VERNER E. SUOMI
University of Wisconsin
Many of the papers in this volume express concern over the possibility of inadvertent climate modification due to the activities of man, particularly through the production of large amounts of energy. It is easy enough to see how these activities of man could influence climate, but it is quite another thing to prove that man has in fact influenced climate. On a global scale, even the most generous estimates of future energy release by man would only represent noise superimposed in the context of the global heat budget. The present uncertainty in the solar constant (1.95 to 2.00 cal/cm2 min) completely dwarfs many of the concerns raised in other parts of this document.
During the GARP Atlantic Tropical Experiment (GATE) the geostationary satellite SMS-1 over the Atlantic showed an enormous dust cloud coming from the Sahara Desert and extending across the Atlantic. Simple calculations show that the effect on the absorption of solar energy of such sources of pollution completely dominate man-made effects. In a global context, nature’s energy sources overwhelm man’s sources. However, regional and local scales are quite another matter. It is possible that regional and local changes due to man’s activities could trigger a global change in the atmosphere’s circulation, but we cannot be certain.
A large part of our difficulty in attempting to assess man’s possible effects stems from the fact that we have such a poor data base that we do not yet adequately under stand natural climatic phenomena. To answer the questions raised in this volume, we need to understand the dynamics of climate. The purpose of this paper is to explain the need for climate monitoring and to outline a way to carry out such monitoring.
The mechanisms that produce and control the earth’s climate are exceedingly complex. One usually considers climate as mainly an atmospheric phenomenon, but it is the atmosphere’s interaction with the ocean, the land, and ice masses, together with the sun and space, that controls the atmosphere’s behavior. If we are to assess man’s possible influence on climate and predict what man’s activities might do to our climate, we must first understand the basic mechanisms and physics of climate well enough to model it. This is beyond our grasp at present.
One might argue that if we cannot presently model climate then we should at least try to measure it. Observa-
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Studies in Geophysics: Energy and Climate 8 The Need for Climate Monitoring VERNER E. SUOMI University of Wisconsin Many of the papers in this volume express concern over the possibility of inadvertent climate modification due to the activities of man, particularly through the production of large amounts of energy. It is easy enough to see how these activities of man could influence climate, but it is quite another thing to prove that man has in fact influenced climate. On a global scale, even the most generous estimates of future energy release by man would only represent noise superimposed in the context of the global heat budget. The present uncertainty in the solar constant (1.95 to 2.00 cal/cm2 min) completely dwarfs many of the concerns raised in other parts of this document. During the GARP Atlantic Tropical Experiment (GATE) the geostationary satellite SMS-1 over the Atlantic showed an enormous dust cloud coming from the Sahara Desert and extending across the Atlantic. Simple calculations show that the effect on the absorption of solar energy of such sources of pollution completely dominate man-made effects. In a global context, nature’s energy sources overwhelm man’s sources. However, regional and local scales are quite another matter. It is possible that regional and local changes due to man’s activities could trigger a global change in the atmosphere’s circulation, but we cannot be certain. A large part of our difficulty in attempting to assess man’s possible effects stems from the fact that we have such a poor data base that we do not yet adequately under stand natural climatic phenomena. To answer the questions raised in this volume, we need to understand the dynamics of climate. The purpose of this paper is to explain the need for climate monitoring and to outline a way to carry out such monitoring. The mechanisms that produce and control the earth’s climate are exceedingly complex. One usually considers climate as mainly an atmospheric phenomenon, but it is the atmosphere’s interaction with the ocean, the land, and ice masses, together with the sun and space, that controls the atmosphere’s behavior. If we are to assess man’s possible influence on climate and predict what man’s activities might do to our climate, we must first understand the basic mechanisms and physics of climate well enough to model it. This is beyond our grasp at present. One might argue that if we cannot presently model climate then we should at least try to measure it. Observa-
