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7—
Research Strategies
The most urgent need for determining solar influences on global
change is reliable, continuous monitoring of solar irradiance over
many decades. Because of the lack of calibration accuracy of
existing solar radiometers, acquiring a record of solar forcing
suitable for global change research will require continuous
monitoring by multiple spacecraft with sufficient temporal overlap
to ensure long term precision by transferring calibration
accuracies. Effort is also needed to improve the long term
precision and the calibration accuracies of existing
instruments.
To fully address the role of solar influences in global change,
additional research will be needed to augment the solar monitoring.
In particular, the terrestrial effects of solar forcing (from the
top of the atmosphere to the surface of the Earth) must be
continuously monitored, also over many decades, and
an understanding developed of the physical feedback
mechanisms responsible for these effects. Ultimately, the ability
to predict past and future solar influences will
derive from improved knowledge of the origins of solar
variability.
This slate of activities, in the broadest sense, encompasses
much of the domain of solar-terrestrial relations. Indeed, our
current knowledge of solar influences on global change has been
derived, for the most part, not from research with this specific
goal but from core solar and atmospheric research programs that
should continue to be supported.
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However, core research has tended to focus along classical
disciplinary lines rather than on the coordinated, long-term,
cross-disciplinary monitoring activities that are essential for
documenting how and why the Earth's environment changes. The
fundamental sources of information will be data bases that are
built up slowly over relatively long periods, and the
interpretation of the changes that occur will involve
cross-disciplinary analyses that use information from many such
data bases. To be effective, global change research must transcend
existing disciplinary barriers and encourage interactions that
cross disciplinary lines. Such interactions are currently difficult
to establish.
This chapter assesses specific ongoing and planned research
activities most relevant to solar influences on global change, and
then discusses some programmatic issues. The core research programs
that exist at present neither accommodate nor foster
cross-disciplinary global change research needs. Measuring and
modeling the variations in energy input from the Sun to the Earth
is essential for research on solar influences on global change. But
it is not a prime goal of existing or planned solar physics
research, since knowledge of solar processes is better achieved
with highly spatially resolved observations of portions of the
solar disk. Nor is it a prime goal of Earth science research, for
which it is an initiator but not an indicator of the physical
processes of interest. As a distinct cross-disciplinary task, the
study of solar influences on global change is championed by neither
the Earth science nor the solar astrophysics community.
Monitoring Solar Forcing
Reliable measurements of solar energy inputs to the Earth system
extend over less than 20 years (which is less than two solar
activity cycles). Existing measurements indicate significant
variability of essentially all solar parameters on essentially all
time scales, from minutes to decades. In acquiring a suite of solar
irradiance measurements with sufficient long term precision for
global change research, important aspects of space based solar
metrology obtained from the experiences of the 1980s must be used
to guide research strategies for the 1990s and beyond.
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Existing solar radiometers have sufficient short term precision
to measure irradiance variations generated by solar rotation over
time scales of days and weeks, but the precision of the
measurements over the 11-year activity cycle is much less secure
because of instabilities in radiometric sensitivity. Critical is
the recognition that true solar irradiance variations cannot be
reliably determined from successive measurements by different
instruments unless they can be intercompared via overlapping flight
epochs. This is because the systematic errors in current
state-of-the-art solar radiometric metrology are of the order of of
the solar cycle variability itself. Improvements in instrument
precision and calibration accuracy are thus important. Furthermore,
it must also be recognized that because satellite instruments can
(and do) fail, concurrent measurements by at least two instruments
are essential to ensure the continuity of the data base. Previous
studies by the National Academy of Sciences (1988, 1991) have also
emphasized the need for overlapping data bases.
Total Solar Irradiance
The detection of solar luminosity variability during solar
cycles 21 and 22, and the interpretation of this variability in
terms of solar magnetic activity, thus far underscores the need to
extend the solar irradiance data base indefinitely with maximum
possible precision. The data are needed for the forseeable future
to reduce the uncertainties in the detection of anthropogenic
climate forcing. A careful measurement strategy will be required to
sustain adequate precision ( < 50 ppm, or 0.005 percent). Due to
the likelihood of instrument degradation, solar monitoring
experiments using current radiometric technology can be expected to
last no more than one decade. Drifts in sensitivity throughout a
10-year mission must also be anticipated and detected. Data gaps
through instrument failure must also be prevented. Therefore,
adequately overlapping experiments and intercomparison of
successive experiments is crucial.
Continuation of the total solar irradiance data base that
extends from November 1978 to the present is in serious jeopardy.
