1.1 Overview

Klaus Hasselmann was born in Hamburg in 1931. His family fled to England in 1934 because of the Nazis, so he grew up in an English-speaking environment, and returned to Hamburg after the war, where he studied physics, started a family, and became an innovative researcher. Later, he spent several years in the United States of America, but always returned to Hamburg, where he became the founding director of the Max-Planck-Institut für Meteorologie in 1975. His Institute soon became one of the world’s leading research facilities in the field of climate science. He retired in 2000, but continued his work in climate science as a “grey eminence” in the background, whilst his heart and mind turned to particle physics. He recently turned 90, and we—a group of former co-workers, scientific friends and colleagues—decided that we had to tell the story of this remarkable man.

One of the challenges we were faced with was the sheer breadth of interest and commitment of Klaus’s career. In this section, we shall attempt to evaluate his achievements as a NaturwissenschaftlerFootnote 1 (Sect. 1.2), an enabler (Sect. 1.3), and as a public figure (Sect. 1.4). Klaus himself will have his say in Chap. 2 in which we reproduce an interview from 2007 as well as a recent brief addition to it, and a chat between two Hasselmann acolytes, Dirk Olbers and Hans von Storch in May 2021. His key scientific achievements, which are summarised in Sect. 1.2, are discussed and evaluated in some detail in Chap. 3 in which a number of his original texts, some in German, are reproduced. In Chap. 4, some of Klaus’ former co-workers and colleagues present their personal accounts and memoirs of what it was like to work with him: the reader will see a certain amount of overlap between the different accounts—recurrent themes include his personal friendliness but also his sometimes-rude insistence on scientific rigour. But the various accounts differ in terms of content, and the scientific issues addressed. Reading these accounts will give the reader an insight into the variety of endeavours, interests and successes of this man. The final Chap. 5 includes an overview of his major publications, awards and a CV.

1.2 The Naturwissenschaftler

When considering how to survey Klaus Hasselmann’s scientific achievements, we were reminded of the old Indian parableFootnote 2 of the wise blind people, who want to understand what an elephant is. One examines a leg, another the trunk, the third an ear and so forth. They all understand exactly what they have in front of then, but none of them sees the whole animal. The authors of this book are the blind people, and Klaus Hasselmann is the elephant.

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We have identified seven major fields, to which Klaus made significant contributions; there may well be more, which we, the blind, have not yet classified and fully evaluated, one of which could be “internal’ waves”—a subject with which Klaus probably became involved during his stay at La Jolla and his collaboration with Walter Munk, and which has much in common with the surface wave problem (generation, wave-wave interactions and dissipation), to which he devoted a great deal of his time. At that time, Walter Munk was working with Chris Garrett to establish the scientific basics of internal wave research in the form of a unified wave spectrum of the three-dimensional oceanic wave field. Klaus himself never published any substantial work on the subject (except for [27]) but he did encourage several PhD students (Kern Kenyon at La Jolla and Peter Müller and Dirk Olbers later at WHOI and Hamburg) to work on the question and provided a number of far-reaching new concepts.

Klaus has always been interested in ocean waves, which gave him his entry point into mainstream science (see Sect. 3.1). His PhD thesis from 1957 was titled “Über eine Methode zur Bestimmung der Reflexion und Brechung von Stoßfronten und von beliebigen Wellen kleiner Wellenlänge an der Trennungsfläche zweier Medien “(A method for determining the reflection and refraction of shock fronts and of arbitrary short wavelength at the interface between two media.) [191]. The first seminal paper he published on the subject was “Grundgleichungen der Seegangsvorhersage” (Basic sea state prediction equations) in 1960 ([3], see facsimile in Sect. 3.1). This paper provided a basic foundation for a reliable, generally applicable method of sea state prediction based on the basic energy balance equation of the ocean wave spectrum. He later published 3 papers entitled “On the nonlinear energy transfer in gravity-wave spectrum” in 1962–1963 [6, 8, 9], and “Propagation of ocean swell across the Pacific” [18] with Walter Munk in 1966.Footnote 3

