Understanding a Complex World – Interpretation of the 2021 Nobel Prize in Physics

At 17:49 Beijing time on October 5, 2021, the Royal Swedish Academy of Sciences announced that the 2021 Nobel Prize in Physics was awarded to Japanese-American scientist Syukuro Manabe, German scientist Klaus Hasselmann and Italian scientist Giorgio Parisi for their "pioneering contributions to our understanding of complex physical systems."

Shuro Makoto, Klaus Hasselman and George Parisi (Source: Nobel Prize website)

Born in 1931 in Shingu, Japan, Makoto received his Ph.D. from the University of Tokyo in 1957 and is currently a senior meteorologist at Princeton University in the United States. Born in Hamburg, Germany in 1931, Hasselmann received his PhD from the University of Göttingen in 1957 and is currently a professor at the Max Planck Institute for Meteorology in Hamburg, Germany. Born in Rome, Italy in 1948, Parisi received his PhD from the University of Rome I in 1970 and is currently a professor at the University of Rome I.

Makoto and Hasselman shared half of the prize totalling SEK 10 million for "physically modeling the Earth's climate, quantifying variability and reliably predicting global warming", while Parisi received another SEK 5 million for "discovering the interaction of disorder and perturbation in physical systems from the atomic scale to the planetary scale".

The research of the three winners all focused on chaotic, random phenomena. The research of Makoto and Haselman laid the foundation for our understanding of the Earth's climate and how humans affect it, while Parrisi revolutionized the theory of disordered materials and stochastic processes.

All complex systems are made up of many different interacting parts. Physicists have been studying them for centuries, but have had a long time difficulty understanding them because they are difficult to describe mathematically—they may have a huge number of components, or be dominated by random factors. They can also be chaotic, and just like the weather, small changes in the initial value can lead to large differences that follow. This year's Nobel Prize in Physics rewards new ways to describe complex systems and predict their long-term behavior.

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Reveal the laws of the Earth's climate

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Among the various complex systems we face, one of the complex systems that is crucial to human beings is the Earth's climate, so it has become the object of long-term exploration by scientists. French physicist Joseph Fourier began studying the effects of solar radiation on Earth 200 years ago and discovered what we today call the "greenhouse effect." Since then, climatologists have continued to add detail to the knowledge of atmospheric science and develop climate models based on models that predict weather.

As a scientific term well known to the public, the greenhouse effect is closely related to our human lives. Carbon dioxide, methane, water vapor and other gases are all greenhouse gases, of which the greatest impact is actually water vapor. However, we cannot control the concentration of water vapor in the atmosphere, and we can only try to control the concentration of other greenhouse gases such as carbon dioxide and methane.

Svante Arrhenius, a Swedish scientist and winner of the 1903 Nobel Prize in Chemistry, was the first to discover the effects of carbon dioxide on temperature. Through his research, he found that if the level of carbon dioxide in the atmosphere was halved, it would be enough to make the earth enter the ice age again, and if the carbon dioxide level doubled, the earth's temperature would rise by 5 °C to 6 °C. In a way, this result is strikingly consistent with current estimates.

In the 1950s, after receiving his Ph.D., Makoto moved to the United States to continue his research career, and his research direction, like Arrhenius, hopes to understand how the increase in carbon dioxide levels in the atmosphere leads to an increase in the earth's surface temperature. In the 1960s, he led the development of physical models of the Earth's climate, incorporating the vertical transport of convective-induced air masses and the latent heat of water vapor.

To make these calculations easier, Makoto chose to reduce the model to a vertical cylinder that stretches from the ground to the 40-kilometer-high atmosphere. After extensive calculations, he found that the effects of oxygen and nitrogen on surface temperatures were negligible, while carbon dioxide had a significant effect: when the concentration of carbon dioxide doubled, global temperatures rose by more than 2°C. Moreover, this model also confirms that this warming is indeed caused by an increase in carbon dioxide concentration. Because this model predicts that when the upper atmosphere cools, the temperature of the atmosphere near the ground will rise, and if the change in solar radiation causes the temperature to rise, then the entire atmosphere should be heated at the same time. With insights from one-dimensional models, Makoto uses insights from one-dimensional models to derive climate models in three-dimensional space, which is a milestone in our process of revealing the laws of Earth's climate and lays the foundation for the development of current climate models.

After about 10 years of Makoto's research, Hasselman found a way to deal with the changing and chaotic weather, successfully creating a model that links weather and climate so that it can answer the question of why climate models are reliable despite their variable and chaotic weather. After completing the climate change model, Haselmann also developed methods capable of identifying specific signals ("fingerprints") that natural phenomena and human activities affect the climate. In doing so, he cleared the way for further research into climate change.

