Syukuro Manabe, Klaus Hasselmann and Giorgio Parisi have won the 2021 Nobel Prize for Physics. The prize is awarded “for groundbreaking contributions to our understanding of complex physical systems”.

The prize is worth 10 million Swedish krona ($1.1 million). Manabe and Hasselmann will share one half of the prize and Parisi takes the other half. Because of the COVID-19 pandemic, there will not be an award ceremony in Stockholm in 2021 and laureates will be presented with their Nobel medals in their home countries.

Parisi bagged his half of the prize “for the discovery of the interplay of disorder and fluctuations in physical systems from atomic to planetary scales”.

Manabe and Hasselmann share their half of the prize equally “for the physical modelling of Earth’s climate, quantifying variability and reliably predicting global warming”.

Common thread

While Parisi’s research is very different from that of Manabe and Hasselmann, the common thread in this year’s award is the study of disorder and fluctuations in complex systems. When announcing the award, the secretary general of the Royal Swedish Academy of Sciences, Göran Hansson, explained: “disorder, fluctuations, and how disorder and fluctuations together, if you understand it properly, can give rise to something you can predict”.

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During the early 1970s Parisi began to work on the theory of phase transitions within solids. He was particularly attracted by the application of field theory techniques  from high-energy physics to condensed-matter physics. Indeed, it is in this area that Parisi says he made his most important scientific contribution – helping form the physical theory behind spin glasses. These are magnets with “frustrated” interactions caused by disorder, which give them a rich set of behaviours.

Steven Thomson, who studies spin glasses at Dahlem Center for Complex Quantum Systems at the Free University of Berlin says, “Parisi has had a hand in inventing some of the most powerful and important theoretical machinery we have for understanding the behaviour of spin glasses and other complex systems. His work has had far-reaching impact on a huge variety of other fields, including particle physics, quantum field theory, neural network theory and other mathematical methods. The depth and breadth of these achievements is remarkable, and it’s fantastic to see all of this work recognized by the Nobel Committee.”

COVID-19 and conference registrations

Speaking to journalists minutes after the prize was announced, Parisi said that he was currently studying the dynamics of how COVID-19 spreads. Indeed, he has looked at a wide range of complex systems, and in 2007 he co-authored a letter in Nature Physics that identified a universal behaviour in the dynamics of how people register for conferences and showed how the final number of attendees could be predicted from early registrations.

When asked how he felt about winning, Parisi said, “I was very happy. I was not expecting the phone call but I knew there was non-negligible possibility [of winning].” Commenting on the work of Hasselmann and Manabe, he said, “It is urgent that we take strong decision on the climate. We are in a situation where we have positive feedback and accelerating increase of temperature. We have to act in a fast way and without delay.”

Pioneering climate research

Manabe is a pioneer in the development of  physical models of the Earth’s climate. In the 1960s he was the first to study how the balance of energy absorbed and emitted by the Earth interacts with the vertical transport of air masses. This research continues to play a crucial role in the development of climate models. In a ground-breaking paper published in 1967, Manabe and colleague Richard Wetherald showed that a doubling of the carbon dioxide content of the atmosphere would result in a 2 °C increase in the temperature of the lower atmosphere.

According to physics Nobel prize committee member John Wettlaufer, Manabe was “gobsmacked” when he heard he had won. “I’m just a climate scientist!” protested Manabe.

About 10 years later, Hasselmann created a model that connects climate and weather, showing that climate can be reliably modelled despite the fact that weather is changeable and chaotic. He showed, for example, that variation in weather on a timescale of days can influence the ocean on a timescale of years.

Hasselmann also developed techniques for identifying signals, or fingerprints, that natural phenomena and human activities create in climate data. These techniques have been used to prove that the increasing temperature of the atmosphere is the result of the human emission of carbon dioxide.

Essential understanding

James Hansen, director of climate science, awareness and solutions at Columbia University’s Earth Institute, says that the Nobel Foundation has made a good choice in awarding the prize to Manabe and Hasselman. “Their award is very well earned – their climate models provide the essential basis for our understanding of global warming,” he says.

Hansen, who pioneered work in the 1980s on climate sensitivity and feedback and who is a  former director of NASA’s Goddard Institute for Space Studies, adds that Wallace Broecker, who died in 2019, would have also been a possible recipient given his foundational work in the paleoclimate and oceanographic studies. “It’s too bad that he did not live to be part of the award,” adds Hansen.

Tim Palmer, Royal Society Research Professor of Climate Physics at the University of Oxford, says that this year’s prize is “well deserved”. “[Manabe] carried out pioneering studies of climate change using 3D climate models,” says Palmer. “They predicted the cooling of the stratosphere, the Arctic hotspot as well as global warming.”

Palmer adds that there are also strong links between the work of Hasselman and Parisi. “How do you represent small-scale events such as clouds and turbulent eddies in a climate model that cannot resolve these processes?” asks Palmer. “Parisi carried out fundamental work in this area and Hasselmann pioneered the use of stochastic models for studying the climate.”

Nobel careers

Born in Rome in 1948, Giorgio Parisi graduated in 1970 with a PhD in high-energy physics from La Sapienza University, during which he studied the Higgs mechanism under Nicola Cabibbo. Parisi then worked at the Laboratori Nazionali di Frascati, on the outskirts of Rome, on the theory of positron and electron collisions, which were being performed at the National Research Council’s Adone accelerator, also in Frascati. From 1981 Parisi worked at the University of Rome Tor Vergata, and in 1992 returned to La Sapienza.

Parisi was awarded the Dirac Medal in 1999 from the Abdus Salam International Centre for Theoretical Physics (ICTP) and the Max Planck medal in 2012 from the German Physical Society. In 2021 he was also awarded the Wolf Prize for his “ground-breaking discoveries in disordered systems, particle physics and statistical physics”.

Syukuro Manabe was born in 1931 in Shingu, Japan. He received his PhD at the University of Tokyo in 1958 before heading to the US to work at the US Weather Bureau until 1997. He then moved back to Japan to work at the Frontier Research System for Global Change, serving as director of the Global Warming Research Division. In 2002 he returned to the US to Princeton University, where he is currently a senior meteorologist.

In 1992 Manabe was the first recipient of the Blue Planet Prize of the Asahi Glass Foundation and in 2015 he bagged the Benjamin Franklin Medal of Franklin Institute. In 2018 he received the Crafoord Prize in Geosciences jointly with Susan Solomon “for fundamental contributions to understanding the role of atmospheric trace gases in Earth’s climate system”.

Klaus Hasselmann was born in Hamburg, Germany, in 1931. He was awarded his PhD in physics from the University of Göttingen in 1957 before moving to the Institute of Naval Architecture at the University of Hamburg, where he remained until 1961. He then moved to the US to work at the Scripps Institution of Oceanography before moving back in 1964 to the University of Hamburg. In 1975 he became a director of the Max-Planck-Institute of Meteorology in Hamburg before retiring in 1999.