Klaus’ work on remote sensing and the satellite ERS-1 (see Sect. 3.2) was also related to the field of ocean wave dynamics. He and Manfred Schieler published a paper on “Radar backscatter from the sea surface” [26] in 1970. Later papers, beginning with [45] from 1978, addressed aeroplane or satellite-based ocean wave spectra measuring methods. The MARSEN experiment (see below) led to another breakthrough when Klaus and his co-authors published the “Theory of SAR ocean wave imaging: A MARSEN view” [75] based on an imaging model, which was fundamental for SAR imaging of the ocean surface from future satellites SARs. Another fundamental paper, which he co-authored with his wife Susanne, was “On the nonlinear mapping of an ocean wave spectrum into an SAR image spectrum and its inversion” [102], a reproduction of which is included in Sect. 4.2. This paper serves as an example of one of the major contributions the Hasselmann couple made to the future of the retrieval of the ocean wave spectrum from the ERS-1 C band SAR on the global scale. Klaus’ latest, and perhaps final contribution to this topic was the extensive review entitled “The ERS SAR wave mode: A breakthrough in global wave observations” [176].

When Klaus Hasselmann took on the responsibility and challenge of running the Max Planck Institute for Meteorology (MPI-M) in Hamburg, most of his time and attention was taken up by the subject of climate change. His initial thoughts on the subject were set out in his seminal “stochastic climate model”, which was published in 1976 ([38], see facsimile in Sect. 3.3), which provided an insight into the formation of long-term internal variations excited by short term random fluctuations. Although this approach was not particularly surprising for a theoretical physicist, the concept did change the way climate scientists thought about the problem. The stochastic climate model firmly established the concept of a stochastic climate system, which included the separation of externally provoked variations (“signal”) and unprovoked internal” variability (“noise”). This led to the emergence of the general concept of “Principal Interactions Patterns” ([86], see facsimile in Sect. 3.4), which included the key idea that the full infinite state space may be split into a low-dimensional “signal” space in which deterministic dynamics hold sway, and an infinite higher-dimensional “noise”-space, which is well approximated by stochastic dynamics. The “Principal Oscillation Patterns” [89] represented a special case. But the most important aspect of this approach was the question of detection and attribution [54, 110], i.e., of detecting the footprint of anthropogenic climate change in the empirical record of climate variation. This approach emerged as a key argument in the Intergovernmental Panel on Climate Change’s (IPCC) assessment that anthropogenic climate change is real and will intensify if greenhouse gas emissions continue unabated.

When the science of the mechanisms of climate variability and change had matured [118] in the early 1990s, Klaus Hasselmann became interested in the interaction between climate and society and how mankind could deal with human-induced climate change. He understood quite early that the anthropogenic climate change problem goes well beyond the domain of climate science. It is not nearly enough to frame climate change research solely within the limits of what is sometimes referred to as “curiosity-driven science”. Instead, research into anthropogenic climate change should support policymaking and coordinated climate action. These ideas prompted Klaus to create the Potsdam Institute for Climate Impact Research as a matter of urgency to deal with the economic dimension of climate change. In collaboration with Dmitry Kovalevsky and others, such as Michael Weber and Volker Barth, Klaus attempted to construct optimal policies to balance the expected costs of damages with the expected costs of mitigation. Section 3.5 provides more information about this field.

Following his retirement, Klaus Hasselmann became less interested in climate science, probably because he thought that he had already contributed everything that he could to the field and that the remaining challenges, such as the economic dimension, would be taken care of by others. Instead, he returned to a topic, which he had been thinking about in his spare time throughout his career. On his 60th birthday, he surprised his guests with the announcement that he would present something new—which would also explain to his family that he had really been thinking about particle physics (Sect. 3.6), when they falsely believed that he was just trying to get out of mowing the lawn. His talk took about two or three hours: “You can ask me, but you cannot stop me!” he said, and most of the audience did not understand a thing but enjoyed the show. Of course, Klaus was serious about the topic and his Metron concept. His wife, Susanne, volunteered to manage his schedule and he began to present his ideas to the physics community, the majority of whom were unfortunately not inclined to listen to him. The full concept has now been documented in a series of articles and in an unfinished book whose introduction we have reproduced in Sect. 3.6. All we can do at present is to wait to see if Klaus ideas’ will eventually rule the waves, as they often did in the past.

