Mason Inman, National Geographic News 19 Jun 08;
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 tomorrow 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 4 to 7 degrees Fahrenheit (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 18 to 22 degrees Fahrenheit (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."
Greenland ice cores show clues to climate change
Michael Kahn, Reuters 19 Jun 08;
LONDON (Reuters) - An analysis of Greenland ice cores shows how atmospheric changes during the last ice age probably spurred wild temperature swings, a finding researchers said on Thursday could help predict future climate change.
The northern hemisphere emerged from the last ice age 14,700 years ago with about a 12 degree Celsius (22 Fahrenheit) spike in just 50 years before plunging back into icy conditions, then suddenly warming again 3,000 years later, the researchers said.
Rapid changes in atmospheric circulation -- such as where storms occurred or where the jet stream was -- coincided with each temperature shift, pointing to a potential trigger for severe climate change, the researchers said.
"We know abrupt climate change happens," said Jim White, a paleoclimatologist at the University of Colorado at Boulder in the United States, who worked on the study.
"We don't know why it happens and we don't know what to look for as a first early warning."
The researchers did not deal with current climate change, which may be rapid for humans but is actually slower than the abrupt changes they were looking into.
They did however say that the findings could give scientists clues to what may trigger sudden severe changes in the future.
The U.N. Climate Panel last year blamed human activities, led by burning of fossil fuels that release heat-trapping gases into the air, for global warming that may disrupt water and food supplies with ever-more droughts, floods and heatwaves.
Knowing more about past changes can help scientists and policymakers better prepare for future temperature rises that the U.N. panel predicts will range from 1.8 to 4 degrees C (3.2 to 7.2 degrees F) this century, the researchers said.
"It tells us that there are some tipping points in our climate system and that it can change very quickly," said Dorthe Dahl-Jensen of Denmark's University of Copenhagen, who led the study. "We should be aware that these abrupt climate changes can happen."
The team, which published its findings in the journal Science, analyzed ice cores drilled between 1998 and 2004 from a two-mile stretch in Greenland to chart past temperature and precipitation swings.
They found different amounts of dust blown across Greenland from around the world as evidence of changing atmospheric conditions right before the abrupt temperature changes, White said.
"We are beginning to tease apart the sequence of abrupt climate change," he said. "Since such rapid climate change would challenge even the most modern societies to successfully adapt, knowing how these massive events start and evolve is one of the most pressing climate questions we need to answer."
(Reporting by Michael Kahn; Editing by Will Dunham and Richard Meares)