The current and proposed total solar irradiance monitoring program
shown in Figure 2.1 relies almost exclusively on one upcoming NASA
mission, and as presently conceived will not satisfy the
requirement for continuous overlapping experiments, nor even for
third party comparisons between
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successive experiments. ACRIM III has been selected for
inclusion on the Earth Observing System (EOS) CHEMISTRY 2003
platform, but this is not scheduled for launch until early in the
twenty-first century. Thus it is very unlikely that the requisite
overlap between UARS and EOS will be achieved, let alone the
multiple measurements crucial for data validation and data loss
prevention. Uncertain funding prospects during the next decade have
already threatened removal of ACRIM III from EOS (Hartmann et al.,
1993). There are no plans for subsequent solar irradiance
measurements.
The European Space Agency (ESA) will contribute to the total
solar irradiance data base with an experiment on the Solar and
Heliospheric Observatory (SOHO) Mission to be launched in mid-1995,
which may be able to provide a third party comparison between UARS
and EOS total irradiance data, although this will require that the
SOHO mission be extended significantly beyond its planned
lifetime.
Achieving a meaningful third party comparison with radiometers
flown on the Space Shuttle will be difficult, if not impossible.
Improved precision may be achieved by deploying the new cryogenic
radiometric technology (Foukal et al., 1990); while it requires
expendable cryogens at present, and is therefore limited to
recoverable or serviceable platforms, it could provide a useful
backup. Aside from whether long term instrument precision can be
demonstrated from one shuttle launch to the next, despite present
absolute uncertainties of more than ± 0.2 percent, is the
impact of significant rotational modulation. Solar irradiance can
change by as much as 0.3 percent per 13 days, precluding the
reliable determination of long term trends of 0.1 percent per
decade from sporadic measurements for a week or so, once per
year.
Rather than including essential solar monitoring instrumentation
as part of complex space platforms that inevitably suffer delays, a
series of small, overlapping satellite missions dedicated to
monitoring solar irradiance variability will likely prove to be a
more reliable strategy for obtaining the requisite data for global
change research.
Solar Spectral Irradiance
Obtaining an unbroken, reliable record of the Sun's UV
irradiance variations will require an approach similar to that
identified for total
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irradiance monitoring. This includes utilization of the overlap
measurement principle whereby new instrumentation has sufficient
temporal overlap with the instrumentation that it is intended to
replace. Having at least two simultaneous measurements of solar
spectral irradiance at any one time will provide this overlap as
well as prevent a break in continuous data should one of the
instruments fail. The sensitivity of space borne solar
instrumentation must also be tracked throughout operation.
The proposed UV irradiance measurement scenario is illustrated
in Figure 3.2. The demise of the SME spacecraft precluded overlap
with measurements from UARS of solar spectral irradiance from 115
to 410 nm. Beyond the UARS mission (i.e., solar cycle 23 and
subsequent cycles), it is presently planned to launch a second
SOLSTICE instrument as part of the EOS CHEMISTRY 2003 platform in
the early twenty-first century. Continuity between the UARS solar
UV spectral irradiance data base and EOS is critical but
improbable, either by direct overlap or third party comparisons.
Uncertain funding prospects during the next decade also threaten
removal of SOLSTICE from EOS. No additional space borne UV
spectroradiometers are planned for overlap or backup or for
missions beyond EOS.
NOAA's SBUV/2 instruments are currently measuring solar UV
irradiances, but only as a subset of their primary goal of
measuring atmospheric ozone and only at wavelengths longer than 160
nm. Furthermore, the SBUV/2 instruments lack the capability of
end-to-end in-flight sensitivity tracking, making it difficult, if
not impossible, to adequately account for instrumental effects in
the data. It is unlikely that measurements of solar UV irradiance
made from the Space Shuttle will be adequate for assessing solar
influences on global change during the coming decades. They are
unable to quantify the contribution of short term irradiance
variability to measured long term trends. With measurement
uncertainties of about 5 percent in the region between 200 and 300
nm, which is thought to be most important for global change, they
are insufficiently accurate, since this is the order of the solar
cycle UV irradiance variability.
Because of the paucity of plans for future solar UV spectral
irradiance measurements, the preferred strategy would be to include
instruments to measure both spectral and total irradiance on a
series of overlapping solar monitoring satellites. If possible,
instruments that measure changes in the
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solar spectrum at wavelengths longer than 400 nm, for example in
the region between 1 and 4 microns, should also be considered.
Despite the importance of this latter infrared spectral region for
the biosphere, the solar cycle changes in this spectral region have
yet to be determined and may even be out of phase with the sunspot
cycle (see Figure 1.1).
An opportunity exists to obtain new measurements of solar EUV
and X-ray spectral irradiances with the Solar EUV Experiment (SEE)
instrument that has been selected for inclusion on TIMED (Woods et
al., 1994) as part of NASA's new Solar Connections Program. Limited
solar EUV monitoring will also be provided by SOHO. An example of a
spacecraft program for developing the necessary understanding of
the EUV irradiance variations is SOURCE (Solar Ultraviolet
Radiation and Correlative Emissions). The intent of this mission
would be to measure EUV spectral irradiance in conjunction with
observations by full disk solar imagers to record EUV emissions
from solar surface magnetic structures, with the goal of developing
reliable empirical (but physically based) irradiance variability
models for use in studying solar forcing of the upper atmosphere
(Smith et al., 1993).
