THE NATURAL world is filled with complexity. The more closely scientists study everything from planets to atoms, the more structure they find and the more detailed their explanations must get. But if you want to predict the behaviour of such systems, how much detail do you need? To understand how Earth’s oceans will behave, say, do you need to track every individual molecule of water within them?
This year’s Nobel prize for physics was awarded to a trio of researchers who have studied complex, chaotic and apparently random systems and developed ways to predict their long-term behaviour. Half of the prize of SKr10m (about $1.1m) was shared by Syukuro Manabe of Princeton University and Klaus Hasselmann of the Max Planck Institute for Meteorology, in Hamburg. The other half went to Giorgio Parisi of Sapienza University, in Rome.
Drs Manabe and Hasselmann laid the foundations of the modelling of the Earth’s climate that led to “quantifying variability and reliably predicting global warming”, according to the citation by the Nobel Committee for Physics of Sweden’s Royal Academy of Science. Dr Parisi was awarded his share for his discoveries around the “interplay of disorder and fluctuations in physical systems from atomic to planetary scales”.
In the 1960s, Dr Manabe, an atmospheric scientist, wove together emerging strands of understanding of the dynamics and thermodynamics of Earth’s atmosphere to make the first reliable prediction that doubling the level of carbon dioxide present would also increase the planet’s surface temperature. His work led to the development of physical models of Earth’s climate and laid the foundation for the climate models used today.
Around the same time, scientists such as Edward Lorenz of the Massachusetts Institute of Technology were beginning to describe weather as a chaotic system—in other words, something that had so many interacting individual components, such as temperature, pressure, humidity and wind speed, that even small variations in initial conditions could result in enormous differences at a later stage. In this description, weather evolved rapidly and became essentially unpredictable even just a few days into the future.
In the 1970s, Dr Hasselmann developed models to show how weather, despite itself being chaotic and unpredictable in the short-term, could nonetheless yield reliable models to predict Earth’s climate over much longer periods. In describing his work he made an analogy to Brownian motion, the jostling movement of pollen grains in water that was first observed down a microscope by Robert Brown, a botanist, in 1827. Almost 80 years later, Albert Einstein posited that the slow zigzagging of the grains could be explained by their continual bombardment much tinier, fast-moving water molecules. The large-scale climate can similarly be seen as a consequence of numerous much smaller events.
Around 1980 Dr Parisi found some of the rules that govern apparently random phenomena. He studied a type of material called “spin glass”, in which, for example, iron atoms are mixed randomly into a grid of copper atoms. The iron atoms each behave as tiny magnets but, unlike the case of a normal lump of magnetised metal, the north-south poles of the iron atoms in a spin glass do not point in a uniform direction. Dr Parisi devised a way to understand how they find their optimal orientations. His mathematical ideas not only help explain some of the complex systems of Earth’s climate, as described by his two fellow laureates, but also other apparently random phenomena in fields as diverse as biology, neuroscience and machine learning.
This year’s physics prize is the first scientific Nobel to be awarded for understanding the Earth’s climate. Asked if this was a not-so-subtle message to world leaders ahead of the upcoming cop26 climate summit in Glasgow, members of the Nobel committee said the prize was meant to celebrate the discoveries themselves. But, they added, it also showed that the modelling of climate and the notion of global warming rested on solid physical science. Human beings can no longer say they did not know how or why Earth was heating up.