Nobel committee honors two pioneers in climate physics

(Axel Timmermann, Director IBS Center for Climate Physics)

This year’s Nobel prize in physics is awarded - in part - to two climate scientists, who made fundamental contributions to our understanding of the climate system and its response to anthropogenic greenhouse gas emissions. Their research marks a milestone in understanding, simulating, and detecting human-induced climate change.

2021 Nobel laureates in Physics – (from left) Syukuro Manabe (Princeton University), Klaus Hasselmann (Max Planck Institute of Meteorology), and Giorgio Parisi (Sapienza University)

Prof. Klaus Hasselmann, emeritus Director of the Max Planck Institute of Meteorology in Hamburg, Germany is recognized for his ground-breaking contributions in understanding how weather and climate interact and how global warming can be unequivocally distinguished in temperature observations from the internally generated variability in the climate system.

His 1976 paper “Stochastic Climate models” has become a milestone in climate physics - a must-read for every graduate student ever since it was published. It adopts the concept of Brownian motion, which was first mathematically described by Einstein in 1905, to the climate system. The idea is that fast weather fluctuations can be regarded as a random forcing for the slowly responding climate system. Variations in the climate system are then generated by the continuous accumulation of fast weather fluctuations, combined with amplifying, or damping coupled feedbacks. This fundamental concept essentially explains 95% of all naturally occurring climate fluctuations. Climate scientists also call it the “Null hypothesis model” of climate because its theoretical predictions match the observed data so well. The stochastic climate model, which does not include the effect of global warming, also highlights that in spite of the seemingly random weather fluctuations (e.g. winds over the ocean) which occur on timescales of days to weeks, the natural climate system (e.g. ocean temperatures) can still react on much slower timescales. This finding also marks the birth of modern seasonal climate predictions. Extensions of Prof. Hasselmann’s theoretical model to more than one mathematical dimension can even be applied to understand and predict climate phenomena such as the El Niño-Southern Oscillation, which wreaks havoc across the globe by shifting weather patterns.

The second breakthrough contribution recognized by the Nobel committee is Prof. Hasselmann’s optimal fingerprinting method, nowadays also nick-named climate-CSI*, which has made it possible to detect the human effect in 20th century temperature observations by an ingenious way of filtering the data in space and time. Originally developed by Klaus Hasselmann in 1979, the application of this optimal fingerprint to temperature observations has been a key component in linking global warming to human activities. Collaborating with Prof. Hans von Storch and Dr. Gabriele Hegerl, formerly at the Max Planck Institute in Hamburg, Klaus Hasselmann laid the foundation for an entirely new research discipline, which is known now under the term “Detection and Attribution” and which played a fundamental role in the assessment reports of the Intergovernmental Panel on Climate Change (IPCC). The optimal fingerprinting methods which have now been applied to various observational datasets have proven “beyond reasonable doubt” that human fossil fuel emissions cause global warming.

The other “quarter” of the Nobel prize in physics is awarded to the founder of modern climate modeling: Dr. Syukuro (Suki) Manabe. Already in the 1960s Dr. Manabe began building the first computer models that solved the complex physical equations of the climate system on the first generation of supercomputers at the Geophysical Fluid Dynamics Laboratory in Princeton, USA. Initially the models described only the atmospheric motion, but later, thanks to a long-lasting collaboration with oceanographer Dr. Kirk Bryan, also the ocean circulation.

These so-called Coupled General Circulation models demonstrated for the first time, that a doubling of atmospheric CO2 concentrations will increase the temperatures of our planet by about 2-4 degrees Celsius and that warming on land and in polar regions will be even higher than the global mean. These predictions still hold today – 50 years after Manabe’s initial computations. Dr. Manabe’s pioneering work, which also included the first ice age simulations, led to the development of climate models in other climate research centers worldwide, including Prof. Hasselmann’s Max Planck Institute in Hamburg. Tested repeatedly by different research teams and later paraphrased by the Intergovernmental Panel on Climate Change, the conclusion from thousands of independent climate model simulations is simple: anthropogenic greenhouse gas emissions cause global warming. If mankind does not stop emitting carbon into the atmosphere, our planet will continue to warm, sea-level will rise and large-scale rainfall patterns will shift.

Dr. Manabe, who rightfully can be called the “father of climate modeling” has made other groundbreaking contributions that changed our understanding of how the climate system, and in particular the Atlantic Meridional Overturning circulation, which contains the Gulf Stream as one important surface branch, operates. Together with his colleague Ronald Stouffer from the Geophysical Fluid Dynamics Laboratory in Princeton, they found in one of their coupled atmosphere-ocean simulations, that for the same environmental conditions, the Atlantic circulation can either remain in an “on” or in an “off” state. The “on state” corresponds to our current climate situation, the “off state” is associated with a dramatic cooling of the northern hemisphere and southward shift of tropical rainbands. When published in 1988, this result caused a major paradigm shift in our understanding of how the ocean circulation works. The model predicted existence of so-called multiple equilibria in the climate system spurred a world-wide hunt for observational evidence. Just several years later in 1993, the scientific community found confirming evidence in ice-cores from Greenland, which showed a rapid flickering of temperatures between two distinct states. Using ocean sediment core data, these so-called Dansgaard-Oeschger events were later linked to similar switches in the strength of the Atlantic Ocean circulation. Again, one of Dr. Manabe’s computer model predictions came true.

Recognizing two pioneers in climate physics, the Nobel prize committee sends a clear message: climate research is part of physics; it has the same solid foundation as other subdisciplines. After all, global warming is caused by a change in earth’s radiation balance, which arises from changing the infrared absorption in air, due to the quantum mechanical properties of CO2 molecules.

Several decades after the awardee’s pioneering discoveries, many key questions in climate research remain unanswered: How will the major ice-sheets and global sea level respond to global warming? Can abrupt climate change occur in future? How will life on our planet react to the increasing climate stress? Is there a way out of this self-inflicted crisis? Walking in the footsteps of giants, such as Prof. Klaus Hasselmann and Dr. Suki Manabe, scientists at the IBS Center for Climate Physics at Pusan National University use one of the world’s fastest computers dedicated mostly to climate research, to address these questions about the future habitability of our planet.

I have had the pleasure and privilege to interact with both Nobel laureates in climate physics; with Prof. Hasselmann as my official PhD advisor at the Max Planck Institute of Meteorology and with Dr. Syukuro Manabe as a colleague, with whom I shared a common interest in North Atlantic Ocean circulation variability. Both impressed and inspired me deeply by their intellectual generosity, their inexhaustible curiosity, and their motivation to make ground-breaking contributions to a research field with far reaching implications for humanity.

*CSI: Crime Scene investigation