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date: 13 November 2018

Climate and Simulation

Summary and Keywords

Climate and simulation have become interwoven concepts during the past decades because, on the one hand, climate scientists shouldn’t experiment with real climate and, on the other hand, societies want to know how climate will change in the next decades. Both in-silico experiments for a better understanding of climatic processes as well as forecasts of possible futures can be achieved only by using climate models. The article investigates possibilities and problems of model-mediated knowledge for science and societies. It explores historically how climate became a subject of science and of simulation, what kind of infrastructure is required to apply models and simulations properly, and how model-mediated knowledge can be evaluated. In addition to an overview of the diversity and variety of models in climate science, the article focuses on quasiheuristic climate models, with an emphasis on atmospheric models.

Keywords: climate research, climate modeling, dynamic meteorology, computer-based simulation, IPCC Assessment Reports, model intercomparison, community models, model thinking, model evaluation, uncertainties


Climate simulation has become synonymous with trying to understand and forecast possible future global developments. No other scientific discipline has gained more attention and media coverage, nor posed such a challenge for politicians, as climate models have. However, neither the scientific concept of climate nor the practice of mathematical modeling and simulation is correlated with everyday experience. Climate, as defined by the World Meteorological Organization (WMO), is “the statistical description of weather variables in terms of the mean and variability of relevant quantities over a period of time” (World Meteorological Organization, 2016). The “period of time” addressed usually covers 30 years of weather observations, the “mean quantities” refer to the averaging of local data in terms of global means, and “variability” marks the changes in anomalies compared to other periods of time. For instance, global mean annual surface temperature exhibited a deviation of about 0.4°C between the 1880s and 1970s and of about 0.8°C by 2010 (GISTEMP [GISS Surface Temperature Analysis], 2016). In other words, we know that we face global warming, although none of us can ever personally experience the trend in global mean annual surface temperature. Furthermore, we know from the projections of climate models that global warming will increase. Based on model-mediated knowledge, we know that societies will have to invest billions of dollars in order to mitigate and to adapt to climate change. But what is a model? What kind of scientific knowledge does it generate? How can model-based knowledge be evaluated?

Models are common tools in science (Black, 1962; Giere, 1999; Hesse, 1963; Magnani & Nersessian, 2002; Morgan & Morrison, 1999; Toon, 2012; for an overview, see Frigg & Hartmann, 2012). In general, scientific models can be classified in two categories: material models and symbolic models. So-called material models, scale models and analog models, mimic objects and processes, respectively, in experiments. Scale models of ships, airplanes, and cars are tested in wind tunnels to help engineers investigate the aerodynamic properties of a scaled-down prototype, based on the underlying hypothesis that upscaling will not change the properties. Analog models “simulate” processes in the laboratory. For instance, hydraulic models materially simulate the behavior of tides and floods. Cloud chambers allow the study of the microphysical processes of clouds in order to gain a conceptual understanding of specific relationships.

Entirely different from material models are symbolic models, which represent objects, processes, and properties symbolically and thus provide information about the state and development of a system (Müller & von Storch, 2004). Usually expressed in mathematical terms, symbolic models can vary from conceptual models based on major simplifications to quasirealistic models containing as much information about processes as possible (von Storch, 2010). Conceptual models, sometimes formulated as only a single equation expressing a specific relationship, are easy to investigate mathematically. They can usually be solved analytically, yielding a solution that illuminates the specific aspect of interest. However, the climate system is a complex system, interweaving numerous processes that constitute its state and development. Thus, conceptual models have gradually been transformed into complex, quasirealistic models, containing information about atmospheric and oceanic processes interacting with land surface, ice, vegetation, chemistry, and anthropogenic effects. Today, comprehensive Earth System Models (ESMs) are the ultimate goal of climate science. The price for increasingly complex mathematical models is that they cannot be solved analytically. Instead, they need to be computed numerically (simulated), which requires advanced computing resources. Because mathematical models represent climate symbolically, such representations can depict either facts or fictions, not only about the climate system, but also about the projected results as well. Thus, climate models have to be thoroughly evaluated, requiring a global infrastructure to supply observational data for model comparison and evaluation.

Based on the specific circumstances of climate and simulation, this article explores how climate became a scientific object and how climate models became subject to simulation, with a focus on meteorology and the atmospheric part of climate models. The article also investigates the global infrastructure that modern climate models require in order to be applied properly. Finally, it discusses models as new “tools” of scientific knowledge production, afflicted with many problems but with the unique advantage of providing insight into complex phenomena, such as climate in the future.

Historical Development of Quasirealistic Climate Models

Advent of Dynamic Meteorology and Model Thinking in the Late 19th Century

Until the late 19th century, meteorology was a purely empirical science. Meteorologists measured the main meteorological quantities (using specific tools): temperature (thermometer), air pressure (barometer), wind direction and velocity (vane, anemometer), and humidity (hygrometer). However, the measurement data they collected gave only a description of the atmosphere, snapshots of its momentary states.

Of greater interest to scientists is understanding which atmospheric changes occur, how they are caused, and how they will develop in the future. Such a theoretical understanding can be developed either inductively or deductively (Lorenz, 1969). The inductive approach is based on measurement data and the attempt to find structures and patterns in the data. It led to the development of the statistical and climatological approaches to weather, respectively, as well as to the synoptic approach. Yet, the theoretical results of the inductive approach were meager: Some synoptic rules were developed, and a single empirical law, Buys-Ballot’s baric wind law describing the relationship between air pressure and wind direction (Buys-Ballot, 1854, 1857). Therefore, the deductive approach based on a physical and mechanical understanding of the atmosphere increasingly gained favor. The consequence was that it turned meteorology into the physics of the atmosphere—also called “dynamic meteorology”—focusing on the dynamics of atmospheric processes articulated by mathematical models.

Since the time of Isaac Newton, a physical and mechanical understanding of nature had been expressed by abstract models, and since the time of Leonard Euler, differential equations had become the lingua franca for conceptualizing the kinetics and dynamics of physical processes mathematically (Darrigol, 2005; Eckert, 2006; Euler, 1755; Newton, 1687). Thus, mechanics and mathematics became synonyms, and the study of natural phenomena was, in an increasing number of scientific fields, replaced by studying abstract mathematical models and their behavior. Celestial mechanics was the first science to use this new research style (Dear, 1995). An important reason for the primary role of astronomy was the intrinsically distant perspective of astronomers on the objects of their research, which supported an abstract model view usually expressed as the linear behavior of two bodies (e.g., planets). How were meteorologists to develop a distant perspective on the extremely complex states of the atmosphere surrounding them in order to arrive at a similar abstract model view?

