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Executive Summary

Is the Sun an agent of global change? Whether variable energy inputs from the Sun have anything to do with the Earth's weather and climate has been debated contentiously for more than a century. In 1982 a National Academy of Sciences Panel on Solar Variability, Weather and Climate studied the issue in detail, concluding that it is conceivable that solar variability plays a role in altering weather and climate at some yet unspecified level of significance. In the decade since, monitoring of the Sun and the Earth has yielded new knowledge essential to this debate. There is no doubt that the total radiative energy from the Sun that heats the Earth's surface changes over decadal time scales as a consequence of solar activity. Observations indicate as well that changes in ultraviolet radiation and energetic particles from the Sun, also connected with solar activity, modulate the layer of ozone that protects the biosphere from solar ultraviolet radiation. This report reassesses solar influences on global change in the light of this new knowledge of solar and atmospheric variability. Moreover, the report considers climate change to be encompassed within the broader concept of global change; thus the biosphere is recognized to be part of the larger, coupled Earth system.

Implementing a program to continuously monitor solar irradiance over the next several decades will provide the opportunity to estimate solar influences on global change, assuming continued maintenance of observations of climate and other potential forcing mechanisms (e.g., greenhouse gases, aerosols, clouds, ozone). In the lower atmosphere, an increase in



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Page 1 Executive Summary Is the Sun an agent of global change? Whether variable energy inputs from the Sun have anything to do with the Earth's weather and climate has been debated contentiously for more than a century. In 1982 a National Academy of Sciences Panel on Solar Variability, Weather and Climate studied the issue in detail, concluding that it is conceivable that solar variability plays a role in altering weather and climate at some yet unspecified level of significance. In the decade since, monitoring of the Sun and the Earth has yielded new knowledge essential to this debate. There is no doubt that the total radiative energy from the Sun that heats the Earth's surface changes over decadal time scales as a consequence of solar activity. Observations indicate as well that changes in ultraviolet radiation and energetic particles from the Sun, also connected with solar activity, modulate the layer of ozone that protects the biosphere from solar ultraviolet radiation. This report reassesses solar influences on global change in the light of this new knowledge of solar and atmospheric variability. Moreover, the report considers climate change to be encompassed within the broader concept of global change; thus the biosphere is recognized to be part of the larger, coupled Earth system. Implementing a program to continuously monitor solar irradiance over the next several decades will provide the opportunity to estimate solar influences on global change, assuming continued maintenance of observations of climate and other potential forcing mechanisms (e.g., greenhouse gases, aerosols, clouds, ozone). In the lower atmosphere, an increase in

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Page 2 solar radiation, like the greenhouse gas increase, is expected to cause global warming. In the stratosphere, however, the two effects produce temperature changes of opposite sign. A monitoring program that would augment long term observations of tropospheric parameters with similar observations of stratospheric parameters could separate these diverse climate perturbations and perhaps isolate a greenhouse footprint of climate change. Monitoring global change in the troposphere is a key element of all facets of the United States Global Change Research Program (USGCRP), not just of the study of solar influences on global change. The need for monitoring the stratosphere is also important for global change research in its own right because of the stratospheric ozone layer. There are no firm plans at present to implement the primary recommendation of this report, a program of continuous monitoring of solar irradiance to provide the data needed to diagnose and interpret solar influences on climate change. Because current solar radiometric techniques are insufficiently accurate, ensuring data continuity over many decades will require a series of space based observations with sufficient temporal overlap for calibration transfer and prevention of data loss from instrument failure. This measurement program may well be precluded by the dearth of access to space. Scientific Conclusions Q: Do solar variations directly force global surface temperature? A: Yes. Inexorable change is predicted for the biosphere, that sphere of the terrestrial global environment where life exists. It is imperative to reliably detect, understand, and predict climate change arising from increasing greenhouse gases and aerosols in the Earth's atmosphere. This requires that natural climate forcing, particularly solar variability, also be detected and understood. In the study of solar influences on global change, determining the extent to which solar influences modify global surface temperature is a matter of the highest priority. Energy from the Sun sustains life on Earth. By far the dominant energy input is the visible solar radiation that heats the Earth's land

