Klaus Hasselmann - Nobel Prize in Physics 2021

The Royal Swedish Academy of Sciences has decided to award the Nobel Prize in Physics 2021 “for groundbreaking contributions to our understanding of complex physical systems” with one half jointly to Syukuro Manabe and Klaus Hasselmann, “for the physical modelling of Earth’s climate, quantifying variability and reliably predicting global warming” and the other half to Giorgio Parisi, “for the discovery of the interplay of disorder and fluctuations in physical systems from atomic to planetary scales”.

Credit: Nobel Prize Outreach. Illustration Niklas Elmehed

The Human Fingerprint in the Climate System — About the pioneer of climate research Klaus Hasselmann

Credit: Victor Ocana/Klaus Hasselmann

Roland Wengenmayr

Today, human-induced climate change is common knowledge. But, how do we know that we are the ones causing it? And that we are heating up the Earth more and more with our greenhouse gas emissions, especially carbon dioxide? We largely owe this knowledge to the pioneering work of Klaus Hasselmann and Syukuro Manabe. Both were honored with the Nobel Prize in Physics in 2021 “for the physical modelling of Earth’s climate, quantifying variability and reliably predicting global warming,” one half of which they shared [Nobelpreis2021]. With their pioneering work, for which they had to develop and introduce completely new concepts, these two scientists played a leading role in founding modern climate research.

A crucial requirement for today’s understanding of global warming is the knowledge of how the atmosphere reacts to a rising carbon dioxide concentration. “Suki” Manabe was able to show with early computer simulations how the radiation budget and convection, the circulation of air masses in the atmosphere, interact and cause the thermometer to rise slowly. But how do you trace the human “fingerprint” in the chaotic, chance-dominated climate system? Klaus Hasselmann succeeded with a criminalistic touch. The metaphor of criminology fits perfectly here, because a fingerprint has a characteristic, unmistakable pattern. In that same way, a certain pattern in the behavior of the climate system ultimately leads to us humans — as perpetrators, as we must admit.

Bjorn Stevens, director at the Max Planck Institute for Meteorology, as whose founding director Klaus Hasselmann was appointed in 1975, compares Hasselmann’s crucial work to the discovery of gravitational waves. The extremely weak signal of two black holes tumbling into each other in the form of minimal oscillations in space-time could only be found because it had been possible to calculate it theoretically beforehand. This allowed astrophysics to search specifically for such tiny signals in the gigantic noise of the cosmos. This was achieved for the first time in 2016, a hundred years after Albert Einstein had predicted gravitational waves.

In a very similar way, Klaus Hasselmann’s concept was in principle based on looking for a certain, pre-estimated pattern in the noise of the climate system. Here, too, there is a connection to Einstein, via the so-called Brownian motion of molecules. In 1905, Einstein succeeded in physically describing the trembling motion of teeming molecules in a warm liquid by capturing their collective behavior with the tools of statistics. This was quite revolutionary in physics at the time.

It was in this spirit that Klaus Hasselmann introduced the “stochastic” methodology to climate research. Stochastics is a field of mathematics that describes systems using the tools of statistics in which chance plays a central role. Sounds pretty abstract, so let’s get specific. In the climate system, processes interact whose time scales range from seconds to extremely slow, that is, centuries and even longer. At the fast end rages the weather that is characterized by a certain degree of chaos. It is no coincidence that a meteorologist, the American Edward Lorenz, developed the foundations of chaos theory in the 1960s. This element of chaos in weather makes weather forecasts difficult, especially when they look beyond a week into the future. It will never be possible to predict exactly what the weather will be like in Hamburg on 1st October three years from now.

However, climate models can now with reasonable confidence predict what an average autumn in Hamburg will be like in 30 years’ time. And this is due to the enormously lethargic actor at the other end of the time scale, the world’s oceans. Water has a large so-called heat capacity, which is why it takes so long to finally bring a liter of water to the boil for tea. Because all of the oceans together form a huge body of water distributed around the Earth, it reacts very slowly to changes in the atmosphere. If the atmosphere tends to get warmer, the oceans buffer this as “climate memory” for a long time, at least until the water has warmed up accordingly.

In 1976, Klaus Hasselmann cracked the problem of the many time scales in a first pioneering act by considering the weather as noise and tackling it with stochastic means — in the spirit of Einstein’s approach to get a mathematical grip on Brownian motion. This allowed him to address a previously unanswered question: What causes natural climate variability? At the time, the widely shared view was that external drivers were the causes of climate variability, including changes in solar activity or large volcanic eruptions. Hasselmann could now show that the noise of the weather alone is enough to cause long-term climate fluctuations. Accordingly, the climate is permanently changing on its own, without any “kick” from the outside.

We can imagine this by picturing a drunken man who wants to go home from the pub with his dog. The man is in a state where he has lost his orientation. The young, untrained dog takes advantage of this passivity and runs erratically back and forth in all directions on the leash, because the world smells equally interesting everywhere. Consequently, he drags his staggering master sometimes here, sometimes there, with the result that the odd couple obviously can’t get away from the spot.

