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•REVIEW•https://doi.org/10.1007/s11430-022-9906-4
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Understanding and building upon pioneering work of Nobel Prize
in Physics 2021 laureates Syukuro Manabe and Klaus Hasselmann:
From greenhouse effect to Earth system science and beyond
Tianjun ZHOU1,2*, Wenxia ZHANG1, Deliang CHEN3, Xuebin ZHANG4, Chao LI5,
Meng ZUO1& Xiaolong CHEN1
1State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics (LASG),
Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China;
2University of Chinese Academy of Sciences, Beijing 100049, China;
3Department of Earth Sciences, University of Gothenburg, Gothenburg 40530, Sweden;
4Environment and Climate Change Canada, Toronto M3H 5T4, Canada;
5Max Planck Institute for Meteorology, Hamburg 20146, Germany
Received February 16, 2022; revised March 1, 2022; accepted March 3, 2022; published online March 15, 2022
Abstract The Nobel Prize in Physics 2021 was awarded jointly to Syukuro Manabe, Klaus Hasselmann, and Giorgio Parisi for
their groundbreaking contributions to our understanding of complex systems. This is the first time that climate scientists were
awarded the Nobel Physics Prize. Here, we present the evolution of climate science in the past ~200 years and highlight the
landmarks of the developments in advancing our understanding of climate change, placing the pioneering contributions of
Manabe and Hasselmann into a historical perspective. The backbone of modern climate science is further discussed in the context
of the development of the discipline from the discovery of the greenhouse effect to the formation of Earth system science.
Perspectives on the future development of climate science are also presented.
Keywords Greenhouse gases, Climate change, General Circulation Model, Detection and attribution, Climate science, Earth
system science
Citation: Zhou T, Zhang W, Chen D, Zhang X, Li C, Zuo M, Chen X. 2022. Understanding and building upon pioneering work of Nobel Prize in Physics 2021
laureates Syukuro Manabe and Klaus Hasselmann: From greenhouse effect to Earth system science and beyond. Science China Earth Sciences, 65,
https://doi.org/10.1007/s11430-022-9906-4
1. Introduction
Syukuro Manabe from Princeton University and Klaus
Hasselmann from the Max Planck Institute for Meteorology
shared the 2021 Nobel Prize in Physics “for the physical
modelling of Earth’s climate, quantifying variability and
reliably predicting global warming”. While the award is a
public recognition of Manabe and Hasselmann’s seminal
personal contributions to “laying the foundations of our
knowledge of the Earth’s climate and its influence by human
activities”, their findings, together with the contribution of
the climate research community, inform the public that the
entire world, including humans and ecosystems, is affected
by climate change and that this will continue.
The pioneering work of Manabe and Hasselmann on the
causes and signs of human-induced climate change are ho-
nored together with Giorgio Parisi’s contribution to dis-
ordered physical systems. The three laureates’ contributions
are combined under the umbrella of complex systems. The
climate system is a prime example of a complex physical
© Science China Press and Springer-Verlag GmbH Germany, part of Springer Nature 2022 earth.scichina.com link.springer.com
SCIENCE CHINA
Earth Sciences
* Corresponding author (email: zhoutj@lasg.iap.ac.cn)
system since it features all the typical characteristics of a
complex system (Slingo et al., 2009). The interactions
among physical, chemical, and biological processes and
various components, at a wide range of spatial and temporal
scales, have generated complex behaviors in the Earth’s
climate. The contributions of Manabe and Hasselmann to the
understanding and predictions of Earth’s climate behavior
are reasonably honored as “groundbreaking contributions to
our understanding of complex systems”.
It has been 195 years since Joseph Fourier uncovered the
greenhouse effect in 1827 (Fourier, 1827). The role of the
greenhouse effect in global warming is now understood in
the context of climate system science and has been extended
to Earth system science. Based on improved observational
datasets to assess historical warming, as well as progress in
scientific understanding of the response of the climate sys-
tem to human-caused greenhouse gas (GHG) emissions, the
UN Intergovernmental Panel on Climate Change (IPCC)
concludes in its 6th assessment report from Working Group I
that human influence has warmed the climate system, in-
cluding the atmosphere, ocean and land, unequivocally
(IPCC, 2021). Climate change is already one of the greatest
challenges facing the world and humanity. This is, however,
the first time that climate scientists have been awarded the
Physics Nobel. Here, we briefly review the history of climate
science and interpret the contributions of Syukuro Manabe
and Klaus Hasselmann from a historical perspective. We also
provide a perspective on discipline development from the
greenhouse effect to the climate system and Earth system
science.
