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3 Panel Discussion The final workshop session was a discussion led by chairs of the sessions, and all participants were encouraged to share their thoughts on the open research questions in these fields. The principal topics for discussion were as follows: • Solar irradiance variability (total solar irradiance [TSI] and spectral). What do we know about TSI variability now and in the past? • Sun-climate connections on different timescales. What do we know about the variability of climate on global and regional spatial scales on timescales of years, decades, and longer? • Mechanisms for Sun-climate connections. Do we understand and have we correctly modeled the top-down, bottom-up, galactic cosmic ray forcings and chemical couplings that determine the system response? The discussion was led by Joanna Haigh; Daniel Baker was the facilitator. The panel members— Joe Giacalone, Ka-Kit Tung, Isaac Held, and Peter Pilewskie—were the chairs of the four workshop sessions (with the exception of Joe Giacalone, who stood in for Peter Foukal). The basic question in understanding the Sun’s role in climate change is a compelling one: How well is past and present total solar irradiance known and understood? As Haigh pointed out, it is certainly an issue of concern that the existing TSI database has been derived from measurements that could not be intercalibrated to the degree of accuracy necessary for climate studies. Giacalone led off the panel discussion by providing an overview of what he felt were the key issues, stating that understanding the Sun’s role in climate is an important and compelling issue. The research community has to understand the Sun’s variability and how it can be tied to proxy measurements, and with what degree of accuracy. Greg Kopp pointed out that understanding the Sun, its output, and the various proxies used to infer its output may inform the ability to specify the accuracy of back-projections of paleo-TSI based on various proxies (including sunspot number). Mark Rast pointed out the difficulty of the issue, given that simplifying assumptions are currently made about the relationship between sunspots, faculae, and TSI when in fact modern observations have indicated that the situation is really quite complex. Baker questioned the audience for key steps that NASA and the National Science Foundation (NSF) can pursue to address these questions. Rast responded that radiometric imaging of the Sun is important not only to extrapolate irradiance, but also to understand the substructure of the magnetic field and its relationship to spectral irradiance. Giacalone and others also pointed out as a research area that needs to be more fully understood the relationship between the galactic cosmic-ray flux and details of the interaction with the variable solar wind and magnetic field configuration. He and other audience members noted that now, as the extended solar minimum ends, would be an excellent time to study this phenomenon. Philip Judge suggested that, at the moment, the research is limited by time series length and that it will be necessary to continue with accurate measurements in the future. Giacalone posed the question of whether other potential proxy records exist, for example from Mars or a meteorite. Tung led a discussion that reflected the talks and comments from the audience on the issues surrounding the modeling of past climate events. He noted that, when viewed from the perspective of the global response, there is a coherent structure of predictable patterns in latitude, longitude, height, and 28

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three-dimensional spatial domains that may change with changing external or internal forcing terms. Much of the emphasis in climate and especially paleoclimate studies has been on the global mean response. There will, however, be significant changes as a function of height, local time, latitude, longitude, and season. The troposphere tends to respond as a whole to changes in radiative forcing owing to the action of moist convection, and there are coherent patterns to high-latitude response (warming) to the movement of storm tracks and to precipitation patterns. Tung also noted that workshop participant Gerald Meehl had shown that there is a huge response in the equatorial Pacific to variations in solar-cycle flux, cooling of almost 1 degree, which is 10 times what might be expected from solar heating, at the solar peak years, relative to the average climatology, and that Meehl could reproduce this finding in global climate models with the mechanism of positive feedback using clouds. Held pointed out that one of the fundamental questions is whether the Pacific climate system plays a role in amplifying the response of Earth’s climate system to changes in the solar signal. He expressed his doubts that the Pacific Ocean El Niño-Southern Oscillation-like dynamic is strongly involved in the response to the solar cycle. Tung summarized another mechanism discussed in Meehl’s presentation, the top-down stratosphere ozone mechanism, in which increased levels of radiation lead to increased ozone heating and ozone production, which modifies the temperature and zonal wind in the stratosphere, which in turn alters wave propagation. In addition, warming in the tropical lower stratosphere changes vertical convection in the tropics, and can shift the Hadley circulation and storm tracks. Some members of the audience and panel felt that answering the question about the effects on the Pacific climate system may require spectrally resolved measurements of the solar irradiance rather than just of the TSI. The role of top-down forcing is an evolving research question. Pilewskie proposed that since this is a multidisciplinary problem, it would be beneficial to establish a cooperative program between NASA and NSF as has been done previously for specific problems. In addition, he suggested a standardization of data used for models as well as wide availability of data to the public. Other participants voiced support for this suggestion as well. A discussion of the nature of the comparison of model predictions evolved from the review of the results