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The Earth as a System- A Global Perspective for Future Planning INTRODUCTIONOBJECTIVES AND GRAND THEMES While rapid progress is being made in several of the earth sciences, outstanding problems will remain in 1995 and persist indefinitely. In this chapter the task group presents a synthesis of objectives, based on four grand themes: the surface, crust, and interior of the Earth; the atmosphere and oceans, including the hydrologic cycle; the biosphere; and the impact of mankind. These themes provide a philosophical basis for the necessary measurements and experiments over the years 1995 to 2015. The themes arise from the fact that in order to understand the inte- grated functioning of the Earth as a system it will be necessary to abandon the conventional subdivisions of earth science for an integrated study of processes. There are elements of each of the traditional disciplines involved in understanding the major ques- tions regarding the Earth, its origin, evolution, structure, and present operation. There are also elements in common in the measurement programs required to address these issues. The complex of scientific issues discussed in earlier sections can be user! to establish a base for a coherent plan of action. The task group draws upon these statements of progress and problems to attempt a synthesis through the identification of grand themes 52

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53 that encompass the many threads of scientific investigation of the Earth. The themes are mostly concerned with changes and interactions, which implies that we must have an understanding of the baselines. Clearly, the Earth is not a steady-state system, and must be viewed as evolving. This evolution can be seen as an ongoing proce - , where the basis of our extrapolation- both forward and backward is the present. The necessity for extrapolation has widely differing time scales and reliabilities for different parts of Earth. The evolution of the Earth can also be viewed as a comprehensive process, starting from its formation out of the solar nebula and leading eventually to a state of stable stratification as internal energy sources run down (as they have on the Moon). This view of the Earth must depend to an appreciable degree on a comparative planetological approach. Conventional wisdom presents the Earth as a roughly steady- state system, with oscillations about its mean state and occasional wild excursions. Nearly all human activities implicitly assume this steady state. But a cursory examination of the historical record indicates we are on a one-time binge of a couple of centuries with respect to population and petroleum, and perhaps in other areas of comparable importance, such as arable soil. I,onger time scales are associated with oscillations in climate (104 to 106 years), and in the solid Earth (106 to 109 years). These characteristics are oversimplifications; the real Earth has appreciable oscillations, both endogenic and exogenic, over a wide range of time scales. Most striking are catastrophic events, such as volcanic eruptions, asteroid impacts, and earthquakes. The geologic record indicates that on a million-year time scale events occur that are thousands of times as energetic as the Mount St. Helens outbursts, with short-term global consequences for the climate. Climate variation on the lOO,OO~year time scale ~ dorn~nated by the waxing and waning of glaciers. Currently the Earth ~ enjoy- ing an unusually warm period. The temperature variation inferred from oxygen isotopes of deep-sea cores appears to be correlated with variations in the Earth's orbit, but to have appreciable non- linear enhancement. This problem may be solvable with global observation of the temporal variations of the Earth's albedo, sea surface temperature, and other relevant parameters in response to the milder seasonal variations in our time. Study of Venus indicates that the present Earth may be radi- cally different from its early environment. It is essential to study

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54 early Earthto determine whether its atmosphere was reducing. When did the oxidizing environment occur? Did it coincide with the development of life? Did this change coincide with the onset of plate tectonism? Are these connected in time? Are they cause and effect? The contemporary behavior of the solid Earth is also anoma- lous in that there are now an exceptional number of continents compared to that typical in the Phanerozoic (the last 600 million years). Geologic evidence also indicates significant variations of plate tectonic rates and patterns on time scales of 10 to 100 mil- lion years. These oscillation are evidence of the mantle dynamic system and its interaction with the lithosphere. Again, under- standing must be advanced by observations of the present state plus extrapolation based on theoretical modeling and the geologi- cal record. Imposed on this general evolution of the natural Earth is the rapidly expanding effect of man on the landscape. Better understanding of this impact is a major scientific interest as well as a matter of great practical concern. The themes identified below grow out of the need for such understanding. Necessity drives earth scientists to ask for a variety of measure- ments simultaneous, continuous, and on a worldwide basis- from the obvious global tool, artificial satellites. Because satellites in orbit are external to the Earth, the answers they give are incomplete and must be supplemented by measurements at closer range or in situ, by laboratory experiments, and by theoretical modeling. Our discussion below identifies the global issues for each grand theme, and then specifies the measurements required. GRAND THEME 1: STRUCTURE, EVOLUTION, AND DYNAMICS OF THE EARTH'S INTERIOR AND CRUST Global Issues About 70 percent of the Earth's mass is mantle, the rocky region between the crust and core. A leading problem of solid earth science can be described as mantle climatology: the description of variations in composition and physical properties of regions of the mantle, how these heterogeneities relate to the dynamics, and the resulting evolution over the eons. The crust ~8 very much dependent on the dynamics and evolution of the mantle. The crust is a region of importance greatly out of proportion to its

