5 January 2007

Fossil Records Show Yo-Yo Effect Of Changing Climate

by Kate Melville

The mid-Permian transition from ice age to an ice-free planet was marked by dips and rises in carbon dioxide and extreme swings in climate, according to University of California, Davis (UC) researchers writing in Science. During the mid-Permian, 300 million years ago, much of the southern hemisphere was covered in thick ice sheets and floating pack ice likely covered the northern polar ocean. But forty million years later, all the ice was gone and the climate hot and dry with sparse vegetation.

UC's Isabel Montanez, lead author on the paper, derived levels of atmospheric carbon dioxide and sea surface temperatures from the fossils of brachiopod shellfish and fossilized plants from the ancient rainforests. They also looked at the scars and clues left by glacial ice sheets that once covered the great southern continent of Gondwanaland, which included most of the land masses of the modern southern hemisphere.

Montanez's analysis shows that throughout the change over millions of years, atmospheric carbon dioxide levels swung back and forth between about 250 parts per million (close to present-day levels) to more than 2,000 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. "We can see a pattern of increasing carbon dioxide and increasing temperatures, with a series of rises and dips," Montanez said.

Previously, it was assumed that as the climate warmed, a tipping point would be reached at which the ice sheets would melt rapidly and for good. Instead, the new data shows that the climate went back and forth between the extremes. Instead of a smooth shift, the transition occurred in a series of sharp swings between cold and hot conditions, occurring during perhaps a half-million to few million years.

Montanez pointed out that these results cannot be directly applied to current global warming. The current rise in atmospheric carbon dioxide is occurring throughout a much shorter timescale, for one thing. But the current work does show that such a major change in climate will likely not proceed in small, gradual steps, but in a series of unstable, dramatic swings. Somewhat worryingly, while the mid-Permian changeover took millions of years, similar events might take place during a much shorter time span. "Perhaps this is the behavior one should expect when we go through a major climate transition," Montanez mused.

Source: University of California - Davis