2. The history of climate science
The development of climate change science has passed
through the following important stages (see Archer and
Pierrehumbert (2011) for a review and a depiction in Figure
1). In 1827, Joseph Fourier found that the atmosphere is
largely transparent to solar radiation but is relatively opaque
to infrared radiation, leading to a warmer planet than it would
have been if sunlight were the only warming factor (Fourier,
1827). Subsequently, the warming effect of the Earth’s at-
mosphere was termed “the greenhouse effect” for the first
time by Nils Gustaf Ekholm in 1901 (Ekholm, 1901). In
1861, John Tyndall demonstrated the greenhouse effect in
the laboratory. His laboratory studies were driven by ques-
tions of the Earth’s climate and the greenhouse effect. His
study implied that most GHG effects in the atmosphere are
due to a few trace gases, such as water vapor and CO2
(Tyndall, 1861).
In 1896, Svante Arrhenius calculated the warming caused
by doubling CO2concentrations based on a one-layer model
under the constraints of energy balance (Arrhenius, 1896),
which is named “climate sensitivity” at present. This paper is
regarded as the birth of modern climate science since the
importance of satisfying the energy balance both at the top of
the atmosphere and at the surface was identified. Arrhenius
also described the water vapor feedback and the ice albedo
feedback. In 1956, Gilbert Plass accurately calculated the
radiative forcing of CO2for the first time and determined the
infrared cooling rate and the net outgoing radiation at the top
of the atmosphere caused by CO2(Plass, 1956). One year
later, Charles David Keeling began to measure the atmo-
spheric CO2concentrations at the Mauna Loa Observatory in
Hawaii. This is the first time that human society recorded
atmospheric CO2concentrations based on instrumental
measurements (Keeling, 1960,1970).
In 1967, Syukuro Manabe and Richard Wetherald made
the first fully sound estimate of the magnitude of warming
caused by a doubling of CO2concentrations based on a
simple radiative-convective equilibrium model (Manabe and
Wetherald, 1967), which represents the modern era of global
warming research. In 1975, Manabe and Wetherald further
developed the first credible three-dimensional atmospheric
climate model (Manabe and Wetherald, 1975), which is
known as the General Circulation Model (GCM). It was the
first GCM to be able to deal with a doubling of CO2, which
reproduced the “polar amplification” phenomenon of global
warming originally indicated by Svante Arrhenius.
In 1979, Jule G. Charney organized and compiled the first
comprehensive assessment of Earth’s climate change due to
fossil fuel CO2emissions, which was entitled “Carbon Di-
oxide and Climate” and is now known as the famous
“Charney Report” (Charney et al., 1979). The synthesis re-
port stated that there was no doubt that a doubling of at-
mospheric CO2concentrations would lead to significant
global climate change, resulting in global warming by
1.5–4.5°C, which is similar to the latest results given by the
IPCC AR6 (Chen et al., 2021). The synthesis report of
Charney yielded new insights and understanding that are
central to the climate sciences.
In 1984, James Hansen provided a quantitative analysis of
climate feedbacks, which was one of the early indications
that cloud feedbacks have significant influences on climate
sensitivity (Hansen et al., 1984). The paper of Hansen et al.
(1984) thus represents a landmark in the development of
climate models with regard to the quantitative analysis of
climate feedbacks. Another important finding of the paper
was the delayed warming of the ocean due to ocean heat
storage (i.e., so-called “committed warming”). In 1986,
strong observational evidence was provided to confirm
global warming. This was typically represented by the Jones
et al. (1986) paper entitled “Global Temperature variations
between 1861 and 1984”.
Despite the recognition of global warming and the green-
house effect, further evidence is needed to link the observed
2Zhou T, et al. Sci China Earth Sci
global warming to human-induced GHG emissions. The
observed climate change is composed of internal variability
and externally (both naturally and anthropogenic) forced
changes. Understanding how a predictable climate system
response emerges from chaotic behavior, detecting climate
change and identifying its natural and anthropogenic causes
have been challenging to climate science. Klaus Hasselmann
developed the optimal detection method in a series of studies
(Hasselmann, 1979,1993,1997) to identify climate change
signals from the noise of internal variability. His methods of
identifying “fingerprints” now allow the IPCC to assess
whether the climate change observed over the past years is
dominated by human activities. Through the years with
growing evidence from detection and attribution studies, the
IPCC WG1 assessment reports have concluded with in-
creasing confidence on the human contribution to observed
global warming, which is assessed as “likely” in TAR (2001),
“very likely” in AR4 (2007), “extremely likely” in AR5
(2013), and “unequivocal” in AR6 (2021). This comes as a
result of multiple factors, including the intensification of
anthropogenic climate change signals associated with the
increasing concentration of GHGs, the increases in ob-
servational data and development in climate models, im-
provements in detection and attribution techniques, and
progress in the physical understanding of climate change.
In summary, the century-long development of climate
science, including the pioneering contributions of Syukuro
Manabe and Klaus Hasselmann, is the fundamental basis for
the breakthroughs in our physical understanding of global
warming. The award of the Nobel Prize in Physics to climate
scientists also indicates that climate change science, with a
solid mathematical and physical basis, is receiving more
attention from the scientific community.
