.

"The Paleocene-Eocene thermal maximum has become an interval of intense interest because it was a result of the addition of methane -- and carbon dioxide through oxidation -- to the Earth’s atmosphere at roughly the same rate as recent human contributions to global warming," Brill said.

Since the early warm period is frequently cited as an analog for modern global warming and it was such an abrupt and notable switch in climate, it was imperative for scientists to resolve the conflicting evidence if they hoped to predict Earth’s future based on what happened long ago, she said.

"Either climate theory and our models were wrong or our geochemical evidence was lacking, which is what turned out to be the case," Brill said. "Our previous isotope data, which suggested no temperature change in tropical seas, was inaccurate, and our climate model and theory were correct."

Fossilized single-celled organisms known as foraminifera, or plankton, which look like microscopic seashells, are recovered by drilling into sediments below the Pacific Ocean and extracting core samples, she said. They are chiefly composed of calcium, carbon and oxygen, with magnesium substituting for calcium depending on water temperature. By studying the ratios of magnesium to calcium, the team produced temperature estimates that clearly showed tropical surface waters warmed by 8 to 9 degrees Fahrenheit.

Other co-authors of the new Science paper are Dr. Margaret Delaney and graduate students Michael Wara and Steven Bohaty of U.C.- Santa Cruz; Dr. Isabella Premoli-Silva and graduate student Maria Rose Petrizzo of the University of Milan; and Dr. Tim J. Bralower, former chair of geological sciences at UNC and now geosciences chair at Pennsylvania State University.

Support for the research came from the National Science Foundation. Core samples came from the Ocean Drilling Program based at Texas A&M University.

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Note: Zachos can be reached at jzachos@es.ucsc.edu, Brill at (919) 412-8159.

Contact: David Williamson, (919) 962-8596