Unprecedented levels of greenhouse gases in the Earth's atmosphere are disrupting normal patterns of glaciation, scientists said.

The Earth's current warm period that began about 11,000 years ago should give way to another ice age within about 1,500 years, according to the study published online Jan. 8 in Nature Geoscience.

However, current levels of carbon dioxide are trapping too much heat in the atmosphere to allow the Earth to cool as it has in its prehistoric past in response to changes in Earth's orbital pattern.

The research team, a collaboration among University of Florida, University College London, and University of Cambridge, said their data indicate that the next ice age will likely be delayed by tens of thousands of years.

"Ice sheets like those in western Antarctica are already destabilized by global warming," said co-author Jim Channell, distinguished professor of geology at University of Florida.

"When they eventually slough off and become a part of the ocean's volume, it will have a dramatic effect on sea level." Ice sheets will continue to melt until the next phase of cooling begins in earnest.

The study looks at the prehistoric climate-change drivers of the past to project the onset of the next ice age. Using astronomical models that show Earth's orbital pattern with all of its fluctuations and wobbles over the last several million years, astronomers can calculate the amount of solar heat that has reached the Earth's atmosphere during past glacial and interglacial periods.

"We know from past records that Earth's orbital characteristics during our present interglacial period are a dead ringer for orbital characteristics in an interglacial period 780,000 years ago," said Channell. The pattern suggests that our current period of warmth should be ending within about 1,500 years.

However, there is a much higher concentration of greenhouse gases trapping the sun's heat in the Earth's atmosphere now than there was in at least the last several million years, he said. So the cooling that would naturally occur due to changes in the Earth's orbital characteristics are unable to turn the temperature tide.

Over the past million years, the Earth's carbon dioxide levels, as recorded in ice core samples, have never reached more than 280 parts per million in the atmosphere. "We are now at 390 parts per million," Channell said. The sudden spike has occurred in the last 150 years.

For millions of years, carbon dioxide levels have ebbed and flowed between ice ages. Orbital patterns initiate periods of warming that cause ocean circulation to change. The changes cause carbon dioxide-rich water in the deep ocean to well up toward the surface where the carbon dioxide is released as a gas back into the atmosphere. The increase in atmospheric carbon dioxide then drives further warming and eventually the orbital pattern shifts again and decreases the amount of solar heat that reaches the Earth.

"The problem is that now we have added to the total amount of CO2 cycling through the system by burning fossil fuels," said Channell. "The cooling forces can't keep up."

Channell said that the study brings to the forefront the importance of atmospheric carbon dioxide because it shows the dramatic effect that it is having on a natural cycle that has controlled our Earth's climate for millions of years.

"We haven't seen this high concentration of greenhouse gases in the atmosphere for several million years," Channell said. "All bets are off."