The answer was the perspective on the atmosphere as a fluid with a global circulation. In 1686, Edmund Halley had realized that solar radiation differs in low and high latitudes, and that this causes a north–south circulation as heated tropical air is replaced by cooler air from polar regions (Halley, 1686/1687a, 1686/1687b). In 1735, George Hadley had pointed out that the atmospheric circulation is deflected by the Earth’s rotation (Hadley, 1735). Because the speed of rotation differs at each point on Earth, the deflection of air masses differs as well. As Dove explained in 1837, this causes a difference in rotational speed between moving air masses and the places to which the masses move (Dove, 1837). Finally, Ferrel accounted for the Coriolis effect of the Earth’s rotation in 1856 (Ferrel, 1856, 1858; cf. Fleming, 2000, 2002). From these considerations, the first three-cell model of the global circulation for each hemisphere resulted.

The three-cell model consists of the polar cells (beyond 60°N and 60°S), the middle-latitude cells (60°N−30°N and 60°S−30°S), and the tropical cells (30°N−0°N and 30°S−0°S)—the latter also called Hadley cells. In the tropical cells, the circulation of the northeasterly and southeasterly trade winds is regularly compared to the complex wind patterns of the westerlies in the middle-latitude cells. The open question in the 19th century was how the circulation in the upper atmosphere transported air from one cell to the other. In particular, James Thomson realized that, “in temperate latitudes, there are three currents at different heights” bringing air from, and back to, the poles (J. Thomson, 1857, p. 38). However, it was Ferrel, rather than Thomson (Maury, 1855; J. Thomson, 1857, 1892), who rooted the three-cell circulation model in a sound physical basis expressed by advanced mathematics (Ferrel, 1877, 1886). Therefore, he was regarded as the “standard authority” for dynamic meteorology in his day (Davis, 1887; Sprung, 1885). His approach changed the view of the atmosphere, which was now seen as a giant engine for the circulation of air masses and heat, driven by solar radiation and gravitational forces.

Methods of Computation Before the Advent of Electronic Computers

An early application of dynamic meteorology for weather forecasting was provided by the Austrian school of meteorology at the late 19th century. Max Margules tried to mathematically describe and compute a conceptual model of the “atmosphere’s tide” and its barometric fluctuations (1890)—a laborious work before the advent of electronic computers. Margules had to find analytically tractable approximations of the governing equations to calculate them by hand. Thus, Margules based his computations on Buys-Ballot’s baric wind law and Laplace’s scheme for calculating tides, and he calculated the work needed to change the state of a quantum of air from motion into equilibrium (Laplace, 1775, 1776; Margules, 1901; cf. Pichler, 2001). At the same time, his colleague Felix Exner developed a conceptual model based on hydrostatic and geostrophic approximations for the thermodynamic equation. He manually computed the advective rate of change in the potential temperature for one layer. His computations are seen as the first numerical forecast in meteorology (Exner, 1902, 1917; Fortak, 2001; Volkert, 2007).

Another way of computing weather forecasts before the advent of electronic computers was to use graphical methods in order to project into the future the course of the cyclones and anticyclones roaming over synoptic maps (Ekholm, 1904). An important contribution was made by the research program of Vilhelm Bjerknes, who developed a graphical algebra for performing computations directly upon the charts. Such a graphical method, he announced, “will be of the same importance for the progress of dynamic meteorology and hydrography as the methods of graphical statistics and of graphical dynamics have been for the progress of technical sciences” (V. Bjerknes & Sandström, 1910/1911, p. 69). As knowledge of the development of cyclones was increasingly discovered and applied—for instance, the polar front theory developed mainly by the Bergen school of meteorology (V. Bjerknes, 1919; Friedman, 1989)—synoptic maps became more reliable prognostic instruments (Fjörtoft, 1952; Scherhag, 1939).

However, synoptic forecasts depended on the subjective experience of the meteorologists and their “intuitive glance” for adjusting data (Anderson, 2005). Therefore, in addition to the conceptual models and synoptic forecasts, an objective forecasting method based directly on the hydro- and thermodynamic equations (the so-called “primitive equations” or “quasirealistic” model, respectively) became the overall aim of dynamic meteorology. As early as 1904, Vilhelm Bjerknes had outlined a fully developed circulation model consisting of seven hydro- and thermodynamic equations expressing the relationship between the seven main state variables of the atmosphere: temperature, pressure, density, humidity, and wind velocity in three directions (V. Bjerknes, 1904; Gramelsberger, 2009; Persson, 2005b). Introducing more than two variables into a model makes it nonlinear and thus, in principle, not analytically solvable, as the mathematician Henri Poincaré had already proven in 1890. A seven-variable problem like Bjerknes’ circulation model, consisting of a set of seven equations, is far beyond the possibility of ever deriving a solution algebraically. Lewis F. Richardson was the first to try to numerically compute a quasirealistic model by hand, but he failed to correctly predict a change in air pressure. However, his book Weather Prediction by Numerical Process (Richardson, 1922) anticipated the mid-20th-century style of weather and climate simulation (Lynch, 1999; Nebeker, 1995; Platzman, 1968).

Thus, in the early 20th century, meteorology was trapped between analytically tractable approximations, which were manually computable but highly idealistic, and fully developed circulation models beyond any computability by hand or by early computers. Vorticity, the core problem of every circulation model, disappeared in idealized models. Such idealized models were outlined by Hermann von Helmholtz and William Thomson (Lord Kelvin) in the middle of the 19th century (W. Thomson, 1867; von Helmholtz, 1858). Because in these simple models density depended solely on pressure, vorticity didn’t occur. However, weather is characterized by the appearance and disappearance of vorticity (storms). Therefore, V. Bjerknes and others tried to derive more complex models, but the complex models were not computable (V. Bjerknes, 1898; Schütz, 1895a, 1895b; Silberstein, 1896; Thrope, Volkert, & Ziemianski, 2003). Thus, treating vorticity in a computable way was the aim of early 20th century dynamic meteorology. In the 1930s, Carl-Gustaf Rossby articulated a linear model that conserved the vertical components of absolute vorticity in currents for the perturbations in the upper westerlies (Rossby, 1939; cf. Beyers, 1960). A little later, Ertel formulated a generalization of Bjerknes’ circulation theorem by conserving potential vorticity (Ertel, 1942a, 1942b). Ertel and Rossby were laying the foundation for today’s weather models and together they “derived another vorticity theorem for barotropic fluids, known as the Ertel-Rossby invariant” (Ertel & Rossby, 1949; Fortak, 2004; Névir, 2004, p. 485). Rossby, in particular, became the leading figure for the simulation style of dynamic meteorology in the 1940s. He introduced numerical weather prediction to Europe at the University of Stockholm as well as to the United States at the University of Chicago (cf. Allan, 2001; Harper, 2008).

