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1 Background THE POTENTIAL SUN-CLIMATE CONNECTION The Sun varies over a broad span of timescales, from its brightening over its lifetime to the fluctuations commonly associated with magnetic activity over days to years. The latter activity includes most prominently the 11-year sunspot cycle and its modulations. The September 2011 workshop summarized in this report explored the connection between this kind of activity and Earth’s climate variability. Research on the connection between solar magnetic activity and Earth’s climate spans the subdisciplines involved in heliophysics, climate science, and the science and engineering of space-borne observatories. Intended to briefly introduce the workshop’s topics, the background information provided here is not a product of the workshop itself. Variations in the total solar irradiance (broad spectral band irradiance: TSI) incident on Earth’s atmosphere can cause imbalances in Earth’s radiation budget that can induce temperature shifts near the surface. The temperature of Earth can be understood to a first approximation as controlled by the balance between the radiative energy received from the Sun and Earth’s thermal emission of radiative energy to space.1 Thermal emission increases with increasing temperature, and Earth can be thought of as settling into an equilibrium by adjusting its temperature so that this thermal radiation balances the solar energy absorbed by the planet. An increase or a decrease in the TSI is expected on this basis to increase or decrease the temperature of Earth. For example, the TSI changes over an 11-year cycle in step with the cycle of sunspots with an amplitude of nearly 0.1 percent, and this variation’s small effects (perhaps an amplitude of a few hundredths of a degree centigrade) on temperatures can be detected, albeit with considerable imprecision, in climate records.2 The sunspot cycle amplitude varies, and during the 17th century the Sun was virtually without spots for about 70 years. This observation has bolstered research on the theoretical underpinning of the solar cycle and the roles played by different features of the Sun’s face (photosphere). These features include the sunspots themselves, the faculae (bright regions surrounding the spots), and the network of magnetic field features over the Sun’s surface.3 Interest is focused on the quantitative relationship between these features and the TSI.4 How can the TSI be extrapolated to past (or future) periods when the Sun appeared to be (or might be) more or less active than now? Related studies include the 1 S. Solomon, D. Qin, M. Manning, R.B. Alley, T. Berntsen, N.L. Bindoff, Z. Chen, A. Chidthaisong, J.M. Gregory, G.C. Hegerl, M. Heimann, et al., Technical Summary in Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (S. Solomon, D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor, and H.L. Miller, eds.), Cambridge University Press, Cambridge, U.K. and New York, N.Y., 2007. 2 C.D. Camp and K.K. Tung, Surface warming by the solar cycle as revealed by the composite mean difference projection, Geophysical Research Letters, 34:L14703, 2007. 3 J. Eddy, The Sun, the Earth, and Near-Earth Space: A Guide to the Sun-Earth System, NP-2009-1-066-GSFC, NASA, Washington, D.C. 4 P. Foukal, C. Fröhlich, H. Spruit, and T.M.L. Wigley, Variations in solar luminosity and their effect on the Earth’s climate, Nature 443:161-166, 2006. 3

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astronomical work on Sun-like stars, whether they exhibit quiet periods, and whether their brightness is diminished during quiet periods.5 The observed sunspot number has been demonstrated to be negatively correlated with the cosmic- ray flux. The cosmic-ray flux reaching Earth’s surface is modulated by the strength of the solar wind. It is now understood that this decrease in cosmic rays is due to changes in the magnetic field geometry in the heliosphere, the bubble blown in the interstellar medium by the solar wind.6 Higher levels of solar activity lead to a decrease in the cosmic-ray flux at Earth. Cosmic rays are potentially implicated in climate change on Earth because as they penetrate Earth’s atmosphere they leave behind an ionized path that could serve as a source of condensation centers that in turn affect cloudiness and Earth’s albedo (reflectivity of solar radiation).7 Cosmic rays may also have an effect on the global electric circuit of Earth’s atmosphere that is caused by thunderstorms separating charge from the surface to the troposphere.8 Research is being conducted on these potential mechanisms and