(AP) - Carbon dioxide levels in the atmosphere fluctuated after the Ice Age, helping to heat up Earth's climate and trigger the spread of deserts thousands of years ago, a study reports.

Scientists say the findings, which were based on an analysis of ice cores drilled from glaciers in Antarctica, could serve as warning of what global warming could do to the Earth in the 21st century.

Carbon dioxide in the atmosphere is called a ''greenhouse gas'' because it traps the sun's heat.

Levels of carbon dioxide fell and rose by small but persistent amounts between 11,000 and 1,000 years ago, said the Swiss and American scientists, who examined the ice cores, in a study published in today's issue of the journal Nature.

They also found that the fluctuations correlate with droughts and the spread of deserts in Africa and Asia during the prehistoric period known as Holocene.

These ancient carbon dioxide levels, while significant, were far lower than the rising concentrations in today's atmosphere that are blamed on industry and motor vehicles.

As a result, the findings raise questions about whether the Earth is headed for rapid and drastic climate changes in the 21st century.

"The carbon dioxide changes over the last few thousand years have been tiny and slow compared to what humans are doing," said glaciologist Richard Alley of Pennsylvania State University, who did not participate in the study. "We are moving into uncharted waters."

"We have tended to view the last 10,000 years as being constant," said ice core expert James White of the University of Colorado. "But carbon levels really haven't stabilized. Humans have continuity built into their thinking, and this study will shock people."

The Holocene's climate swings were a natural phenomenon. But during the past 200 years, the burning of coal, gasoline and other fossil fuels has added more than 80 ppm of carbon dioxide to the atmosphere. The total amount is now above 370 ppm and is expected to double in the 21st century.

The 1990s are the warmest decade on record. Many scientists fear that human activity is the driving force behind the warming.