Even if adequate flight opportunities can be established for
solar irradiance instrumentation, a data base with sufficient
accuracy for establishing the variability of solar UV energy inputs
to the Earth will only be achieved through a continued commitment
to innovative radiometric programs dedicated to the improvement of
absolute measurement accuracies. Needed are extensive absolute
radiometric cross-calibrations using a variety of laboratory
irradiance sources, verification and improvement of the irradiance
and detector standards maintained by the National Institute of
Standards and Technology (NIST), laboratory intercomparisons of the
flight instruments, and provision for end-to-end calibration
monitoring during integration and flight.
Energetic Particles
The observational needs for energetic particles are long term
monitoring of relativistic electrons and particle fluxes, with more
emphasis on the higher energies (> 100 MeV) than has been the
case in the past. The recently launched UARS and Solar, Anomolous
and Magnetospheric Particle Explorer (SAMPEX) missions are
currently measuring some
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particle energy inputs, and further measurements are planned for
the upcoming NASA International Solar-Terrestrial Program
(ISTP)/Global Geospace Study (GGS) and the continuing Department of
Defense (DoD) Defense Meteorological Satellite Program (DMSP).
However, simultaneous measurements of the atmospheric response to
these inputs will only be made from a few selected ground stations
as part of the National Science Foundation's (NSF) Coupled
Energetics and Dynamics of Atmospheric Regions (CEDAR) program.
Additional measurements of energetic particle input and the
atmospheric response should continue to be made with the suborbital
and NOAA programs.
Ground Based Solar Variability
Indicators
A number of existing ground based solar observing programs have
proven, over the past decade, to be extremely valuable and cost
effective for studying solar influences on global change, and these
programs should be continued indefinitely. Two types of ground
based solar observations are important: 1) measurements of the
relative solar flux in specific regions of the visible, infrared,
and radio spectrum that reflect the integrated variations caused by
magnetic brightness sources in the solar atmosphere, and 2)
spatially resolved observations of the solar disk from which the
active region features that contribute to the irradiance variations
can be identified and characterized. These data (Figure 6.3) are
utilized to generate surrogates for solar activity modulation of
the total solar irradiance (Figure 6.4) as well as for the UV and
EUV fluxes. The 10.7 cm radio flux, available since 1947, is a
uniquely long solar flux time series which must be continued.
Of the ground based solar observing programs, perhaps the most
important is the NSF-funded program at the National Solar
Observatory (NSO) Kitt Peak and Sacramento Peak facilities for
measuring spectral irradiances of chromospheric lines (He I 1083 nm
and Ca II K) and coronal lines (Fe IV 530.3 nm) in units relative
to the background continuum. The NSO data base is especially
valuable because it now extends for 20 years, from 1974 to the
present, longer than any of the continuous satellite records of
irradiance variations. Extending the data base will further
increase the possibility of statistically meaningful correlations
with climate parameters. The usefulness of the NSO ground based
solar monitoring for
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global change research could be enhanced by making measurements
on a daily basis and with photometric calibration. Improvements
such as these have been detailed in the Proceedings of a Workshop
on Solar Radiative Output Variations (Foukal, 1987) and
incorporated in NSF's Radiative Inputs of the Sun to the Earth
(RISE) initiative.
A number of solar observatories record full disk solar images
detailing magnetic field strengths and photospheric and
chromospheric magnetic structures. These measurement programs
provide basic data (frequently daily) on the evolution of the
details on the solar surface. With improved analysis, these images
could provide quantitative characterizations of the active regions
that generate solar energy output variations and that are needed to
construct full disk irradiance variations.
Also relevant are observations of variability in Sun-like stars.
Existing programs now include data collected over more than a
decade; they should be continued indefinitely. The use of this
larger stellar sample can yield important clues about solar
variability on otherwise inaccessible time scales, such as, for
example, the propensity of brightnesses of stars similar to the Sun
to oscillate regularly or irregularly. Where possible, spectral
irradiance observations should also be made -- for example,
observations of the UV emission of these Sun-like stars.
Indirect records related to solar variability have been provided
for many decades by measurements of the Earth's magnetic field from
global ground based magnetometer networks. In many cases, these
ground based data have been used to construct global geomagnetic
indices, represented, for example, by Kp, Ap, and Dst indices, that
describe amplitude and directional changes in the varying field of
the Earth. As noted by NAS (1988), because ground based geomagnetic
activity indices extend continuously from before the space age to
the present, they are of great importance to long term
solar-terrestrial research. Ground magnetometers also provide the
long term record used to monitor the secular variation of the
Earth's internally generated magnetic field. Maintaining the ground
based magnetic records entails keeping in operation a sufficiently
dense and properly distributed network of ground magnetic
observatories.