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Studies in Geophysics: Energy and Climate tions of a whole host of parameters describing climate have been collected for centuries. These observations and other indirect but equally valid ones definitely show that climate varies on a short time scale and can change significantly on longer time scales. These facts have been especially well summarized elsewhere in this volume and in two recent reports, Understanding Climatic Change: A Program for Action (U.S. Committee for the Global Atmospheric Research Program, 1975) and The Physical Basis for Climate and Climate Modelling (WMO/ICSU, 1975). These documents recommend many new monitoring observations from satellite platforms. Space technology has made truly global observations possible for the first time. Because spacecraft carry the same instruments over different parts of the earth, regional differences and variations should be more easily detected. Despite these important advantages there are also significant limitations. Obviously, spacecraft orbit well above the earth’s atmosphere, and certain common climatic parameters such as temperature can only be inferred from the electromagnetic radiation emanating from the atmosphere below. One can hardly expect to achieve the same intrinsic accuracy that an in situ thermometer would achieve. On the other hand, other quantities such as the extent of sea ice and possibly even the thickness of sea ice can, at least in principle, be far better determined from space than is economically feasible using observers on the earth. With these new possibilities for climate monitoring, will we be able to measure man’s influence on climate? Perhaps, but most likely not. The difficulty arises from the fact that changes in climate that are significant in their effects on man may be scarcely detectable on a global scale. The situation is not quite so difficult on the regional scale. Regional changes are often larger in magnitude but compensated for by changes in the opposite direction in other areas. Thus small global changes that are difficult to measure may be manifested in regional shifts that we can observe quantitatively. Even though our ability to monitor the climate of the earth is very much better now than ever before, and even though this new capability will make it possible to obtain a better set of observations of many key climatic parameters, it does not appear possible at present to measure how man is changing the global climate. On the other hand, it may be possible to measure regional climate changes because these changes are larger. We are already certain that we can observe changes in local climate because that has already been done without space platforms. We probably can obtain certain local observations even better with them. On the local scale, we may even be able to separate the changes caused by man from those caused by nature. The most productive approach to devise a climate observing system is through a combination of modeling and monitoring. One of the most successful accomplishments of the Global Atmospheric Research Program (GARP) so far is the clear specification of what is required to describe the initial state of the atmosphere so its short-term transient behavior (i.e., the weather) can be predicted. Numerical atmospheric simulation schemes, often called numerical models, require that the state of the atmosphere be specified at some initial time for several levels over several thousand grid points spaced over the earth. The specification of the atmospheric state can be real (from measurements) or fictitious (from guesses) or both, but the data set cannot be empty or only partly filled. A model that simulates atmospheric behavior is an especially powerful tool not only because it can predict the future weather, but, equally important, it specifies what observations are the key ones. Moreover, sensitivity tests with the model can be used to learn how good, how often, and how closely space d these observations must be. These modeling tools have had a great influence on our meteorological satellite program. What can be measured from space must not only be novel, it must also be useful. Sensitivity tests indicate just how useful the observations will be. For example, our ability to observe the atmosphere’s initial state is now considered promising enough—on the basis of model experiments—to warrant a large international cooperative program—the First GARP Global Experiment (FGGE)—which will be conducted in 1978–1979. The status of climate modeling in the late 1970’s is not so fully developed as weather modeling was in the late 1960’s when GARP was first proposed. The problem is that we do not have our theory of climate in as good order as our theory of weather was at that time. Stated simply, the short-term future behavior of our atmosphere depends on its present physical arrangement and almost unchanging boundary conditions. But the present state of the atmosphere is unimportant for climate because the atmosphere soon “forgets” its present state. Its statistical behavior depends more on the boundary conditions, and the transient behavior of the atmosphere can slowly change the boundary conditions. Such feedback mechanisms can be positive or negative. In summary, models give us the basis for determining what observations are required for the study of climate and how well they must be obtained. These same questions have been looked into in detail at several study conferences. What follows has been extracted from those conference reports at which the author was a participant. These have been summarized and added to in an attempt to present the latest consensus on the observing requirements. In doing this, the author acknowledges the contributions of the other participants in these meetings (listed in the referenced reports) and assumes full responsibility for any change in emphasis, deliberate or inadvertent, that such a synopsis may incur. If one takes the time to read all the recent documents on the requirements for a global climate observing system, one comes away with two strong impressions. First, these documents do not read like novels. Secondly, there is a considerable difference between what the modelers want and what has been proposed as feasible by those familiar with how the observations might be obtained. Part of this confusion stems from the lack of a satisfactory theory of climate and part from our ignorance of the best approach