1.3 The Enabler

When we talk about Klaus Hasselmann as an “enabler”, we are referring to his ability to set things in motion, to create a scientific environment, which enables individuals to realise their full potential. Enabling activities may not leave a scientific footprint, but they do make an indirect contribution to the scientific process whether by creating a well-functioning working environment or by providing access to crucial empirical evidence. Klaus was an enabler in multiple ways.

His most important achievement was certainly the Max-Planck-Institut für Meteorologie, which provided many (at that time) young scientists with an environment in which to develop their skills. The various personal accounts included in Chap. 4 illustrate this breeding ground convincingly. “Do something that you consider interesting”—this Klausian request sounds like a recipe for disaster, an invitation to a hoard of intelligent young scholars to develop without coordination, without reference to a programme, and in various directions. But that did not happen. The young researchers all converged on the same problems but from their own unique angles. Of course, an Ernst Maier-Reimer would not take orders from anybody, but he would always have a suitable FORTRAN code for most problems in one of his desk drawers. In short, the Institute was a marvellous incubator, which was sometimes compared to an aquatic ecosystem populated with bottom feeders, primary producers, and gracious predatory fish. At the top of the chain there was just the one big fish, Klaus himself, who somehow managed to steer the dynamics within the incubator, with unconditional scientific rigour, personal friendliness and an endless reservoir of ideas.

One may well ask whether the MPI was organised in any way? There were a number of “Zwischenkapazitäten” (something like “lieutenants”), such as Dirk Olbers, Jürgen Willebrand, Mojib Latif, Martin Heimann, Peter Lemke, Ernst MaIer-Reimer and Hans von Storch, who acted as mentors to the younger researchers, but ultimately it was Klaus who guided and managed ideas, often by rejecting, replacing, or rectifying them. Even the management functioned smoothly. Klaus served as Managing Director throughout most of 1975–2000; only once did he wish to take a short break, and someone else took the helm and attempted to install a certain amount of administrative order, counting pens and the like. One of the then unhappy Zwischenkapazitäten went to the elder statesman, Reimar Lüst, at the end of the hallway to ask if we had misunderstood something? Nope, said Lüst, you’ve understood perfectly well; don’t give in, it’ll soon be over. And, really, Klaus was back after just a few days, and there was no more counting pens. Scientific paradise was re-established. Some of his co-workers, who later became Institute directors themselves, tried to copy his approach with some success.

As Institute Director, Klaus was also responsible for the financial side of things and had a special way of looking at this challenge, possibly guided by his colleague Hans Hinzpeter’s famous dictum “a number is not a number”, which played upon the empirical fact that any assertion about the financial situation would be preliminary, and prone to significant changes at short notice. Klaus spoke of “flying in fog”, to suggest that knowing whether money would be available for hiring someone or making a capital purchase was largely based on a feeling. In a sense it was also a signal-to-noise problem. It worked out well and broke the potential spell of financial details involved in running a scientific Institution.

As outlined in his interview (Sect. 2.1) Klaus spent most of the first ten years trying to clarify the basic dynamic aspects of climate variability and change. Although the Institute has been founded to conduct research into climate change, most people there failed to notice this link. He also decided against purchasing a big computer, as most people had expected him to do but he worked instead with a relatively small group and a modest suite of hardware. But when the need for quasi-realistic climate models had become obvious by the early 1980s, Klaus established a close link to Günter Fischer’s group at the Meteorological department of the University of Hamburg, which was located in the so-called in the Geomatikum some 10 m below the MPI. One member of that group, Erich Roeckner, was the then leading expert on atmospheric modelling. The first step was to replace the dynamical core that had been developed in-house with the model of the European Center for Medium Range Forecast, ECMWF, with parametrizations of the Hamburg model. The model was dubbed ECHAM—EC + HAMburg.

