The IAMAS-CNC Early Career Scientists Nobel Prize Online Interpretation Workshop

The 2021 Nobel Prize for Physics was awarded to three scientists for their contribution to the physical understanding of complex systems (The Nobel Committee for Physics, 2021). Two of the laureates, Dr. Syukuro MANABE and Dr. Klaus HASSELMANN, are climate scientists. This is the first time a climate scientist has won a Nobel Prize for physics and is thus a great encouragement to the entire climate science community, especially young scientists. Because the two winners' groundbreaking contributions that led to their award were achieved decades ago, young people may not be very familiar with these works. Therefore, to help young scientists better understand the scientific merit of the fundamental works and to inspire them in their future research careers, on 20 October 2021, the young scientist working group of The China National Committee of the International Association of Meteorology and Atmospheric Sciences (IAMAS-CNC) organized an online Nobel Prize interpretation workshop. Over 400 participants attended the online workshop, and more than 120 000 participants watched the replay.

Five invited speakers delivered insightful talks on the history of climate change research, progress and philosophy in climate modeling, the optimal finger printing method and the disciplined development of atmospheric science. They also shared interesting personal interactions with the prize winners.

The workshop was opened by the president of IAMAS-CNC, Prof. Mu MU from Fudan University. Prof. MU emphasized that this milestone event, i.e., a climate scientist winning the Nobel Prize for physics, is an excellent demonstration of interdisciplinary research. It will promote the joint research between atmospheric science and many other fields, such as physics, chemistry, public health, and artificial intelligence. It will also contribute to the balanced and sustainable development of atmospheric and climate sciences. He also hoped that atmospheric science research in China would benefit from learning the experiences of the Nobel Prize winners and stimulate ongoing efforts to produce more world-class research.

Two invited speakers, Prof. Yongyun HU and Prof. Jianhua LU, offered informative and insightful reviews of how the investigation of climate change and, in particular, global warming has developed.

The greenhouse effect was first discovered by Joseph FOURIER in 1827 when he calculated the Earth's temperature from a mathematical and physical perspective. His calculation showed that the climate would have been much colder if the Earth were only heated by the incoming solar radiation. He thus proposed the greenhouse effect to account for the temperature difference. John TYNDALL measured the infrared absorption and emission of various gases. He revealed that only certain gases or vapors (e.g., carbon dioxide, water vapor, ozone, etc.) have greenhouse trapping characteristics, all of which consist of triatomic molecules. Svante ARRHENIUS first quantified this effect in 1896 with a single-layer atmospheric model. He further estimated an increase of global mean surface temperature by ~6°C with a doubling of carbon dioxide concentrations, close to the upper limit (~4°C) given by IPCC AR6. To upgrade the oversimplified single-layer models, Karl SCHWARZCHILD and Subrahmanyan CHANDRASEKHAR made profound contributions to radiative transfer, the key to multiple-layer models. SCHWARZCHILD established the fundamental theory of radiative transfer, while CHANDRASEKHAR fully theorized and systematized this process. In the last two decades, the development of quantum mechanics and computer technology also played essential roles in climate research by advancing our understanding of the molecular absorption spectrum and enabling the calculation of complex physical systems.

Prof. LU specially mentioned C. G. ROSSBY's contributions to the research on CO₂-induced global warming. In the first chapter of "The Atmosphere and the Sea in Motion", which was titled "Current Problems in Meteorology" (Rossby, 1959), ROSSBY discussed the radiative effect of CO₂. This work was also his last paper. From 1870 to 1956, CO₂ in the atmosphere increased from more than 280 ppm to 320 ppm. ROSSBY and other scientists began to consider the global warming caused by CO₂ at that time, although many feedbacks were still unclear. In 1954, they proposed that CO₂ be monitored and suggested that observational stations be deployed in the Arctic, the Antarctic, North America and South America, and on islands in the Pacific. This formed the basis of 'KEELING's observation at Mauna Loa, which started in 1957, the International Geophysical Year.

In the 1970s, reliable global numerical models began to develop with the efforts of Syukuro MANABE and other scientists. The Charney Report (Charney et al., 1979), announced in 1979, marked the first comprehensive assessment of global warming prediction. Subsequently, new generations of models have attempted to better quantify this effect with more comprehensive considerations. By now, the latest IPCC AR6 suggested an increase in global temperature of 2.5°C–4°C with doubling carbon dioxide concentrations.

The research of atmospheric science constantly involves the interaction with many other disciplines, including but not limited to mathematics, physics, mechanics, astronomy, computer science, etc. Atmospheric science is not an isolated subject; rather, it is developed together with many other subjects to form an essential part of modern science.

The three invited speakers, Prof. Yongyun HU, Prof. Shian-Jiann LIN, and Prof. Yanluan LIN explained the work of Syukuro MANABE in detail.

Prof. Yongyun HU mentioned that Syukuro MANABE led the development of climate models to quantify climate variability and estimate the degree of global warming. To be specific, Manabe took account of convective adjustment and involved positive water vapor feedback effects by fixing relative humidity in climate models. These improvements made his radiative transfer schemes applicable to three-dimensional climate models. Based on his model (Manabe and Wetherald, 1967), MANABE calculated that if carbon dioxide concentrations were cut in half, this would induce a surface temperature decrease of 2.28°C; and if the carbon dioxide concentration were doubled, it would result in a temperature increase of 2.36°C.