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ss mass since it is the intermediary of the solid Earth for the several interactions with the hydrosphere, atmosphere, and biosphere. Somewhat more separate is the core, mostly fluid, whose principal manifestation is the Earth's magnetic field. The geoid and detailed seismic imaging show that the mantle is inhomogeneous, both radially and laterally. Geochemical data indicate that there are ancient reservoirs in the mantle, but their locations and relation to the seismic inhomogeneities are unknown. The geophysical and geochem~cal data must constrain the style of mantle convection and contribute to the understanding of earth evolution and the nature of the energy sources. The lithospheric plates are the cold, surface boundary layers of mantle convection cells, but several aspects of mantle convec- tion are poorly understood. These include the energy source- primordial heat or radioactivity and its distribution. Many plate boundary phenomena are also ill understood; most important are those associated with subduction how subducted material produces magma and how this magma rises to the sur- face. Furthermore, the nature of subduction zones varies greatly, apparently influenced by the natures of both the overlying and sum ducted materials: oceanic-under-oceanic (e.g., Tonga), oceanic- under-continental (e.g., the Andes), and continental-under-con- tinental (e.g., the Himalayas). Continents evidently grow by the accretion of island arcs, but it ~ unknown whether they grew predominantly by this pro- cess in the past. The stabilization of continental crust and litho- sphere, and the control by crusta] thickness are the consequence of mantle-crust interactions not yet well identified. Mass balance calculations based on isotopic data require appreciable recycling of crustal material; the proportion recycled by subduction of sed- iments, delamination of lower crust, or other mechanisms is also as yet unknown. The extent to which continental basalts and the associated upper mantle arise from a reservoir distinct from the source of m-ocean ridge basalts needs to be inferred much more precisely. Planetary magnetic fields measured to date show a wide range of behaviors, plausibly arising from major differences in funda- mental characteristics of the planets. However, these plausibilities are not yet proven and, as in most complicated problems, the so- lution is to be found in the examination of details. Fundamental to the strong magnetic fields of the Earth, Jupiter, and Saturn

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56 (and the nonfield of Venus) are planetary dynamos: interactions of convection, electromagnetic induction, and rotational dynamics occurring in fluid interiors of high electrical conductivity. Prom lems that are likely to persist beyond 1995 are the energy sources for these dynamos, the scales and patterns of the motion of fluids, the temporal evolution of the flow, the boundary layer interaction with overlying material, and the values of key physical properties. The Earth offers the best opportunities to observe details relevant to these processes. Pursuit of this branch of earth science requires a combination of new satellite data to be used in conjunction with those existing from previous systems, plus ground-based observa- tions. The two quantities of great interest and importance are the vertical magnetic field and its first time derivative, or its con- tinuous time dependence, measured everywhere over the Earth's surface. The Measurements Required Structure and Chemistry A number of specific measurements are required to describe, in three dimensions, the variation of physical parameters and chem- ical/m~neralogical composition at all depths within the Earth's interior. These include global seisrn~c wave propagation studies to describe lateral heterogeneities up to at least spherical har- monic degree and order 20. Regional seismic wave propagation measurements are also required to provide detailed images of ma- jar features such as subduction zones, fine structure of the crust and lithosphere, and selected areas near the core-mantle bound- ary. Smart ground stations, portable seismic stations, and ocean bottom systems will be needed for these measurements. In addition, gravity observations (global and regional), geo- logic mapping using space techniques, geochemical and petro- logical analyses, and high-pressure, high-temperature laboratory experiments to understand the properties of terrestrial materials under these conditions will be necessary. Dynamics To understand mantle convection and the resulting motion and deformation of the surface plates, it is necessary to study