From Weather Forecast Models to Climate Models

The situation changed in the late 1940s, with the development of electronic computers. However, due to the limited performance of early computers, the simulation of weather with numerical models started with extremely simplified barotropic models comparable to von Helmholtz’s and Thomson’s models of the 1860s. In a barotropic model, pressure is solely a function of density and fields of equal pressure running parallel to fields of equal temperature, thus reducing the effort of computation by modeling wind independent of altitude (geostrophic wind). The very first weather model ever electronically computed—Charney and his colleagues’ simulation of pressure development at a height of about 5,500 m for a 15 × 18 grid representing Northern America processed on the ENIAC computer (Charney, Fjørtoft, & von Neumann, 1950)—was a barotropic model (cf. Harper, 2008; Nebeker, 1995; Persson, 2005a). In order to give rise to cyclones and anticyclones (a quasigeostrophic model), an instability process had to be introduced (Charney & Phillips, 1953; cf. Lewis, 2000; Phillips, 1995). Thus, it was not surprising that the new approach of tackling the forecast problem with numbers was heavily criticized in the beginning. In particular, the “tricks” used to simplify the models in order to reduce computations caused major suspicions. For instance, averaging out errors by relaxation methods was seen as a major problem (Thompson, 1954, p. 320). Or, more generally, it was criticized that “500 mb geopotential is not weather” (Norbert Wiener quoted in Arakawa, 2000, p. 6). Nevertheless, the numerical approach gained ground. In 1954, Solomon Belousov computed a barotropic model on the Russian BESM computer (Blinova & Kibel, 1957; cf. Marchuk, 1974; Wiin-Nielsen, 2001). Also in 1954, a group of Scandinavian meteorologists carried out a barotropic forecast on the Swedish BESK computer (Persson, 2005a). Similar efforts took place in other countries (cf. Guillemot, 2011), and as early as 1948, the British Meteorological Office held a workshop, The Possibilities of Using Electronic Computing Machines in Meteorology (cf. Persson, 2005b; Walker, 2011), which was followed by two symposia on the development in numerical prediction, one in Stockholm in 1952 and the other in Frankfurt in 1954. However, it took years before meteorological offices could afford computers and numerical weather forecasts became operational for everyday use.

The very early barotropic models could hardly be considered weather models. The same is true for the first electronically computed “climate model.” In the mid-1950s, geophysicist Norman Phillips computed a general circulation model on von Neumann’s computer at the Institute for Advanced Studies (IAS) at Princeton (Phillips, 1956; cf. Goldstine, 1972; Lewis, 2000). His two-level quasigeostrophic model “predicted the easterly-westerly-easterly distribution of surface zonal wind, the existence of a jet, and the required net poleward transport of energy” (Phillips, 1956, p. 157). Phillips’ computations are considered to be the crucial evidence that simulations can represent large-scale dynamic patterns of the atmosphere as conceived in the three-cell-model a hundred years earlier. The pioneering work of barotropic models was later called by Akio Arakawa the “epoch-making first phase” of numerical modeling (Arakawa, 2000), although the early models were proofs of concepts rather than quasirealistic weather or climate models.

This first phase was followed by the “magnificent second phase” in the 1960s of global circulation models (GCMs), which enjoyed fewer restrictions than the barotropic models (cf. Arakawa, 2000; Edwards, 2000, 2010; Spekat, 2001). Arakawa collaborated with Yale Mintz to open up the second phase with their General Circulation Model at the University of California Los Angeles (UCLA), applying Jakob Bjerknes’ program of investigations into the general circulation of the atmosphere (J. Bjerknes & Mintz, 1955; Mintz, 1955, 1958; cf. Johnson & Arakawa, 1996; Randall, 2000). The Mintz-Arakawa model was a global, two-level model based on the primitive equations of hydro- and thermodynamics, accounting for realistic land−sea distributions, surface topography, and moisture by introducing a moisture convective adjustment. Furthermore, the model included long-wave cooling and seasonal changes in solar radiation. An initial study with the GCM was carried out in 1965 on an IBM 709 computer at UCLA (Mintz, 1965). Mintz and Arakawa used the model as an experimental test bed, changing boundary conditions artificially to study the behavior of their model by comparison across various numerical experiments. This opened up the “in silico” experimental style of using simulation models as test beds that characterizes modern computational sciences, requiring enormous computing resources.

The GCMs of the second phase increasingly included information about the oceans (swamp ocean) and finally developed into coupled atmosphere−ocean models (AOGCMs). Thus, they transformed into quasirealistic climate models because climate is influenced mainly by oceans and sea ice coverage (cf. Dahan, 2010; Heymann, 2010; Laprise, Lin, & Robert, 1997). The reason is that the top few meters of the oceans hold more heat energy than the entire atmosphere; therefore, the energy exchange between atmosphere and ocean as well as the ocean’s circulation are crucial for long-term climatic integrations. The first coupling of an atmosphere and an ocean model was carried out by Syukuro Manabe and Kirk Bryan at the Princeton Geophysical Fluid Dynamics Laboratory (GFDL) in 1969. The tradition in numerical modeling at GFDL was rooted in von Neumann’s and Charney’s meteorology project of numerical weather forecasting. However, Manabe and Bryan had climate calculations in mind, taking into account “the entire fluid envelope of the earth, consisting of the atmosphere and the hydrosphere” as well as of the cryosphere—sea ice and land ice (Manabe & Bryan, 1969, p. 786). Furthermore, water vapor, carbon dioxide, and ozone were considered for the transfer of terrestrial radiation. Simulations were carried out, using an UNIVAC 1108 computer, on nine levels for a 500-km grid resolution for the atmosphere, and on five levels for the ocean with the same horizontal resolution, with extra rows of grid points added to the western boundary. The results confirmed the hypothesis that oceanic currents have a substantial effect on the distribution of temperature, humidity, and precipitation patterns.