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Page 3 surfaces and the oceans. The atmospheric composition and the distribution of oceans and land masses combine with the solar energy input to determine the radiative balance, and hence the climate, of the Earth's biosphere. A change, DS W/m2, in solar radiation received by the Earth causes climate forcing of 0.7DS/4 W/m2 which perturbs directly the equilibrium global temperature by an amount DT=l-1(0.7DS/4)°C where the climate sensitivity, l-1, is currently estimated to be in the range 0.3 to 1.0°C/(W/m2) (e.g., Wigley and Raper, 1990). While the primary radiative perturbation of ± 3 percent is indisputably the changing insolation generated by the annual cycle of the Earth's elliptical orbit around the Sun, intrinsic changes in the Sun's radiative output also occur on decadal and possibly longer time scales. The Sun's total radiative input to Earth decreased by about 0.1 percent during 1980–1986 then increased by about the same amount during 1986–1990 (Willson and Hudson, 1991). A 0.1 percent solar irradiance change produces a climate forcing of 0.24 W/m2. For comparison, the climate forcing by increasing greenhouse gases from 1980 to 1986 was about 0.25 W/m 2 (Hansen et al., 1990). Concomitant increases in atmospheric aerosols may have reduced the net anthropogenic climate forcing to almost half that arising from greenhouse gases alone (Hansen et al., 1993). Thus, during the recent descending phase of the 11-year solar activity, solar forcing canceled much of the net anthropogenic forcing. The climate system's response to various forcings depends on the history, altitude, and latitude of the forcing and the climate sensitivity, l-1. While the change in equilibrium global surface temperature associated with a steady climate forcing of 0.24 W/m2 is estimated to be in the range 0.1° to 0.2°C, the transient response to a periodic 11-year forcing of the same magnitude is assumed to be much less than the equilibrium response because the response time of the climate system is of the order of decades or more (Hansen and Lacis, 1990). However, the true extent to which the climate system's response diminishes or amplifies solar forcing compared with anthropogenic forcings is uncertain. As records of paleoclimate and historical solar activity have improved, the possibility that variations in solar radiative forcing played a role in past climate change continues to be raised (see, for example, The Royal Society, 1990). There is now clear corroborating evidence from 14C in tree rings and 10Be in ice cores that solar activity during past millennia

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Page 4 exhibited a series of minima, each of 40 to 100 years duration, roughly every 200 to 210 years, and that these minima appear to be associated with colder-than-average global temperatures on Earth (Eddy, 1976; Wigley and Kelly, 1990). The coincidence of the Sun's Maunder Minimum with the lowest temperatures of the Little Ice Age is the best documented of such associations in the recent past. That a physical mechanism might be responsible for the similarity of the historical climate and solar activity records has become more plausible because of observational proof that the Sun's radiative output varied throughout the only 11-year activity cycle during which it has thus far been monitored (Willson and Hudson, 1991). Furthermore, circumstantial evidence suggests that solar irradiance variations may not be limited to the 0.1 percent change detected by contemporary solar monitoring. Observations of Sun-like stars indicate a greater range of activity levels than yet detected in the contemporary Sun (Baliunas and Jastrow, 1990; Lockwood et al., 1992). Also, decreases in irradiance in the range 0.2 percent to 0.3 percent, consistent with the stellar data, are simulated for the Sun's Maunder Minimum by altering the distribution of magnetic features in the solar atmosphere, known to cause much of the 11-year cycle change, within limits defined by independent, spatially resolved solar observations (White et al., 1992; Lean et al., 1992a). Taken collectively, the above evidence, although circumstantial, does suggest that solar variability could influence future global change, which requires that solar irradiance be properly monitored, understood, and, if possible, predicted. Lack of knowledge of solar influences will limit the certainty with which anthropogenic climate change can be detected. But it is unlikely that solar influences on global change will be comparable to the expected anthropogenic influences. Were solar irradiance to decrease by 0.25 percent over the next 200 years, a value speculated for the Maunder Minimum, the equilibrium global surface temperature is estimated by a general circulation model to decrease 0.46°C (Rind and Overpeck, 1993). This decrease would be too small to offset greenhouse forcing which, by the mid-twenty first century, is expected to have caused a global temperature increase in the range 1.5 to 4.5°C.