3. Syukuro Manabe’s contributions
To address the global warming problem since the industrial
revolution, scientists need to answer two core questions. One
is how to physically elucidate the role of CO2in global cli-
mate warming and exactly predict the impact. The other is to
what extent human activities contribute to global warming.
Manabe’s studies settled the first question, being the first to
credibly compute the global temperature change under a
Figure 1 Key landmarks in the history of climate change science.
3
Zhou T, et al. Sci China Earth Sci
doubling of CO2concentration. Here, two of his major aca-
demic contributions are highlighted.
First, Manabe, together with his cooperators, credibly
predicted global warming under a doubling of CO2con-
centration by modeling the equilibrium between radiative
and convective processes and involved the greenhouse effect
of water vapor.
In 1967, Syukuro Manabe and Richard Wetherald colla-
borated on a paper entitled “Thermal equilibrium of the at-
mosphere with a given distribution of relative humidity”,
published in the Journal of the Atmospheric Sciences, op-
erated by the American Meteorological Society (Manabe and
Wetherald, 1967). In this paper, they developed a one-di-
mensional radiative-convective equilibrium model based on
the latest advances in atmospheric radiation transfer theory.
The energy in each layer from the bottom to top is redis-
tributed by the adjustment between radiative cooling and
convection; therefore, the observed atmospheric temperature
profile is realistically reproduced in the single-column
model. Based on the model, they calculated the near-surface
temperature changes when setting CO2concentrations from
150 to 300 ppm and to 600 ppm and found approximately
2.3°C warming under every doubled CO2concentration
(1 ppm=1 μL L–1). This work pioneered realistic modeling of
the temperature profile response to radiative forcing and
reasonably estimated CO2-induced global warming. They
creatively used outgoing longwave radiation curves to in-
tuitively explain how CO2changes could lead to surface
warming and how water vapor feedback could increase cli-
mate sensitivity.
Second, Manabe, together with his cooperators, developed
the first three-dimensional sophisticated GCM of the globe,
which was the starting point of the modern climate model.
This work initiated the development of a three-dimensional
climate model based on global fluid dynamics and thermo-
dynamic principles, which play indispensable and crucial
roles in climate change research (Figure 2).
Manabe and Wetherald’s radiative-convective model
treated the Earth as a single column, in which the tempera-
ture distribution is subjected to vertical heat exchange from
radiation and convection. However, the temperature change
on Earth’s surface is far from uniform. To obtain a complete
picture of climate change, we need to predict the geographic
distribution of warming and precipitation changes, and so on.
To this end, the hydrological cycle and complex feedbacks
related to water vapor, sea ice and snow cover should be
included comprehensively. A three-dimensional GCM,
which can adequately represent the above processes, is re-
quired to solve the three-dimensional fluid dynamic and
thermodynamic equations that control the global heat,
moisture and momentum transportations, as well as to realize
the coupling between fluid equations and other equations
that control physical processes such as radiation, convection
and surface heat exchanges.
In 1975, Manabe and Wetherald published a paper titled
“The effects of doubling the CO2concentration on the cli-
mate of a General Circulation Model” in Journal of Atmo-
spheric Sciences (Manabe and Wetherald, 1975). This paper
is credited with the birth of the General (Global) Circulation
(Climate) Model (Figure 2). In this work, they constructed an
atmosphere-ocean coupled model with an ideal land-sea
distribution, in which the ocean was simplified as marsh and
oceanic circulation and heat transport were ignored. By using
such an idealized GCM, they demonstrated that CO2-induced
global warming was stronger than that in the simple radia-
tive-convective model. They also showed the polar ampli-
fication phenomenon and intensification of the global
hydrological cycle under global warming. The birth of the
first GCM opened a new era of climate modeling. The pre-
sent-day climate models are becoming increasingly sophis-
ticated, but their ancestries are traced back to the model
created by Manabe and his colleagues.
4. Klaus Hasselmann’s contributions
Klaus Hasselmann has made fundamental contributions to
understanding climate change from two aspects.
First, Hasselmann developed stochastic climate models to
link weather and climate, in which the slowly varying cli-
mate variability on long time scales is interpreted as the
response that emerges from rapidly varying weather dis-
turbances on short time scales. By connecting the chaotic and
random weather system and slow climate variations, his
theory explains the origin of internal climate variability.
Inspired by his research experience in turbulence and
ocean waves, as well as using the analogy of Brownian
motion, Hasselmann proposed a generalizable stochastic
description of climate variability. In 1976, he established
stochastic climate models to explain the origin of internal
climate variability, demonstrating that rapidly varying
weather disturbances (i.e., the white noise of the atmosphere)
can integrate to form slow variations in climate, thus al-
lowing us to understand natural climate variability as red
noise (Hasselmann, 1976). Taking the ocean response to the
atmosphere as an example, the rapidly varying atmospheric
system can disturb the temperature in the ocean mixed layer.