The development of global models during these years created, as Paul Edwards called it, a “family tree” of GCMs in the United States and Europe (Donner, Schubert, & Somerville, 2011; Edwards, 2000; Kasahara & Washington, 1967; Leith, 1964; Manabe, Smagorinsky, & Strickler, 1965; Messinger & Arakawa, 1976; Smagorinsky, 1963; Weart, 2010). The models influenced each other and sometimes one model was the direct ancestor of another as the latter model inherited parts of the software code. But not just the quasirealistic models yielded insights into the climate system. Regional and mesoscale models, too, as well as cloud-resolving models, showed promise for studying the whole spectrum of atmospheric phenomena. With the diversity of models, increasingly prominent questions concerning anthropogenic climate change could be addressed, when human carbon dioxide (CO2) production came under suspicion as having potential to irreversibly change climate. Evidence for the “vast geophysical experiment” of mankind (Revelle & Suess, 1957) was delivered by Charles D. Keeling, whose measurements at the Mauna Loa Observatory in Hawaii clearly demonstrated the increase in CO2 concentration at a site that was thought to be otherwise unpolluted (Keeling, 1958, 1978; Plass, 1956; Weart, 2003). The Keeling Curve has become the icon of human-induced climate change. It led to the core question of climate change: How much increase in the annual mean temperature of the surface would result from doubling of CO2 concentrations? However, the answer to the CO2 doubling question can be given only by climate simulations, and the first answer computed was already alarming: “Doubling the existing CO2 content of the atmosphere has the effect of increasing the surface temperature by about 2.3°C” (Archer & Pierrehumbert, 2011; Manabe & Wetherald, 1967, p. 254; Möller, 1963; Stehr & von Storch, 2010; Weart, 2003). Thus, in silico experiments were urgently needed in order to better understand the vast geophysical experiment of mankind, or to be more precise, of the highly industrialized nations in the Western world. “But such [in silico] investigations [were] in danger of becoming mere exercise, due to the lack of observations to supply the initial conditions and to check the calculations” (National Academy of Sciences, 1966, p. 4). During the 1960s, numerical simulation had outstripped the evaluation potential of meteorological and climatological observations.

Global Infrastructure for Climate Modeling and Simulation

WMO’s Global Infrastructure of Climate Science

Originally focused on weather monitoring, the WMO quickly expanded its work from the late 1960s on, due primarily to concerns about human influence on the climate (cf. Young, 1997). Understanding the climate, and climate change in particular, requires globally collected data on relevant processes within and between the components of the climate system: the atmosphere, the hydrosphere (oceans, lakes), the cryosphere (land and sea ice), the pedosphere, the terrestrial and maritime biosphere, and the anthroposphere. Intensifying observation of these processes has led to an outstanding global observation infrastructure. Along with the United Nations Environment Programme (UNEP), the WMO has installed and organized many global programs. The concerns about climate change reached a first peak with the report on the Study of Man’s Impact on Climate (SMIC) in 1971 (SMIC, 1971) and the United Nations Conference on the Human Environment (UNCHE) in 1972 (cf. Demeritt, 2001; Howe, 2014; Weart, 2003, 2014). The WMO responded to the concerns by setting up programs like the Global Atmosphere Watch (GAW), combining the Global Ozone Observing System (GO3OS) and the Background Air Pollution Monitoring Network (BAPMoN). From 1966 until 1979, the Global Atmospheric Research Program (GARP) addressed both requirements–understanding climate and climate change, respectively, with the goal of “advancing the range of deterministic weather prediction and understanding the physical basis of climate” (Barron, 1992, p. 1).

The first efforts were rooted mainly in scientific concerns and studies on ongoing developments, but at the end of the 1970s the situation changed. Attempting to answer the CO2 doubling question more robustly, Charney et al. carried out numerical studies using two GCMs that represented the state of the art at the time. They concluded that “our best estimate is that changes in global temperature on the order of 3°C will occur and that these will be accompanied by significant changes in regional climatic patterns” (Charney et al., 1979, p. 17). The highly influential “Charney Report,” as it became known, and the first World Climate Conference (WCC-1) in Geneva in 1979, marked a watershed in climate science. Climate change turned into a public policy issue, increasingly interlinking climate science and politics through expanded international conferences, research programs, working groups, and committees (cf. Depledge, 2005; Grover, 2008; Halfmann & Schuetzenmeister, 2009; Hare, Stockwell, Flachsland, & Oberthür, 2010; Jasanoff, 2011; Jasanoff & Martello, 2004). The aims of the international activities coordinated by the WMO were threefold: a better understanding of the current state of the atmosphere (observation), a framework for negotiating an adequate response to climate change (climate politics), and a better understanding of future trends (simulation; for details, see Edwards, 2010).

Ever since the first sputnik collected data from space for the International Geophysical Year from 1957 to 1958, the goal of the WMO was to gain better knowledge of the current situation by installing a global observation infrastructure (see Table 1). Through the global observation infrastructure, more than 50 different essential climate variables (ECV) have become observable.

Table 1. World Meteorological Organization Global Infrastructure of Climate Science

Since 1979

World Climate Programme (WCP)

World Climate Impact Assessment and Response Programme (WCIRP)

Since 1980

World Climate Research Programme (WCRP)

Since 1992

Global Climate Observing System (GCOS), including the Global Observing System (GOS) and the Global Atmosphere Watch (GAW)


GOS collects data from 1,000 land stations, 1,300 upper-air stations, 4,000 ships, ~1,200 drifting and 200 moored buoys, and 3,000 Advanced Research and Global Observation Satellite (ARGOS) profiling floats, as well as 3,000 commercial aircraft, five operational polar-orbiting meteorological satellites, six geostationary meteorological satellites, and several environmental research and development satellites.


GAW coordinates data from 26 global stations, 410 regional stations, and 81 contributing stations to produce high-quality data on selected variables characterizing the chemical composition of the atmosphere.

The second goal of the WMO was to install a framework for negotiating an adequate response to climate change. In particular, the view of Earth from space—in December 1972, the crew of the Apollo 17 spacecraft sent back the famous Blue Marble image of a cloud-surrounded blue sphere embedded in the blackness of space—increased awareness about the uniqueness and vulnerability of the planet (cf. Cosgrove, 2001; Jasanoff, 2001; Poole, 2008). This view, combined with alarming reports in the 1970s and early 1980s on environmental catastrophes, such as droughts, floods, air pollution, acid rain, and depletion of the ozone layer, caused calls for action to increase dramatically. Thus, in 1985, the Villach Conference on the assessment of the role of CO2 and other greenhouse gases (GHGs) emphasized the need to institute a property rights regime for human use and modification of the carbon cycle by establishing the Advisory Group on Greenhouse Gases (AGGG). Inspired by the successful negotiations of the Montreal Protocol on Substances that Deplete the Ozone Layer in 1987 (cf. Andersen & Sarma, 2002), these activities became a role model for environmental governance of climate change. They led to the appointment of the Intergovernmental Negotiating Committee on Climate Change (INC), the United Nations Framework Convention on Climate Change (UNFCCC), and finally the adoption of the Kyoto Protocol in 1997 (cf. Agrawala, 1999; Aykut & Dahan, 2015; Boehmer-Christiansen, 1994; Bodansky, 1995; Dahan & Aykut, 2013; Elzinga & Landström, 1996; Franz, 1997; Gupta, 2014; Jasanoff & Martello, 2004; Miller & Edwards, 2001; Shackley & Wynne, 1995; Siebenhüner, 2003; Skodvin, 2000; Stehr & von Storch, 2010; van Asselt, 2014).

