From data collector to climate modeller

Frerk Pöppelmeier is working towards a better understanding of the Atlantic circulation. He’s combining his knowledge of environmental proxies with the unique capacities of the Bern 3D model.

When Frerk Pöppelmeier talks about his field of research, he has to disappoint lay people sooner or later. If asked how far the world is from the tipping point he’s investigating, he replies, “It’s not that simple”. The postdoctoral researcher in the OCCR group “Earth System Modelling: Climate Dynamics” is investigating the weakening of the Atlantic thermohaline circulation, one of the 15 or so tipping points that could abruptly send the climate system into a new state. The catastrophic scenario of ocean circulation coming to a halt has even been the subject of Hollywood films – because the ensuing collapse of the Gulf Stream could bring ice and cold over cities like New York, and also over central and northern Europe.

Frerk Pöppelmeier studied physics at the University of Heidelberg and subsequently earned a doctorate in geosciences. As part of his dissertation, he reconstructed changes in the Atlantic circulation since the last ice age. He investigated geochemical proxies from sediment cores, especially neodymium isotopes. The ratio of the different isotopes to each other provides information about the current strength and origin of the water masses in the Atlantic. But as the young researcher has realized, “In our area of study, data alone will not get us anywhere. To get a little closer to the truth, you have to link measurement results to models”. At the OCCR, Frerk Pöppelmeier has found exactly the expertise in climate modelling that he needed to advance his research.

Harmonizing reconstructions and model simulations

Since taking up his postdoctoral position at the start of 2020, he has been working with the so-called “Bern 3D model”. Frerk Pöppelmeier emphasizes that it is “very efficiently” programmed. Because the model does not offer a particularly high resolution, it can model long periods of time quickly while incorporating additional parameters. “We are also able to take most geochemical processes into account in the simulations,” says Pöppelmeier. Equipped with the model modified according to his needs, he is now trying to find out under which conditions the tipping point of the Atlantic circulation has already been reached – or would be reached again in the future. “With the help of the model, we are investigating how the disruptive factors affect the Atlantic circulation and trying to reconcile this with the reconstructions.”

It has long been clear that the Atlantic circulation is weakened by large amounts of inflowing freshwater. Over the past 100,000 years, the circulation has collapsed several times. The best-studied collapses are those at the transition from the last ice age to the present warm period, i.e. around 15,000 years ago. However, it is not quite clear what halted the circulation in the past. Why did so much freshwater reach the Atlantic in such a short time? One of the possibilities discussed: The Greenland ice sheet grew so immense that it became unstable under its own weight. One thing is clear, however; global warming is the reason for today’s rapid melting of the Greenland ice sheet.

Complex interaction of processes

And what does this mean for the future? Is it possible to predict when the Atlantic circulation will reach the point at which it will irrevocably tip over into another state? This is where Frerk Pöppelmeier has to remind us that things aren’t that simple. First, there are a multitude of factors that influence surface circulation: wind, sea level, the state of the oceanic roads – that is, whether the Bering Strait, for example, is open or covered with ice. Also of great importance is the direction in which the freshwater from Greenland flows out into the Atlantic Ocean and, conversely, how the supply of salty water from the Southern Ocean behaves. In addition, processes outside the Atlantic Ocean can also have an impact on it. Depending on the case, this can reinforce or compensate for the circulation.  “How all these processes interact and how they develop is very difficult to say,” says Frerk Pöppelmeier.

So is it impossible to make any quantitative statements about the future of the Atlantic circulation? The geoscientist says that “initial indications show that the circulation has weakened by 15-30% over the past 100-200 years”. When the tipping point will be reached, however, cannot be determined at present. “We still know too little about all these variables.” But one thing can be said with certainty: The collapse of the Atlantic circulation is a “low-risk, high-impact event”. In other words: “The probability of a collapse is rather low compared to other tipping points, such as the disappearance of the Amazon rainforest”. But as Frerk Pöppelmeier emphasizes, the influence on the climate of the northern hemisphere would be extremely severe.

(October 2020)