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Studies in Geophysics: Energy and Climate to obtain the understanding needed to synthesize such a theory. The author proposes to use as a framework the latest document on this subject, the report of the October 1975 Tokyo meeting of the Joint Organizing Committee for GARP (ICSU/WMO, 1975). First, it represents in my view the clearest statement of the best strategy to be used in developing an understanding of climate. The strategy, of course, will control the observation program. Secondly, it provides a useful definition of the difference between observations that are strictly climate monitoring and those observations needed to understand and test the models, i.e., their representation of the physical basis of climate. Some detailed proposals for components of a monitoring program will then be made where appropriate. As a basis for observing climate and its determining factors, we must adopt some definition of the climate system. One conceptual scheme that is widely accepted distinguishes between an internal system and an external system. The internal system includes those variables whose interrelationships are well enough understood to be modeled quantitatively. Specifically, the following may be termed internal variables: The atmosphere’s dynamic quantities (wind, temperature, pressure, humidity); Clouds and precipitation; The motions of the world ocean; The formation and motion of sea ice; The hydrological cycle; The biomass. In contrast, the external system consists of those quantities that cannot now be modeled and predicted quantitatively. They must therefore be monitored and specified as fixed conditions in numerical models. Some of these are truly external for time scales up to a century or so: Solar flux; Surface characteristics such as land or ocean bottom, topography roughness, vegetation albedo, ice-sheet configuration, etc. Other factors should really be dealt with as part of the internal system but will, for the time being, be considered as external parameters in the climate models either because of inadequate knowledge at present about their proper treatment as internal variables or because such a separation of the problems for the time being appears feasible. These include the following: Atmospheric aerosols; Optically active minor constituents in the atmosphere, particularly carbon dioxide and ozone. It is understood that these working definitions will be modified in the future when our understanding of sources, sinks, and transport mechanisms will allow prediction of the time variations of these factors as part of the internal system. If we are concerned with the shortest time scale, i.e., annual and interannual variations, it may be sufficient to consider only the uppermost part of the oceans as belonging to the internal system. In such a case even the basic characteristics of the deep-sea circulation would be assumed not to change and to be given as external parameters in the model. These internal and external elements form a single climatic system. We define the climatic state as the average behavior of this system, as characterized by the statistics of its variables over some specified period of time in some specified domain of the earth-atmosphere system. The time interval is understood to be considerably longer than the life span of individual weather systems and longer than the period over which the behavior of these sy stems can be predicted. A climate monitoring system must really observe both the external system and the internal system. The reasons for the first have already been given. The reason for the second is that we must monitor the climate state to assess how the model is performing and how it might be improved. The documents we have referred to earlier call this category of observations global data sets. Now if we can refine our specification of both the external parameters and those that account for the climate state into two unique classes, man-originated and nature-originated, we have a basis for separating the inputs and thus for determining the magnitude of change in the outputs of our model. These outputs might be very sensitive to man-made inputs if there is positive feedback in the model (and in nature), or man-made inputs might result in a trivial change in output if man’s contribution compared with that of nature is small or if there is negative feedback in this part of the model mechanism. There are several ways in which man’s input can be monitored. One can add a source inventory to the data base. This approach has been successful in pollution control. In other instances, trace gases or particulates in the atmosphere are uniquely man-made and can be directly measured. The global inventory is still difficult in this latter case because of the enormous dilution of the atmosphere. In still other instances one can institute patrols from space platforms. Forest fires, industrial particulate plumes, thermal pollution in waterways, and other small-scale sources might be detected from space because concentrations are so large and meaningful signals can be obtained even from the distances needed for space surveillance. The idea here is to measure the parameters before they are diluted by the atmosphere. The advantage of the patrol approach is that the signals are large on the regional and local scale, but they are exceedingly small on the global scale. An extremely important aspect of the entire climate monitoring activity is the data-processing effort required. It is possible for the secrets of nature to be hidden in a flood of data as well as in nature. Clearly, we need information more than we need data.