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Page 5 Q: Do solar variations modify ozone and the middle atmosphere structure? A: Yes. The biosphere, a fragile region in the troposphere where weather and climate are experienced, is protected from solar ultraviolet radiation by a layer of ozone that resides about 30 km above it in the Earth's middle atmosphere. The middle atmosphere is significant for global change not just because of the ozone layer embedded in it; this region is also the upper boundary of the troposphere. Changes in the middle atmosphere, which are known to occur in response to variable solar energy inputs, are suspected of impacting the weather and climate. Determining the extent to which solar variability modifies ozone and the middle atmosphere is therefore the second highest priority in the study of solar influences on global change. The Earth's middle atmosphere absorbs the Sun's ultraviolet (UV) radiation. Were this radiation able to penetrate to the biosphere, it would damage life on Earth; instead it creates our protective ozone shield. Ozone forms when solar UV radiation (at wavelengths less than 242 nm) dissociates molecular oxygen into oxygen atoms that combine with molecular oxygen to make ozone. Extending outward from the Earth's surface to about 100 km, the ozone layer has its peak concentration at about 30 km. The Sun's UV radiation also creates many of the radical species that subsequently destroy ozone. Most notably, the chlorine (Cl) atom is a product of UV photodissociation of chlorofluorocarbons (CFCs) that have risen to the lower stratosphere following their release near the Earth's surface. Ozone is also destroyed by solar radiation at longer wavelengths and by catalysts produced by energetic particle precipitation. Both the solar ultraviolet radiative energy and the energetic particle output are modulated by solar activity. The Sun's UV radiation is an order of magnitude more variable than the visible solar radiation that penetrates to the Earth's surface, and these variations generate natural changes in the ozone layer. Specifying natural ozone variability is essential for untangling anthropogenic effects in the long term ozone record. Observational studies (Stolarski et al., 1991; Hood and McCormack, 1992; Randel and Cobb, 1994) signify the response of ozone to solar forcing. From 1986 to 1990 the increase in

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Page 6 solar UV radiation in concert with the Sun's 11-year activity cycle is estimated to have increased global total ozone by about 1.8 percent. This approximately offset the suspected anthropogenic decrease of 1.35 percent over the same period (-0.27 percent/year). Other studies suggest that changes in the precipitation of relativistic electrons that penetrate into the middle atmosphere may also play a role in natural ozone variations (Callis et al., 1991). Current atmospheric loading of chlorine and other anthropogenic radicals is expected to deplete global ozone until the beginning of the twenty-first century. Elimination of CFCs is expected to reverse this downward trend. Determining whether an observed ozone recovery in the twenty-first century is the consequence of successful CFC mitigation or, instead, of increased solar activity will require continuous, reliable monitoring of solar energy inputs to the middle atmosphere. As well as the UV irradiance, sporadic solar influences, such as energetic particles that may destroy ozone for periods of days to many months, must also be understood. This has been clearly demonstrated by a series of high surges of solar activity throughout 1989 (near the peak of the current activity cycle) that may have depleted ozone in the Antarctic (Stephenson and Scourfield, 1991) and at lower latitudes (Reid et al., 1991). Q: Do solar variability effects in the Earth's upper atmosphere couple to the middle atmosphere and the biosphere? A: Possibly. The Earth's lower and middle atmospheres are surrounded by the neutral and ionized medium of the upper atmosphere and its embedded ionosphere, which shelters the biosphere from highly energetic, dramatically varying solar radiation and particles. In the upper atmosphere, temperature, density, and winds are highly responsive to variations in solar energy input. Furthermore, adjacent layers of the Earth's atmospheric envelope are intimately connected. Highly variable solar inputs to the Earth's upper atmosphere in the form of energetic photons at wavelengths of less than 180 nm and energetic particles cause the global mean exospheric temperature of the thermosphere to vary by about 700 K, from about 600 K during solar cycle minimum to about 1300 K during solar cycle maximum. Physical

