Nobel Prize in Physics recognises climate modelling breakthroughs
Image credit: Pontus Lundahl/TT News Agency via REUTERS
The Nobel Prize in Physics 2021 has been awarded to three scientists from Japan, Germany and Italy for their work modelling complex physical systems, including Earth’s climate.
The Royal Swedish Academy of Sciences recognised Syukuro Manabe, 90, and Klaus Hasselmann, 89, for their work in “the physical modelling of Earth’s climate, quantifying variability, and reliably predicting global warming.” They will each receive a quarter of the 10 million kronor prize money (£840,000). The other half of the prize goes to Giorgio Parisi, 73, for “the discovery of the interplay of disorder and fluctuations in physical systems, from atomic to planetary scales.”
The work of the three physicists can be described broadly as describing and predicting 'complex systems', such as Earth’s climate. These systems have many interdependent factors that cause what appear to be random and disordered behaviour.
The physicists used complex mathematics to explain and predict what seemed like chaotic forces of nature in computer simulations, called modelling. That has given scientists such a solid understanding of those forces that they can accurately predict weather a week in advance and warn about the state of the climate decades in advance, based on factors such as anthropogenic CO2 emissions.
Specifically, Manabe and Hasselmann were credited with laying “the foundation of our knowledge of Earth’s climate and how human actions influence it”.
In the 1950s, atmospheric physicist Manabe left Tokyo – which had been devastated by war – and continued his work in the US. He hoped to investigate how increased levels of CO2 can cause increased temperatures. While previous efforts focused on radiation balance, in the 1960s, Manabe led the development of physical models to incorporate the vertical transport of air masses due to convection, as well as the latent heat of water vapour. To make these calculations manageable, he reduced the model to a single dimension: a vertical column of air 40km up into the atmosphere. He found that oxygen and nitrogen had negligible effects on surface temperature, while CO2 had a clear impact; when levels of CO2 doubled, global temperature increased by over 2°C.
Approximately a decade later, Hasselmann developed a model that helped explain why climate models can be reliable in spite of the seemingly chaotic nature of the weather. The planet has vast shifts in weather because solar radiation is so unevenly distributed (spatially and temporally). Around 1980, Hasselman demonstrated that chaotically changing weather can be described as rapidly changing noise, placing long-term climate forecasts on a firm scientific foundation for the first time.
He also developed techniques for identifying influences on the climate; this requires incorporating rapid changes in weather into calculations as noise and showing how noise affects the climate. Once this model for climate variation was complete, he developed methods for identifying human and natural 'fingerprints' imprinted in the climate. This made it possible to prove that global warming is due to human emissions of CO2.
The Royal Swedish Academy of Sciences said: “[Manabe and Hasselmann] have contributed to the greatest benefit for humankind […] by providing a solid physical foundation for our knowledge of Earth’s climate. We can no longer say that we did not know - the climate models are unequivocal. Is Earth heating up? Yes. Is the cause the increased amounts of greenhouse gases in the atmosphere? Yes. Can this be explained solely by natural factors? No. Are humanity’s emissions the reason for the increasing temperature? Yes.”
In around 1980, Parisi built a deep physical and mathematical model that made it possible to comprehend complex systems in fields as diverse as neuroscience, machine learning, and mathematics. His work was originally focused on spin glass, a type of metal alloy with magnetic properties which appeared to change randomly with the arrangement of atoms; for instance, the position of iron atoms in a grid of copper atoms. Each iron atom behaves like a small magnet (spin) which is affected by the other iron atoms nearby. While in an ordinary magnet, all spins point in the same direction, but in a spin glass they are 'frustrated': some pairs show an inclination to the same direction and some to opposite directions. In 1979, Parisi discovered a hidden structure to explain this behaviour. Since then, his method has been applied to many complex systems.
Although Parisi himself did not turn his professional attention towards climate modelling, he added his voice to calls for climate action following the announcement. He said: “It’s very urgent that we take very strong decisions and move at a very strong pace in tackling global warming. It’s clear for future generations that we have to act now.”
Thors Hans Hansson, chair of the Nobel Committee for Physics, said: “The discoveries being recognised this year demonstrate that our knowledge about the climate rests on a solid scientific foundation, based on a rigorous analysis of observations. This year’s laureates have all contributed to us gaining deeper insight into the properties and evolution of complex physical systems.”
German climate scientist Stefan Rahmstorf commented: “Physics-based climate models made it possible to predict the amount and pace of global warming, including some of the consequences like risings seas, increased extreme rainfall events and stronger hurricanes, decades before they could be observed. Klaus Hasselmann and Suki Manabe were pioneers in this area and personal role models for me. We now witnessing how their early predictions are coming true one after the other.”
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