Prof. Shian-Jiann LIN especially emphasized MANABE's modeling philosophy, which was to keep the model as simple as possible. MANABE has the talent for simplifying complex problems and grasping the key point. Therefore, he is a sketcher of atmospheric science, sketching out important things with a few simple strokes. He has the amazing ability to isolate the relevant mechanisms in a complex problem, and then represent them in the climate model. Manabe once said, "If you can not describe well an important process, use a constant; if you understand it a bit more, try a linear approximation." This concept is very similar to that utilized in numerical methods, in which the finite-volume method uses large-scale information to assume subgrid distributions. There are many super-simplified models that Manabe likes to use, such as the "Swamp ocean", "Bucket land model", and "Moist convective adjustment". His research always focuses on simplifying complex systems.

Prof. Yanluan LIN elaborated upon the radiative-convective equilibrium (RCE) model that MANABE first developed and shared his own understanding of RCE. He briefly reviewed the history of RCE and mentioned that MANABE's work fundamentally settled the debate about whether the increase of CO₂ leads to global warming. Manabe quantitatively predicted the temperature changes with the increased levels of CO₂ through a simple one-dimensional (1D) RCE model, which shows that the global warming caused by the increase of CO₂ is indisputable. Prof. LIN also talked about many modern developments of the RCE, including its evolution from 1D to 3D, from f-plane approximation to a rotating sphere, incorporating the temperature gradient and seasonal migration of the ITCZ, parameterization of the surface, etc. Prof. LIN finally emphasized that convection is by itself a complex system and that the ability to simplify is critical in solving problems in these complex systems.

Profs. Shian-Jiann LIN and Yanluan LIN also shared their interesting experiences with Syukuro MANABE at NOAA's Geophysical Fluid Dynamics Laboratory (GFDL). MANABE is frank and unassuming and enjoys communicating with other people. When teaching, he prefers talking at length about his own insights and reflections on scientific works rather than rigidly following the textbook. He often gives the students many papers to read and then discusses them in the next lesson to broaden their horizons and develop their reasoning abilities. He advises students to start with simple models (such as the linear model) rather than complex climate models. A deeper understanding of complex systems can only be achieved once simple principles are understood. MANABE is a good thinker and is usually the first to ask a thought-provoking question about a talk in the GFDL lecture hall, often the most exciting and anticipated part of the presentation. This ability to think greatly boosted the progress of his team.

Klaus HASSELMANN shared one-quarter of the Nobel Prize in Physics 2021 for his pioneering contribution to the attribution of climate change. The most fundamental contribution of Klaus HASSELMANN is the development of the optimal fingerprinting method, which allows separating the anthropogenic component from natural variability in climate change (Hasselman, 1993). Using this method, he has proven that the observed global warming can be largely attributed to human emissions, especially CO₂. The invited speaker, Dr. Ying SUN, explained HASSELMANN's work in detail.

HASSELMANN realized that the predictability of atmospheric general circulation systems would rapidly decrease because of its intrinsic chaos. With a solid physics and mathematics background, he developed a random climate model, proving that the random and high-frequency fluctuations of small-scale weather had long time-scale regularities so that the prediction of a long-term climate system was reliable (Hasselmann, 1976).

HASSELMANN further investigated the attributes of climate change and developed the optimal fingerprinting method, which made it possible to quantify the influence of human activities on the climate system. HASSELMANN treated the response of the climate system to external forcings as the signal and the natural variability as the noise, and he resolved the time series of meteorology fields into several vectors. He managed to amplify the signal-to-noise ratio by rotating the vector space so that the change due to external forcing could be extracted out of the natural phenomena. This fingerprinting method laid a solid physical foundation to understand the impacts of human activities on climate. Previous IPCC assessment reports point out that the confidence level of anthropogenic contribution to climate change has continually increased, and today it is well recognized that, since industrialization, human activities have caused the warming.

Looking back to Klaus HASSELMANN's academic career, his creativity, enthusiasm, and curiosity made him an outstanding scientist. Moreover, his early career as a physics major prepared him with a solid mathematical and physical background, which is an excellent example that highlights the importance of interdisciplinary research in climate science.

As mentioned earlier, the awarding of the Nobel Prize of Physics to climate scientists is a milestone event in atmospheric and climate research. The experiences of Drs. MANABE and HASSELMANN are very inspiring to young people for their career development. Professor MU and the five invited speakers, in addition to the audience, shared their thoughts in both the talks and the following question and answer session.

Most importantly, to carry out original research, young scientists should learn to identify the truly fundamental and meaningful problems rather than following temporary research hot spots. It requires us to read widely, think deeply and creatively. Persistency is also an important quality when pursuing a scientific goal. In particular, we need to think about what our world will be like in the next 50 years and beyond, just like those great scientists had done. Also, when facing the complexity in the climate system, we need to develop the capability of multi-level understanding and decompose the questions into many simple aspects. Last but not least, good scientific taste and scientific appreciation should be cultivated during the research process.

Despite the substantial development of climate change research by the two Nobel Prize winners and many other scientists, it remains a challenge to accurately project future climate change. With respect to future challenges, many key problems in atmospheric sciences have yet to be solved, such as Earth's climate sensitivity, cloud feedbacks, the nature of turbulence and convection, tipping point problems, weather modification and control, etc. However, we are already facing many global warming penalties, such as sea level rise, extreme weather, wildfires, etc. Therefore, understanding the mechanisms and inter-relationship of these changes and how they may evolve in the future is a fundamental task for the young generation.

The Nobel Prize Interpretation Workshop ended with a warm discussion and question and answer session. Prof. MU and the invited speakers patiently answered questions about the science related to MANABE and HASSELMANN's works and gave valuable advice on scientific research and career development. The audience felt inspired and motivated by the pioneering work of the two scientists and benefited from learning about the history of climate change and the development of atmospheric science research.

All workshop presentations can be viewed at Sina scientific and technology channel (https://zhibo.sina.com.cn/tech/physics) and the Koushare platform (https://www.koushare.com/lives/room/850721).