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Page 7 processes by which this profound solar influence on the Earth's upper atmosphere might impact the biosphere are not established, but radiative, chemical, dynamical, and electrical mechanisms have been identified that couple the upper atmosphere with the middle and lower atmospheres. For example, large variations in both solar high energy photons (X-rays and extreme ultraviolet radiation) and energetic particles initiate significant changes in upper atmosphere odd-nitrogen, which can destroy ozone if transported down to the high-latitude middle atmosphere by the mean circulation pattern (e.g., Huang and Brasseur, 1993). Also, the ionosphere is connected to the troposphere via the global electric circuit. Solar influences on the upper atmosphere do affect our society in other ways. Activities related to navigation and rescue, defense, and communication rely increasingly on spacecraft technology. Solar energy inputs control the state of the Earth's upper atmosphere where spacecraft orbit, and efficient use of space requires operational understanding of the variability of the upper atmosphere, a part of the global Earth system that is highly sensitive to solar forcing. In the context of global change within the upper atmosphere, knowledge of solar forcing is essential. Since the emission of carbon dioxide infrared radiation is the dominant cooling mechanism throughout the mesosphere and lower thermosphere, releases of trace greenhouse gases from human activity potentially could cause significant changes in the structure of the Earth's upper atmosphere. Whereas the greenhouse effect will cause the troposphere to warm by a few degrees, the global mean thermosphere has been predicted to cool by as much as 50 K in response to projected doublings of carbon dioxide (CO2) and methane (CH4) concentrations from present levels (Roble and Dickinson, 1989). Accompanying redistributions of major and minor constituents may decrease satellite drag by up to 40 percent; affect the propagation of atmospheric tides, gravity waves, and planetary waves into the thermosphere from the lower atmosphere; modify the thermospheric circulation; and change the electrodynamic structure. Such anthropogenic effects on the upper atmosphere will be superimposed on large natural variability caused by solar forcing. It is unknown whether these anthropogenic changes could alter the couplings between the upper, middle, and lower layers of the atmosphere. Monitoring the upper atmosphere in the light of its natural variability is therefore important.

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Page 8 Q: Do solar variability effects in the Earth's near-space environment couple to the biosphere? A: We don't know. Surrounding the Earth and its atmosphere is the geospace environment of the magnetosphere, composed of solar and space plasmas and energetic particles. Shaped primarily by the Earth's magnetic field and its interaction with the solar wind, the magnetosphere is the primary receptor of the highly variable mass, momentum, and energy from the solar wind. It is tightly coupled to the upper atmosphere and is also involved in the global electric circuit. Any study of solar influences on global change must therefore consider potential coupling to the biosphere. The relatively self-contained magnetosphere extends from the upper atmosphere to altitudes of about 10 Earth radii on the Sunlit side of the Earth and to more than 1000 Earth radii on the nightside. The global topology of this region is organized by the dipole magnetic field intrinsic to the Earth, which extends far into space and serves to deflect the onrushing plasma, or solar wind, that emanates from the solar corona. The solar wind flows continually over, around, and into the terrestrial magnetosphere, and in so doing continually imparts mass, momentum, and energy to the system. The added energy must then be dissipated either continuously or sporadically. An example of such dissipation is geomagnetic storms, major disturbances in the magnetosphere that manifest themselves by large variations (for periods of hours to days) in the magnetic (and electric) fields surrounding the Earth. Whether solar forcing of the Earth's near-space environment couples through the upper atmosphere to the biosphere in a way that would be important for global change remains unknown. The extent to which energetic particles couple into the Earth's system depends on the geospace medium through which they must travel. High energy solar protons have been observed to modify ozone concentrations in the middle atmosphere; relativistic electrons precipitating from the magnetosphere may also play a role (Baker et al., 1987). Human activities, such as navigation and resource exploration, can be significantly affected by magnetic field variations associated with geomagnetic storms. In particular, bursts of