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Monitoring Terrestrial Solar
Effects
Lower Atmosphere
Continuous monitoring of the solar irradiance over the next
several decades, as discussed in the previous section, would
provide the opportunity to quantify its potential impact on the
climate system, assuming that observations of climate and other
potential forcing mechanisms (trace gases, aerosols, ozone, etc.)
are maintained as well. In the lower atmosphere, it is expected
that an increase in solar radiation will, like increasing
greenhouse gases, warm the Earth's surface. In the stratosphere,
however, the two effects would produce temperature changes of
opposite sign. Augmenting long term observations of tropospheric
parameters with similar observations of stratospheric parameters
would constitute a monitoring program that could separate these
diverse climate perturbations and help isolate a greenhouse
footprint for climate change. Monitoring global change in the
troposphere and understanding climate forcings and feedbacks is the
specific focus of the Climate and Hydrologic Systems science
element of the USGCRP and a key element of its other facets, not
just of the study of solar influences on global change. The need to
monitor the stratosphere is also important for global change
research in its own right within the Biogeochemical Dynamics
science element, because of the existence of the stratospheric
ozone layer, and this is discussed more extensively in the
following section.
Global monitoring of the solar ultraviolet radiation reaching
the biosphere is relevant for studying solar influences on global
change as well as for the Ecological Systems and Dynamics science
element of the USGCRP; both involve changes in middle atmosphere
ozone. A much needed step toward a credible ground based UVB
measurement program is the development of absolutely calibrated UVB
photometers and high resolution spectrometers, presently being
sponsored by the Department of Agriculture. While improved
monitoring techniques are being developed in the U.S. by programs
within the Department of Agriculture, the Environmental Protection
Agency (EPA), and NIST and by similar endeavors in other countries,
these capabilities will still not meet all of the needs for global
monitoring. Neither the accuracy nor the adequacy of the
measurements will be sufficient to determine global anthropogenic
trends
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Representative terms from entire chapter:
solar irradiance
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or natural solar-induced cycles, especially over oceans. At
selected sites, accurate, detailed measurements of the UV radiation
as a function of wavelength are needed to verify the monitoring
program and for model verification studies. Also necessary at these
sites are accompanying measurements of other parameters, such as
transparency, total ozone, and aerosol loading. If techniques for
determining incident UV radiation from space can be developed,
these satellite data in combination with the ground based
measurements could provide the needed monitoring capability.
Middle Atmosphere
Both in situ and remote measurements of as many of the chemical
constituents in the middle atmosphere as possible are needed to
understand the processes affecting ozone and other important
constituents. Achieving these measurements is a focus of the
Biogeochemical Dynamics science element of the USGCRP, and a
substantial effort in this area is underway, much of it embodied in
the National Ozone Plan. Such missions as the Upper Atmosphere
Research Satellite (UARS) and the implementation of the
ground-based Network for Detection of Stratospheric Change (NDSC)
should add greatly to our knowledge as well as provide a suite of
middle atmospheric observations that should better define the
response of the middle atmosphere to solar forcing.
However, it must also be recognized that determining solar
influence on the middle atmosphere requires global measurements
over not just one but several solar cycles, at least. In addition
to ozone, measurements of nitrogen oxides (NO, NO2, HNO3)
are particularly important in determining the influence of solar
radiation, energetic particle, and cosmic ray variations over the
11-year solar cycle. NOAA continues to monitor atmospheric ozone
through its operational polar orbiting satellite series (albeit
with uncertainties arising from the instrument diffuser
degradation), and calibrated measurements of individual profiles
are being made by the Stratospheric Aerosol and Gas Experiment
(SAGE) II.
It is doubtful that continuous measurements of the parameters
necessary for adequately assessing solar influences on the middle
atmosphere will continue beyond the UARS time frame. This region of
the atmosphere is not the focus of EOS. With the continuing
reduction in scope of the EOS program, there is no assurance of
continuous records of
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certain species measured by UARS -- such as temperature, Ox, NOx ,
HOx , and Clx— that are key to the long term data
base needed for global change studies. Some critical constituents,
such as OH, are not even measured by UARS; the only measurements of
OH planned will be from a few Space Shuttle flights of the Middle
Atmosphere High Resolution Spectrographic Instrument (MAHRSI)
experiment. Ground based networks, such as NDSC are therefore of
major importance.
A program that combines both satellite measurements for global
coverage and supportive suborbital data is needed to guarantee the
science, calibration, collaboration, validation, and in situ
studies of specific middle atmospheric processes relating to
energetic particles. Neither is in place. The UARS Particle
Environment Monitor (PEM) experiment and the SAMPEX mission are
providing some information on particle energy deposition to the
middle atmosphere that may allow the relevant particle energy
inputs to be sufficiently well defined that proxy measurements by
the Geostationary Operational Environmental Satellite (GOES), DoD
and ISTP programs will be adequate for providing the long term data
base.
