Understanding Climatic Change A Program for Action (1975) / Chapter Skim
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PHYSICAL BASIS OF CLIMATE AND CLIMATIC CHANGE
Pages 13-34

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From page 13...
... The properties of the climatic system may be broadly classified as thermal properties, which include the temperature of the air, water, ice, and land; kinetic properties, which include the wind and ocean currents, together with the associated vertical motions, and the motion of ice masses; aqueous properties, which include the air's moisture or humidity, the cloudiness and cloud water content, groundwater, lake levels, and the water content of snow and of land and sea ice; and static properties, which include the pressure and density of the atmosphere and ocean, the composition of the (dry) air, the oceanic salinity, and the geometric boundaries and physical constants of the system.
From page 14...
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From page 15...
... The upper layers of the ocean interact with the overlying atmosphere on time scales of months to years, while the deeper ocean waters have thermal adjustment times of the order of centuries. The cryosphere, which comprises the world's ice masses and snow deposits, includes the continental ice sheets, mountain glaciers, sea ice, surface snow cover, and lake and river ice.
From page 16...
... Physical Processes of Climate The climate at any particular time represents in some sense the average of the various elements of weather, along with the state of the other components of the system. The physical processes responsible for climate (as distinct from climatic change)
From page 17...
... Although the equations referred to above are fundamental in that they form the basis of our ability to simulate numerically the climate with dynamical models, they are not in themselves particularly revealing as far as the more subtle physical processes of climate are concerned, to say nothing of the processes of climatic change. The heating rate is itself highly dependent on the distribution of the temperature and moisture in the atmosphere and owes much to the release of the latent heat of condensation during the formation of clouds and to the subsequent influence of the clouds on the solar and terrestrial radiation.
From page 19...
... Climatic variation. This is defined as the difference between climatic states of the same kind, as between two Januaries or between two decades.
From page 20...
... It should be noted that we have included the oceans in the definition of a climatic state, as well as information on other aspects of the physical environment. The ensemble of statistics required to completely describe a climatic state is presently available for only a few regions and for limited periods of time.
From page 21...
... Just how much of the recorded paleoclimatic variations may eventually be accounted for by such effects, however, is not known, and applying climatic models to the systematic reconstruction of the earth's climatic history prior to about 10 million years ago is an important component of the research program recommended in this report (see Chapter 6)
From page 22...
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From page 23...
... , must also be considered an external cause of climatic variation. These considerations lead to the "physical" definition of climate as the equilibrium statistical state reached by the elements of the atmosphere, hydrosphere, and cryosphere under a set of given and fixed external boundary conditions.
From page 24...
... , is probably the best known example of a semipermanent positive feedback process, although other positive feedbacks of climatic importance may be noted. One of these is the snow cover -- albedo -- temperature feedback, in which an increase of snow extent increases the surface albedo and thereby lowers the surface temperature.
From page 25...
... Climatic Noise Climatic states have been defined in terms of finite time averages and as such are subject to fluctuations of statistical origin in addition to the changes of a physical nature already discussed. Since these statistical fluctuations arise from the day-to-day fluctuations in weather (the autovariation of the atmosphere identified in Figure 3.3)
From page 26...
... This warm surface layer represents a large reservoir of heat and acts as a significant thermodynamic constraint on the atmospheric circulation. The exchange of the ocean's heat with the atmosphere occurs over a wide range of time scales and largely determines the relative importance of other physical processes in the ocean for climatic change.
From page 27...
... Modeling the Oceanic Circulation The systematic examination of the various mechanisms and feedbacks by which the oceanic thermal structure and circulation are maintained on various time scales is largely a task for the future. In this research, it will be necessary to conduct intensive observational programs in order to gain greater understanding of the various oceanic physical processes themselves and to construct numerical models of the oceanic circulation in which these processes are correctly represented.
From page 28...
... Until the dynamics of this oceanic boundary layer are better understood, our ability to model climatic variations on any time scale will remain seriously limited. SIMULATION AND PREDICTABILITY OF CLIMATIC VARIATION Climate Modeling Problem From the above remarks it is clear that the problem of modeling climatic variation is fundamentally one of constructing a hierarchy of coupled atmosphere -- ocean models, each suited to the physical processes dominant on a particular time scale.
From page 29...
... useful in the calibration of the simpler models, but they are essential to the detailed diagnosis of the shorter-period climatic states that are in approximate statistical equilibrium with slowly changing boundary conditions. A fundamental approach to the problem of modeling climate and climatic variation must proceed through the consideration of dynamical models of the coupled components of the climatic system.
From page 30...
... regarded as the sole component of the climatic system, and with all external boundary conditions held fixed, there is, in spite of our physical expectations, no assurance that there will be a climate in the sense that time series generated by the atmospheric changes will settle into a statistically steady state; and no assurance that the climate, if it exists, is unique in the sense that the statistics are independent of the initial state. It is therefore useful to define a random time series (or the system generating such a series)
From page 31...
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From page 32...
... . What may appear to be a climatic transition on one time scale may become the natural noise level of a climatic state defined over a longer interval.
From page 33...
... While these considerations do not directly address the physical basis of climatic change, they are nevertheless basic to our view of the predictability of climatic change. What parts of climatic variations on various time scales are potentially predictable, and what parts are just climatic noise?
From page 34...
... The related problem of forecasting specific seasonal and annual climatic variations rests upon the same physical basis and may prove more difficult to solve. To reach these goals will require the coordinated use of all our research tools, whether they be observational, numerical, or theoretical.


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