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s7 the dynamics of the Earth's interior. Although it ~ generally accepted that the lithospheric plates are the cold, near-surface boundary layers of mantle convection cede, many aspects of mantle convection are still uncertain. Interactions between plates are responsible for a large fraction of the Earth's seismicity, volcanism, and mountain building. Many of the fundamental processes are poorly understood. Episodic ac- cumulation and release of stress at plate boundaries are responsible for great earthquakes. Lines of volcanoes are generally associated with subduction. But what happens to subducted material to produce magma? How does this magma rise to the surface? Why are some plate boundaries broad, such as in the western United States and China, and why are others relatively narrow, as in the Andes? What processes are responsible for the elevation of major mountain belts? With nearly real-time transmission of data through satellites, seismologists are now prepared to derive advanced quantitative models of faulting within an hour after an event has occurred. Exercise of such a capability has clear implications for society and for science. Post-seismic rebound instrumentation, for example, can be rapidly deployed to a hypocentral region. The Global Positioning System will greatly facilitate these projects and will be a key element in these studies. A global array of digital seismometers and geodetic devices telemetering via satellites to central ~observatoriesn is the solid Earth equivalent of a versatile, multispectral telescope or a large- aperture radio telescope. The inside of the Earth is now a candidate for imaging just as are other objects in the universe. An important aspect of the study of the 800] Earth is its rheology, commonly parameterized as viscosity. It is significant for problems ranging from mantle convection to wobble and polar wander. Radial variations of viscosity are poorly constrained; lateral variations, while necessarily large because of temperature variations, are virtually unquantified. The correlation of seismic tomographic and geiod data with the heat flow from the Earth makes it possible to place bounds on the viscosity variations. Specific measurements of the surface gravity field and geoid are required to provide information on the interior density distribution within the Earth. Satellites have provided a large fraction of our current data base. At present, our primary need is improved data over remote mountain areas such as the Andes and Himalayas.

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s8 Full understanding of the geoid requires seismic studies of the interior. A Magnetic Satellite Mission dedicated to measuring the long- wavelength (400 km or more) components over several years and preferably decades would be particularly useful. The satellite could be rather simple; its orbit should be polar and about 1000 km in altitude, to assure both long lifetime and sensitivity to the long wavelength of the field. Substantial progress in understanding the origin of the Earth's magnetic field can be expected once we have detailed maps of the velocity and density variations In the outer core and in "region D,~ the mantle-core transition region. In addition, seismic studies and seismic tomography can pro- vide detailed three-dimensional images of the Earth's interior. These studies require measurements of travel times and the free oscillations of the Earth. They require a wide distribution of digital surface seismographs. Anisotropy, related to flow directions, can also be measured. Geodetic observations at the centimeter level could provide a wealth of information on tectonic displacements. A direct measurement of the plate motions would be obtained, and active tectonic processes could be studied in detail. A number of other measurements are also needed. These in- clude: electromagnetic measurements (satellite studies of the time variability of the electromagnetic field can be used to obtain the distribution of electrical conductivity within the Earth's mantle, electrical conductivity being a sensitive measure of the tempera ture within the mantle); ground deformation measurements (GPS, corner reflectors, readily deployable strainmeters); space mapping; space and ground chemical analyses; and measurements from air- craft and balloons. Geological Mapping Geological maps are perhaps the most fundamental data set in solid earth geoscience. The spatial distribution of rock types, when added to chronological and compositional data, allows de- tailed reconstruction of the geological evolution of a region. Only by mapping all of the continents to a uniform resolution can the record of the evolution of the Earth from 3.8 billion years be estate fished. Thus, the importance of accurate geological maps cannot be overemphasized. Key operations for understanding the tectonic

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59 history of a region often center on the rates and magnitudes of pro- cesses such as faulting and uplift. Good geological maps afford an opportunity to compare estimates of short-term rates derived from geophysical techniques to long-term geological rates. Geological maps are also critical in that they supply con- straints to models. For example, it is important to develop modem that relate strain buildup, fault slip, and earthquake occurrence to rheological properties of the crust and lithosphere. The spa- tial Attribution of fault planes and the width over which shear is distributed across a fault zone are important parameters. It is apparent that for many of the problems discussed above, highly detailed maps, coupled with extensive chronological data, are required. Such maps cannot be generated with space-based techniques alone, but require detailed ground investigations. Nev- ertheless, an important background data set, particularly for many poorly surveyed areas outside the United States, can be generated with remote sensing techniques. Both multispectral, optical-band stereo imagery, and synthetic and real aperture radar unagery can provide useful data for regional investigations. Although many remote sensing data have already been gener- ated by NASA, a surprisingly small amount has been used by the geological/geophysics community. The cost and time involved in acquiring and processing remote sensing data in its present form often make it prohibitively expensive for the average geologic map- ping program. The time involved in searching the large variety of data archives also tends to limit accessibility. A centralized facility that would catalog, process, and make available such data to the geological/geophysical community would be an important step. For many geological problems, spatial resolution of 10 m or better is required to adequately map the distribution of critical lithological units. Present space-based sensors are thus not ade- quate for many tectonic problems. Nevertheless, they can provide important constraints for regional problems and afford an op- portunity to look at large terrains in a new, synoptic manner. Improved spatial resolution would greatly enhance applicability to other problems. The photographs of the Large Format Camera on the Shuttle have now established the extreme usefulness of Am resolution. There can now be no going back to 3(>m resolution. The present spectral resolution of the thematic mapper is a great improvement over other Landsat sensors. It nevertheless does not allow discrimination of most lithologic units. Higher