At about that time, Klaus had decided to set up a separate large computing centre, the Deutsches Klimarechenzentrum (DKRZ), which was to be led by Wolfgang Sell. This move made it possible to carry out gigantic simulations using a climate model based on Erich Roeckner’s atmospheric ECHAM and Ernst Maier-Reimer’s ocean LSG models. The computer system was updated regularly, with generous funding from the German Federal Ministry of Education and Research, whilst the costs for running the DKRZ were shared between a the MPI (the majority shareholder), the University of Hamburg, the Alfred Wegener Institute in Bremerhaven, and the GKSS in nearby Geesthacht. This system continues to run smoothly in 2021.

This process was eventually completed when Lennart Bengtsson was persuaded to become co-director of the MPI, where he would focus on atmospheric modelling (see Sect. 3.7). The ECHAM has become one of just a few leading quasi-realistic climate models, which are used in various institutions all over the world.

The incorporation of the DKRZ and the Bengtsson-department within the MPI were completed in the 1990s when the Potsdam Institute of Climate Impact Research, headed by Hans-Joachim Schellnhuber, was set up. This completed Klaus’ original vision. He went on to think about metrons (Sect. 3.6).

Gathering data

Although Klaus Hasselmann was really a theoretical scientist, he often got involved in the practical challenge of gathering the relevant data, whether in preparation for satellite missions, or for setting-up and managing various campaigns (in atmospheric and oceanographic science jargon: experiments).

The first experiment he ran as the lead scientist was the JONSWAP project in 1969 (see Sect. 3.1). Wind stress, atmospheric turbulence and swell attenuation were monitored in the German Bight, and eventually the JONSWAP spectrum was derived.

Following the success of JONSWAP, Klaus apparently felt secure enough to initiate the IWEX (Internal Wave Experiment) mooring campaign during his stay at the Woods Hole Oceanographic Institution (WHOI) which he visited alongside Mel Briscoe, Terry Joyce, Claude Frankignoul, Peter Müller and Dirk Olbers. To our knowledge, the IWEX tripod was the first mooring capable of measuring current cross-spectra with sensors at horizontal and slanted separations. The experiment was carried out in the Sargasso Sea in 1973.

This culminated in the international Marine Remote Sensing Experiment, MARSEN which Klaus coordinated. It was carried out in the North Sea between the 16th of July and the 15th of October 1979 and was designed to achieve the following two objectives: (1) to investigate the use of remote sensing technology for oceanographic applications and (2) to utilise remote sensing technology in concert with in-situ oceanographic measurements to investigate oceanic processes in finite-depth water in the near-shore zone. MARSEN made use of 6 remote sensing aircraft including the NASA CV-990 with the JPL SAR. 60 scientists from 6 countries took part in the experiment, which spawned a plethora of papers, 14 of which were published in a special issue of the Journal of Geophysical Research in 1983.

In his later career, Klaus no longer participated in long-term empirical observational campaigns, perhaps because his attention was increasingly focused on the climate issue. Experimental work in ocean science is more about understanding and parametrizing various processes in ocean models—and the JONSWAP spectrum is an excellent example of this—whilst climate science mostly depends upon ongoing monitoring efforts. So, Klaus became involved in remote sea state monitoring technologies, which also, of course, had to do with his interest in the predictive potential of ocean waves. By the 1980s he had already become a key member of the ESA High Level Advisory Committee (EOAS), which had been set up by the ESA DG (Sect. 3.2). His commitment to the preparation of the ERS-1 satellite among other things was honoured by ESA, who invited him to join the launch of ERS-1 on the 17th of July at the Guiana Space Centre in French Guiana.

When Klaus became aware of the societal urgency of the climate problem in the 1990s, he founded the European Climate Forum (ECF), which was later expanded to cover a broader geographical range and renamed as the Global Climate Forum (GCF).Footnote 4 Klaus made an active contribution to various ECF/GCF operations, where was vice-chairman and a member of the board for many years.