In particular, the Intergovernmental Panel on Climate Change (IPCC), established in 1988, and the IPCC Assessment Reports on Climate Change have become core instruments for the supranational governance of climate change. Since 1990, five IPCC Assessment Reports have been released and a sixth is in preparation (see Table 2). All of the reports have three parts of each working group (WG): the Physical Basis (WGI); the Impacts, Adaptation, and Vulnerability (WG2II); and the Mitigation of Climate Change (WG3III). Every report is the product of several hundred lead authors and contributing authors who consider tens of thousands of comments from the scientific and government community. Each report starts with a Summary for Policymakers (SPM), “reviewed at final plenary sessions, where governments have to approve the SPM text, tables and figures in detail, that is, line by line” (Petersen, 2011, p. 100). The final plenary sessions are laborious, days-long meetings attended by government officials and scientists. They document the interlinking of climate science with politics and the establishment of global climate governance (cf. Agrawala, 1998a, 1998b; Bolin, 2007; Beck, 2009; Hulme & Mahony, 2010).

Table 2. IPCC Assessment Reports on Climate Change


First Assessment Report (FAR)


Second Assessment Report (SAR)


Third Assessment Report (TAR)


Fourth Assessment Report (AR4)


Fifth Assessment Report (AR5)

The third goal of the WMO was to achieve a better understanding of future trends based on climate modeling and simulation. This led to a conjoint infrastructure in environmental sciences coordinating global climate modeling. Among others, the mission of the World Climate Research Programme (WCRP) was, and still is, to develop and evaluate climate system models. In accord with this mission, the Working Group on Numerical Experimentation (WGNE) was established in 1980, followed by the Working Group on Coupled Modeling (WGCM) in 1997. The working groups organize the numerical experimentation and evaluation for the IPCC Assessment Reports and have an important influence on the practice of climate modeling and simulation—an impact unique in science (cf. Mahony & Hulme, 2016). They have coordinated and synchronized model development, numerical experimentation, and evaluation for all participating modeling groups since the first IPCC Assessment Report in 1990.

Model Intercomparison and Reanalysis Data

In particular, model intercomparison has shaped the rhythm of model development for the IPCC Assessment Reports. Model intercomparison is coordinated by the Coupled Model Intercomparison Project (CMIP) under the auspices of the WGCM and is carried out at the U.S. Lawrence Livermore National Laboratory. Its mission is to provide “a community-based infrastructure in support of climate model diagnosis, evaluation, intercomparison, documentation and data access” (CMIP, 2015; Meehl et al., 2005). In past years, CMIP3 (AR4), CMIP5 (AR5), and CMIP6 (AR6) have replaced the Atmosphere Model Intercomparison Project (AMIP) of the first, second, and third IPCC Assessment Reports. Thus, “virtually the entire international climate modeling community has participated in this [intercomparison] project since its inception in 1995” (CMIP, 2015). The timeline of CMIP5 documents how the work steps are coordinated (see Table 3).

Table 3. Timeline of CMIP5 for the Physical Basis (Working Group I) Part of IPCC AR5

2006, 2007

Discussion of model improvements and a preliminary set of CMIP5 numerical experiments.


Community-wide consensus about improvements and experiments.


List of requested model outputs available. Final set of CMIP5 experiments approved by the WCRP Working Group on Coupled Modeling, including decadal hindcasts and prediction simulations, “long-term” simulations, and “atmosphere-only” simulations for especially computationally demanding models.


Modeling groups participating in AR5 begin simulation runs with their improved models for CMIP5 and deliver their results to the Lawrence Livermore National Laboratory.

In November 2010, work on the AR5 begins with the First Lead Author Meeting of Working Group I.


In February 2011, the first model output becomes available to the climate science community for analysis.


By the end of July 2012, papers based on the CMIP model output have to be submitted for publication to be eligible for assessment by WG1.


By March 2013, papers cited by WG1 have to be published or accepted for publication (with proof of acceptance).

The standard protocols of model simulations defined by CMIP5 combine three types of simulations: Decadal hindcasts, for evaluating how realistic the models are in simulating the recent past; forecasts, providing projections of future climate change on two time scales, near term (out to about 2035) and long term (out to 2100 and beyond); and finally, scenarios for model intercomparison in order to understand some of the factors responsible for differences in model projections (e.g., those involving clouds and the carbon cycle). In particular, the decadal hindcasts are core experiments to evaluate the quality of climate models. If a model does not represent today’s climate variables correctly, the model is deemed unable to forecast future states. Thus, decadal hindcasts are the classic test beds for climate model evaluation.

Initializing standardized simulation runs of climate models requires standardized observational data. For instance, the decadal hindcasts of CMIP5 consisted of 10-year hindcasts initialized from climate states in the years 1960, 1965, and 1970. The experiments were based on so-called reference data sets, which were used for standardized numerical experiments in order to generate comparable and reproducible results. Usually, every data product is based on its own assimilation methods, which change over the years. Therefore, reference data sets are reanalysis data “with a ‘frozen’ data assimilation system (one that would not change during the reanalysis)” (Edwards, 2010, p. 324; cf. Parker, 2011).

Data assimilation is required because measurement data exhibit great uncertainties and irregularities due to their inhomogeneous characteristics. The inhomogeneity results from the diversity of measurement platforms and methods (weather stations, buoys, radiosondes, satellites, rockets), their irregular spatial distribution, and their diverse accuracy and error characteristics. On the one hand, observational data have to be placed into a gridded model space, as climate simulations are usually performed on regular and global computing grids. For instance, satellites cover swaths of only several hundred to a few thousand kilometers of the Earth’s surface and produce distorted data at the edges of their focus. Therefore, data from various satellites have to be assimilated and composed in order to gain a global, straightened, regular data set (“making data global,” Edwards, 2010). On the other hand, reanalysis data “improve” measurement data by combining information on the actual state (measurement) with physical laws (model) accounting for observation error as well as model error.

Creating reference data sets is laborious work. In particular, two major reanalysis projects have been carried out: the NCEP/NCAR Reanalysis I and II data sets (RA-I, RA-II) of the U.S. National Centers for Environmental Prediction (NCEP) and the National Center for Atmospheric Research (NCAR), and the ERA-15 and ERA-40 data sets of the European Centre for Medium-Range Weather Forecasts (ECMWF). The NCEP/NCAR Reanalysis I and II data sets cover the period from 1948 to 2002 (Kalnay et al., 1996), while the ERA-40 data cover the period from mid-1957 to mid-2002 (Uppala et al., 2005). However, reference data sets can be afflicted with biases, as “deficiencies in the analysis method or assimilating model could introduce significant biases in the resulting analyses, and could invalidate the conclusions drawn from them” (Uppala et al., 2005, p. 2962). For instance, ERA-15 data sets showed shifts in humidity and temperature resulting from problematic assimilation of satellite data (Trenberth, Stepaniak, Hurrell, & Fiorino, 2001). Because the data sets are widely used by the modeling community, biases propagate into many simulation results. However, neither in situ measurements nor reanalysis data provide a true image of the world’s states. Reanalysis data document that in situ and in silico data are increasingly merging (cf. Feichter, 2011).