Upper Atmosphere
Like the state of knowledge of the relevant solar energy inputs,
global information on the responses of the thermosphere and
ionosphere system to solar forcing faces a dearth of observational
programs. Different types of measurements are needed to elucidate
aspects of the upper atmosphere relevant to global change: its
extensive, natural, solar-driven variability, the predicted
anthropogenic forcing, and the coupling of changes of both natural
and human origin to the lower layers of the atmosphere. The
emphasis of these measurements must be on a global spatial scale
and for long time scales, with the goal of defining the present
structure from which future trends in global change can be
derived.
No existing or planned long-term spacecraft programs address any
of these issues. NASA is currently pursuing initiatives to launch
an extensive investigation of the upper atmosphere with the
Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED)
Mission as part of its Solar Connections Program. While the chief
goals of the program are oriented toward process studies rather
than global change issues, the observations should provide
important new knowledge about the mesosphere
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to regions far from where the energy is deposited, and the
feedback mechanisms that they invoke. The processes by which
changing solar energy inputs impact the lower stratosphere, as
implied by observations, require particular attention, as do the
influences of solar soft X-rays and energetic particle
precipitation on the production of nitrogen oxide molecules. These
studies are essential to establish a solid definition of the
background against which human-related effects on the atmosphere
need to be measured. As noted previously, observations of middle
atmosphere trends may allow separation of solar from anthropogenic
forcing in the troposphere.
With respect to understanding solar influences on the upper
atmosphere and possible indirect forcing of global change,
numerical models of physical and chemical processes and global
circulation of the coupled thermosphere-ionosphere-mesosphere
system should be developed and used to investigate the effects of
solar terrestrial couplings on global change. These models should
also consider the effects of anthropogenic forcing, such as CO2 and CH4
increases, on the properties and dynamics of the upper atmosphere.
Techniques for coupling these models to models of lower atmosphere
chemistry and dynamics should also be explored.
A quantitative global climate model of the entire coupled Earth
system is likely to be needed to understand, the global changes
expected in the twenty-first century, despite the difficulty of
constructing such a model. Examples of the types of studies that
could be pursued are investigation of the possibility that
information on the chemical coupling between the upper atmosphere
and the biosphere on historical time scales might be revealed by
deposits of odd-nitrogen species in ice cores (a topic that has
obvious links to the Earth System History science element of the
USGCRP), and the extent of dynamical coupling that might generate
solar/QBO signatures common to a wide range of altitudes within the
atmosphere.
A number of laboratory studies are needed to support global
change research. Photodissociation processes play an important role
in determing g the production and loss rates of many of the
constituents in the middle and upper atmospheres. However, there
are few observational data to verify the photodissociation rates
determined and used in models of atmospheric photochemistry.
Relevant laboratory studies include, for example, measurements of
the O-CO2 vibrational exchange
coefficient, of the branching ratio of N2 dissociation by electron impact to
N(4 S) and
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N(2 D), and of the rate
coefficients of a number of reactions involving metastable
species.
Records of the Past
It is clearly necessary to understand natural variability and
other forcings, including possible solar influence on the
decadal/century time scale, to be able to understand and predict
the likelihood of anthropogenic-induced climate change in the next
several decades. Concerning the influence of solar variability and
past climate, much needs to be done from both the observational and
the modeling perspectives. Clearly, there is significant overlap in
the focus on paleo- and recent climate with similar goals of the
Earth System History science element of the USGCRP.
For relatively recent climate variations, such as the purported
Little Ice Age, more quantitative and global coverage of the
climate (temperature in particular) is obviously needed. This will
require a program to use and combine many paleoclimate indicators,
such as those derived from tree rings, glaciers, pollen, and
corals. Surrogates for solar irradiance variations are needed here
in particular, if any connection between the decadal to century
scale climatic oscillations and solar variability is to be proven.
Models of the climate system should be used with potential solar
irradiance (and other) perturbations, and compared with
paleoclimate observations over different centuries. Both the
magnitude and the spatial patterns of the effects will help in
assessing the likely solar irradiance contribution and model
sensitivity.
A joint NSF/NOAA funded program on the Analysis of Rapid and
Recent Climatic Change (ARRCC) is in progress, with the intention
of using all available climate indicators to develop global climate
assessments for several cold epochs in the eighteenth and
nineteenth centuries. Combined with estimates of potential forcing
factors (e.g., solar variability, volcanic aerosols, and ocean
circulation changes), modeling studies will attempt to reproduce
the observations, perhaps implicating one or another of the
mechanisms. Modeling studies are already under way to assess the
impact of recent solar variations and should be carried to
fruition.
