By 1965, three groups in the United States had established ongoing efforts in general circulation modeling:
At this point, AGCMs and modeling techniques began to spread by a variety of means. Commonly, new modeling groups began with some version of another group's model. Some new groups were started by post-docs or graduate students from one of the three original AGCM groups. Others built new models from scratch. An AGCM family tree offers a visual map of these relationships, with links to some of the models and modeling groups.
Among the important AGCM groups established in 1965-75 were:
Each group initially borrowed an existing model, but subsequently made significant modifications of its own.
Two important innovations of the 1965-75 decade were coupled atmosphere-ocean models and spectral transform techniques.
Coupled atmosphere-ocean models
The ocean's enormous heat capacity makes it a key component of the total climate system. Because of their short forecast times, AGCMs used for numerical weather prediction need not take ocean effects into account. When used as climate models, however, AGCMs must somehow include ocean effects. The ocean also has a global circulation, which may be modeled.
GFDL was among the first groups to attempt coupling of an atmospheric GCM to an ocean model. Initially, highly simplified ocean models (1-layer "swamp" oceans) were used. These were succeeded by 2-level "mixed-layer" ocean models. In 1969, Manabe and Bryan published the first results from a coupled ocean-atmosphere general circulation model (OAGCM).[1] However, this model used a highly idealized continent-ocean configuration. Results from the first coupled OAGCM with more realistic configurations were published in 1975.[2]Spectral transform techniques
Spectral methods are an alternative to finite difference schemes, the method used by all of the first-generation primitive-equation AGCMs. They express the horizontal variation of dynamic model fields in terms of orthogonal spherical harmonics. This technique simplifies the solution of many of the nonlinear partial differential equations used in general circulation modeling. Essentially, elements of the models are converted from physical grid had been identified as early as 1954.[3] Heavy calculational demands made them unsuitable for use in early AGCMs. Faster computers, and improvements in spectral methods that reduced their calculational intensity, led to their adoption in AGCMs around 1970.[4]
During this period, studies of the effect of changing carbon dioxide concentrations on the Earth's radiative equilibrium began in earnest, as data from Mauna Loa continued to show steady CO2 increases. The first studies used simpler models, rather than AGCMs.[5] Responses to CO2 doubling became the standard form of this experiment. The first use of an AGCM to study the effects of carbon dioxide doubling came in 1975.[6]
During this period, anthropogenic effects on climate were usually considered under the rubric of weather modification, which had been among the stimuli for early efforts in weather modeling. Literature on the subject frequently uses the phrase "inadvertent climate modification" in discussing anthropogenic climate change, to make the parallel.[7]
SCEP and SMIC
With the rise of the environmental movement in the early 1970s came early interest in world-scale environmental problems. Two important studies, both prepared as input to the 1972 UN Conference on the Human Environment, noted the possibility of "inadvertent climate modification." The Study of Critical Environmental Problems (SCEP) focused on pollution-induced "changes in climate, ocean ecology, or in large terrestrial ecosystems." It cited AGCMs as "indispensable" in the study of possible anthropogenic climate change.
The Study of Man's Impact on Climate (SMIC) also endorsed AGCMs. (The section on this subject was drafted by Syukuro Manabe.) Both SCEP and SMIC recommended a major initiative in global data collection, new international measurement standards for environmental data, and the integration of existing programs to form a global monitoring network. These reports are widely cited as origin of public policy interest in anthropogenic climate change.[8]Other Issues
In the early 1970s, several other large-scale atmospheric issues rose to public awareness. Notable among these were stratospheric ozone depletion, acid rain, and upper-atmosphere pollution problems raised by the supersonic transport.
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[1]
S. Manabe and K. Bryan, "Climate Calculations with a Combined
Ocean-Atmosphere Model," Journal of the Atmospheric Sciences
26 (1969): 786-789.
[2]
S. Manabe, K. Bryan, and M.J. Spelman, "A Global Ocean-Atmosphere
Climate Model: Part I. The Atmospheric Circulation," Journal of
Physical Oceanography 5, no. 1 (1975): 3-29.
[3]
G.W. Platzman, "The Spectral Form of the Vorticity Equation,"
Journal of Meteorology 17 (1960): 653-644.
I.S. Silberman, "Planetary Waves in the Atmosphere," Journal of
Meteorology 11 (1954): 27-34.
[4]
A.J. Robert, "The Integration of a Spectral Model of the Atmosphere
by the Implicit Method," in Proceedings of the WMO IUGG Symposium
on Numerical Weather Prediction in Tokyo, Japan, November 26-December
4, 1968, eds. World Meteorological Organization and International
Union of Geodesy and Geophysics (Tokyo: Meteorological Society of
Japan, 1969), VII-9-VII-24.
S.A. Orszag, "Transform Method for Calculation of Vector-coupled
Sums: Application to the Spectral Form of the Vorticity Equation,"
Journal of Atmospheric Sciences 27 (1970): 890-895.
W. Bourke, "A Multi-Level Spectral Model. I. Formulation and
Hemispheric Integrations," Monthly Weather Review 102 (1974):
687-701.
E. Eliasen, B. Machenhauer, and E. Rasmussen, "On a Numerical Method
for Integration of the Hydrodynamical Equations with a Spectral
Representation of the Horizontal Fields" (Institut for Teoretisk
Meteorologi, Köbenhavns Universitet, Denmark, 1970).
[5]
S. Manabe and R. Wetherald, "Thermal Equilibrium of the Atmosphere
with a Given Distribution of Relative Humidity," Journal of the
Atmospheric Sciences 24 (1967): 241-259.
[6]
S. Manabe and R.T. Wetherald, "The Effects of Doubling the CO2
Concentration on the Climate of a General Circulation Model,"
Journal of Atmospheric Sciences XXXII, no. 1 (1975): 3-15.
[7]
National Research Council, "Weather and Climate Modification"
(Washington, D. C.: National Academy of Sciences, 1966).
Study of Man's Impact on Climate, Inadvertent Climate
Modification (Cambridge: MIT Press, 1971).
[8]
Study of Critical Environmental Problems, Man's Impact on the
Global Environment (Cambridge, MA: MIT Press, 1970).
Study of Man's Impact on Climate, Inadvertent Climate
Modification (Cambridge: MIT Press, 1971).
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