The ocean mixed layer, working as a damper, delays the
response of ocean temperature to the atmosphere, conse-
quently resulting in slow variations in the ocean on long
(even on decadal) time scales, such as the Pacific decadal
oscillation (PDO; Yang et al., 2004). The ocean variations,
subsequently, can lead to long-term variations in the atmo-
sphere. His simple but elegant model explains how slow
variations in the ocean emerge from chaotic weather beha-
vior in the atmosphere, which has important implications for
4Zhou T, et al. Sci China Earth Sci
short-term climate predictions in theory.
Second, Hasselmann established the theoretical foundation
of climate change detection and attribution. He developed
methods for identifying the ‘fingerprints’ of external for-
cings, including greenhouse gases, on climate systems,
which then allows detecting and attributing observed climate
change to human influence.
The existence of internal climate variability means that any
attempt to detect climate change is essentially a question of
signal versus noise, i.e., whether the observed change
(‘signal’) is outside the ‘noise’ of internal variability. In the
book ‘Meteorology Over the Tropical Oceans’ published in
1979, Hasselmann suggested that the identification of at-
mospheric responses to external forcings from atmospheric
internal variability can be treated as the detection of dis-
tributions. With the release of the IPCC FAR in 1990, the
detection and attribution of climate change became increas-
ingly important. In his work published in Journal of Climate
in 1993, he found that external forcings, including green-
house gases, leave unique signals —fingerprints—in climate
series, which can be identified. In this work, he proposed the
concept of optimal detection, i.e., using ‘optimal’ techniques
to increase the signal-to-noise ratio by looking at the com-
ponent of the response away from the direction of highest
internal variability, which can consequently increase the
detectability of forced climate changes. Through years of
development, this method was further extended to climate
change attribution. To implement optimal detections, in ad-
dition to the optimal fingerprints, other approaches, includ-
ing optimal weighting and optimal filtering, have also been
developed (Hegerl and North, 1997). Despite different
mathematical forms, these approaches share the same es-
sential concept (Hegerl and North, 1997). With the concept
of multiple regression being introduced later (Allen and Tett,
1999), the optimal detection method became easier to un-
derstand and more widely used, including the detection and
attribution studies of Chinese climate changes (Ma et al.,
2017;Sun et al., 2014;Zhou et al., 2020a;Zhou and Zhang,
2021;Sun et al., 2021, among many others).
The two major contributions of Hasselmann, i.e., ex-
plaining the origin of the slow-varying internal variability of
the climate system and developing methods for identifying
climate change signals from noise, laid the foundation for
estimating human contributions to climate change. The
methods of climate change detection and attribution provide
strong scientific support for the conclusion reached by the
IPCC AR6 that “It is unequivocal that human influence has
warmed the atmosphere,ocean and land.The likely range of
total human-caused global surface temperature increase
from 1850–1900 to 2010–2019 is 0.8°C to 1.3°C,with a best
estimate of 1.07°C” (Figure 3).
From the establishment of stochastic climate models to the
development of detection and attribution methods, Hassel-
mann has made fundamental contributions to addressing to
what extent human activities have affected climate change.
The conclusion that human influence has unequivocally
contributed to the observed climate warming provides a solid
scientific basis for international actions targeting climate
change adaptation and mitigation.
In addition, from a methodological perspective, the es-
Figure 2 Past, present and future of Earth system model development. The figure shows the development from the climate system model to the Earth
system model, which will be coupled with the ecological environmental system and human system in the future. The height of the cylinder indicates the
complexity and improvement of the component (modified based on Figure 1.13 of IPCC AR5).
5
Zhou T, et al. Sci China Earth Sci
tablishment of stochastic climate models was inspired by his
research in turbulence, and the development of the detection
and attribution method was inspired by the process of signals
in information science. As such, Hasselmann has perfectly
shown the benefits of multidisciplinary research efforts.
5. The backbone of climate science develop-
ment
Supporting the evolutionary development of the discipline
from the greenhouse effect to the climate system and Earth
system are the following interrelated foci that drive climate
science forward: observations, the theory of climate physics,
the concept of the climate system, the emergence of Earth
system science, synthesis such as IPCC assessment reports,
and computer simulations of the climate system from the past
to the future. They form the backbone of modern climate
science.
First, the observations have provided solid evidence of
global warming and have served as a testbed of theory and
prediction. From John Tyndall (1861) and Svante Arrhenius
(1896) to Manabe and Wetherald (1967) and the release of
the famous Jule G. Charney Report in 1979, the fundamental
physical processes that govern the role of GHGs, particularly
CO2, in causing global warming are mostly well understood.