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From the paleoclimate perspective, orbital-induced insolation
variations are now thought to serve as the principal pacemakers of
glacial cycles. Nevertheless, the mechanisms through which changes
in the distribution of insolation can influence climate are as yet
undefined and controversial. The long term climate change proxies
from marine data were found quite by accident to exhibit orbital
periodicities, contrary to the prevailing view of most
climatologists. Periodicities and phase lags in the climate system
during the Pleistocene need better definition. For interpreting the
orbital-induced insolation variations, it is important to clarify
the absolute dating of the paleoclimate record, including the deep
sea record, without the aid of assumptions on orbital parameter
influence. Additional calcite veins from as diverse a geographical
area as possible are needed to better understand the local/global
signals as well as hydrologic cycle influences. It is also
necessary to assess the newly available ice core data, including
the critical comparison of Greenland and Antarctic results.
From the modeling end, additional GCM experiments should be made
to quantify the solar insolation changes needed to support low
elevation ice sheets. The models used must have sufficient
horizontal and vertical resolution to address the problem of
surface layer effects in specific areas and must be able to produce
a reasonable cryospheric climatology for the current climate (i.e.,
correct seasonal variation of snow cover and reasonable mass
balances for current ice sheets). These modeling experiments should
be encouraged to explore all possible feedback mechanisms whereby
solar radiation changes may influence global climate. As orbital
variations represent our most quantifiable solar insolation
changes, they provide tools both for quantifying climate
sensitivity and for validating climate models on long time
scales.
Understanding And Predicting Solar
Variability
Acquiring a reliable data base of solar energy inputs to the
Earth through the next decades is essential for monitoring and
understanding solar forcing of global change. Understanding and
predicting solar forcing on the much longer time scales needed for
global change research will require additional effort. In
particular, the origins of solar energy variations must be
understood in terms of the variable Sun to allow
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historical solar variability to be reconstructed from proxy
records, for tying contemporary solar variability measurements to
each other and to stellar analogs, and for forecasting irradiance
variations expected to arise from solar magnetic activity.
It is hoped that the acquisition of a reliable, long term data
base of solar irradiance measurements from the UARS and Yohkoh
spacecraft will facilitate improved understanding of the origins of
the irradiance variations through analysis of these data in
conjunction with auxiliary solar data, such as spatially-resolved
magnetic field maps, and Ca II and He I images and fluxes. The
irradiance variations must be physically connected to the
fundamental cause of solar variability, which is solar magnetic
activity, to achieve the ultimate goal of prediction.
Until the data base of solar EUV spectral irradiance
observations has been augmented substantially, significant
improvements in the simple empirical models that predict EUV
irradiance variations from solar activity proxies will not be
possible. These models are derived from correlation analysis of a
chosen solar proxy with existing data and are thus constrained by
the inadequacies of those data. Some progress may be possible by
developing more sophisticated models based on the physical solar
processes that drive solar irradiance variability. By incorporating
independent knowledge of the characteristics of active region
emission and other magnetic features on the solar disk, such models
can potentially provide improved predictions of irradiance
variability as well as a tool for investigating the origin of the
variability. Concepts for models of this latter type have recently
begun to emerge, but are currently unfunded, partly because
studying the variability of the Sun-as-a-star has not enjoyed high
priority focus within the solar physics research community.
To reconstruct past changes in solar radiative energy inputs to
the Earth, it is necessary to determine the validity of empirical
models relating irradiance variations to surrogate phenomena of
solar activity. Existing empirical models (see Figure 6.4),
although essential for verifying and interpreting the variations
recorded by satellite instrumentation, are nevertheless
rudimentary. Still needed, for example, is the ability to quantify
the solar irradiance variations of the past several thousand years,
this will necessitate the incorporation of solar activity
indicators, such as tree-ring 14C
or ice-core 10Be, and studies to
define their physical relationships to changes in the Sun's
radiative input to the Earth.
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For understanding the origins and mechanisms of solar radiative
output variations, an important proposed research program is RISE
(Radiative Inputs of the Sun to Earth) which includes both space
based and ground based measurements and analyses that must be the
foci of solar variability studies from a global change perspective.
With the exception of some NSO ground based observation programs,
RISE is the first research program with the goal of understanding
the variations of the Sun-as-a-star. One component of RISE is now a
program in the global change initiative at NSF within the USGCRP.
This includes an effort to obtain precision photometric images of
the solar photosphere and chromosphere using a dedicated basic
telescope system designed specifically for photometric
observations. If adequately funded, RISE could also provide future
support for the analysis and interpretation of the historical solar
image data base of contemporary ground based data relevant to
understanding the Sun as a variable star and of the terrestrial
pathways and processes through which solar variations might impact
global change.