Community Models and Platforms

The IPCC Assessment Reports serve as unique documentation of the history of climate models since 1990 (Le Treut et al., 2007). They began with eleven CGMs and AOGCMs from seven countries in 1990 (IPCC, 1990, pp. 81−82), and the recent, fifth report is based on more than 40 AOGCMs and ESMs as well as 15 Earth Models of Intermediate Complexity (EMICs) from a total of 15 countries (IPCC, 2013, p. 747). The global models are supplemented by regional models, integrated assessment models, and special interest models (such as high-resolution cloud models). Modeling centers and university departments are responsible for these models (cf. Easterbrook & Johns, 2009; Krueck & Borchers, 1999; Randall, 1996). When work on climate models started, such as the UCLA Department of Meteorology’s Mintz-Arakawa model, only a few modelers were involved and not many researchers outside a given modeling center worked with the model. In fact, the Mintz-Arakawa model was one of the first distributed models used by the RAND Corporation for numerical studies of climate dynamics (Gates, Batten, Kahle, & Nelson, 1971).

Today, many modelers are collaboratively involved in generating, improving, and maintaining climate models, which have become community models. Thus, in 1983 NCAR began distributing a freely available Community Climate Model (CCM) of the atmosphere to the climate research community (Washington, 1982), followed in 1994 by a Community Climate System Model (CCSM) including models of the atmosphere, land surface, ocean, and sea ice (Kiehl, Hack, & Bonan, 1998) and the Community Earth System Model (CESM) in 2010 (Hurrell, Holland, & Gent, 2013). More than 300 researchers are involved in developing the CESM. The development requires an advanced software design and collaborative tools for community development, such as software repositories, version control systems, and procedures for introducing code into the repositories, coding standards, and testing infrastructure (Drake, Jones, & Carr, 2005). Furthermore, the community contributions have to be evaluated scientifically by the Change Review Board.

Besides community development, community-wide use has been eased by increased traffic performance, allowing online platforms and gateways to be established for downloading models and simulation data. Thus, in addition to modelers, the numbers of model users and simulation data users have increased enormously, involving more and more scientific disciplines, such as biology, agriculture, and economics. Model users apply models from others for their own experiments, while simulation data users analyze the output of the in silico experiments with models, either for scientific purposes or for policy purposes. A new type of data users are climate prediction services, which tailor simulation data and predictions for sector-specific purposes, including agriculture, environmental politics, and industry. Like daily weather forecasts by weather services, regional and seasonal temperature and precipitation projections as well as forecasts of extreme weather changes and unusual seasonal variations have become real-time products of climate prediction services all over the world (Dutton, 2002; National Research Council, 2001). Last but not least, the media and the public use simulation data (Asrar, Hurrel, & Busalacchi, 2013).

Against this backdrop, an important goal of today’s climate science is to provide a fully flexible plug-and-play modeling framework as “the community moves toward componentization and shared utility infrastructures” (Drake et al., 2005, p. 180). Such a framework has been developed by the U.S. Earth System Modeling Framework (ESMF) (Collins et al., 2005) as well as by the British Grid ENabled Integrated Earth System Model (GENIE). The vision is that a registered user can construct a model from a library of components (ocean, atmosphere, ice, etc.; Price et al., 2005). With such plug-and-play frameworks, anybody could, in principle, conduct an earth system experiment. Nevertheless, because there is no single best model or best approach, evaluating which data from which model or method to use is difficult. Furthermore, evaluation of models is a complex task and “it is well known that in many situations climate models obtain the right result for the wrong reason” (de Elía, 2014, p. 1004). Thus, the lack of a dominant model, of confirmation, and of good causality complicates the application of climate models for model users and simulation data users (Aroonruengsawat & Auffhammer, 2011; Landström et al., 2011).

Model Evaluation, Uncertainties, and the Complexity of Climate Models

Sources of Uncertainties

Meteorology is a field paradigmatic for its use of models and simulation in science. It was been among the first of the sciences to take advantage of electronic computers and today it is leading the way in building a conjoint infrastructure of model intercomparison, standardized reference data sets, community models, and data platforms. In the process, meteorology has turned back into an “experimental” science. However, the term experimental refers here to in silico experiments with only mathematical models symbolically representing the atmosphere, since experimenting with the real atmosphere is not feasible. Since mathematical models are purely representational, evaluating these models is anything but trivial.

The quasirealistic models presented in this article are deterministic models based on “first principles” of hydro- and thermodynamic theory. Although this type of model is predestinated for expressing processes, the main problem is that many important physical processes are only partly resolved in the models, either because of lack of knowledge about the processes or because of lack of computing resources to increase spatial resolution. The limit in spatial resolution divides the model in resolved and unresolved processes. For instance, the first IPCC Assessment Report was based on a 500-km grid (T21, corresponding to a 500-km grid). Such a grid does not resolve major cyclones, and small countries are represented by only a few grid points. Therefore, the image presented was extremely coarse for climate projections. The fourth IPCC Assessment Report employed a 110-km grid (T106, corresponding to a 110-km grid) and 10-min time steps, which led to 52,560 iterations to simulate a year of climate development, and more than five million iterations for a century. However, an extremely high spatial resolution (T1279, corresponding to a 16-km grid) is needed in order to simulate the circulation regime structure of the northern hemisphere (Dawson, Palmer, & Corti, 2012). Even today, such a resolution can be computed for only a limited area and short-term predictions, but not for the globe and long-term forecasts. Thus, small-scale (unresolved) processes have to be explicitly parameterized, because they have an important influence on the resolved scale of quasirealistic models. Unfortunately, subscale parameterizations are a major source of uncertainties. In particular, parameterized clouds have become prominent for afflicting climate models with uncertainties, because many cloud processes are not known and those known are often insufficiently parameterized (parameter uncertainties).