The WAM group

In the spring of 1984 Klaus invited a number of ocean wave researchers to a meeting in Hamburg. There he proposed to jointly work on the development of an advanced numerical ocean wave prediction model. He wrote a new acronym (WAM, for Wave-Modelling group) on the blackboard and opened the discussion. The meeting supported the idea, because previous collaborations had created a shared feeling of urgency: model improvement was needed. Klaus recruited the participating Gerbrand Komen as chairman of the new group. He and Klaus would collaborate closely to “create a scientific environment, which enables individuals to realise their full potential”, but which also enabled Klaus to achieve his goals.

One of the first challenges was to develop a scientific strategy. Some members focused on model development, but others simply wanted to collaborate on ocean wave research. This led to a number of subprojects, one of which was aimed at the development and implementation of global and regional versions of the model. Other subprojects focused on growth curve reanalysis, directional effects, shallow water effects and data assimilation.

Annual meetings were hosted at participating institutes on a rotational basis. They all opened with a review of the group objectives and what had been achieved in the past year. This was then followed by one or more formal presentations by guest speakers and a tour de table where each participant could say what he or she had done and was planning to do. After discussions, the tasks and commitments for the coming year would be listed. Klaus usually played a rather passive but very inspiring role in these meeting. Of course, some stricter co-ordination was later agreed upon in smaller groups, especially when the model was implemented at ECMWF.

Initially, there was no special funding. Most people contributed because the objectives of the WAM group aligned with the objectives of their home institutes. Once the research got underway the group became successful in acquiring additional funding. Outreach and enlargement of the group played an important role in generating support. Outreach took the form of many (invited) lectures at participating institutes and specialist conferences, sometimes as a showcase of a successful collaboration. The group was made truly international when it merged with a newly established working group (Working group 83, ‘Wave Modelling’) of the Scientific Committee on Oceanic Research. This brought in participants from China, the Soviet Union and elsewhere, to everyone’s mutual benefit. The group ultimately included about 70 people from 15 countries.

One characteristic of the group was the spirit of collaboration. Group identity was reinforced by a newsletter in addition to informal contacts during working visits and the annual meetings. One of the participants wrote some alternative lyrics to the well-known Beatle song “Those were the days, my friend”, which became “Those were the waves, my friend”, which included the unforgettable line “Comparisons have shown // The physics are unknown”. It was sung loudly after dinner during a meeting in Canada, which created a warm feeling of solidarity. In hindsight one may well wonder whether there was too much of a warm feeling, as it may have blocked some healthy dissidence. But so it goes.

Understandably, given his many other activities, Klaus was strongly focused on model development, which culminated in the implementation of the WAM model at the ECMWF. The other subprojects were also successful and resulted in several publications. One of the outreach highlights was a five-week course on ocean waves and tides at the International Centre for Theoretical Physics with support from the World Meteorological Organization and about 100 participants, mainly from developing countries. Another one was the successful completion of a jointly written monograph (Dynamics and Modelling of Ocean Waves [244]) which was published by Cambridge University Press in 1994. After that the group was dissolved. Mission completed.

1.4 The Public Figure

Klaus Hasselmann’s work on ocean waves, which included remote monitoring and predictions was relevant and represents elegant science, which, however, hardly attracted public interest. The climate issue was completely different: his two major achievements, namely the detection of the signal of anthropogenic change against the background noise of internal variability, and the attribution of this signal to human greenhouse gas emissions, as well as the provision of a scientifically first-class climate simulation platform, had an enormous impact on the relevant public discourse, not only in Germany but around the world. Nevertheless, he remained mostly unknown to the general public. This was not a matter of bad luck but of his own deliberate intent—he was simply not “interested in informing the public, [he was] always interested in basic research”, as his wife Susanne put it in Sect. 2.3. Thus, he was happy when others offered to do the job for him. These others were Hartmut Graßl and Mojib Latif, who both became very well known to the German public. Both excelled in explaining complex dynamics and perspectives in a way that lay persons could readily understand. Interestingly, they did so without coordination between themselves or with Klaus, which obviously caused no problem.

Asked for his opinion on the public perception that the most important climate researchers in Germany were Graßl and Latif, whilst he himself was barely known, he responded (Sect. 3.3): “I was very pleased about that.”