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60 spectral resolution, particularly in the infrared, is required to obtain even crude lithologic discrimination capability. Current coverage of thematic mapper imagery is, to some extent, limited by ground receiver capability, though this is expected to improve when another TDRS satellite becomes operational and as more ground receiving stations come on line. Present coverage with synthetic aperture radar imagery is extremely limited. It cannot be emphasized enough that the strength of space- borne sensors lies in their global, synoptic coverage; hence, Shuttle deployment is of limited use. Global coverage is required to attack many of the significant problems in tectonics. Global Topography Global, digital topographic data are required for a number of geological and geophysical investigations. At present, data at adequate resolution are available only for the United States and a selected number of Western European countries. Topographic data are required for proper analysis of gravity data, in order to deconvolve the contribution of topography to a given gravity signal. More generally, analysis of coupled topography and grav- ity data allows the determination of subcrustal structure, gravity compensation models, and crustal theological properties. Clearly, adequate topography data must be an integral part of any gravity mapping mission. Sufficient topographic map coverage ~ lacking for many crit- ical regions, including much of Africa, South America, and the Himalayas. Digital topographic data for the continents are useful for an astonishing range of purposes, including geophysics (for ex- ample, gravity compensation modeling), civil engineering (for site surveys), and botany (for example, species distribution and health, estimated from optical sensing techniques, as a function of alti- tude). It also has obvious applications in geology/geomorphology, and would aid remote sensing in general because registration of digital topography with other kinds of image data wouIc} allow correction of albedo effects and layover distribution in optical and radar data, respectively. Finally, altimetry data over the polar ice caps would allow calculation of ice-flow-driving stress and would aid in monitoring the long-term health of ice sheets. Topography data with moderate resolution can be obtained economically with a dedicated Topographic Mapping Mission on

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61 the Space Shuttle. Global coverage can be obtained in three m~s- sions. The system would employ a microwave altimeter with a phased array antenna. The long dimensions of the antenna would generate a small footprint in the cross-track dimension (500 m) for the required spatial resolution. Electronic beam steering of the phased array would allow the appropriate swath width for complete global coverage. Synthetic aperture techniques would ensure adequate spatial resolution (500 m) in the along-track di- mension. Real aperture techniques may allow the same coverage in one extended mission. Height resolution should be better than 5 m. Higher resolution altimetry could be obtained with a scanning laser altimeter. Higher power requirements for such a system, in the range 2 to 5 kW, dictate deployment on a large permanent platform such as EOS. A pulsed laser with a pulse duration in the range 5 to 20 ns and a pulse repetition frequency In the range 2 to 4 kHz could generate global coverage in about 1 year with 100 m spatial resolution and 1-m height resolution. Technical improvements in the long-time reliability of lasers are needed for this purpose. Surface Imaging and Sounding Space-borne Synthetic Aperature Radar (SAR) systems have proven to be very useful for a variety of geological, botanical, and agricultural applications, as well as selected oceanographic and ice monitoring studies. Current generation space-borne SAR is re- stricted to single-frequency, single-polarization instruments. How- ever, multifrequency and multipolarization capability and utility have been tested on aircraft, and are expected to be demonstrated before 1995 on the Space Shuttle with the SIR-C experiment. Multifrequency radar can potentially be used to map parame- ters such as soil moisture, vegetation mass and health, and possibly the amount of snow pack. In arid regions, multifrequency SAR can be used effectively to distinguish and map shallow subsur- face layers. Multipolarization capability at a given wavelength effectively maps volume-scattering properties. Perhaps the most obvious applications of multipolarization SAR are in the fields of botany and agriculture. Here, the orientation and volume density of plant leaves and stalks deterrn~ne the relative proportions of

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62 backscattered energy at the various polarizations. Thus, multipo- larization SAR can be used to map vegetation type ancI monitor vegetation health. A variety of unaging and sounding instruments on geosynchronous and polar platforms wait be needled to obtain uniform global coverage. GRAND THEM1: 2: ATMOSPHERE, OCEANS, CRYOSPHERE, AND HYDROLOGIC CYCLE Global Awes The central theme 2~! be to establish and understand the structure, dynamics, and chemistry of the ocean, atmosphere, and cryosphere, and their interaction with the solid Earth, including climate, the hydrological cycle, and other biogeochemical cycles. The Earth is unique in possessing an ocean and living organ- isms. There are growing realizations that the hydrosphere and biosphere, while constituting tiny fractions of the planet's mass, are crucial in establishing the character of the Earth in several ways. The ocean, to a visitor from another planet interested in physics, would be most quickly recognized as the controller of water and heat, and the relative sluggishness of its circulation makes it the buffer to the variation of the atmosphere on time scales ranging from days to seasons. It also imposes its own pattern on decadal and longer time scales, as manifest in such phenomena as El Nina. The ocean and the cryosphere aLso are the main control on solar inputs to climate and weather. On longer time scales- 102 to 106 years the ocean, glaciers, and their distribution with respect to the land vie with volcanic inputs and solar variations in influencing climate. The relative roles of these different effects are still is-understood; many observations remain that could improve our insight into these phenomena that are so important to human welfare. It would be evident to a visitor interested in biology that the ocean would be essential to the development of life. Its margins have offered such stable riches as light, nutrients, perches, and pro- section from ultraviolet radiation through a reducing atmosphere. As life has evolved, its symbiosis with the ocean has made it a phenomenon covering the Earth's surface, as discussed below. The influence of the ocean on the behavior of the solid Earth

