Violent swings in weather patterns occurred after Earth's climate crossed "tipping points" thousands of years ago, a new study argues.

In as little as three years, patterns in the atmosphere have suddenly shifted and flipped into a new state, apparently contributing to rapid warming of the Northern Hemisphere, according to the new analysis of an ice core from northern Greenland.

The study focused on two quick warming periods - 14,700 and 11,700 years ago - that together pushed our planet out of the last ice age.

During such ice ages, Earth's climate was more variable overall than it has been relatively recently; spanning the past 11,700 years, our planet's current climatic period - known as the Holocene - has been marked by an unusually warm and stable climate.

But it's possible this could change quickly, the new study suggests.

"We are changing the climate," with greenhouse gas emissions, said study co-author Dorthe Dahl-Jensen, an ice and climate scientist at the University of Copenhagen, Denmark.

"We are moving out of that stable state now." Dahl-Jensen adds that scientists can't say that similar rapid changes will result from human-caused global warming.

But the new study, appearing in the online edition of the journal Science, does "tell us about a capacity in our weather systems to change so fast," she said. "I think that's worth drawing attention to."

CLIMATE RECORD

Snow falling on central Greenland lays down a distinct layer each year, trapping bubbles of atmospheric gas, dust, and other impurities and gradually compacting into ice that captures an ancient climate record stretching back tens of thousands of years.

In their study, researchers focused on an ice core drilled by the North Greenland Ice Core Project.

They measured several features of the ice core, including changes in the levels of deuterium, a heavy version of the element hydrogen and dust trapped in the ice core.

Using new measurement techniques, the team could distinguish not only individual years of climate data, but seasonal fluctuations.

For some of the measurements, "we measure basically month to month," Dahl-Jensen said. "We can see the seasons - is it winter, summer?"

"We were very surprised, because one of the parameters - called the deuterium excess - can switch modes basically from year to year," Dahl-Jensen said. This shift in deuterium reflects a "dramatic change" in the source of moisture that creates snowfall in Greenland.

It appears that this moisture suddenly started coming from another Atlantic Ocean region that was about 2 to 4 degrees Celsius warmer.

The researchers argue that this change points to a major shift in atmospheric circulation, mostly likely in the so-called Intertropical Convergence Zone, a low-pressure area of the atmosphere that controls tropical storms and monsoons.

These shifts seem to have triggered rapid warming that was more gradual, which heated Greenland by 10 to 12 degrees Celsius over 50 years.

CLIMATE "FLIP"

Some previous studies have found evidence of fast changes in the climate, says Richard Alley, a geoscientist at Pennsylvania State University.

He notes that Greenland ice cores have shown abrupt shifts in snowfall occurring within as little as three years. The new findings bolster "the strong evidence for very fast changes having occurred," Alley said.

Jeffrey Severinghaus, a geoscientist at the Scripps Institution of Oceanography in La Jolla, California, said the quick shift in Greenland's ancient climate shows "it really has to be something about the atmosphere that's causing the changes."

However, it's not clear what exactly is causing those changes, he noted, adding that realistic computer models that simulate Earth's climate don't show such abrupt changes.

"There's something missing from the models," Severinghaus said. "It's important to know that, especially in the light of ongoing human changes to the climate."

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