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Studies in Geophysics: Energy and Climate FIGURE 8.1 Design of a typical environmental data system. Even a superficial assessment of the data presently needed for adequate climate monitoring is staggering. A single day’s worth of images from only one geostationary satellite will yield 1011 bits of information. This number is large enough to record the name and address of every person on earth with enough space left over to give each one a telephone number besides. Figure 8.1 shows the design of a typical environmental data system that is used to screen data gathered mainly for other purposes, i.e., as weather forecasting. In the design of a climate monitoring system there are two things wrong with the diagram. First, in typical use the flow is from left to right. However, in the design of a system, the task we are considering now, the order should be just the reverse. What is desired, i.e., the output requirements of the system, should be considered first. Then and only then will it be possible to say what data are required to get needed information. Designing a system in the form of Figure 8.1 does not provide any assurance of getting the needed information. Secondly, the data-processing system cannot be independent of the data-collection system. In the case just discussed, an imaging geostationary satellite, there are extremely large quantities of data with low information density. However, for most purposes, small amounts of data having high information density are required. To assemble such subsets of data, it is necessary to sort through the entire data set, retaining information along a certain “path” through it Obviously the various paths that can be taken to increase the information density depend on the information desired, i.e., time, space, parameter. In fact, as will be shown later, knowing what data are desired can greatly simplify the climate monitoring systems. In the system depicted in Figure 8.1, one typically uses large computers and batch processing. In a well-designed data-processing system, a small minicomputer or microprocessor can be an integral part of the data system. The cost and usefulness differences between these two routes can be enormous. The cost ultimately controls what is actually possible. Requirements for a viable climate monitoring system must take account of this fact. Debate and negotiation between the modelers who want the information and the observers who will design and operate the system that will collect it is absolutely essential. Some requirements may since have been relaxed, while others may have been strengthened. We have made these remarks to indicate the formidable task that faces us. We do so to warn the technologists who are anxious to get on with the task and to gain some sympathy and understanding from the modelers who are so anxious for the information. Clearly, the dialogue that has started in the climate program planning sessions must be continued; the interface between what modelers desire and what technology can provide is not sharp and clear. In some instances, it is foggy and some might even say murky. Members of each group must make the effort to reach some distance into the other’s area. Only if effective negotiation goes on between the two basic groups will there evolve a system that meets the needs of the program and that can be held within the bounds of available resources. We have already demonstrated a capability to do this in preparation for the FGGE. The requirements for a monitoring system are exceedingly complex. This complexity is demonstrated in Table 8.1, which was adapted from the summary of the Joint Organizing Committee (JOC) on the formation of data sets needed for studies in climate dynamics. We have simplified the table to show which data need be included in the global data sets and where in GARP Publications Series No. 16 (WMO/ICSU, 1975) these requirements and possible solutions can be found. TABLE 8.1 Adapted from Summary of the JOC Recommendations on the Formation of Data Sets Needed for the Climate Dynamics Subprogram Variable Reference to Tables in GPS No. 16a Total solar flux 6.2 Solar uv flux 6.2 Net radiation budget 6.2 Cloudiness 6.2 Sea-surface temperature 6.3 Surface albedo 6.3 Precipitation over oceans 6.4 Soil moisture 6.4 Water runoff 6.4 Heat content of the upper layer of ocean 6.5 Wind stress 6.5 Sea level 6.5 Near-surface currents 6.5 Deep-ocean circulation 6.5 Extent of snow 6.6.1 Extent of sea ice 6.6.1 Sea-ice melting 6.6.1 Drift of sea ice 6.6.1 Thickness of polar ice sheets 6.6.2 Deformation of polar ice sheets 6.6.2 Change of boundary of polar ice sheets 6.6.2 Water vapor 6.7 CO2 6.7 Ozone distribution 6.7 Tropospheric aerosols 6.7 Atmospheric turbidity 6.7 Stratospheric aerosols 6.7 aWMO/ICSU (1975).
