For his diploma-thesis, Jörg Lippold focussed on X-ray astronomy. Today, with the help of a new geochemical method, he wants to find out how ocean currents changed in the past. This knowledge may help us to understand the transport of heat and carbon in the sea – and also the entire climate system.
Jörg Lippold is a researcher with a flair for eye-catching images. “For example, the large overturning circulation in the Atlantic steals heat from the south Atlantic and transports it to the north, which is the principal effect of the Gulf Stream,” he says. For his research project, Jörg Lippold wants to create a map of the Ocean flow velocities over the past 140,000 years. This information will help to better understand the various factors that have influenced the past climate. “In order to predict the evolution of the climate,” says the physicist, “it is crucial to detangle the other interwoven processes that drive the climate system.”
Lippold analyses marine sediment cores that were taken from various locations several kilometres below the ocean floor. Many researchers before him have worked with these climate archives, but what is unique about Lippold’s project is that it will, for the first time, provide not only qualitative but also quantitative estimates of past ocean circulation – a research fields where experts have different opinions.
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.
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.
The big challenge in the OCEANQUANT project is also a technical one. Using the mass spectrometer of the Geological Institute of the University of Bern, Jörg Lippold measures in the trillionths of a gram range. “To measure these concentrations reliably, and also to correctly apply corrections for radioactive decay, is quite challenging,” he says. In fact, only a handful of specialists across Europe are able to do this because, preparing and handling the measurements requires a lot of experience.
Jörg Lippold has acquired exactly this specific knowledge over the course of his career. He graduated with a degree in astronomy following his physics studies at the University of Tübingen. Then he worked as a safety expert for nuclear facilities. Later, he did his doctorate in environmental physics at Heidelberg University – his first step in climate research, which eventually brought him to the Oeschger Centre on a Marie Curie grant. “The beauty of climate research is that it brings so many fields of research together. Through our project we bring another piece of the puzzle towards understanding what drives the climate system of our planet.”
* People Programme (Marie Curie Actions) of the European Union's Seventh Framework Programme (FP7/2007-2013) under REA grant agreement n° 622483.