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63 is important as well. It has a major effect on the chemistry of the continental crust through the intermediacy of its sediments. The hydrosphere may be important to island arc volcanism by fluxing magmat~c activity In subduction zones. Just how hydrated sedi- ments influence this process of continent-building is not clear, and has been much debated for decades. In addition, the ocean may significantly influence the mechanical behavior of the lithosphere; a relatively small proportion of water can weaken rocks so they are more easily subducted. The ocean is also the most pervasive connecting medium for global biogeochemical cycles. The magnitudes of most chern~cal reservoirs and their rates of accumulation are strongly controlled by the ocean, which is significantly older than the ocean basins beneath it. It has become apparent that the atmosphere, oceans, and the hydrologic cycle cannot be considered in isolation, but rather as 1 ~ _ , ~ a more complete system that includes interactions between the biosphere, solid Earth, and perturbations caused by solar variabil- ity and orbital changes. Many of the individual components of the system wild have been investigated by 1995, and many of the techniques needed to address the Earth as a planet will have been developed. The Measurements Required In order to address this grand theme we will need to monitor and eventually understand the processes involved in global change of atmosphere, oceans, and their interaction with land. We need long-term (on decadat time scales)' consistent' an] precise mea- surements of geophysical parameters such as the solar constant, stratospheric ozone, stratospheric temperature and aerosol, atmo- spheric trace compounds, surface albedo, land biomass, sea surface temperatures and topography, concentration of chlorophyll in the oceans, global cloudiness, and rainfall patterns and soil moisture. Because the coverage has to be global and repetitive space satel- - , , lites are, in principle, ideally suited to provide these data con- sistently over time. Today a variety of satellites exist that are measuring some of these parameters routinely. As we look beyond 1995, we see that the results from the 1985 to 1995 decade can be used to develop a cost-effective, long- term measurement scheme with a mix of satellite and in situ

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64 _ _ ~ ~ ~ ~ ~ _ 1 ` , measurements. A program of space observation wid therefore have to be designed that will provide unique global data vets mace up of Q-~mu~taneous observations of the atmosphere, land, and oceans for two principal purpose-: to facilitate the setting of parameter" for the various fluxes in the model-, and to check the mode] predictions on a global scale. In order to achieve this, an observing program can be visualized that: ~ Provides long-term and consistent data on some of the key parameters such as sea surface temperature-, ice cover, albedo, stratospheric ozone, and solar constant, so that we can begin to test the modem at least on a decadal time scale. Develops new techniques for monitoring those parameters that are important in climate research but cannot be measured by the current space system-: rainfall, evapotranspiration, biomass. ~ Assures the compatibility and continuity of some of the current observing systems: operational versus research satellites and U.S. versus non-U.S. satellites. Organizes field experiments that would help validate and authenticate the space observation-. . At the same time, we will have to build a research community that is conversant with space technology and ~ drawn from a num- ber of traditional disciplines of earth sciences such as volcanology, agronomy, geology, oceanography, meteorology. ~laciolo~v. and 1~ ~ 1 ~ 1 1 1 ~ . . . ~ _ _ _ ololOgy so that a coherent attack on the climate predictability problem can eventually be mounted. The task group expects to see a continuation of the World ~limz~t.a R"Q^=r~L PI AL ~L ~ 11 ~ . ~ ~ ,^~_v_ vat ~ ~V61 ~111~ ~111~11 Will De operating in Ernest. her AL Or lOC]t~ _~ ~ ~ ___ ~1~_ ~ _ ~7 _ ~ lo ~ v ~~~ undo ~~ 1,~ 8 EVE ~11~ At :~ -file oe- l~mn~n~ of the Intern at.inn n1 ( ~ ~r~or~h^~_R;~o~l-~ ~~ t _ involve: ~ ~ ~ _- A- ~&WiJll~l ~ 1 1 Reroll- 1 lie 1 ~ ~ I ~ . _ latter Will tOCUS on interactions in physics, chemistry, and biology. The major thrusts for atmospheric science beyond 1995 will Development of a global measurement system for precipi- tation and evapotranspiration to define the latent heat budget for the atmosphere. Continuation of intensive studies of severe storms; their generation, steering, and dissipation. . Development of a detailed understanding of the role of the biota in influencing the atmosphere through trace gas up- take and emanations, through albedo influences, and through

