‘Rollercoaster’ temps prevailed as iceberg flotillas invaded North Atlantic
By studying chemical tracers in seafloor sediments, scientists have been able to show that periods of abrupt climate change during the last ice age are somehow linked with dramatic changes in key ocean currents, especially the Atlantic meridional overturning circulation, which carries heat from the tropics to the northern latitudes.
Specifically, the study looked at series of abrupt climate changes that occurred between 60,000 and 25,000 years ago, ending as the last ice age peaked. A press release on the study describes it as an era when “temperatures in the Northern Hemisphere went on a rollercoaster ride, plummeting and then rising again every 1,500 years or so.”
“People have long supposed this link between overturning circulation and these abrupt climate events. This evidence implicates the ocean,” said L. Gene Henry, the lead author of the study and a graduate student at Columbia University’s Lamont-Doherty Earth Observatory. The findings, published in the journal Science, show for the first time that the ocean’s overturning circulation slowed during every one of those temperature plunges — at times almost stopping.
The issue has received research attention lately because some scientists suspect that a big massive of fresh, cold water from the melting Greenland ice sheet could have a similar effect, potentially leading to very sudden and disruptive climate change on a relatively short time scale that would be very difficult to adapt to.
The Atlantic meridional overturning circulation, distributes heat as it moves warmer surface water from the tropics toward Greenland and the high northern latitudes and carries colder, deeper water from the North Atlantic southward. The chemical analyses used in the new study enabled the researchers the relative speed of the Atlantic meridional overturning circulation during each abrupt climate change during the last ice age.
The evidence suggests that the speed of the ocean overturning circulation changed first, and that sea surface temperature changed a while later. That suggests that cooling may start with changes in the ocean circulation, influencing the northern sea surface and atmosphere, said co-author Jerry McManus, a professor at Lamont-Doherty Earth Observatory.
Evidence from ice cores and deep-sea sediment has shown that the northern climate also cooled before the southern climate during these abrupt changes, creating a “bipolar seesaw,” with the north cool while the south was warm, and the south cooling as the north warmed.
The scientists said they’re still facing a chicken-or-egg question that needs more research to be answered, namely whether changes in ocean circulation initiated the abrupt climate changes or were an intermediary effect initially triggered by something else.
“Our study supports the view that changes in ocean circulation were at least in part responsible for causing abrupt climate changes. However, what in turn caused those changes in circulation remains a mystery,” Henry said.
The study also doesn’t explain why there haven’t been recent similar sudden climate shifts. The instability appears to occur only in certain temperature ranges, and when there is a large amount of land ice that could contribute freshwater.
“We would all like to understand better how the earth’s climate operates,” McManus said. “This demonstrates the crucial role that global circulation can play. The dynamics of the deep ocean directly influence the earth’s climate.”
Each of the abrupt climate shifts during the ice age followed a general pattern in the Northern Hemisphere: The cooling happening over hundreds to 1,000 years, then the frigid temperatures persisted for a few hundred years in what is known as a stadial, McManus said. Once warming started, it happened very rapidly, with a rise of 3 to 6 degrees Celsius in average sea surface temperature and larger changes over Greenland within a span of decades.
During every cold northern stadial, the overturning circulation had slowed, so it wasn’t bringing as much heat northward from the tropics and Southern Hemisphere, the study shows.
The chemical tracers also suggest that circulation slowed almost to a halt during certain stadials known as Heinrich events, when massive amounts of icebergs broke off and drifted away from the Laurentide ice sheet, which covered a large part of North America at the time. Icebergs carry freshwater that can affect ocean circulation, and computer models have suggested that adding that much freshwater to the Atlantic could shut down circulation. Exactly what influence the icebergs had during these periods will be the target of future research.
Axel Timmermann, a professor of oceanography at the University of Hawaii who studies abrupt climate changes and was not involved in this study, called it a “breakthrough analysis.”
“Large changes in the North Atlantic meridional overturning circulation are thought to have played a major role in generating millennial-scale global variability, known as Dansgaard-Oechger events, during the last glacial period. The paper by Henry, McManus and colleagues finally provides supporting evidence for this fundamental scientific hypothesis,” Timmermann said.