Initial and boundary conditions are another source of uncertainties. Weather models extrapolate the current state of the atmosphere into the future; therefore, they are heavily influenced by the initial condition problem of uncertain measurement data and intrinsic nonlinearities, quickly leading to unstable results after three to seven forecasted days (initial condition uncertainty; Lorenz, 1963). Different climate models, which extrapolate averaged states of the atmosphere into the future, thus average out the initial data uncertainties after a while. However, simulation results of climate models are sensitive to the boundary conditions, such as GHG concentrations forecast for the next decades by socioeconomic scenarios. Such scenarios have been developed in the course of the IPCC Assessment Reports (SA90 scenarios, IPCC, 1990, Appendix 1; IS92 scenarios for FAR, Pepper et al., 1992; SRES scenarios, Nakicenovic & Swart, 2000; and RCP scenarios, van Vuuren et al., 2011, and Meinshausen et al., 2011). They cover a wide range of possible changes in future anthropogenic GHG emissions and technological development (boundary condition uncertainty). Since the first scenarios, a “business as usual” scenario and several ecologically friendly stories have been developed, leading to temperature projections by climate models for 2100 that vary between + 0.3°C and + 4.8°C.

There are many other sources of uncertainties for climate and simulation. An uncertainty typology has been developed by Arthur Petersen (2006, 2012) that differentiates the location of uncertainty (models, input data, model implementation, and output interpretation) from the nature of uncertainty (epistemic uncertainty due to incompleteness and fallibility of knowledge, and ontic uncertainty due to the intrinsic character of a natural system) and the range of uncertainty (statistical and scenario uncertainty). The typology yields an awareness of the limits of predictability, about the adequacy or inadequacy of methods, and the value-laden choices of decisions. Nevertheless, climate simulation literature often refers to other types of uncertainties, differentiating structural uncertainty (a mix of Petersen’s epistemic and ontic uncertainty), from parameter uncertainty (including tuning) and observational uncertainty. All the sources of uncertainty contribute to the forecasting uncertainty of models, in general, compelling the IPCC to introduce a “likelihood talk” for its Assessment Reports and to suggest speaking about climate projections, rather than predictions or forecasts (Bray & von Storch, 2009; IPCC, 2005).

Evaluation of Climate Models

Nevertheless, the core topic of climate simulation is the question about the “truth” of a model: How can climate models afflicted with many sources of uncertainties be evaluated? Does a model “truly” grasp the relevant aspects of a real system for which it has been conceived? The questions are particularly important because climate models are purely representational models and because there is no single best model or best approach (cf. Hulme & Dessai, 2008; Lahsen, 2005; Parker, 2009b, 2011, 2014). Against this backdrop, Oreskes, Shrader-Frechette, and Belitz (1994) differentiated main concepts, such as the verification, validation, and confirmation of models. Verification ensures the “truth” of a statement, but this absolute certainty is only deductively achievable for, and within, first-order logical statements and linear systems. With some of the conceptual models, an exact solution can be algebraically deduced for a specific context, but none of the quasirealistic climate models discussed qualifies, so the concept of verification can’t be used. Validation, often used synonymously with verification, is similarly misleading, because the term valid “might be useful for assertions about a generic computer code but is clearly misleading if used to refer to actual model results in any particular realization” (Oreskes et al., 1994, p. 642). Such an assertion can refer to the consistency of a code, designating the correct implementation of the algorithm of a model (Sargent, 2013) or a method. Similarly, the comparison of numerical results with an analytical solution, if it exists, does not say anything about the “truth” of a model, only about the convergence of the discrete approximations to the solution. For complex (nonlinear) models, not even such a comparison is possible, because an analytical solution is missing and therefore is replaced by “semi-empirical” convergence tests. In other words, it checks the stability of numerical results of a simulation run by doubling the resolution of a control run. If both runs behave stably, it is assumed that the discrete approximations converge toward the unknown analytic solution. However, such an assumption is legitimate only for linear problems (Lax & Richtmyer, 1956).

Thus, we can only talk about confirmation of models. The crucial method for confirmation is the quantitative comparison of simulation results with observational data to assess model performance. Complex models like GCMs, AOGCMs, and ESMs are usually evaluated with observational data on the system level as well as on the component and parameter levels (Randall, Khairoutdinov, Arakawa, & Grabowski, 2003). The basic tests of the system level are decadal hindcasts (for instance, for known extreme-value statistics), as well as transient climate simulations (usually 1850 to 2100) to reproduce observed climate change. If simulation results match observational data, it is said that the model is reliable.

According to AR5, “a significant development since the AR4 is the increased use of quantitative statistical measures, referred to as performance metrics,” enabling the assessment of model improvements over time (IPCC, 2013, p. 753). Performance metrics are standardized measures of benchmark experiments established by WGNE and carried out by CMIP, which allow the strengths and weaknesses of a given model to be assessed in comparison with in situ data and other models (Gleckler, Taylor, & Doutriaux, 2008; Mearns, 1997). By comparing in silico and in situ data sets of many variables of atmospheric fields, a detailed analysis of model performance is possible, and comparing the performance of various models yields a model performance index. Such an index of a multimodel ensemble (MME) reveals that the median model outperforms every other model (Pennell & Reichler, 2011; Reichler & Kim, 2008). However, by comparing many climate models, CMIP5 does pose the question of model comparability. This has sparked a debate on model weighting (Weigel, Knutti, Liniger, & Appenzeller, 2010) versus “model democracy” (Knutti, 2010). Averaging model results has led to another ensemble method—perturbed parameter ensembles (PPEs)—which tests single models. Perturbing particular parameters of a model can yield knowledge on the behavior and uncertainty of parameters (Allen, 1999; Lambert et al., 2013; Stainforth et al., 2005). Initial condition ensembles, a variation of PPEs, change the initial conditions of a climate model slightly in order to gain knowledge about the sensitivity of a model and to average the results. Combining both methods, MME and PPE, may eventually help to better assess the structural and parameter uncertainties of models (Sexton, Murphy, Collins, & Webb, 2012), but ensemble methods are computing-intensive (cf. Parker, 2010, 2013).

Problem of Tuning

The “reality check” of climate models on the system, component, and parameter levels is laborious and beset by many problems. First, fulfilling the requirements of a specific set of numerical experiments, which is always purpose-driven, does not imply any general quality. The second problem is model tuning, which is “the idea that models need to be harmonized with observations . . . to improve the representation of some aspect of the climate system” (Mauritsen et al., 2012, p. 1). Parameter fitting and adjusting, also known as model tuning or model calibration, are issues that have been long practiced and discussed in climate modeling, although not many publications highlight this aspect of modeling (Gramelsberger, 2010; Guillemot, 2010; Mauritsen et al., 2012; Petersen, 2011; Randall & Wielicki, 1997). Furthermore, some numerical parameters lack observational data and have to be adjusted (tuned). For instance, flux corrections of the top of the atmosphere (TOA) radiative imbalance have been a well-known example of tuning for many years, although they are barely accepted in the climate modeling community today. Models without flux corrections demonstrate progress in modeling, although such models have been replaced by cloud-related parameter tuning and other methods.