of the talks. The principal issue is that when model results and observations are compared, three effects have to be clearly delineated: (1) variation in the TSI, (2) variation in the shape and magnitude of the spectral distribution of the solar flux, and (3) the time-dependent behavior of changes caused by aerosol events (e.g., eruptions of volcanoes). The need for coherent and well-defined model intercomparisons was also stressed. Held then reviewed the results of session 2 of the workshop, summarizing four themes that he saw as emerging from the talks: (1) there is a spatially coherent picture of past climate emerging from the proxy-based studies; (2) there should be a broad-based study of the available proxy data to look at the amplitude and phase of the deduced solar flux variation; (3) trying in historical studies to separate the effects of changes in TSI from the effects of volcanoes and greenhouse gases in historical studies is fraught with difficulty, and the community needs to be very careful in understanding the error budget associated with this process; and (4) modeling studies are needed of the stratospheric response, the ozone response to the solar cycle, and how those responses penetrate into the troposphere. With respect to point 1 the spatially coherent picture emerging from proxy studies was one of cooler temperatures at high northern latitudes, weaker monsoons, and reduced seasonal movement of the intertropical convergence zones. Caspar Ammann felt that, when considering the correlation of the response of different regions to changes in TSI, if the community is interested in looking at a solar response in the system, the general response of the internal variability must be considered: What is systematic in how the internal variability might be modulated, either in phase or in amplitude, or even in some of its spatial expression, through the solar cycle? Pilewskie summarized the fourth session as consisting of an investigation of the role of energetic particles of both solar and extrasolar origin, with a contribution from Dan Lubin on the need for understanding solar spectral variability through observations of other Sun-like stars. In his presentation, Charles Jackman had focused mostly on solar particles and their influences on the middle atmosphere and potentially on climate, and Jeffrey Pierce’s talk described the influence of galactic cosmic rays on clouds 29

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via two mechanisms—nucleation impacts and changes in the fair weather electric circuit of the atmosphere. A central question is then, Where does the role of energetic particles fit in the effort to understand the influence of the Sun on climate? The discussion among several workshop participants indicated that although TSI is the main driver of response, there is the question of whether smaller (in terms of energy) perturbations can have a tangible effect on climate and by what mechanisms, and whether that impact might show a significant location dependence. The panel discussion concluded with a general discussion of whether the workshop had addressed the issues raised by NASA and NSF—what is known about the role of the Sun in determining climate, and what are the future research directions? During the discussion period, several workshop participants stated that many of the major issues had been aired in the workshop but that significant and important work remained. In addition, participants discussed what sorts of investments agencies could make now with available resources in, for example, improving existing data sets, versus continuing measurements or starting new measurements or new observations, or making new investments in modeling efforts to address these open issues. Suggestions included looking at model studies in a systematic way; examining the paleoclimate record to see whether there were natural oscillations in the system that could result in the system transitioning from one mode to another; developing an understanding of the inherent timescales in the system and the feedbacks that might amplify effects; determining through model or data studies if there are certain regions that are more susceptible to solar influence on climate; evaluating the quality of the past record of environmental response to establish a better chronology; or developing a better understanding of the issues associated with some of the proxies used in studying the Sun-climate linkage. At the end of the panel discussion Gerald North summarized the issues that were developed during the workshop. He made three principal points: 1. NASA has led the way in providing a model for ready access to data from many sources—the challenge is to provide better access to paleoclimate data while recognizing the effort it takes to acquire and archive those data in a form accessible to the community. 2. Coupled models, with their inherent complexity, are the future and need to be used more widely for well-designed studies. It is fortunate that climate modeling has advanced to the point that such projects can be undertaken with some confidence. 3. The directly measured record is limited and not without its issues, and so the challenge is to make sure that a means is developed to infer the time history of TSI variability and the limitations on the ability to specify that past behavior. North also summarized other issues that he felt had been addressed during the workshop and that were particularly noteworthy. Those issues included, among others, the need to be careful in making inferences from the isotope record, which may reflect influences of atmospheric circulation; the need to understand the role galactic cosmic rays may play in cloud nucleation; and the influence of variations in geomagnetic field on the paleo-climate record. North noted that Peter Foukal’s discussion of the Sun was particularly interesting because of the unresolved issues with understanding variability and the sources of variability in TSI arising from the details of the quiet network. Also, a better understanding is needed of how solar brightness, TSI, and the spectral and spatial distribution of energy are affected by the faculae and the dynamics of the Sun. 30