The ongoing Global Oscillation Network Group (GONG)
helioseismology program (Harvey et al., 1987) continues to be a
high-priority international effort that is also related to the
study of solar variability. The National Solar Observatory directs
this effort to build, install, and operate optical
helioseismographs at a network of sites around the world. This
project will yield almost continuous observations of velocity and
brightness oscillations over the full solar disk starting in 1994,
and promises a major advance in understanding the structure and
dynamics of the solar interior , where solar activity is thought to
be generated and modulated.
Experiments on the SOHO spacecraft will also provide valuable
helioseismic and solar atmosphere structure measurements beginning
in 1995. If they can be made with sufficient long term precision,
measurements of the solar diameter may also prove beneficial,
insofar as they serve as a proxy for other significant solar
changes. Further research is needed not only to measure and model
solar processes on the fundamental scale of the magnetic flux tube,
but also to convert the actual magnetic fluxes into radiative and
particle outputs from the full solar disk. The Mechanisms of Solar
Variability (MSV) program, a recently conceived Solar Research Base
Enhancement within NASA's Space Physics Division, may provide some
progress in this area, providing that the proposed high spatial
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resolution investigations are demonstrated to be directly
relevant to understanding disk-integrated solar emission
variations.
Programmatic Approach
Need for Interdisciplinary
Efforts
Clearly no physical walls separate the various parts of the
coupled Sun-Earth system — the heliosphere, the magnetosphere,
the ionosphere and upper atmosphere, the middle atmosphere, and and
the climate system — from one another. However, separate
disciplines have developed over the past several decades to study
the various parts of this system. This situation has led to
intellectual and administrative walls delineating distinct
scientific communities that make it difficult to study the entire
coupled system.
Yet to successfully investigate the influence of solar effects
on global change requires a program that compasses all of these
areas of research. The need for this interaction is clearly
demonstrated by the following hypothetical examples. A search for
connections between solar and atmospheric (or oceanic) behavior
might first proceed with correlation studies between some broad
indicative measures — for instance the solar radio flux at
10.7 cm and terrestrial surface temperature. A next step, though,
might be to look for correlations between parameters hypothesized
to be involved in mechanisms for such efforts — for instance,
cosmic rays and cloudiness. A parallel effort would comprise cloud
physics experimentation to see if some of the hypothesized crucial
cloud nucleation mechanisms actually take place in the laboratory.
These steps involve at least four separate sub disciplines.
As another example, a search for the effects of solar UV
variations on the atmosphere requires a model properly formulated
to include both direct and indirect UV effects on the middle
atmosphere the lower atmosphere and the couplings between. This in
turn calls for interdisciplinary collaboration.
Clearly, then, for research in this area to succeed, scientists
in various disciplines need to focus some of their activities on
this specific area of research and to interact in formulating
research approaches.
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Connections to Other Areas of the
USGCRP
Because the Sun is the dominant source of energy for the Earth,
the need to understand solar influences on global change pervades
almost all other areas of the USGCRP. This is illustrated in Table
7.1. It is clear that solar influences have the potential to affect
Climate and Hydrological Systems. As has been mentioned, U.S.
seasonal winter forecasts are already being implemented with
consideration of solar influences. There is also a clear
relationship to Biogeochemical Dynamics. Solar variations are known
to affect the middle atmosphere, and these effects must be
considered when looking for trends in stratospheric ozone. There is
an obvious relationship between solar influences on global change
and Ecological Systems and Dynamics. Solar effects on climate, if
shown to be important, can have ecological impacts, and of course
solar effects on stratospheric ozone play a role in modulating the
UV-B flux into the biosphere. In Earth System History, the record
of past solar variations must be considered. Less direct, but
possibly present, are solar influences on Human Interactions
through the climate connection, the impacts of changing UV
radiation on human health, the disruption of society caused by
power and communication failures and the reliance of society on
satellite technology to meet many needs, including defense.
Agency Roles
The USGCRP is an attempt to better coordinate research on global
change among various U.S. agencies. For instance, NOAA, NASA, NSF,
and DoD all have research activities involving the Sun and aspects
of its influence on the Earth system. In fact, the products of the
basic research programs funded by the various agencies have
provided the basis for current understanding of solar influences on
global change. These activities have quite different goals,
however, even within different programs in an individual agency.
Improved coordination is needed within and among U.S. agencies to
focus existing research and to more effectively direct new research
on solar influences on global change.
NOAA, for example, has traditionally been concerned with the
long term monitoring aspects of the problem. A substantial data
base of energetic particle fluxes has been built up from particle
monitors on its
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Table 7.1 Connections between Solar Influences on Global Change
and other science elements identified by the U.S. Global Change
Research Program.
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operational satellites, and routine monitoring of solar
irradiance has been carried out by the ERBE and SBUV/2 instruments,
albeit with less than adequate temporal resolution (in the case of
ERBE) and long term instrument monitoring (in the case of SBUV/2).
