In a phone interview accompanying the announcement that Giorgio Parisi had been awarded one half of the 2021 Nobel Prize in Physics, the new Laureate was asked by a journalist whether he had been expecting the award. Parisi offered a response characteristic of this year’s awardees: “I was not expecting [to win], but I knew there was some non-negligible possibility.”
Parisi and co-recipients Sykuro Manabe and Klaus Hasselmann share a fascination with systems characterised by randomness and disorder. Their ground-breaking contributions have provided new methods for describing and predicting the long-term behaviour of such complex systems – not least the effects of climate change.
More specifically, Manabe and Hasselmann shared one half of the Prize “for the physical modelling of Earth’s climate, quantifying variability and reliably predicting global warming”. Parisi was awarded the other half “for the discovery of the interplay of disorder and fluctuations in physical systems from atomic to planetary scales”.
How Greenhouse Gases Affect Climate
Manabe – who left Japan in the late 1950s to continue his career at the U.S. Weather Bureau – was the first of the three Laureates to make a defining contribution towards distilling the complexity of physical systems. In the 1960s, he and his collaborators took the first step towards predicting how greenhouse gases – carbon dioxide, methane, water vapour and other gases that absorb radiation from Earth and then release it to heat the air and ground – affect climate.
Manabe considered a simple, vertical column stretching from the ground to the stratosphere. He then created computer simulations of how carbon dioxide affects the flow of heat and air within the column. Though unsophisticated by today’s standards – largely due to the computing power available sixty years ago – extrapolating the column to the whole globe in 1967, Manabe’s model made it plain that carbon dioxide has an unequivocal impact: while levels of oxygen and nitrogen in the simulation did not affect surface temperature globally, doubling carbon dioxide levels increased temperature by 2.3 °C (a gauge now referred to as ‘climate sensitivity’).
By 1975, Manabe had refined his model and calculations to simulate for the first time the three-dimensional response of temperature and the hydrologic cycle to increased carbon dioxide. This model gave a global climate sensitivity of 2.93 °C – well within the bounds of highly sophisticated modern climate models which predict 2.5–4 °C increases.
Long-Term Climate Forecasts
Around the same time, German physicist Hasselmann was working on what would be his first big breakthrough: developing climate models that incorporate weather events. As the climate of a particular location is in essence the long-term prevailing weather, the link between the two appears obvious. But before Hasselmann’s work, no one had managed to incorporate the rapid, chaotic nature of weather changes into long-term climate forecasts. To bring the two together, Hasselmann built a slowly evolving climate model that responded realistically to fast, random fluctuations, i.e. weather. Using this model, he showed that weather fluctuations on a timescale of days can influence the ocean on a timescale of years.
Having integrated weather events into climate models, Hasselmann was also the first researcher to tease apart weather events and other natural changes from human-caused climate change. Assessing models, theories and observations, he built a framework of techniques over two decades to extract the ‘fingerprints’ of human-caused global warming. Further studies over recent decades have refined and extended these techniques to the point that it is now certain that humanity’s emissions are having a significant impact on the world’s climate.
Urgent to Take Strong Decisions on the Climate
While Manabe and Hasselmann were still taming the complexity of the global climate system, Italian theorist Parisi began his exploration of an entirely different group of complex systems – spin glasses. Spin glasses are metal alloys in which magnetic particles like iron atoms are randomly mixed into a grid of copper atoms. In a spin glass, each iron atom acts like a small magnet, but instead of all pointing in the same direction like magnets would normally, their direction or ‘spin’ is influenced by other nearby atoms.
In this case, the atoms cannot find the optimal spin configuration because they are being pushed and pulled by counteracting forces, so they end up in the least bad one, a state aptly named ‘frustration’. Echoing Hasselmann’s work but at a much smaller scale, Parisi elucidated how the rapid random back-and-forth flipping of atomic spins affected the slow dynamics of the spin glass as a whole.
Given many systems exhibit frustration, Parisi’s deep insights into the phenomenon have been applied in fields as diverse as mathematics, biology, neuroscience and machine learning. Moreover, Parisi himself has gone on to apply his ability to distinguish how simple behaviours give rise to complex collective actions in a host of different areas – from quantum chromodynamics, string theory and fluid dynamics to how patterns arise in murmurations of thousands of starlings and even why the Earth experiences periodic Ice Ages.
Though delighted that the Nobel Committee recognised his achievements, Parisi could not ignore the timeliness of his co-recipients’ awards – the first climate change-related Nobel Prizes in Physics. “It is urgent that we take strong decision[s] on the climate”, he said. “We are in a situation where we have positive feedback and accelerating increase of temperature. We have to act in a fast way and without delay.”
A Clear Message
Manabe and Hasselmann join a small but distinguished group of Laureates whose awards relate to climate change. Former US Vice President Al Gore and the Intergovernmental Panel on Climate Change (of which Manabe was one of several thousand members) received the 2007 Nobel Peace Prize “for their efforts to build up and disseminate greater knowledge about man-made climate change, and to lay the foundations for the measures that are needed to counteract such change”. And William Nordhaus received half of the 2018 Prize in Economic Sciences “for integrating climate change into long-run macroeconomic analysis”.
Like Gore and Nordhaus before them, they used the Nobel platform to send out a clear message, too. “Climate change not only involves our environment, but also energy, agriculture, water, everything you can imagine, and when these major problems of society are interwoven, you can understand how difficult it is to sort this thing out”, said Manabe in a post-award Princeton University press conference. “We have to think about how to mitigate climate change but also … how we adapt to climate change, which is happening right now.”
“There are many things we can do to prevent climate change”, said Hasselmann in his Nobel interview. “It’s a whole question of whether people will realise that something which will happen in 20 or 30 years is something which you have to respond to now.”