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65 evapotranspiration and the way in which these influences depend on environmental parameters. For these purposes we will need an extensive program of in situ observations of processes in large land ecosystems, of tro- pospheric chemistry, of oceanic biogeochemistry, and of severe storms. Satellites will play a major role in precipitation mea- surements and complementary roles for severe storm, biota, and atmospheric chemistry investigations. We expect that the most cost-effective program for oceanog- raphy will continue to be the relatively low-cost, single-purpose satellite missions that are properly intercalibrated. There is an important role for the Space Station, including polar platforms, in local and regional measurements that require high power for the sensors. Ground-based studies of high-deposition-rate pelagic sediments are also required. The major science thrust for 1995 to 2015 will continue to be climate prediction for longer and longer time periods. As we move from interannual, E] Nino-type events to long-term changes caused by increasing carbon dioxide, we must include the interactions of biology in the system. Understanding biology will be a major thrust for the 1990s and beyond. To do this we will need a global satellite network together with major in situ programs to measure: Ocean currents and mixing. This includes a network of polar-orbiting satellites to measure sea surface topography, build- ing on the results from the Ocean Topography Experiment (TOPEX) and the altimeters on the Earth Observing System (EOS). A larger in situ program, including moored and drifting stations, will be required to monitor mixing and sinking rates, as well as to validate the altimeter measurements and to measure currents below the surface. . Ocean-atmosphere interaction. This includes a network of polar-orbiting satellites to measure sea surface topography and sea state, building on the ESA Remote Sensing Satellite (ERS-1) and EOS results. In situ programs of moored and drifting stations again will be required to calibrate the satellite data. . Ocean chemistry. New satellite techniques will most likely be available for monitoring ocean chemical parameters from space, especially salinity. These will be measured by multispectral tech- niques from polar-orbiting satellites, and must be calibrated by in

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66 situ measurements. In addition, chemistry measurement must be made In the bulk of the ocean by standard techniques to monitor long-term change. Precipitation and the hydrological cycle. These are funda- mental to the physical processes of climate and to the studies of climate variations. The flux of latent heat in the form of water vapor from the surface to the atmosphere, and its subsequent re- lease through the condensation/precipitation process, constitutes the largest single heat source for the atmosphere. Current rain gage networks on land are generally adequate to measure precipitation in heavily populated regions, but con- siderable standardization in worldwide observing and reporting practices is necessary. It is principally over unpopulated land areas and the oceans that precipitation data are lacking. Studies are under way to investigate measurements of rainfall over land through remote sensing via satellite. The measurement of precipitation from space on a global scale is a formidable problem because as yet there are no methods that can be relied upon to perform under all circumstances around the world. Nevertheless, we already have some visible and infrared techniques that provide climatologically useful data. Also, over the oceans we are quite confident that by 1995 these methods can be extended by means of improved microwave radiometers. The use of combinations of measurement systems should be most valuable in filling the great gaps in our knowledge of oceanic precipitation, and it would serve to give us a better understanding of the sampling requirements and the adequacy of current surface observations. The potential of space-borne radar as the ultimate too} for making direct precipitation measurements over the entire globe must be seriously considered. A number of approaches can be taken that involve conventional pulsed radar, coherent Doppler, dual wavelength, and polarization, among others. All these possi- bilities must be subjected to detailed feasibility studies. An impor- tant consideration is the possible combination of active and passive microwave techniques, and hybrid schemes involving visible and infrared channels. The goal is to overcome the long-standing oh stacles to obtaining reliable global precipitation data. In the area of climate research we will have to spend the next few decades improving global models in which atmosphere, land,

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67 and ocean interact by exchanging energy, mass, and momentum on a variety of spatial and temporal scales. We wiD need data on fluxes at the boundaries rather than just on the state of the atmosphere or of the oceans. We emphasize again the role that ice-core and pelagic sediment studies can play in extending the record. GRAND TlIEME 3: LIVING ORGANISMS AND THEIR INTERACTION WITH THE ENVIRONMENT Global Issues The overall goal for the study of global biogeochemical cycles is to improve understanding of the geologic, atmospheric, oceanic, and biotic reservoirs and their interactions in order to mode! and predict changes important to the biosphere and climate. What must be known to permit us to understand the global balance of these cycles? Uncertainties in our understanding of the carbon cycles lead to serious difficulties in balancing the current budget of atmospheric carbon dioxide. There are a number of problems that must therefore be addressed: the extent of major terrestrial blames and their carbon contents; the factors controlling the internal routes for uptake and release of carbon; the processes that control the exchange of carbon (both oxidized and reduced) between the interior, the atmosphere, biota, and oceans; and finally, the response of the carbon cycle to human perturbations. While the amount of nitrogen fixation controlled by man an- nually ~ significant compared to natural fixation, it is still small compared with the exiting fixed nitrogen pools in the soil and in the oceans. These pools therefore will be influenced only slowly. It will take at least several decades before significant global changes may be expected due to human activities; changes in particular lo- calities, such as soil and water systems, may appear much sooner. But for the very reasons that it wiB be several decades before any significant global changes could be apparent, it will also take an equally long time for conditions to return to an earlier balance once a change Is detected. Specific issues are the following: 1. The elucidation of the storage and exchange of the principle elements in living things, in and between different components of the biosphere the "biogeochemical cycles" of carbon, nitrogen,

