The Nobel Prize in Physics 2021

Syukuro Manabe was awarded the 2021 Nobel Prize in Physics for his continuous work on climate modeling. He shared the award with climate scientist Klaus Hasselmann and theoretical physicist Giorgio Parisi. The Nobel Committee cited “groundbreaking contributions to our understanding of complex physical systems” and, in case of the two climate scientists, specifically their achievements in “physical modelling of Earth’s climate, quantifying variability and reliably predicting global warming.”

In his Nobel Prize lecture, Syukuro Manabe sketched his achievements and their significance for science, the public, and policymaking.

This Nobel Prize also has wider implications. Scientific disciplines evolve following the evolution of scientific thought and the path of discovery, in a process that in the beginning is simply interdisciplinary, implying collaboration and the exchange of ideas and information, but that may evolve into a real convergence, when methods, principles, and strategies are fused in a different set. The early 21st century has seen convergence happening in the field of genetics and computer science, but it is a process that has been going on for many decades earlier in climate science. Climate science requires expertise not only in meteorology and oceanography but also in computer science and chemistry, biology, geology and, now that the human system is fully integrated in the climate system, also economics and the social sciences. However, the global research system has been slow in recognizing this evolution, and it has been anchored to traditional (historical) disciplines. So, it was unsurprising that when the Nobel Prize was announced, some took it as recognition that climate science was finally accepted as a full scientific discipline on a par with the historical ones like physics (e.g., Hegerl, 2022).

Thus the prize was particularly significant for the status of climate science within the scientific and academic context, and it was important for the image of climate science in the general public. Raising the status of climate science has made the results and information on global change more authoritative, making it more difficult to discredit and belittle them. Therefore, the prize also acquired a special meaning in the global conversation regarding climate change and the policy measures required to moderate it.

The Early Years

Like many scientists of his generation, Manabe was first interested in the dynamics of the atmosphere to address the prediction problem. He was part of a group of Japanese scientists around Professor Syono at the University of Tokyo who were pursuing weather research in Japan. The group was energized by Jules Charney’s paper on baroclinic instabilities of the westerlies (Charney, 1947), which had a strong influence on the direction of their research, introducing strong collaborative ties with the United States.

At the same time, John von Neumann at the Institute for Advanced Studies in Princeton was organizing a group tasked with the goal to realize the first numerical weather prediction of the atmosphere, showing in the process the full potential of the new electronic calculator that von Neumann had conceived. Solution of hard nonlinear hydrodynamical equations was possible putting together recent Charney’s theoretical insights and the ability to perform a large amount of calculation in a short time. Charney headed the group and pioneered the development of the dynamical model of numerical weather prediction that has become indispensable in daily life. The papers resulting from that project, Charney et al. (1950) and Charney and Phillips (1953), were also influential in the formation of the early ideas of the young Manabe.

Realizing that his group at the institute would be terminated soon, von Neumann convinced Harry Wexler, the director of research at the U.S. Weather Bureau, that it was desirable to form a group at the bureau to develop a climate model as a natural extension of the dynamical model of numerical weather prediction. Following the advice by von Neumann, the U.S. Weather Bureau created a research group for the development of a climate model in 1955, extending the dynamical model of numerical weather prediction. Manabe was invited to join this group by Joe Smagorinsky, the first director of the group, which then became the geophysical Fluid Dynamical Laboratory of National Oceanic and Atmospheric administration (S. Manabe, personal communication, 2022).

In the early 1950s, Manabe was among the graduate students majoring in dynamical meteorology at the University of Tokyo. Smagorinsky remembers that Manabe came to the group before he earned his doctorate, and he worked on the air mass dynamics over the Sea of Japan, but Manabe’s interests were shifting to the need to include radiative effects in the climate models (Smagorinsky in Lewis, 1993). That was particularly attractive to Smagorinsky who was starting to put together the idea that a numerical model of the entire global circulation was indeed possible. Since then, the story of Manabe and of GFDL (Geophyical Fluid Dynamics Laboratory) would never separate. In Manabe’s words (personal communication, 2022):

A general circulation model of the atmosphere consists of prognostic equations of state variables such as wind, temperature, specific humidity, and surface pressure. Each prognostic equation usually consists of two parts. The first part is based upon the laws of physics, such as the equation of motion, the thermodynamical equation, Kirchhoff’s law of radiative transfer, Planck’s function of black body radiation, and the Clausius-Clapeyron equation of saturation vapor pressure. The second part includes parameterizations of various sub-grid scale processes such as moist and dry convection, the formation and disappearance of cloud in the atmosphere, the budget of snow and soil moisture at the continental surface, and the formation of disappearance of sea ice at the oceanic surface.

