San Francisco, Calif. -- About 8,200 years ago, the world climate suddenly got colder and stayed that way for a few hundred years before temperatures returned to normal, according to a team of paleoclimatologists.
"This event, which we are calling the 8k event, was short compared to other, more distant events, lasting only about 200 years" says Anna Maria Agustsdottir, graduate student in geosciences, Penn State. "We see it in the Greenland ice cores as one of the biggest dips during the Holocene."
The Holocene is the geologic period beginning about 10,000 years ago at the end of the last glaciation and continuing up to today. Unlike other events, the 8K takes place in what for geologists is the very recent past.
"Temperatures abruptly decreased about 11 degrees Fahrenheit during the 8K event," Agustsdottir said today (Dec. 10) at the fall meeting of the American Geophysical Union in San Francisco.
The change in climate during this period can be seen not only in the temperature record from the Greenland ice cores, but also in ice accumulation, in the indicators of forest fires and in the amounts of methane found in the atmosphere.
"Methane is not just an indicator of local climate change," says Agustsdottir, "But it indicates a global change in climate."
The researchers, who include Agustsdottir; Richard Alley, professor of geosciences, Penn State; and Peter J. Fawcett of the University of New Mexico, note that during the 8K, Greenland became cold, dry and windy, Canada became cold and the North Atlantic Basin cold and fresh. Asia and Africa also showed colder, dryer climate while South America and North America were wetter.
"This event appears to be very similar to, if some what shorter than, the Younger Dryas event that occurred about 12,000 years ago," says Agustsdottir. "We are trying to find the underlying cause for these sudden temperature drops."
The researchers believe that these events occur when the ocean conveyor system shuts down. This system is a series of currents that normally move warm water from the equatorial zone to the north. This water cools as it moves northward and the colder, saltier water sinks and flows back toward the equator to replace water moving north. Temperature, water density and salinity control ocean currents. When the ocean conveyor shuts down, deep, cold water formation stops in the north and the cyclical flow of water halts, cooling Europe and its surroundings.
"We do not know what shuts down the conveyor, but one possibility is an increase in fresh water in the North Atlantic that would decrease salinity and prevent the water from sinking," says Agustsdottir.
Using a climate simulation model called GENESIS, the researchers are trying to model events leading up to the 8K event to simulate an ocean conveyor shut down and temperature decline. The Penn State researchers have used this method on the Younger Dryas event with some success.
"Using conditions similar to today's oceans, the model response to a conveyor shutdown does not match data from the 8K event," says Agustsdottir. "However, shutdown from an ocean with a more vigorous conveyor does match observations. This indicates that things were different in the early Holocene."
She considers the mechanism behind these sudden cold spells important because the climate changes are so rapid. While we cannot predict the future, we can learn from events in the past and see how they occurred.
"If change is gradual, animals, plants and humans can adapt to the new environment," Agustsdottir says. "If change is abrupt, crops fail, rains do not come or come too frequently and people do not have time to adjust."