Haselmann's method was used to prove that the rise in atmospheric temperature was due to human emissions of carbon dioxide. Well-established climate models clearly show the accelerated greenhouse effect: the amount of carbon dioxide in the atmosphere has increased by 40% since the mid-19th century. Never before in the last hundreds of thousands of years has the Earth's atmosphere had so much carbon dioxide. Temperature measurements indicate that global temperatures have increased by 1°C over the past 150 years. This seemingly modest warming, in addition to bringing direct visible glacier melting and rising sea levels, has already had a fatal impact on many ecosystems. The Nobel Prize Committee used four concise questions and answers on its website to highlight the significance of the study of Shuro Makoto and Hasselman, which is a conclusion that the climate model clearly tells us.

- Is the Earth heating up?

- Yes.

- Is it because of the increase in the amount of greenhouse gases in the atmosphere?

- Can this be explained by natural factors alone?

- No, you can't.

- Are human emissions the cause of the increase in temperature?

<h3>Discover the secrets of the disordered system</h3>

Around 1980, Parisi discovered hidden patterns in disordered complex materials. His discovery is one of the most important contributions to the theory of complex systems. His discoveries allow us to understand and describe many different, completely random materials and phenomena, not just in physics, but also in other very different fields, including mathematics, biology, neuroscience, and machine learning.

The study of complex systems began in the second half of the 19th century with James C. Maxwell, Ludwig Boltzmann, and J. Gibbs. The statistical mechanics developed by Willard Gibbs et al. gives us the means to describe and study systems made up of large numbers of particles. This method must take into account the random motion of particles, so the basic idea is to calculate the average effect of particles rather than studying each particle individually. Statistical mechanics has been a huge success by providing microscopic explanations for the macroscopic properties of gases and liquids, such as temperature and pressure.

Particles in a gas can be thought of as pellets, and when the temperature drops or the pressure rises, the system of pellets becomes first liquid and then solid—usually crystals, in which the pellets are arranged in a regular manner. But if this change occurs very quickly, the pellets form an irregular arrangement. If this rapid process is repeated, the pellets will take on a new arrangement, even though the system undergoes exactly the same changes.

Why do you produce different results? Parisi understood the complexity of another system called spin glass while studying it and discovered the secrets of disordered systems. Such a system is a special metal alloy, such as an alloy obtained by randomly mixing iron atoms into a network of copper atoms. Although it contains only a small number of iron atoms, it can change the magnetic properties in a confusing way. Each iron atom behaves like a small magnet (or spin) and is affected by other iron atoms around it. In ordinary magnets, all spins point in the same direction; but in spin glass, the spin state is frustrated: some spin pairs want to point in the same direction, while others spin pairs are opposite, so how do they find the best direction?

Spin glass and its peculiar properties provide a model for complex systems. In 1979, Parisi made a decisive breakthrough, showing a solution to the spin glass problem. The mathematical correctness of his solution was not proven until many years later. Since then, his method has been widely used in many disordered systems, becoming a cornerstone of complex systems theory.

Parisi also studied many other phenomena in which stochastic processes play a decisive role in the formation and development of structures. For example, how the grunts of thousands of starlings form specific patterns. This problem may seem far removed from spin glass, but in Parrissi's view, much of his research focuses on how simple behavior leads to complex overall behavior, which applies equally to spin glass and starling flocks.

Thors Hans Hansson, chairman of the Nobel Committee on Physics, said: "The findings that won this year show that our understanding of the climate is based on solid scientific foundations, based on rigorous analysis of observations. This year's winners have contributed to a deeper understanding of the properties and evolution of complex physical systems. ”

From a realistic point of view, carbon peaking and carbon neutrality have been incorporated into the overall layout of China's ecological civilization construction, China has put forward the goal of carbon peaking by 2030 and carbon neutrality by 2060, and some other countries in the world are also proposing various measures to reduce emissions and cope with climate change. The pioneering research of Makoto and Hasselman is a representative example of how scientific research has changed human life, influencing today's policymaking and future action plans. And in such a critical period of human development, in the face of an increasingly complex world and emerging problems, Parisi's study of complex systems has had a profound impact on many aspects of our lives.

Alfred Nobel set up the Nobel Prize in his will to reward those who "made the greatest contribution to humanity," and by that standard, Makoto, Hasselman, and Parisi deserved the 2021 Nobel Prize in Physics.

Southern Weekend contributed to Ju Qiang