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68 phosphorus, sulfur, hydrogen, calcium, potassium, and oxygen- together with sources and sinks of elements that are present as minor components in various forms of life. 2. The determination of the rates of organic production and respiration on land and in sea. How does production on land change with the climate and with changes in the chemical compo- sition of the atmosphere? What is the relationship between ocean circulation and organic production in the sea? 3. Biological systems are currently experiencing changes that are rapid in comparison to evolutionary changes. These changes represent a perturbation of biological systems, the results of which may give an important insight into the relationship between biota and the Earth. 4. Does the increase of nitrogen and sulfur in rain act as fertil- izer in forests? Will the increased concentration of carbon dioxide stunulate biotic production? If so, will the carbon-to-nitrogen ra- tio of plants increase? Will the resulting litter decompose more slowly, thereby locking up critical nutrient supplies and leading to a decrease in biotic production? Or will the reverse occur? The Measurements Required The external information needed to model these processes in- cludes the major biological sources and sinks of organic carbon and active nitrogen, and inputs of sulfur and other compounds from volcanic activity. Urban pollution is a topic all by itself, but Is a major regional source of tropospheric ozone, oxides of nitrogen, sulfur dioxide, and other ingredients of larger-scale problems like acid rain. Also required are the transportation and mixing capac- ity of the atmosphere. Clouds play a key role in catalyzing certain reactions and in scavenging water-soluble products in precipita- tion. Although, in principle, the atmospheric transports and cloud fields are available as part of the modeling and data base of the physical climate system, in practice, considerable additional effort is required to make them useful for chern~cal purposes. Just as important are the internal measurements that give guidance as to which chemical processes are most significant and confidence that they are being modeled correctly. The following measurements are therefore desirable:

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69 1. Global measurement of changes with tune in the minor constituents both isotopes and elements of the atmosphere, oceans, and outer-earth layers. 2. A global inventory, as a function of surface slope, of soils of different texture, and water- and nutrient-retaining capacity. 3. Measurements of the quantitative Attribution of biomass on the land surface of the Earth. GRAND THEME 4: INTERACTION OF HUMAN ACTIVITIES WITH THE NATURAL ENVIRONMENT Global Ares Human activities since the beginning of the industrial revo- lution have increased to such an extent that they must now be regarded as important factors in changing the environment. The effects are approaching a significant stage in altering the concen- tration of ozone and carbon dioxide in the atmosphere, in changing the surface properties by deforestation and erosion, and in other industrial and agricultural activities. Man ~ a major force now in the chemistry of the atmosphere and in the allocation of resources on land, and increasingly an influence on the ocean. Moreover, the influence can be subtle, as illustrated by the potential vulnerability of stratospheric ozone. It has become apparent within the last decade that mankind has the ability to alter ozone, and to thus change the level of harm- fu} ultraviolet radiation penetrating to the ground. We can do so by the direct injection of exhaust gases of high-flying aircraft into the stratosphere, by release of chlorinated gases used as aerosol propellants, as industrial solvents, and as working fluids in refrig- eration systems, and by complex perturbations to the global nitro- gen cycle. These activities lead for the most part to reduction in ozone, but they are offset to some extent by thermal disturbances due to enhanced leveb of carbon dioxide, causing a rise in ozone. Assessment of human impact is hampered by lack of understand- ing of the underlying physical, chemical, and biological influences regulating ozone in the natural state. This matter is critical bet cause the gases responsible for change in ozone the man-made chIorofluorocarbons and biologically formed nitrous oxide have lifetimes ranging from 50 to 200 years. The self-cleansing function

