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The Earth as a System-
A Global Perspective for Future Planning
INTRODUCTION—OBJECTIVES 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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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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early Earth—to 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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(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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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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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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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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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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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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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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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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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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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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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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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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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
cycles—nitrogen, 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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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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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.
Representative terms from entire chapter:
mantle convection