their possible relevance as a climate-forcing agent. Furthermore, solar energetic particle (SEP) events, created at the shock front of coronal mass ejections (CMEs), for example, can influence the composition of the upper atmosphere. During a SEP event, solar protons and electrons follow Earth’s magnetic field lines toward the poles. The higher-energy particles can penetrate well into the stratosphere where they ionize the atmosphere, producing nitrogen oxides, whereas lower-energy particles can create nitrogen oxides in the lower thermosphere and mesosphere that then descend into the polar stratosphere. These nitrogen oxides can destroy ozone, thus altering not only the chemistry but also the radiative balance of that region. THE MEASUREMENT RECORD FROM SPACE The TSI and the spectral solar irradiance are the two most important measurements of the Sun’s output as it impacts climate. The continuous 33-year record of total solar irradiance from space-based observations is shown in Figure 1.1. This data record is the result of overlapping measurements from several instruments flown on different missions. Measurements made by individual radiometers providing the data shown in Figure 1.1 exhibit a spread of nearly 1 percent that is of instrument rather than solar origin (the upper panel) and far exceeds the 11-year or rotational solar variability. These radiometers’ measurements have a reproducibility of roughly 0.03 percent over decadal timescales.9 A 2005 workshop conducted at the National Institute of Standards and Technology in Gaithersburg, Maryland,10 sparked investigations into the effects of diffraction, scattered light, and aperture area measurements on the differences between instrument results. Evident in this combined, recalibrated record is an 11-year cycle with peak-to-peak amplitude of approximately 0.07 percent and variations greater by a factor of two to three that are associated with short-term transits of sunspots due to solar rotation (the lower panel of Figure 1.1). Measurement continuity has enabled successive radiometric time series obtained from different space missions to be intercalibrated to produce a 33-year-long composite TSI record. The need for such intercalibration makes 5 P.G. Judge, and S.H. Saar, The outer solar atmosphere during the Maunder Minimum: A stellar perspective, The Astrophysical Journal 663:643, 2007. 6 L.A. Fisk, K.P. Wenzel, A. Balough, R.A. Burger, A.C. Cummings, P. Evenson, B. Heber, J.R. Jokipii, M.B. Krainev, and J. Kota, et al., Global processes that determine cosmic ray modulation. Space Science Review 83:179- 214, 1998. 7 R. Harrison, The global atmospheric electric circuit and climate, Surveys in Geophysics 25:441-484, 2004. 8 L.I. Dorman and I.V. Dorman, Possible influence of cosmic rays on climate through thunderstorm clouds, Advances in Space Research 35(3):476-483, 2005. 9 M. Fligge and S.K. Solanki, The solar spectral irradiance since 1700, Geophysical Research Letters 27:2157, 2000. 10 J. Butler, B.C. Johnson, J.P. Rice, E.L. Shirley, and R.A. Barnes, Sources of differences in on-orbital total solar irradiance measurements and description of a proposed laboratory intercomparison, Journal of Research of National Institute of Standards and Technology 113:187-203, 2008. 4

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FIGURE 1.1 Space-borne measurements of the total solar irradiance (TSI) (top) span the last 34 years. Offsets between measurements are the result of calibration differences between instruments. Measurement continuity allows construction of a composite solar climate data record (bottom) showing ~0.1 percent variations with solar activity on 11-year and shorter timescales. SOURCE: Courtesy of Greg Kopp, University of Colorado; high-resolution EPS plots are available at http://spot.colorado.edu/~koppg/TSI. 5

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this record vulnerable to loss in the event of a gap in measurements. Proxy records of radioisotopes provide evidence of long-term change in solar activity, but these must be tuned and extrapolated from the existing TSI data record; however, based on present understanding, the irradiance variations inferred from them are no greater than those observed radiometrically over recent solar cycles. New evidence now suggests that secular variations of larger amplitude may have occurred on multi-decadal to millennial timescales. The Intergovernmental Panel on Climate Change Fourth Assessment Report estimated the direct radiative forcing due to changes in solar output since 1750 to be ~0.12 W m−2 (0.06 to 0.30); corresponding to a change in TSI of ~0.5 