Despite the recent trend toward global warming, scientists have long wondered whether the Earth is nearing another ice age - and an end to the 12,000-year temperate spell in which civilizations arose.
Some have said that such a transition is overdue, given that each of the three temperate intervals before this one lasted only about 10,000 years.
But in an eagerly awaited study, a group of climate and ice experts say they have new evidence that the Earth is not even halfway through this warm era. That evidence comes from the oldest layers of Antarctic ice ever sampled.
Some scientists had proposed similar hypotheses, basing them on the configuration of the Earth's orbit, which seems to dictate the ice ages. Temperature patterns deciphered in sediments on sea bottoms in recent years also supported the theory.
But experts say the new data are by far the strongest evidence, revealing many similarities between today's atmospheric and temperature patterns and those of a 28,000-year warm interval that reached its peak 430,000 years ago.
The findings are described today in the journal Nature by the European Project for Ice Coring in Antarctica.
The evidence comes from a shaft of ice extracted over five years from Antarctica, composed of thousands of layers of ice that were formed as each year's snowfall was compressed over time.
The deepest ice retrieved comes from layers 10,000 feet deep and dates back 740,000 years. The relative abundance of certain forms of hydrogen in the ice reflects past air temperatures.
Many ice cores have been cut from various glaciers and ice sheets around the world, but none had reached back beyond 420,000 years, making this core the first to fully capture the conditions during that long-lasting warm period, called Termination V.
"It's very exciting to see ice that fell as snow three-quarters of a million years ago," said Dr. Eric W. Wolff, an author of the study who is an ice core expert with the British Antarctic Survey.
Several independent researchers familiar with the project said there is now a strong case that the current warm period would be prolonged. Yet even with this new evidence, they said, it is based on a sketchy view of the climatic past.