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Studies in Geophysics: Energy and Climate Some of these parameters that may have a man-made component and a few ideas on how they could be measured are discussed below. Net Radiation Budget All components of this can be measured accurately from space. To detect variations significant for climate, emphasis should be placed on stable instruments and vehicles with long useful lifetimes. Man contributes to the radiation budget by release of heat and by changing surface characteristics, topics treated elsewhere in this volume. These can be monitored to some extent from space. Although trivial in terms of energy, artificial light may be statistically related to total energy release with sufficient reliability to be a useful monitoring tool, since artificial light can be easily detected from space at night. This approach might provide a simple means for monitoring changes in human energy consumption patterns. Surface Albedo Man changes surface albedo through deforestation, urbanization, grazing, agriculture, etc. These changes also affect evapotranspiration and surface roughness. All of these factors can be monitored by multispectral remote sensing. Soil Moisture This can be roughly determined from space by passive microwave radiometry. Irrigation, agricultural practices, and large-scale hydrological works of man have significant impact on soil moisture, which in turn affects the albedo, surface temperature, and moisture flux relevant to climate. Both human activities and their consequences in terms of soil moisture should be monitored. Water Runoff River flow statistics are sensitive indicators of climate variations over the continents and also are influenced by man-made changes in the land. It is difficult to acquire such data directly from space, but satellite communication may make feasible the collection of data from isolated locations. Carbon Dioxide It is not possible to identify uniquely the contribution of man except by source monitoring and spectral analysis. Of particular value in carbon dioxide monitoring will be measurement of the vertical distribution on a global basis with a view to identification of sources, sinks, and transport mechanisms. This knowledge might clarify the roles of the land biota and the oceans in the atmospheric carbon cycle. Ozone There is no significant contribution of ozone by man on a global scale. However, ozone is critical in the human environment because of its role in screening out damaging ultraviolet components of the solar system. Other trace gases can act as catalysts to reduce ozone concentration. It is therefore important to monitor this gas. Tropospheric Aerosols These represent a mixture of natural and man-made particles and can be monitored from space. Some estimates of the proportion due to man may be possible by relating contrast changes due to aerosol loading with known patterns of human activities. Atmospheric Turbidity This can be monitored through the depolarization of sunlight by aerosols [see, for example, page 208 of Remote Measurement of Pollution (NASA, 1971)]. It is also possible to monitor turbidity from the changes in apparent contrast of surface targets of known intrinsic contrast [McLlellan in NASA (1971)]. Stratospheric Aerosols Concern has been expressed on possible increases in stratospheric aerosols due to high-altitude aircraft operations, Limb-scanning techniques in the infrared can be used to search for features due to aerosols. Measurements of the solar disk and the aureole in two wavelengths can give both the real and the imaginary parts of the refractive index, together with particle size. Trace Gases Limb scanning with an interferometer-spectrometer as done by Rudolf Hanel of the NASA Goddard Space Flight Center (personal communication) in planetary investigations can provide information. REFERENCES ICSU/WMO (1975). Report of the Eleventh Session of the Joint Organizing Committee, Tokyo, Oct. 1–8, 1975, Global Atmospheric Research Programme. National Aeronautics and Space Administration (1971). Remote Measurement of Pollution, Scientific and Technical Information Office, NASA, Washington, D.C. U.S. Committee for GARP (1975). Panel on Climatic Variation, Understanding Climatic Change: A Program for Action, National Academy of Sciences, Washington, D.C. WMO/ICSU (1975). World Meteorological Organization and International Council of Scientific Unions, The Physical Basis of Climate and Climate Modelling, GARP Publ. Series No. 16, Geneva.