W m−2 from its baseline value of 1360 W m-2) with a factor-of- two uncertainty.11 Shortly before 1750, the Maunder Minimum may have caused greater changes in solar forcing. Continuous measurements of solar ultraviolet radiation began in 1978 with the Nimbus-7 Solar Backscatter Ultraviolet (SBUV).12 These measurements were followed by those from the Solar Mesosphere Explorer,13 NOAA-9 SBUV/2, NOAA-11 SBUV/2, the Upper Atmosphere Research Satellite Solar Stellar Intercomparison Experiment (SOLSTICE),14 and the Solar Ultraviolet Spectral Irradiance Monitor.15 The present-day Solar Radiation and Climate Experiment (SORCE) SOLSTICE and SORCE Spectral Irradiance Monitor (SIM) extend this continuous (albeit differing in spectral coverage, resolution, and instrument accuracies and stabilities) record of the solar ultraviolet and its variability. Although the ultraviolet region of the spectrum provides only a small fraction of the TSI, ultraviolet irradiance can change by several percent over the solar cycle, and thus represents an important source of modulation of the energy deposition and composition in the middle and upper atmosphere. Ultraviolet irradiance both changes the radiative balance of the atmosphere and affects the shape of the spectrum of radiation reaching the lower atmosphere. Such variations are thought to drive the top-down coupling mechanism. The record of measurement of the continuous, full solar irradiance spectrum, which is much shorter than the record of TSI, commenced with measurements by the SIM on the SORCE satellite in 2003. Results have indicated that ultraviolet trends during cycle 23 were larger than those observed in previous cycles, and were compensated by trends in other bands that increased with decreasing solar activity.16 Spectral observations from SIM suggest a very different response in Earth’s atmosphere because of this compensating spectral behavior, suggesting that further modeling studies and analysis of existing atmospheric observations may be needed, as well as continued validations of these new observations. POTENTIAL PERTURBATIONS OF CLIMATE DUE TO SOLAR VARIABILITY Research into possible mechanisms of Sun-climate coupling has taken several paths. Progress is hampered by incomplete understanding of solar variability, climate, and their complex interaction. 11 S. Solomon et al., Technical Summary in Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, 2007. 12 B.M. Schlesinger and R. P. Cebula, Solar variation 1979-1987 estimated from an empirical model for changes with time in the sensitivity of the solar backscatter ultraviolet instrument, Journal of Geophysical Research 97:119-10,134, 1992. 13 G.J. Rottman, Observations of solar UV and EUV variability, Advances in Space Research 8:53-66, 1988. 14 G.J. Rottman, T. Woods, M. Snow, and G. DeToma, The solar cycle variation in ultraviolet irradiance, Advances in Space Research 27:1927-1932, 2001. 15 L.E. Floyd, D. K. Prinz, P. C. Crane, and L. C. Herring, Solar UV irradiance variation during cycles 22 and 23, Advances in Space Research 29:1957-1962, 2002. 16 J. Harder, J.M. Fontenla, P. Pilewskie, E.C. Richard, T.N. Woods, Trends in solar spectral irradiance variability in the visible and infrared, Geophysical Research Letters 36: L07801, 2009. 6

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The changes in TSI over the solar cycle provide a good starting point for discussing these challenges. Periodic, or quasi-periodic, forcing17 provides invaluable information on climate dynamics. Other than the seasonal variability on a yearly scale and the precession of the equinoxes (the change of the season in which the minimum Sun-Earth distance occurs) with scales of 20,000 years, the only quasi- periodic forcing term is the 11-year solar cycle. Based on the climate community’s best estimates of global climate sensitivity, the solar stimuli are much smaller than would be required to dominate the temperature record on decadal timescales.18 The search for the solar cycle signal in the temperature record, albeit small, continues to motivate much of the climate research in this area, and so far two basic mechanisms have been modeled. In the first, the 11-year cycle may affect the climate system via the bottom-up total solar irradiance path through which solar cycle effects can manifest themselves at the surface and its nearby environment. In general, this bottom-up driver is strongest in the tropics, where there are feedbacks (from clouds, ocean currents, sea surface temperature, and so on) present in the climate system that strengthen the effect and even show up at higher latitudes. A second