However, theoretical studies and predictions are only high-
brow academic questions. When expensive decisions related
to climate adaptation and mitigation activities are balanced,
we need observations from the real world to test both the
theory and the prediction. For this purpose, high-quality and
reliable past and contemporary changes evinced by ob-
servations at a wide range of spatial and temporal scales are
crucial. The famous Keeling Curve, which is an ongoing
measurement of atmospheric CO2concentration at the
Mauna Loa Observatory, Hawaii, started in 1957, has de-
picted the continuously increasing CO2concentrations and
underpins the observational evidence of how humans are
influencing the climate (Keeling, 1960,1970;Pales and
Keeling, 1965). The global average temperature of the Earth
compiled by Jones et al. (1986) has shown evidence that the
temperature rise began in the 1970s. The use of proxy
measurements of temperature to extend the climate record
back through the historical era of the last millennium and
Figure 3 (a) Observed changes in annual mean global surface temperature from 1850 to 2020 (black line) with respect to 1850–1900 climatology compared
to CMIP6 climate model simulations driven by both human and natural forcings (brown line and shading) and only natural forcing (solar and volcanic
activity, green line and shading). Solid lines are the multimodel means, and shadings are the very likely range of simulations. (b) Observed global warming in
2010–2019 relative to the 1850–1900 mean (gray bar) with its very likely range (whiskers). (c) Contributions of human influence, natural forcings and
internal variability to 2010–2019 warming relative to the 1850–1900 mean, assessed from attribution studies. Whiskers show very likely ranges (cited from
Figs. SPM.1 and SPM.2 of IPCC AR6).
6Zhou T, et al. Sci China Earth Sci
slightly further pioneered by Mann et al. (1998) provides
evidence that the present-day temperatures are warmer than
they have been over 1000 years. Based on evidence from
both observations and proxy data, it was assessed in IPCC
AR6 that “Global surface temperature has increased faster
since 1970 than in any other 50-year period over at least the
last 2000 years (high confidence).Temperatures during the
most recent decade (2011–2020) exceed those of the most
recent multi-century warm period,around 6500 years ago
[0.2°C to 1°C relative to 1850–1900] (medium confidence)”
(IPCC, 2021).
Second, the recognition of climate change is built upon the
development of the theory of climate physics. If we look at
the timeline illustrating the development of climate physics
from the beginning of the 19th century to 2021, when climate
scientists were awarded the Nobel Prize in Physics for the
first time, it has taken 195 years. Milestone studies during
this period include the following (see also the summary in
Figure 1). The greenhouse effect was first discovered by
Joseph Fourier in 1827 and then demonstrated in the la-
boratory by John Tyndall in 1861. Having identified the
greenhouse warming effect, scientists were devoted to esti-
mating the warming that would result from a doubling of
CO2concentrations, with the first estimate by Svante Ar-
rhenius in 1896 based on a one-layer energy balance model
and the first sound estimate by Manabe and Wetherald
(1967) based on a simple radiative-convective equilibrium
model. With Manabe and Wetherald (1967), the study of
global warming entered the modern era. A few years later,
the work by Manabe and Wetherald in 1975 landmarked the
birth of the GCM, with which they for the first time showed a
surface warming pattern due to a doubling of CO2con-
centration. All these efforts were synthesized into the famous
Charney report in 1979, which concluded that there is no
reason to doubt that doubling atmospheric CO2would lead to
a significant change in the global average temperature.
Hansen et al. (1984) presented landmark work with regard to
the quantitative analysis of climate feedbacks. This provided
the earliest indications that cloud feedbacks had the potential
to greatly affect climate sensitivity. All these achievements
have fostered international concern about climate change and
eventually led to the establishment of the IPCC in 1988.
Since the discovery of the greenhouse effect in 1827, nearly
200 years ago, the time is right for the resurgence of climate
science.
Third, the concept of the climate system has emerged as a
new scientific endeavor, triggered by the growing recogni-
tion that we need to understand global warming based on
how the atmosphere, ocean, land surface, and cryosphere
operate as an integrated system. Climate has been tradi-
tionally defined as a long-term average of meteorological
elements, including temperature, pressure, humidity, etc.
Starting from the 2nd half of the 20th century, with the re-
cognition of the climate system concept, the study of climate
change has entered the modern era. By the late 1960s, sci-
entific concern about climate change began to mount due to
the increasing carbon dioxide concentrations evident from
the early observations at Mauna Loa and the devastating
Sahelian drought of the 1960s. This brought the implications
of climate variability and change back to the attention of the
United Nations. In 1974, the sixth special session of the
General Assembly called on the World Meteorological Or-
ganization (WMO) to undertake a study of climate change.
