Jan. 5, 2007


Maps courtesy of the PALEOMAP Project.
Click here to read the research paper (pdf).

DALLAS (SMU) – Long-held beliefs that the earth made a gradual, steady transition from the Ice Age to a relatively ice-free planet are being turned upside down by a study published in today’s edition of the journal Science.

Analysis of ancient soils, rocks and fossils shows the change, which began 300 million years ago and ended 40 million years later, was marked by pronounced dips and rises in carbon dioxide, extreme swings in climate and drastic effects on tropical vegetation, according to Neil Tabor of the Department of Geological Sciences at Southern Methodist University and Isabel Montañez of the University of California, Davis, lead authors of the study funded by the National Science Foundation.

Their study has “unequivocally documented a strong coupling of atmospheric partial pressure of carbon dioxide and surface temperatures with the changing global ice volume. Although the precise mechanistic link between atmospheric greenhouse gases and climate is debated, there remains little doubt that high concentrations of atmospheric carbon dioxide have strongly amplified Earth’s past climates,” Tabor and Montañez said.

Some 300 million years ago, the Earth was in an ice age and miles-thick ice sheets covered much of the southern continent and floating pack ice likely covered the northern polar ocean. The tropics were dominated by lush rainforests. Forty million years later, all the ice was gone and Earth was a hot, dry place where vegetation was sparse and soils amounted to little more than drifts of wind-blown dust.

The study’s results may not relate to current global warming, Tabor said, because the current rise in atmospheric carbon dioxide is occurring more rapidly. However, it does show that climate changes will most likely occur in a series of unstable, dramatic swings. While the study’s data covered millions of years, similar events might take place over a much shorter period of time.

Further, the authors said, the record of fossil plants shows rapid changes in tropical plant communities as the climate changed. Over a few thousand years, lush forests of tree ferns in cool, wet periods alternated with conifers and other plants adapted to a climate that was warmer, drier and harsher. Tropical forests now are under pressure from human use and settlement, and ecological researchers have recorded species moving north or south, likely driven by current global warming.

Historically, scientists assumed that as the climate warmed, a tipping point was reached at which the ice sheets melted rapidly and for good. Instead, the new data shows that the climate became warmer by going back and forth between extremes until the ice sheets were gone by 260 million years ago.

Tabor and his coauthors derived records of atmospheric carbon dioxide from ancient soils that have been preserved as rocks, from coal and from fossils of plants. They extracted a record of sea surface temperatures from the fossils of brachiopod shellfish, and looked at the extensive records of past plant life from fossils of the ancient rainforests. To see how the glaciers advanced and retreated, they looked at the scars and clues left by ice sheets that once covered the great southern continent of Gondwana.

The new data show that over millions of years, atmospheric carbon dioxide levels swung back and forth between about 250 parts per million, close to present-day levels, to over 2000 parts per million. At the same time, the southern ice sheets retreated as carbon dioxide rose and expanded again when levels fell, a pattern compatible with the idea that greenhouse gases caused the end of the late Paleozoic ice age.

The other authors on the paper are: William DiMichele, Smithsonian Museum of Natural History; Tracy Frank, Christopher Fielding, Lauren Birgenheier and Michael Rygel, University of Nebraska, Lincoln; and John Isbell, University of Wisconsin, Milwaukee.

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Gary Shultz
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