In the 1960s and 1970s, when the early versions of GCMs were developed at various institutions, electronic computers were in the early stages of their development and their capability were limited, making it necessary to make the parameterization of these sub-grid-scale processes as simple as possible. This was the main reason why my job was so challenging.

Manabe had an early vision of what numerical models could do, and he developed his career consistently trying to understand how the climate’s intricate feedbacks and processes could be described in the simplest possible way and included in a numerical model. Among his enormous scientific production, one can recognize three steps that shaped the development of modern climate science (Manabe, 2019; Manabe & Broccoli, 2019).

Radiation Balance and Water Vapor

The first one is a paper published in 1964 together with his collaborator Strickler (Manabe & Strickler, 1964). The paper contains the first clear numerical representation of the processes involving radiation balance and convective balance in the atmosphere, producing a practical method to include them in numerical models. The result is the basis for convective adjustment in its dry and moist versions that allow the treatment of these fast processes in a physically consistent way. The schemes became the cornerstone for several generations of numerical models until the growing capacity of electronic computers allowed for more detailed treatments to become feasible. The debate at the time centered on the evaluation of the combined effect of carbon dioxide and water vapor in regulating the temperature at the ground. The method proposed in 1964 assumed a fixed absolute humidity, namely, a fixed concentration of water vapor in the atmosphere without considering the dependence of the concentration on temperature.

The temperature dependence can be very strong, as it is described by the Clausius-Clayperon relation, a highly nonlinear equation. These difficulties were overcome in 1967, in the second step, when Manabe succeeded in formulating a consistent version of the radiative-convective adjustment process that would relax the absolute humidity requirements and instead used a fixed relative humidity assumption (Manabe & Wetherald, 1967). The relative humidity accounts for the temperature dependence of the concentration of water vapor in the air. Warm air can support more water vapor in absolute terms than cold air, increasing the opacity of the atmosphere and raising the effective radiative level of the atmosphere, resulting in a slower decay of temperature with heights that correspond much better to the observation of the temperature vertical profile. It was of course known that water vapor was a powerful absorber of long-wave radiation, but the 1967 paper showed that it was possible to compute the water vapor feedback with a simple method that would fit with the observations (Figure 1). The paper also contains an estimate of the equilibrium climate sensitivity (defined as the warming achieved at equilibrium by a doubling of CO₂) of 2.3 degrees, a value that can be compared with most recent estimates of about 3 degrees (Masson-Delmotte et al., 2021).

Open in new tab

Figure 1. The mechanism of relative humidity vs. absolute humidity explained in Manabe and Wetherald (1967). Considering the relative humidity allows the raising of the effective radiative level.

Source: Author’s picture from personal copy of original Manabe’s lecture notes.

Toward an Earth System Model

The objective of creating the first model of the entire climate system was closer after these papers, but the model was still missing a crucial component: It was not possible to conceive a climate model that would not take into consideration the oceans. The role of the oceans is paramount in regulating the heat, momentum, and water budgets, and it is necessary to include it in a general climate model. This step was finally reached in 1969 when together with Kirk Bryan (Manabe & Bryan, 1969) it was demonstrated that it was feasible to build a numerical atmosphere-ocean model coupled together, that is, a model in which information between the atmosphere and the oceans will be exchanged in real time during the calculation. The paper is marvelous in its simplicity—in its concise four pages contains all the conceptual development that would be utilized in successive years.

The conceptual design of a comprehensive climate models was the base for the following developments that led to the formulation of the climate models with realistic geography suitable for global change experiments. A series of papers 20 years later (Manabe et al., 1991, 1992; Stouffer et al., 1989) launched the methodology and the strategy for using general circulation models of the climate system for investigating the effect of increasing greenhouse gases in the atmosphere. As the models became more detailed and computing power was available to enrich the spectrum of processes and systems, it was also possible to make deeper investigations of the hydrological cycle and the impact on the water cycle (Manabe, Milly, & Wetherald, 2004; Manabe, Wetherald, et al. 2004).

Epilogue

There is another major contribution of Manabe to the development of science that sometimes is under-recognized compared to his other triumphs. In his relentless pursue of scientific facts and truth, Manabe contributed to shape a new methodological approach to the scientific enterprise. It is an approach that fuses numerical aspects and simulations with the classic scientific theoretical and experimental modes, recognizing that the numerical representations of large, multicomponent, highly nonlinear physical systems represent themselves a laboratory—a laboratory in which experiments can be designed, discovery can be achieved, and scientific progress can be made. He decisively contributed to the evolution of the definition of what is a solution to a scientific problem, establishing in no uncertain terms that numerical solutions are as factual and as reliable as the classic analytical solutions that used to be the mainstay of quantitative science. By doing so, he propelled science and all of the world in the 21st century.