Later, the First World Climate Conference (WCC-1) was
held in Geneva from 12 to 23 February 1979 as “a world
conference of experts on climate and mankind”. The WCC-1
is regarded as one landmark that has greatly promoted stu-
dies of climate variability and change and their implications
for society and the environment. It called on all nations to
strongly support the proposed World Climate Programme.
Closely following the recommendations of the WCC-1, it
formally established the World Climate Programme with
four components, including the World Climate Research
Programme (WCRP) (initially entitled Climate Change and
Variability Research Programme) (Zillman, 2009). In the
past 40 years, the WCRP has been a leading initiative dedi-
cated to coordinating international climate research. This
was achieved as a result of the efforts of the international
scientific community organized through four WCRP core
projects, including CliC (Climate and Cryosphere), CLIVAR
(Climate and Ocean-Variability, Predictability and Change),
GEWEX (Global Energy and Water Exchanges), and SPARC
(Stratosphere-troposphere Processes And their Role in Cli-
mate). The WCRP has played a crucial role in promoting our
understanding of climate variability and change from the
perspective of climate system science. The implementation
of the WCRP fostered the establishment of the joint WMO-
UNEP (the United Nations Environment Programme) IPCC
in 1988. In the third assessment report (TAR) of the IPCC,
the climate system is clearly defined as “an interactive sys-
tem consisting of five major components: the atmosphere,the
hydrosphere,the cryosphere,the land surface and the bio-
sphere,forced or influenced by various external forcing
mechanisms” (IPCC, 2001). The concept of the climate
system has connected traditional disciplines, which typically
examine the component changes in isolation, to build a
unified understanding of the complex climate system, in-
cluding their coupling processes and feedback mechanisms.
Fourth, the emergence and evolution of Earth system sci-
ence have extended the research scope of climate change
from physical science to interdisciplinary and transdisci-
plinary research. The emergence of Earth system science is
built upon the recognition that life exerts a strong influence
on the Earth’s physical and chemical environment. The In-
ternational Geosphere-Biosphere Programme (IGBP) ad-
dressed the challenge of disciplinary integration in 1986 by
7
Zhou T, et al. Sci China Earth Sci
organizing several core projects on biogeochemical aspects
of the Earth system. The implementation of IGBP core
projects has fostered the convergence of disciplines, which in
turn accelerated the evolution of Earth system science. This
is evident in the transition from isolated process studies to
interactions between these processes and increasing global-
level observations, analyses, and modeling. The concept of
the Anthropocene, introduced by P. J. Crutzen, the laureate of
the Nobel Prize in Chemistry in 1995 for his work on the
ozone layer, to describe the new geological epoch in which
humans are the primary determinants of biosphere and cli-
mate changes, has built the foundation for deeper integration
of the natural sciences, social sciences, and humanities
(Crutzen and Stoermer, 2000). The 2001 Amsterdam con-
ference, “Challenges of a Changing Earth”, cosponsored by
the four international global change programs, including the
IGBP and WCRP, introduced the Amsterdam Declaration
and finally led the IGBP to define the term “Earth system” as
the suite of interlinked physical, chemical, biological and
human processes (Steffen et al., 2020).
The famous Bretherton Diagram, a landmark of the Earth
system concept, was the first systems-dynamics representa-
tion of the Earth system to couple the physical climate sys-
tem and biogeochemical cycles through a complicated array
of forcings and feedbacks. It provides a clear visual re-
presentation of the interacting physical, chemical and bio-
logical processes that connect components of the Earth
system. The diagram highlights that human activities are
significant driving forces for change in the system. In the
World Climate Research Programme Strategic Plan (2019–
2028), the classical Bretherton Diagram is used as a sche-
matic to depict the components of the Earth system and their
interactions.
Although there is still no generally accepted definition of
an Earth system, in the WCRP Strategic Plan (2019–2028),
the Earth system is clearly defined as “Earth’s interacting
physical,biogeochemical,biological,and human systems,
including the land,the atmosphere,the hydrosphere,and the
cryosphere”, while the climate system is defined as “the part
of the Earth system that is relevant to climate;that is,the
atmosphere,ocean,land surface,and cryosphere,their
coupling processes and feedback mechanisms” (WCRP,
2019). While the concept of the climate system focuses on
the internal dynamics, external forcing and feedbacks among
the components, the concept of the Earth system highlights
the flows of carbon, energy, and water, in particular bio-
geochemistry cycle processes and their feedback (Figure 4).
The definition by the WCRP focuses on the planetary sur-
face, where the majority of energy, water cycle, and materials
are cycled within the Earth system at the time scales con-
sidered by the WCRP. The emergence of the Earth system
concept has extended the scope of climate research from
ocean-atmosphere interactions, land-air interactions, and
climate feedback processes associated with energy and water
cycle exchanges to carbon feedback associated with complex
biogeochemistry cycles.