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70 of the atmosphere proceeds slowly, therefore, and the effects of our actions today will persist for centuries into the future. Carbon is the largest single waste product of modern society. We have added, by the burning of fossil fuel, over 100 billion tons of carbon to the atmosphere as carbon dioxide since the industrial revolution, with perhaps a quantity of similar magnitude transferred from the biosphere to the atmosphere over this same period as a consequence of land clearance for agriculture. The increase in the burden of atmospheric carbon dioxide is readily detectable. Approximately half of the carbon added to the system remains in the atmosphere and the remainder Is presumed to have been taken up by the ocean on its way to the depths of the oceanic abyss, and eventual subduction into the Earth's interior. Attempts to model the process encounter difficulties, however, due in part to deficiencies in our knowledge of the nature of concurrent changes in the global biosphere, interactions with other nutrient cyclesnitrogen, phosphorus, and sulfur, for exampIc and lack of understanding of the processes of oceanic mixing. The time scales are such as to require a mode! for the atmosphere, ocean, and biosphere as a coupled system. The matter assumes some urgency since the rising level of carbon dioxide can lead to a change in climate, with associated change in the patterns of rainfall. The ozone and carbon questions are but two examples of many global issues affecting the environment that must be faced in the years to come. Changes involving soil erosion, loss of soil or- ganic matter, desertification, deforestation, overgrazing, diversion of freshwater resources, and increasing levels of air pollution and acid rain affect the physics, chemistry, and biology of the Earth. The Measurements Required Human hnpact Tropical deforestation has recently become a scientific issue of major concern, not only because it significantly decreases bio- logical diversity, and leads to soil erosion and lo" of productivity, but aLso because it is quite possible that it modifies the regional cInnate in a substantial manner. In addition, the changing global biomass has direct bearing on the carbon cycles and on ocean productivity. An accurate assessment of the rate of change of the forest cover around the globe is therefore becoming an important

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71 datum that is needed for a number of disciplines in earth sciences. Ideally, space measurements should be well-suited to document such a change globally, quantitatively, and routinely. However, instruments flown on satellites can only measure radiances either reflected or emitted by the Earth's surface. These measurements have to be rectified for the alterations made by the atmosphere and eventually interpreted in terms of the changes in the biomass or in the properties of the surface cover. It is because of these difficulties that no systematic effort has yet been made to derive quantitative estunates of the rate of deforestation from satellite measurements. The current ground-based estimates range all the way from no "net" change in the biomass to as much as 1 percent per year decrease in forest cover around the world. A narrowing of the range of uncertainty should be the principle objective of any global change monitoring program. Deserts such as the Sahara may be expanding at a significant rate. There are suggestions that, once the process of desertification starts by the baring of the soil due to human encroachments, the climate becomes drier and the process is self-feeding. The mech- anism suggested involves an increase in the albedo of the surface which inhibits convection, thus reducing rainfall. In order to de- termine whether this mechanism ~ really at work on a global scale, one needs to measure the change in the surface albedo as a func- tion of tone and change in the precipitation Attribution around the world. None of these are currently available to authenticate the hypothesis of runaway desertification. Satellite observations integrated with observations on land and the oceans can provide basic data and can monitor soil erosion, desertification, fresh water depletion, variations in the concentration of carbon dioxide and ozone, and the occurrence of acid rain. Hazards The larger and larger conurbations that absorb much of the population increase enhance man's vulnerability to natural haz- ards, such as hurricanes and earthquakes, through dependence on longer and more complex systems of transportation and larger habitational structures. Earthquakes and their associated trunk mats and volcanic eruptions are major hazards to life on this planet. Seismic and geodetic studies have been successful in predicting

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72 some volcanic eruptions. It is unport ant to unprove these predic- tions and to apply the techniques globally. It is also important to monitor the effluents from major volcanic eruptions. Predictions can then be made of influences on the global climate. Active voIca noes, instrumented with geodetic devices such as Global Position- ing System (GPS) receivers, can be monitored prior to eruption. Our knowledge of earthquakes is much more primitive. Extensive studies of stress, strain, and other observables are required to oh tain an understanding of basic mechanisms. Successful prediction remains a goal; however, it Is not yet clear whether it will be possible to predict earthquakes with a high degree of reliability. Satellites adore the opportunity to observe geodetic strain changes in the detail essential to improve our understanding. Eventually, this probably will be an important ingredient in any successful pre- diction program. The space-based geodetic observations must be integrated with a variety of surface observations, including seismic studies. Tsunamis are often generated by major earthquakes. A global network to monitor and provide tsunami warnings with a high reliability and long lead-time is clearly feasible and desirable. Tsunamis can also be tracked in the open ocean from orbital and ocean floor measurements. Finally, severe storms constitute yet another major hazard to mankind. Although great progress has been made in predicting and monitoring severe storms such as hurricanes and tornadoes, much remains to be done. Satellite observations, coupled with ground- and ocean-based observations, already provide a much more accurate basis for predicting the occurrence and severity of storms. These studies should also provide the basis for timely warnings of severe flooding.