Thus, not surprisingly, Klaus left few traces in the media. Among the few examples he did leave were:

  • Klaus Hasselmann: Die Launen der Medien. ZEIT 32/1997

  • Johann Grolle: Wieviel ist der Wald wert?–Interview mit Klaus Hasselmann. Spiegel, 41/1992, 271–274

  • Johann Grolle: Nobelpreis? Nee, daran hab‘ ich nie gedacht–Spiegel-Gespräch. Spiegel, 41/2021, 110–111

  • Pieter Sartorius: In Sandalen die Welt von morgen suchen. Süddeutsche Zeitung, 31.10.1997s

The last piece is reprinted in the next Sect. 1.5. It has not been translated, as it is a wonderful example of living German whose special charm would hardly survive translation.

Klaus and two of his Zwischenkapazitäten, Ernst-Maier Reimer (left) and Mojib Latif (right).

(c) Günther Menn, Lea La Greca; mit freundlicher Genehmigung

1.5 In 1997, A Visitor Told His Perceptions When Visiting the MPI

Freitag/ Samstag/Sonntag, 31 Oktober/1./2. November 1997.

Süddeutsche Zeitung Nr 251 / Seite 3.

© Süddeutsche Zeitung GmbH, München. Mit freundlicher Genehmigung von Süddeutsche Zeitung Content ( www.sz-content.de ).

Das Foto auf der vorhergehenden Seite wurde mit dem Text zusammen veröffentlicht. Urheber: Günther Menn; Genehmigung des Nachdrucks durch die Nachlassverwalterin Lea De Greca.

Peter Sartorius

ln Sandalen die Welt von morgen suchen

Klimaforschung: Welche Auswirkung hat die vom Menschen beschleunigte Aufheizung der Erde?lm Hamburger Max-Planck-Institut arbeiten 150 Wissenschaftler an Formeln, mit denen sich der Zustand des Planeten in 100 Jahren errechnen läßt

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A critical inspection of the older sister Almut. Hamburg, shortly before leaving for England in 1934 (left), and In Welwyn Garden City, England, shortly before leaving for Hamburg, 1949 (right).

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With Karl Wieghardt, diplom thesis advisor and later post-doc employer in Institute for Naval Architecture, at inauguration ceremony, 1975.

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With Susanne and two oldest children, Meike and Knut, in La Jolla, 1963.

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At Woods Hole Oceanographic Institution, in front Research Vessel Knorr, 1972.

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With Bob Stewart, Brian Tucker and Australian sheep during break of the Joint Organizing Committee meeting of the Global Atmospheric Research Programme in Melbourne, 1974.

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With Reimar Lüst, President of the Max Planck Society, at inauguration ceremony of the Max Planck Institute, 1975.

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With Peter Fischer Appelt, President of the University of Hamburg, Senator Biallas of the City of Hamburg and Reimar Lüst during the inauguration ceremony, 1975.

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Explaining ocean wave prediction, 1982.

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In the new prefab building (“pavillon”) behind the Geomatikum, after creation of the DKRZ, 1989.

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Making a point, 1988.

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Robertson Memorial Lecture Award, US National Academy of Sciences, 1990 (proposed by Carl Wunsch, second row, first left).

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With Hartmut Graßl, 1996.

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Explaining the multi-agent aspects of a coupled climate-economy model, 2002.

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Explaining the detection of an anthropogenic climate signal at 95% statistical confidence level, with the Federal Minister of Research and Technology, Jürgen Rüttgers, 1992.

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60th birthday, Rissen 1991

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With Walter Munk, during Hasselmann’s 60’th birthday symposium, 1991.

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With Wolfgang Sell, Lennart Bengtsson and wife Susanne during emeritus dinner, November 1999.

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Ola M. Johannessen, Walter Munk and Klaus Hasselmann sailing the fjords near Bergen. Ola comments: “Walter and I are discussing my CO2-Ice paper from 2008, too simple for Klaus, who took a nap”

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Rissen, 2011.

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with Hans von Storch

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Susanne, Klaus and Dirk Olbers 2012 in Fischerhude, discussing ocean physics