Fifth, the IPCC has prompted the development of climate
science by providing regular assessments of the scientific
basis of climate change, its impacts and future risks, and
options for adaptation and mitigation. The IPCC was created
by the WMO and the UNEP in 1988. From 1990 to 2021, the
IPCC produced 6 assessment reports, as well as a number of
special reports and technical papers, by using increasingly
rigorous and comprehensive assessment and review pro-
cesses. Although the assessment of the IPCC is policy re-
levant and does not have a remit to prescribe policies, the
science in the report has clearly influenced policy develop-
ment as a gold standard for our current understanding of
climate science. Meanwhile, the policy sectors have also
prompted new research approaches for the climate sciences.
The IPCC report has acted as a broker between the scientific
and policy-making communities, facilitating new directions
in climate research following feedback from the policy-
making sectors (Zillman, 2009;Steffen et al., 2020). The
IPCC was awarded the Nobel Peace Prize in 2007 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”.
Sixth, climate modeling supported by the quick develop-
ment of supercomputers has driven modern climate science
forward together with observations and theoretical in-
vestigations. Since the birth of the first GCM in 1975
(Manabe and Wetherald, 1975), climate models based on the
fundamental mathematics, physics and chemistry of the cli-
mate system have since developed rapidly (see Figure 2 for a
Figure 4 Climate relevant aspects of Earth system science (cited from
WCRP Strategic Plan 2019, Figure 1).
8Zhou T, et al. Sci China Earth Sci
historical view). (1) Atmospheric General Circulation Model
(AGCM) has been developed quickly since the mid-1970s.
(2) AGCM has been coupled with land surface model since
the mid-1980s. (3) AGCM was coupled with slab ocean
model at the beginning of the 1990s and was used in climate
projection in the IPCC First Assessment Report. (4) Starting
from the mid-1990s, both sulfate aerosols and volcanic
aerosols were included in AGCM. Additionally, the Oceanic
General Circulation Model (OGCM) was used to establish a
fully coupled model, and the fully coupled ocean-atmo-
sphere-land-sea ice model was used in climate projections in
the IPCC Second Assessment Report. (5) Since the begin-
ning of the 2000s, the land surface model has improved in the
context of runoff representation, the aerosol module in the
AGCM has improved in the treatment of both direct and
indirect effects, and the Atlantic meridional overturning
circulation has improved in the OGCM. The fully coupled
models used in climate projection supporting the IPCC Third
Assessment Report have been generally improved in terms
of physics. The carbon cycle process was included in some
models. (6) Dynamic vegetation module was included in the
land surface model, while atmospheric chemistry module
was included in the AGCM since 2007, and the technique of
direct ocean-atmosphere coupling without the employment
of flux adjustment was used in establishing a fully coupled
climate system model. (7) Earth system models with the
inclusion of terrestrial and oceanic biogeochemical cycle
processes have emerged in climate modeling since 2013. (8)
Higher resolution climate models with an atmospheric hor-
izontal resolution of 100 km and oceanic horizontal resolu-
tion of 75 km are widely used in climate models and
projections that support IPCC AR6. A high-resolution cli-
mate system model with both AGCM and OGCM compo-
nents employing a horizontal resolution of 10–25 km
emerged. The convection-permitting model (CPM) was used
in some pioneering climate modeling centers (see Figure 2
and Zhou et al., 2019 for a review; the most recent progress is
in Chen et al., 2021).
The development of the Earth system model (ESM) from
the global climate model is a landmark of model develop-
ment. The global climate model generally represents the
physical climate system components and considers biogeo-
chemical and human systems as external forcings or impacts.
Historically, these models were restricted to the physical
aspects of the atmosphere and ocean. The ESM represents
both the physical climate and the biogeochemical systems
and their interactions as a single coupled system. In most
ESMs, the human system is treated as external, as is natural
forcing (solar and volcanic eruptions) (WCRP, 2019).
Following the improvement of climate and Earth systems
models in the representation of physical and chemical pro-
cesses, the representation of biogeochemistry, including the
carbon cycle, and the increase in model resolution, the
number of climate centers or consortia that engage in model
development has also increased. The wider international
community engaged in climate modeling is evident if we
contrast the Charney report with its successors, the various
IPCC reports. When the famous Charney report was pub-
lished in 1979, the review of results was only based on two
models from Hansen and Manabe. The number of climate
centers or consortia that carry out global climate simulations
and projections has grown from 11 in the first CMIP to 19 in
CMIP5 and 28 in CMIP6. The recently published IPCC AR6
is built upon the output of 39 climate model versions, among
which 11 state-of-the-art Earth system models include car-
bon cycle processes (IPCC, 2021). The climate models de-
veloped by Chinese institutions or universities have made
great contributions to all six rounds of IPCC assessment
(Figure 5). In CMIP1, the climate model developed by the
Institute of Atmospheric Physics, Chinese Academy of Sci-
ences, was the only model from China, while in CMIP6,
there are 8 climate/Earth system models from the mainland
of China (see Zhou et al., 2019,2020b for reviews).
