Climate change: "It’s absolutely vital to act now"
Freigeist Fellow Niklas Boers develops novel prediction models for the earth and climate system. In this interview, he explains what findings they are already delivering - and what should be concluded from them.
Niklas Boers is carrying out his Freigeist project titled "Predicting Abrupt Transitions and Extreme Events in the Earth System" at the TU Munich, where he has held the professorship for Earth System Modeling since October 2021.
Professor Boers, please give us an insight into your research.
Together with my team, I am researching abrupt transitions and extreme events in the Earth system and the climate system. The aim is to arrive at better predictions of such events by extending the purely physics-based models with data-driven models and, in particular, neural networks. This helps us to make more accurate predictions, both for climate models as well as for weather forecasting.
For instance, we were recently able to show that the forecast horizon for the start of the Indian summer monsoon can be doubled from around 40 to between 80 or even 90 days in this way2.
What other insights have been opened up by these new methods?
Recently, I was able to show that an ocean current in the Atlantic, the Atlantic Meridional Overturning Circulation (AMOC), is not only slowing down but also losing stability. Before I elaborate further, though, I would like to put the record straight on an unfortunate misunderstanding: Some media reports state that my research indicates that the Gulf Stream will soon disappear. This is not correct – the Gulf Stream will be there for as long as the Earth continues to spin.
Can you tell us more about the AMOC current?
The AMOC is an ocean current that flows from south to north in the Atlantic Ocean, driven by differences in density. As the surface is constantly subject to evaporation along the way, the salinity of the water steadily increases. This warm, by now more highly salinized water arrives in the North Atlantic where it progressively cools down until it becomes quite cold. As the increased salt density makes the water body heavier, it sinks to a lower depth. This motion acts as an engine for ocean circulation. The result is a positive feedback loop: The more salt is transported north by the current, the stronger the current becomes. Unfortunately, though, this is being offset by a counter development: The more freshwater that enters the northern Atlantic due to ice melt, for example, the greater the weakening effect on the circulation.
What has been the effect on the AMOC?
Over the millennia, the strength of the flow has always been subject to change. The flow has been in the strong mode for about 10,000 years, but the current flow state is still the weakest in at least 1,500 years. The weakening over the last one hundred years most probably has something to do with global warming. We now want to find out whether this is a purely gradual weakening or a destabilization toward a critical transition. At this so-called tipping point, the flow would weaken abruptly. However, in this context "abrupt" means that it would still take several decades – which from a climatological point of view is incredibly fast! The Nature study "Observation-based early-warning signals for a collapse of the Atlantic Meridional Overturning Circulation"3 shows that we are moving closer toward a potential tipping point.
What could be the impact of such a weakening of the ocean circulation?
Should the AMOC approach and even exceed this tipping point – in other words, should it become quite significantly weaker – this would have far-reaching effects on the global climate and on several other components that make up the Earth system. More specifically, it would lead, for example, to a significant drop in average temperatures, especially in northern Europe, but also to changes in tropical monsoon systems.
How does this weakening of the ocean circulation come about?
On the one hand, the higher temperatures we are currently experiencing causes the water masses to expand and prevents them from sinking quite so quickly in the North Atlantic. On the other hand, through the inflow of fresh water – in particular melt water from the Greenland Ice Sheet – plays an important role here. In another study4 we have shown that part of the Greenland Ice Sheet has also lost stability over the last century. The increased melting is causing huge amounts of freshwater to enter the North Atlantic, diluting the saltwater there. Our hypothesis is that the physical cause of the weakening of the AMOC is at least partly due to the melting of the Greenland Ice Sheet and Arctic Sea ice. The density-driven "engine" of circulation is slowing down due to this dilution.
What conclusions do you draw from these findings with regard to the recently held COP26, the Climate Change Conference in Glasgow?
Regarding my area of expertise: In the current report of the Intergovernmental Panel on Climate Change (IPCC), and in particular the so-called "Summary for Policy Makers" on the basis of which the conference in Glasgow was held, the risks of abrupt transitions in the Earth system, such as the weakening of the AMOC, the destabilization of the Greenland ice sheet, or even a possible transition of the Amazon rainforest into a savannah, do not play a central role. This is partly due to the – certainly praiseworthy – consensus-oriented way the IPCC works. Notwithstanding, I think we urgently need an IPCC Special Report on tipping points, which will then hopefully serve as further basis for future negotiations. We are also working toward such a Special Report in the context of our Horizon2020 project "Tipping Points in the Earth System".
Climate change is a highly complex phenomenon, so we cannot predict everything that might happen with absolute precision. But the basic understanding is: The more we contribute to global warming – and with every tenth of a degree – the probability increases that we will trigger abrupt transitions and extreme events, and thus also virtually irreversible processes. There is no other option other than to act now, without delay, to reduce as many greenhouse gas emissions as possible. We need to cut back human intervention in the Earth system as quickly as we possibly can. The time is not going to come when we can say we know all we need to know in order to take action. What must act right now.
What milestone are you currently working toward with your own research?
It won't be long before we can build a completely new generation of Earth system models and climate models that integrate machine learning and neural networks. I'm looking forward to that, because I'm very excited about what else we're going to find out with these new kinds of models when we really get going. However, as is so often the case in science, we are talking about a time horizon of about 5 years.