A paper in the October edition of the journal Geology suggests that a continental ice sheet did not form on Earth during the Late Cretaceous Period more than 90 million years ago, a position that goes against what scientists have thought for years.

By analyzing levels of carbon and oxygen present in fossils of 90-million-year-old, salt-grain sized single-celled organisms, University of Missouri geological sciences professor Ken MacLeod contends that the conditions in the Late Cretaceous were not suitable for ice sheet formation.

MacLeod said the research has modern day applications as well, because if the world's carbon dioxide levels continue to rise, it could spell the end for the Earth's polar ice.

"Currently, carbon dioxide levels are just above 400 parts per million (ppm), up approximately 120 ppm in the last 150 years and rising about 2 ppm each year," MacLeod said. "In our study, we found that during the Late Cretaceous Period, when carbon dioxide levels were around 1,000 ppm, there were no continental ice sheets on earth. So, if carbon dioxide levels continue to rise, the earth will be ice-free once the climate comes into balance with the higher levels."

MacLeod and his team measured the ratios of different isotopes of carbon and oxygen in planktic and benthic foraminifera, tiny microorganisms that lived 90 million years ago.

The research team found the fossils in Tanzania, and after analyzing their carbon and oxygen rations they found no evidence of cooling or changes in local water chemistry that would have been expected if a glacial event had occurred during that time period, the University of Missouri said in a statement.

"We know that the carbon dioxide levels are rising currently and are at the highest they have been in millions of years. We have records of how conditions have changed as carbon dioxide levels have risen from 280 to 400 ppm, but I believe it also is important to know what could happen when those levels reach 600 to 1000 ppm," MacLeod said. "At the rate that carbon dioxide levels are rising, we will reach 600 ppm around the end of this century. At that level of carbon dioxide, will ice sheets on Greenland and Antarctica be stable? If not, how will their melting affect the planet?"

MacLeod said that previous research has indicated that a doubling of Earth's carbon dioxide levels could raise global temperatures by 3 degrees Celsius (6 degrees Fahrenheit), but by his calculations MacLeod suggests that the 90-million-year-old fossils lived in a world where carbon dioxide levels are double what they are today, which he says indicates temperatures as much as 6 degrees C (11 degrees F) higher.

"While studying the past can help us predict the future, other challenges with modern warming still exist," MacLeod said. "The Late Cretaceous climate was very warm, but the Earth adjusted as changes occurred over millions of years. We're seeing the same size changes, but they are happening over a couple of hundred years, maybe 10,000 times faster. How that affects the equation is a big and difficult question."