Climate and Earth system models are among the most
sophisticated simulation tools developed by humankind.
They are based on the mathematical formulations of the
natural laws that govern the evolution of climate-relevant
systems, including the atmosphere, ocean, cryosphere, land,
biosphere, and carbon cycle (Flato, 2011). Climate models
are built on the fundamental laws of physics (e.g., Navier-
Stokes or Clausius-Clapeyron equations) or empirical re-
lationships established from observations and, when possi-
ble, constrained by fundamental conservation laws (e.g.,
mass and energy) (Chen et al., 2021). The availability of
climate models helps us to compute the evolution of climate-
relevant variables numerically using high-performance
computers. Since the birth of the first climate model in 1975,
more than 40 years ago, the resurgence of climate modeling
has resulted in more complex and sophisticated climate
models. Climate modeling has and will continue to play
crucial roles in understanding past climate change, the de-
tection and attribution of anthropogenic climate change, and
the projection of future change under various scenarios.
6. Concluding remarks
Since the discovery of the greenhouse effect in 1827, almost
200 years have passed. Building upon the achievements of
the scientific community in global warming and climate
change relevant studies in the past ~200 years, including the
pioneering work of Nobel Prize in Physics 2021 laureates,
Syukuro Manabe and Klaus Hasselmann, we now have solid
physical knowledge of Earth’s climate. Combined evidence
from observations, theoretical studies, climate modeling,
climate change detection and attribution studies has con-
9
Zhou T, et al. Sci China Earth Sci
firmed the unequivocal role of human influence on the ob-
served climate warming (IPCC, 2021).
What is next? Certainly, this will not be the last time that
climate science is recognized by the Nobel Prize. Looking
forward, there is a high probability that the next question to
the climate community from the public will be “how should
we adapt to and mitigate the ongoing climate change?”.
Reliable information on climate change prediction from
global to regional and even local scales is needed to support
the full range of planned adaptation and mitigation actions
(Martin et al., 2021). Climate models will provide critical
information for both mitigating and adapting to climate
change, including understanding the implications of various
mitigation strategies (e.g., Zhang and Zhou, 2020) and in-
forming adaptation strategies and long-term resilience.
Nonetheless, reliable simulation and prediction of pre-
cipitation as well as climate extremes at a regional scale
remain a challenge to current state-of-the-art climate models
(Zhou, 2021). Higher model resolution is an important step
forward for a further increase in confidence in simulations.
Added values are expected from very high-resolution con-
vection-permitting models. New generations of kilometer-
scale, global storm-resolving climate models are expected to
revolutionize the quality of information available for miti-
gation and adaptation, from global climate and regional cli-
mate impacts to risks of unprecedented extreme weather and
dangerous climate change (Slingo et al., 2021).
The climate system is a complex, adaptive system (Slingo
et al., 2009). The extension of the climate system to the
Earth’s system calls for understanding the coevolution of the
biosphere, including both terrestrial and marine ecosystems
and human activities as social-ecological systems (Steffen et
al., 2020). To achieve this, the science of climate prediction
should be extended to a more multifaceted Earth system
prediction that includes the biosphere and its resources
(Bonan and Doney, 2018). In the near future, an Earth life
system simulator that can predict the multiple relationships
between the physical and natural environments, and poten-
tially with society, at both global and national levels, is en-
visaged (Slingo et al., 2021).
Climate science is built upon physical science and extends
to mathematics, chemistry, and the biosphere. Observations,
theoretical studies, high-speed computing, and new tech-
nologies such as remote sensing, artificial intelligence, and
machine learning have powered climate science forward.
The extension of climate science to Earth system science has
necessitated interdisciplinary and transdisciplinary research.
As all these fields are developing rapidly, it is not hard to
picture climate science in the spotlight again. To understand
the complex Earth system and to further promote future
Figure 5 Global distribution of the modeling centers contributing to CMIP and CORDEX. Different colors indicate participation in different phases of the
CMIP, more colors indicate a longer participation history (modified based on Figure 1.20 of IPCC AR6; the contribution of climate models developed by the
Chinese Academy of Sciences were affiliated with “IAP-LASG” from CMIP1 to CMIP3 and were not included in the original figure of AR6 that only
reviewed the contribution of climate models affiliated with the “Chinese Academy of Sciences”).
10 Zhou T, et al. Sci China Earth Sci
climate predictions that enable society to plan for sustainable
development and to keep human and natural ecosystems
safe, we expect more achievements from the climate science
community to come in the near future.
Acknowledgements This work was supported by the National
Natural Science Foundation of China (Grant No. 41988101) and K. C.
Wong Education Foundation.
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