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Page 9 geomagnetic activity precipitated by solar events can induce current surges that may disable power grids, as was the case in March 1989 (Allen et al., 1989). Q: Do we need to improve our knowledge of the variable Sun to understand and predict solar influences on global change? A: Yes. Solar variability potentially can influence global surface temperatures and middle atmosphere ozone concentrations. The goal of ultimately predicting this influence makes it essential that we learn how and why the Sun varies as it does. Relatively high priority must be given to acquiring knowledge of the origin of the solar energy variations that force global change in the Earth's lower and middle atmospheres. Our Sun is one of many variable stars in the cosmos. Changes in both its radiative and particle outputs originate in what is actually rather common stellar behavior: a cycle in the emergence of magnetic activity with, in the case of the Sun, a period of 11 years. It is the Sun's magnetic flux that generates the dark sunspots and bright faculae that modulate total solar irradiance, providing radiative forcing of climate change. Extensions of magnetic active regions into higher layers of the Sun's atmosphere are enhanced in shorter wavelength, higher energy radiation. The appearance and disappearance of bright active regions throughout the 11-year activity cycle control the solar radiative output variations that perturb the Earth's ozone layer and also, more dramatically, the upper regions of the Earth's atmosphere. Energetic particles traveling from the Sun to the Earth are guided by lines of the solar and terrestrial magnetic fields, while the solar wind continually transports plasmas and magnetic fields to the Earth's near-space environment. For studying solar influences on global change, a continuous record of the variable energy input that reaches the Earth from the Sun is essential. The observational record is, however, intermittent and extends over only a few solar cycles. Direct measurements, which must be made from space, are difficult and frequently devalued by instrumental uncertainties. Ideally, our record of the Sun's variable energy input to the Earth should extend over all possible time scales and be predictable into the future. It will ultimately be extended through reliable understanding of how and why

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Page 10 the Sun, as a star, varies as it does. This knowledge is needed to unravel possible solar forcing in the paleoclimate record and to assess future solar forcing of global change. It may be obtained through analysis and interpretation of solar images and surrogates that connect the changes in global solar energy output to the fundamental physical parameter changes underlying all solar variability, the magnetic field. Records of solar activity during the past few thousand years can serve as surrogates for solar energy inputs to Earth, providing the physical connections are adequately understood. Preliminary analysis of these records indicates that extrema of solar radiative output variations may indeed have been larger than the changes during the few recent cycles of activity for which direct measurements exist. Over time scales of stellar evolution, observations of Sun-like stars can help to provide limits on solar variations that might have occurred in the past and may be expected in the future. We cannot presume from our limited monitoring of the contemporary Sun over little more than a decade, and during an epoch of relatively high solar activity, that we have yet sampled the range of variability of which the Sun is capable. But we must nevertheless comprehend this variability to reliably determine solar influences on global change. Recommendations The highest priority and most urgent activity for determining solar influences on global change is to:

1. Monitor the total and spectral solar irradiance from an uninterrupted, overlapping series of spacecraft radiometers employing in-flight sensitivity tracking. So that the long term value of present solar monitoring is not lost, adequate temporal overlap to permit cross-calibration with future observations is critical. This goal must be achieved in an era of decreasing access to space. In addition, the following activities will be needed to properly monitor, understand, and predict solar influences on global change.

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Page 11 Pursuit of recommendations 2 to 6 is essential to the interdisciplinary research effort needed to provide an adequate scientific basis for global change policymaking. The actions of recommendations 7 to 12 are essential to ensure a complete understanding of all potential coupling mechanisms. 2. Conduct exploratory modeling and observational studies to understand climate sensitivity to solar forcing. 3. Understand and characterize, through analysis of solar images and surrogates, the sources of solar spectral (and hence total) irradiance variability. 4. Monitor, without interruption, the cycles exhibited by Sun-like stars and understand the implications of these observations for long term solar variability. 5. Monitor globally, over many solar cycles the middle atmosphere's structure, dynamics, and composition, especially ozone and temperature. 6. Understand the radiative, chemical, and dynamical pathways that couple the middle atmosphere to the biosphere, as well as the middle atmosphere processes that effect these pathways. 7. Monitor continuously, with improved accuracy and long term precision, the ultraviolet radiation reaching the Earth's surface. 8. Understand convection, turbulence, oscillations, and magnetic field evolution in the solar plasma so as to develop techniques for assessing solar activity levels in the past and to predict them in the future. 9. Monitor continuously the energetic particle inputs to the Earth's atmosphere.

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Page 12 10. Monitor the solar extreme ultraviolet spectral irradiance (at wavelengths less than 120 nm) for sufficiently long periods to fully assess the long term variations. 11. Monitor globally over long periods the basic structure of the lower thermosphere and upper mesosphere so as to properly define the present structure and its response to solar forcing. 12. Conduct observational and modeling studies to understand the chemical, dynamical, radiative and electrical coupling of the upper atmosphere to the middle and lower atmospheres.