The dog embodies the noise in the weather, thus the chaos part, and the master the climate: The man reacts very sluggishly to the movements of the wild dog, but under its impulses he then stumbles a little here, a little there. So, the movement of the master is caused only by the random running around of the dog. It is exactly such a connection that Hasselmann could show in his work of 1976: The “jolts” of the noise in the weather alone cause the climate to change — so this happens only from within this system, without any external influence.

As a next step, let us now imagine that we, as observers, expect the following: In his alcohol-fogged brain, it does dawn upon the man that he actually wants to go home. We know approximately where he lives. So, the man will try to move slowly in the direction of home. Even though he has great difficulty, he manages to direct the wild dog in the desired direction in tiny steps. However, this shift of the involuntary pair dance is so subtle that this signal only becomes recognizable if you know exactly where to look for it in the turmoil. But how do you find it?

Such metaphors always have their limits when abstract mathematical work is involved. But this is roughly how Hasselmann’s second feat can be illustrated. Here, the road home symbolizes the long-term tendency of the climate to become warmer overall, for example, and the weak will of the dog owner symbolizes the influence of humans on the climate system. Hasselmann’s stochastic method is equivalent to an analysis of the couple’s wandering in a phase immediately after leaving the pub, in which no tendency in any particular direction actually seems to be discernible yet in the random movements. But Hasselmann can already detect such a tiny tendency with his method, because he knows where it is pointing and where he has to look for it in the enormous noise.

This is how one can imagine the detection of the human fingerprint in the climate system. Hasselmann’s work, which laid out the basic concept for this, was published as early as 1979, but it took until the mid-1990s for a team led by Hasselmann to identify this fingerprint with certainty. This concept evolved into what is now an important scientific field of climate research, dedicated to the “detection and attribution” of climate change.

Jochem Marotzke, Director at the Max Planck Institute for Meteorology, however, points out that there was a competitor for Hasselmann’s first work, with both scientists apparently unaware of their respective work. The American Cecil “Chuck” Leith had simultaneously and independently developed what Marotzke considered an equivalent concept to Hasselmann’s stochastic climate model [Marotzke2021]. “So, if you focus on the stochastic climate models, you would definitely have to discuss Chuck Leith’s contribution as well,” Marotzke feels, “That’s where I might differ from others in my assessment of what’s worthy of a Nobel Prize.” Marotzke’s intention is not to diminish Hasselmann’s achievement, but simply to point out in a fair manner that two researchers have come up with comparable ideas without knowing about each other. This happens quite often when something new is in the air.

“But his work on the ‘fingerprint’ makes Hasselmann a solitaire!”, Marotzke emphasizes what he sees as the Hamburg scientist’s outstanding achievement: “If you look at this publication from 1979 — this comes absolutely out of nowhere!” That this achievement alone is worthy of a Nobel Prize is also shown by the history of the creation of the “Intergovernmental Panel on Climate Change” (IPCC).

“Much of the IPCC is really constructed around the framework that Hasselmann has set,” Bjorn Stevens points out, ” It’s in the increasingly strong adjectives used by the IPCC from one report to the next to describe the likelihood of human influence on the climate.” The IPCC’s first assessment report in 1990 still assumed that most of the observed temperature increase could be due to natural variations, while not ruling out human influence. The second report, in 1995, remained cautious, arguing that the scientific ability to quantify human influence on climate was still limited — yet there was every indication that there was a discernible human influence on global climate. The 2001 report then already made the more concrete statement that mankind was “likely” — with a probability of more than 66% —the cause of global warming since the middle of the 20th century. By the 4th Assessment Report of 2007, the finding was already “very likely,” with over 90% probability, and by the 5th Report of 2013, “extremely likely,” with 95 to 100% probability. “So, it’s these adjectives that are getting more and more powerful,” Stevens says: “All of these adjectives are coded probabilities that emerge from Hasselmann’s work!” So, the “fingerprint” of humans in the climate system is emerging more and more clearly in these reports. The 2021 Sixth Assessment Report says: “It is unequivocal that human influence has warmed the atmosphere, ocean, and land.” So, the Intergovernmental Panel on Climate Change is one hundred percent certain that humans are causing global warming.

This outstanding scientific achievement has also long been recognized by Hartmut Graßl. The long-time scientific companion and former director at the Max Planck Institute for Meteorology stated as early as 1995 at a press conference in the presence of the then Federal Minister of Research: “The signal has been discovered”. To the press present, he said, “If there is a binding agreement on this under international law, then I am sure that Klaus Hasselmann is a candidate for the Nobel Prize in Physics.“

This prediction came true in 2021. It is important to note that Klaus Hasselmann and Syukuro Manabe received the Nobel Prize in Physics for their conceptual work, emphasizes Stevens. It is a misconception, he says, that this was in recognition of the development of climate models. “Modeling is a collective enterprise,” he says. It’s clear that many more contributors should have been considered in this case. The Nobel Prize in Physics could not possibly do justice to that, since it is limited to a maximum of three heads. But in a way, the Nobel Committee has already acknowledged the collective effort of model development by awarding the 2007 Nobel Peace Prize to the IPCC. Indirectly, Manabe and Hasselmann have also contributed to this Nobel Prize with their life’s work.

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Learn more about Klaus Hasselmann and his research here.

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Source:

https://mpimet.mpg.de/en/institute/the-nobel-prize-in-physics-2021