Climate information from the last Interglacial
Jörg Lippold works primarily with Protactinium (231Pa) and Thorium (230Th). The concentration ratio of the two isotopes provides information about the strength of the flow, because they behave very differently in the ocean. “While Thorium drops more or less like a stone to the sea floor, Protactinium sinks slowly and drifts away.” Despite how well this ratio serves as an indicator of the circulation strength, 231Pa has a disadvantage. There is no stable isotope thereof. It can not be detected in sediment samples older than 140,000 years, because of radioactive decay. “Our method is reaching temporal borders,” Jörg Lippold admits, “but 140,000 years practically corresponds to the time back to the last interglacial period, which is of particular interest with a view of current global warming.”
But what exactly has the varying strenght of Ocean circulation to do with climate change? “This is relatively clear,” says Jörg Lippold as he sketches a graph with pen and paper. It shows two curves: past water temperatures and the strength of the AMOC (Atlantic Meridional Overturning Circulation). “According to our results, the AMOC follows the temperature curve pretty well, an important indication that the two things are connected to each other.” In other words, if the flow was strong in the past, the water in the North Atlantic was always warmer. But an important point remains unclear. Was it the temperature that changed first or was it the other way around? “This is exactly the open research question,” says Jörg Lippold.
Atlantic Meridional Overturning Circulation was more stable than expected
Lippold’s EU-funded* project OCEANQUANT (Quantification of Past Ocean Circulation) is not yet finished, but results from it have already appeared in a “Nature” study . Their central message? In the past, the Atlantic Meridional Overturning Circulation was more stable than previously thought. Only during brief extreme phases of the last ice age it was weaker than today in which there were huge ice sheets reaching far to the south. During the relatively short melting phase of the last ice age, large amounts of freshwater poured into the north Atlantic. Because these enormous ice sheets no longer exist, the ocean circulation is probably less endangered than previously thought. “Even during an accelerated melting of Greenland’s ice, it seems to be unlikely that the Ocean circulation will collapse and that there will be a sudden drop in temperature,” says Jörg Lippold.
