A bumpy shift from icehouse to greenhouse.

• Position: Climate Trends

The transition from an ice age to an ice-free planet was highly unstable, marked by dips and rises in carbon dioxide, extreme swings in climate, and drastic effects on tropical vegetation, according to a study by the University of California, Davis. "This is the best documented record we have of what happens to the climate system during long-term global warming following an ice age," notes lead author Isabel Montanez, professor of geology. However, she adds, these findings cannot be applied directly to current global warming trends.

In the mid-Permian period, some 300,000,000 years ago, the Earth was in an ice age. Miles-thick frozen sheets blanketed much of the southern continent, and floating pack ice likely covered the northern polar ocean. The tropics were dominated by lush rain forests, now preserved as coal beds. Some 40,000,000 years later, all the ice was gone. The world was a hot, dry place; vegetation was sparse, soils little more than drifts of wind-blown dust. "You'd have to be a reptile to want to live there," says Montanez.

The new data shows that, throughout millions of years, atmospheric carbon dioxide levels swung back and forth between about 250 parts per 1,000,000, close to present-day levels, to more than 2,000 parts per 1,000,000. 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. "eVe can see a pattern of increasing carbon dioxide and increasing...