Ahmad Gamal Saad-Eddin

This article was originally published on October 5, 2021, as part of Nature Arabic Edition’s coverage of the Nobel Prizes for that year.

The 2021 Nobel Prize in Physics honors three scientists from Germany, Italy, and the United States, who have brought clarity to the chaos of complex systems, revealing their hidden patterns and crafting models that help us anticipate their behavior.

The first half of the prize was awarded to Syukuro Manabe, the Japanese-American scientist, and Klaus Hasselmann from Germany for their work in developing physical models of Earth’s climate, which provide precise predictions about global warming. The Nobel committee stated that Manabe and Hasselmann’s work laid “the foundation for our current understanding of Earth’s climate and how humans affect it.”

Since the 1960s, Manabe’s research has demonstrated that an increase in atmospheric carbon dioxide levels leads to a corresponding rise in global temperatures—an idea that current climate models rely on. Several years later, Hasselmann developed a model linking weather and climate, offering clear evidence that we can rely on climate models, even if daily weather appears chaotic and unpredictable. He also devised a method to detect specific signals that indicate human influence on the climate.

The other half of the prize was awarded solely to Giorgio Parisi, the Italian physicist, for his discovery of the continuous and reciprocal interaction between disorder and fluctuations in physical systems—ranging from the atomic scale to planetary systems. Parisi constructed a mathematical-physical model that enables the understanding of complex systems in various fields, including mathematics, biology, neuroscience, and machine learning, as well as predicting their future behavior. However, his fundamental achievement lies in uncovering hidden patterns within irregular, complex systems. Many experts consider Parisi’s contribution one of the most significant advancements in complex systems theory, as it has allowed us to describe and comprehend many chaotic materials and phenomena.

Complex Systems

By nature, complex systems resist rigid definitions. However, to simplify, we can say that complex systems consist of numerous interacting elements that continuously influence one another. Notable examples include the climate system, the human brain, the behavior of large human crowds, or even urban traffic systems. These are vast systems comprising numerous components, each behaving differently while interacting with the others.

The challenge with such systems is that they are incredibly difficult to mathematically model or describe in a way that allows for reliable future predictions. This is either due to the sheer number of interacting components or because chance plays a major role. Sometimes, the influence of chance escalates into chaos—especially in the domains of climate and weather—where even a minor change in initial conditions can lead to massive variations later on. The most well-known cultural reference to this idea is the “butterfly effect” from chaos theory, coined by mathematician and meteorologist Edward Lorenz. He illustrated how a butterfly flapping its wings in China could set off a chain of events leading to a hurricane striking a distant part of the world.

The scientific contributions of this year’s laureates aid in developing a deeper understanding of these systems, analyzing their variables, and—at least to some extent—predicting future developments based on past occurrences.

Climate

Earth’s climate is one of the most intricate systems known to us. For years, Manabe and Hasselmann worked on refining climate models to better understand this system, its components, and the factors influencing it. Parisi later contributed theoretical solutions to numerous challenges within complex systems, particularly those related to climate.

Over two centuries ago, the French physicist and mathematician Joseph Fourier studied the balance of energy between the sunlight striking Earth and the radiation reflected back. He recognized the role of this balance in determining global temperatures and the importance of Earth’s atmosphere, which traps heat in a highly intricate process known as the greenhouse effect.

Subsequent generations of scientists continued investigating the mechanisms behind this complex system. Their efforts have culminated in today’s climate models—fundamental tools that explain the climate’s nature, influencing factors, and even help us comprehend modern phenomena such as the undeniable rise in global temperatures and humanity’s role in it.

These models rely on the laws of physics—albeit in a simplified manner—along with calculated statistical elements to generate an overall picture of the climate at any given moment, based on the data and statistics available to us today.

This year’s Nobel Prize in Physics celebrates complex systems and mathematical models in their diverse forms, honoring a long tradition in physics of formulating foundational frameworks for the systems it studies and making predictions whose accuracy and effectiveness in understanding our intricate world become more evident with each passing day.