Since NOAA's operational satellite program is expected to continue
indefinitely, it could provide the logical platform for many of the
monitoring activities discussed above. NOAA's global change
research program has been substantially enhanced since the
inception of the Climate and Global Change program, and enhancement
of the solar component of that program would be appropriate.
NASA, since its inception, has supported solar and terrestrial
research. Several NASA research satellites have been dedicated to
these areas of research; however, the most recent mission dedicated
to solar research, the Solar Maximum Mission (SMM), was launched a
decade ago and ceased operation fully five years ago. In contrast
to NOAA, NASA's programs in solar and space physics are oriented
toward short term research; that is to say, NASA's programs use
newly developed instrumentation that gives new types of data for a
time but with little long term commitment. Even within NASA,
different research divisions have quite different approaches to the
study of solar influences on the Earth system. The main part of
solar and space physics resides within NASA's Space Physics
Division in the Office of Space Science. Here, the main concern is
to understand the workings of the Sun-solar
wind-magnetosphere-ionosphere system. NASA's Office of Mission to
Planet Earth monitors the Sun with a view to elucidating its role
on the lower atmosphere, especially climate change; the three solar
instruments on its recently-launched UARS are the prime source of
current solar monitoring, and future solar monitoring is planned as
part of the EOS. Then, of course, there is the NASA operational
interest in predicting solar activity effects (e.g., orbit decay)
on spacecraft operations as recognized by the recently formulated
Space Environment Effects Program.
NSF has long supported basic research into Solar-Terrestrial
Physics — how solar changes affect the terrestrial
environment. NSF also supports theoretical and observational solar
research, primarily using ground based techniques. The primary U.S.
program for investigating the relationship between solar
variability and climate change is the Solar Terrestrial Research
Program in NSF's Division of Atmospheric Sciences. Research
activities involve solar physics as well as potential solar-induced
climate
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variations on all time scales. Included are paleoclimate
investigations involving records in tree rings, and ice cores, as
well as Sun-weather relationships. Funding for the core program in
this area is diminishing, however, and will need greater emphasis
to strengthen the terrestrial component of the program. NSF lists
three programs as part of its global change initiative; Coupling,
Energetics and Dynamics of Atmospheric Regions (CEDAR); Geospace
Environmental Modeling (GEM); and Radiative Inputs of the Sun to
Earth (SunRISE).
DoD also has extensive research activities in solar and space
physics. These activities involve both theory and ground and space
based observations (both from short term research and long term
monitoring points of view). The relevance of solar variability for
DoD research is that its effects on the Earth environment must be
properly taken into account in military operations and particularly
in communications and surveillance. A requirement for operational
models of upper atmosphere variability to track and predict
satellite trajectories, and of ionospheric variability that affects
communications, has led DoD to support, over the past decade, what
exists today of upper atmosphere research, focusing especially on
the solar influences that determine both its neutral and ion
composition. The recently implemented Strategic Environmental
Research and Development Program (SERDP) encourages use of DoD
resources for global change research. In this regard, the Defense
Meteorological Satellite Program could provide regular access to
space for solar monitoring and global change endeavors.
Because solar influence on global change is a distinctly
cross-disciplinary endeavor, cooperative research by DoD, NASA,
NSF, and NOAA is essential for mutual benefit to both the USGCRP
and the individual agencies. The research effort is hampered by the
lack of a lead agency in this area. One challenge to the U.S.
program of research into solar influences on global change is to
develop a strategy for agency leadership and coordination in
pursuing the overall national effort.
International Apects
Two programs within the International Council of Scientific
Unions (ICSU) relate to solar influences on global change. One is
the Solar-Terrestrial Energy Program (STEP) of the Scientific
Committee on
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Solar-Terrestrial Physics (SCOSTEP). The STEP program seeks to
better understand the coupling of energy and mass throughout the
various parts of the solar-terrestrial system. A related ICSU
program is Stratospheric Processes and their Relation to Climate
(SPARC), an adopted program of the World Climate Research Program
(WCRP). Its emphasis is on understanding the role of the
stratosphere in the climate system. Both of these international
programs include components involving solar influences on global
change.
The International Solar Terrestrial Physics Program (ISTP) is a
cooperative effort involving the U.S., Japan, and Europe. The
program consists of several spacecraft to be launched in the 1990s
by NASA, ISAS, and ESA. The overall scientific objectives of ISTP
are to develop a comprehensive, global understanding of the
generation and flow of energy from the Sun through the
interplanetary medium and into the Earth's space environment, and
to define the cause and effect relationships between the physical
processes that link different regions of this dynamic environment.
The ISTP will provide major contributions to the understanding of
the energy flow between the Sun and the Earth's magnetosphere, but
its principal objectives do not, at present, include study of
energy flow into the lower atmosphere.