Besides the fictive elements of modeling, it has to be mentioned that a good fit between a model and in situ data does not necessarily make for a “good model” (cf. Edwards, 1999; Heymann, 2012; Lenhard, 2011; Shackley et al., 1998, 1999). In situ data usually do not fit model requirements (resolution, distribution, etc.), and many interactions lack observational methods and data, most prominent among them being climate variability. Furthermore, in situ data exhibit a range of uncertainties; because they are samples of incomplete spatial and temporal coverage, they propagate the limits of instrumental capabilities. Satellite data are particularly model-laden in order to make it possible for the required indirect properties to be translated and derived into useful data products. Thus, the empirical basis contains its own data uncertainties. This has led to a new evaluation method: the inverse treatment of data and models by what is called the instrument simulator. Instead of converting satellite data into “model equivalents,” “observation equivalents” are computed and compared with satellite data by simulating a virtual satellite and its records in a model (Bodas-Salcedo & Webb, 2011). On the component and parameter levels, the particular schemes are tested in isolation against in situ data with box or column models and, afterward, within the whole model (Phillips et al., 2004).


Climate models are afflicted with many uncertainties. They can’t be verified. At best, they can be evaluated only more or less properly. Nevertheless, models are the only tools that can provide insights into complex phenomena and that can extrapolate past and current trends into the future. Therefore, they have become indispensable for science and, in particular, for climate science and society facing climate change. Although quasirealistic circulation models provide the dominant, deterministic view on climate, other views on climate have been developed. Generally speaking, besides the deterministic view, the statistical and probabilistic views can be employed (Lorenz, 1969). Statistical models resample observed records and project them into the future (Slingo, 2013). Stochastic models don’t represent the development of processes that deterministic models aim at, but they try to cover the influence of processes of various scales on each other (Hasselmann, 1976; for an overview, see Franzke, O’Kane, Berner, Williams, & Lucarini, 2014). Conceptually speaking, climate models can be complex and comprehensive, like GCMs, AOGCMs, and ESMs, but they can also be more simple one- or two-dimensional energy balance models (EBMs), conceptual models, or box models (Müller & von Storch, 2004; von Storch, 2010). For instance, EMICs have been developed to bridge the gap between conceptual and comprehensive models (Claussen et al., 2002). EMICs replace some interactions of GCMs by prescribing them as external forcings.

Thus, many modelers advocate a hierarchy of models as a proper strategy for dealing with climate and simulation, because “on the one hand, we try to simulate by capturing as much of the dynamics as we can in comprehensive numerical models. On the other hand, we try to understand by simplifying and capturing the essence of a phenomenon in idealized models, or even with qualitative pictures” (Held, 2005, p. 1609; Henderson-Sellars & McGuffie, 1987, 1999; Schneider & Dickinson, 1974). An increasingly important variety of climate models results from the goal of regional and seasonal climate projections. In 1981, Charney and Shukla proposed seasonal studies on the predictability of monsoons. With regional climate models, interactions can be studied in greater detail—for example, effects of lakes and mountains (high-resolution forcings). However, studies show that finer temporal (days) and spatial (local) scales do not necessarily deliver reliable results (Dickinson, Errico, Giorgi, & Bates, 1989, Giorgi, 1990; McGregor, 1997; Takle, 1995).

Whatever type of model is chosen, it is obvious that models are, in general, purpose driven. Different purposes lead to different models by taking specific views on climate and climate change into account, as integrated assessment models, paleoclimate models, data-driven models, and carbon dioxide models do (cf. Ackerman et al., 2009; Nordhaus & Boyer, 2000; Parker, 2006; Parker et al., 2002).

Although a hierarchy of models is important, it must be asked whether the diversity and variety of model approaches are in accordance with the overall aim of science to produce reliable and comparable results. Besides the variety and diversity of models, the interdependency of the models is obvious. In the case of CGMs, they share not only the same hydro- and thermodynamic equations, but also many common ideas about parameterizations and modeling techniques. In particular, the heredity of GCMs as well as the standardization for model intercomparison are accompanied by a notable unification of models, in addition to model diversity and variety. Le Queré has identified three phases in modeling: the illusion, the chaos, and the relief phase (Le Queré, 2006). During the early phase of illusion, modelers have to tackle the lack of observations in order to evaluate the models properly. Therefore, they came along with many different modeling approaches. This leads to the chaos phase, which base modeling on observational data. Thus, the diversity of approaches exhibits a lack of accordance with observational data. Nevertheless, “the chaos phase is the most creative and beneficial period in the development of models. . . . Modelers are, on the contrary, driven by the possibility of exploring new dimensions, unconstrained even by observations” (Le Queré, 2006, p. 499). Finally, the relief phase leads to good understanding of the underlying concepts and good agreement with measurements, as well as increasing unification of the formerly diverse modeling approaches.

From this course, the interdependency of models results. Although a lot has been written about climate and simulation—in climate science itself, but also in philosophy of science and environmental humanities—the process of modeling, how progress in model-mediated knowledge is gained, and whether model interdependency strengthens or weakens knowledge production have not been sufficiently explored. The explanation for this lack is rooted in the novelty of numerical modeling and simulation, influencing progress of scientific knowledge since the 1970s, but effectively since the 1990s (Barbrousse, Franceschelli, & Imbert, 2009; Dowling, 1999; Giere, 2009; Heymann, 2010; Morrison, 1998; Winsberg, 2001, 2010). However, it is also rooted in the increasingly abstract work of scientists who sit at computers and program models, conduct observational experiments with remote devices, and analyze empirical as well as in silico data stored in the same database. Even field studies have inevitably become dependent on computers and (data) models. Hence, climate and simulation are a good example of how science has been changed by the use of computer-based simulation (Heymann, Gramelsberger, & Mahony, 2017), and the changes are not easily understood by non-modelers (Kitcher, 2010; Oreskes & Conway, 2010; Somerville & Hassol, 2011). As Paul Edwards claimed, there are three types of models that enable research on climate: “simulation models of weather and climate; reanalysis models, which recreate climate history from historical weather data; and data models, used to combine and adjust measurements from many different sources” (Edwards, 2010). Considering this, it becomes clear that climate models are just one side of the coin, and data models are the other side. Both constitute the currency of a simulation-driven science, but in the case of climate science, this is leading to major sociopolitical debates (Edwards, 2001; Hulme, 2009, 2011; Kuik et al., 2008; Otto, Frame, Otto, & Allen, 2015; Pielke, 2007; Polley, 2010; von Storch, 2009).


The author thanks Martin Mahony, Matthias Heymann, and Johann Feichter for helpful comments and many discussions on this topic during past years. Thanks, also, to the reviewers for their beneficial comments.


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