The Climate Jump Heard 'Round The World
About 12,900 years ago, the North Atlantic region quickly--in years to a decade--fell into a deep freeze. The ice sheets on Greenland, North American, and Eurasia advanced. Forests in eastern North America, Europe, and Scandinavia turned to tundra, and subarctic animals such as caribou migrated southward into these new landscapes. Then, after about 1,300 years, the area rapidly warmed again. In Greenland, the jump was 15 degrees in just a decade.
The unusual and rapid climate event, called the Younger Dryas, has been well-studied by climatologists seeking understanding of how, and how quickly, climate can shift dramatically. They’ve discovered over the last 15 years that the Younger Dryas was felt not just in the North Atlantic, but over half the globe. But it’s only been in the last two years that scientists have begun to suspect why.
A World of Change
Generally, the world became colder, drier, and more dusty during the Younger Dryas. Dust blown from increasingly stormy deserts in Asia scattered over glaciers in Greenland, revealed by ice core records taken there. By analyzing methane concentrations in air trapped in ice cores, scientists realized that tropical wetlands, which produce methane naturally, had significantly shrunk. Ocean sediment records from waters off California, Venezuela, and Pakistan demonstrated that the Younger Dryas’s climate jumps impacted the ocean and atmosphere of these regions in significant ways.
The temperature decreases and increases in the North Atlantic that marked the beginning and end of the Younger Dryas also appeared in other areas of the globe. Determination of the ratio of the isotope oxygen-16 to oxygen-18 in stalagmites in China’s Hulu cave revealed a temperature record for 75,000 “ 11,000 years ago. Its ups and downs matched those in the North Atlantic, as revealed in ice cores there. (Learn how oxygen isotopes act as paleo-thermometers.) The data also suggest that the strength of China’s monsoon rains shifted suddenly during the Younger Dryas.
Understanding the System
“Since the beginning, the trigger for the Younger Dryas’s climate jumps was attributed to reorganizations of the Atlantic Ocean’s conveyor-like circulation,” says paleoclimatologist Wallace Broecker of Columbia University’s Lamont-Doherty Earth Observatory. Broecker himself first suggested the idea during the 1960s. (Learn how changes in global ocean circulation can induce rapid climate change.) “The idea is that shutdowns and startups of the circulation somehow drove large changes in climate,” says Broecker. “For at least a decade, scientists struggled in vain to characterize the ‘somehow.’ Then two years ago, two breakthroughs occurred.”
Broecker points first to a field study in Greenland’s mountainous eastern fjord region organized by Gary Comer, the founder of Land’s End clothing. Comer, a frequent financial backer of science research, himself participated. He took climatologists Richard Alley of Penn State and George Denton of the University of Maine aboard his yacht and floatplane to a deep inlet called Scoresby Sund. Aerial and ground analysis revealed new information about a set of previously studied glacial moraines: that they were likely deposited during the Younger Dryas. The positions of the moraines, which depend on the positions of the glaciers from which they were deposited, indicated to the group that Greenland temperatures during the Younger Dryas were only 4 to 6 degrees C colder than they are now.
“This stunned them,” says Broecker. The temperature differences were expected to be much greater. In fact, Jeff Severinghaus of the Scripps Institution of Oceanography had shown from isotope measurements of air trapped in ice on Greenland’s summit, 500 miles to the west of Scoresby Sund, that the mean annual temperature during the Younger Dryas was not a mere 4 to 6 degrees C, but 16 degrees colder than now. Then Comer’s team realized that the moraines must have formed in the summer. The 4 to 6 degree C, then, was the shift in summer temperature, not the winter temperature. This means that Greenland winters must have been much colder--perhaps 30 degrees or so--than they are now.
“The only way in which such frigid winter temperatures could be maintained was to cut off all heat release from the surrounding ocean by freezing over the entire northern Atlantic,” Broecker notes. “This created the Siberian-like winter conditions extending from Canada to northern Europe.” In other words, the formation of sea ice “amplified” the cooling to create the extreme winter conditions extending from Canada to northern Europe.
A second breakthrough came from climate models that suggested the large cooling caused by sea ice cover in the northern Atlantic would push the tropical rain belt further to the south and also weaken Asian monsoons. These changes in atmospheric circulation caused cooling throughout much of the northern hemisphere, as evidenced exactly in the Hulu Cave and ocean sediment climate records. So when the Atlantic’s conveyor circulation shut down, it was the formation of sea ice that not only amplified the cooling in the north Atlantic but also caused the climate signal to be transmitted via the atmosphere to the tropics.
“These findings have important implications for the future,” says Broecker. “Some ocean models suggest that, if we were to add enough greenhouse gases to the atmosphere to warm the planet by 4 to 6 degrees C, then the extra precipitation at high northern latitudes would lead to a shutdown of conveyor circulation.” Since shutdowns seem to accompany cold periods in the past, some scientists have warned that a future shutdown could plunge northern Europe into an ice age. “Not likely,” says Broecker. “A 4 to 6 degree C warming would eliminate the possibility of sea ice formation.” And without sea ice, there would be no way to amplify a cold signal throughout the North Atlantic!