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.
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.
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
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
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
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.
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
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
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.
Monitoring Terrestrial Solar Effects
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
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.
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
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.
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
and upper atmosphere. Observations are planned by the DoD Remote Atmospheric and Ionospheric Detection System (RAIDS) spacecraft experiment during the present decade, if the Space Test Program can identify a suitable launch vehicle. But neither RAIDS nor the limited upper atmosphere data from UARS, by themselves, will be sufficient to detect and understand processes important for global change.
A long term data base may become available from UV remote sensing instruments planned for DMSP satellites beginning in the late 1990's. While the goal of these sensors is to provide real time monitoring information on thermospheric and (F-region) ionospheric weather, the data base should extend beyong a decade. However, with the planned merge of DMSP and NOAA weather satellites, a final payload configuration has not been defined.
The dearth of long term studies of the Earth's upper atmosphere from space platforms places greater responsibility on ground based observations and in particular on NSF's CEDAR program, which was originally identified with NSF's Global Geoscience Program and is, again, principally focused on process studies. Thus the activities pursued as part of CEDAR should be extended enough to study the long term and global scale phenomena important for understanding and predicting global change. In particular, cooperative measurements by the CEDAR community could enhance the scientific products of the UARS, RAIDS, and TIMED missions.
Understanding Solar Influences on
Current understanding of solar influences on global change is largely the result of studies initiated by a few interested researchers, working largely within their own institutions, on problems ancillary to those of global change, and without a broad community base. The disciplinary differentiation between middle and upper atmosphere and climate studies limits the opportunities for combined-region research in this area (and others). In addition, the traditional skepticism that often greets solar/weather relationships means that such studies generally receive low priority. Existing atmospheric and climate modeling programs should be
encouraged to become more integrated and to give higher priority to solar forcing in addition to anthropogenic effects that are their primary focus.
Studies of Present Day Behavior
Proper specification of the response of the climate system to contemporary solar forcing requires the identification of all relevant pathways and feedbacks operable over decadal time scales. This will ultimately require combining climate and middle atmosphere models to construct extended three-dimensional models with complete, coupled radiation, chemistry and dynamics. Investigations using improved models may then lead to new understanding of the potential solar cycle/weather relationships that are not simulated by existing climate or middle atmosphere models alone.
Given that the most direct impact of varying solar radiation is on the middle atmosphere, there must continue to be detailed observational and modeling studies of the relationship of the solar cycle to middle atmosphere phenomena, such as stratospheric warmings and mean wind and temperature fields. The effects must then be followed into the troposphere -- for example, alteration of planetary wave energy propagation from the troposphere by modification of zonal winds in the stratosphere. Direct and indirect tropospheric effects, such as the relationship of solar activity to cirrus clouds, should also be pursued. Statistical tests of the relationships must be continued, and the perspective of the NOAA Climate Analysis Center, with its solar cycle-influenced forecasts, should be brought to bear.
The relationship of the QBO to the whole question has to be studied, both through the observation of altered energy pathways in the troposphere associated with the changed tropical lower stratosphere zonal winds and through its interaction with solar cycle effects. GCMs of the troposphere and stratosphere should be used in combination to model the direct solar cycle influence, the direct QBO influence, and the combined processes. The ability of models to reproduce results such as those shown in Figure 2.6 would not only validate the solar cycle relationships, but would go a long way toward improving our confidence in the capability of models to predict the regional effects of global change.
Atmospheric modeling studies of trends in ozone, temperature, and winds need to more accurately consider the combined effects of variations in solar flux, cosmic ray, and energetic particle influences, their transport
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
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.
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
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.
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
resolution investigations are demonstrated to be directly relevant to understanding disk-integrated solar emission variations.
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.
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.
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
Table 7.1 Connections between Solar Influences on Global Change and other science elements identified by the U.S. Global Change Research Program.
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
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.
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
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.