avenue of inquiry is the top-down mechanism that makes use of the modulated absorption of ultraviolet radiation in the stratosphere. Top-down mechanisms operate through changes in the more energetic, shorter-wavelength components of the solar spectrum that influence stratospheric temperatures and winds directly and through absorption by stratospheric ozone. Early work by Karen Labitzke and Harry Van Loon on interactions of the solar cycle and the quasi-biennial oscillation of the equatorial stratosphere helped direct attention to the top-down pathway.19,20 The modulation of stratospheric temperatures is clear from observations. Climate models also take this modulation as input and have demonstrated significant perturbations on tropospheric circulations. If borne out by future studies and shown to be of sufficient magnitude, this mechanism could be an important pathway in the Sun-climate connection, particularly in terms of regional impacts. However, it is important to realize that, unlike the bottom-up mechanism, it can in itself contribute very little to global temperature variations. The effects on climate of centennial timescale variations in TSI have been an even more difficult and contentious issue. Since the work of Jack Eddy in 1976,21 the claim that the lower temperatures of the Little Ice Age from roughly 1600 to 1850 are connected to the secular changes in the Sun, as reflected in paleoclimate data derived from cosmogenic isotopes in sediments and the observed record of sunspots, remains an unresolved research topic (Figure 1.2). Recent findings that removal of small-scale photospheric fields could dim the Sun more than previously expected increase the likelihood of such variations in secular irradiance.22 It remains to be shown whether or not the field decreased significantly below levels observed during normal 11-year activity minima. Ongoing discussion of the role of solar variations in the early 20th century has given rise to the unfounded conjecture that the observed increase in temperature in the last half century could also be due to changes in TSI rather than to anthropogenic 17 Forcing, or radiative forcing, denotes an externally imposed perturbation in the radiative energy budget of Earth’s climate system. As defined in S. Solomon et al., Technical Summary in Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, 2007. 18 D.R. Marsh and R.R. Garcia, Attribution of decadal variability in lower-stratospheric tropical ozone, Geophysical Research Letters, 34: L21807, 2007. 19 H. van Loon and Labitzke, The Southern Oscillation. Part V: The anomalies in the lower stratosphere of the northern hemisphere in the winter and a comparison with the Quasi-Biennial Oscillation, Monthly Weather Review 115:357-369, 1987. 20 K. Labitzkeand and H. van Loon, Associations between the 11-year solar cycle, the QBO and the atmosphere, Part I: The troposphere and stratosphere in the northern hemisphere in winter, Journal Atmospheric and Solar- Terrestrial Physics, 50:197-206, 1988. 21 J.A. Eddy, The Maunder Minimum, Science 192:1189-1202, 1976. 22 P. Foukal, A. Ortiz, and R. Schnerr, Dimming of the 17th Century Sun, The Astrophysical Journal Letters 733:L38, 2011. 7

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FIGURE 1.2 The yearly averaged sunspot number for a period of 400 years (1610-2010). The Maunder Minimum is shown during the second half of the 16th century. SOURCE: Courtesy of NASA Marshall Space Flight Center. influences. The Intergovernmental Panel on Climate Change Fourth Assessment23 and the recent National Research Council report on climate choices24 agree that there is no substantive scientific evidence that solar variability is the cause of climate change in the past 50 years.25 However, the mechanisms by which solar variations can affect climate over longer timescales remain an open area of research. THIS WORKSHOP REPORT Chapter 2 of this report summarizes the workshop presentations, ordered according to broad science topics. Chapter 3 summarizes the panel discussion session. Appendix A contains the statement of task and work plan for the project. The full workshop agenda is included in Appendix B, and workshop presentation abstracts, prepared by the workshop speakers, are included in Appendix C. This report summarizes the views expressed by individual workshop participants. Although the committee is responsible for the overall quality and accuracy of the report as a record of what transpired at the workshop, the views contained in the report are not necessarily those of all workshop participants, the committee, or the National Research Council. 23 S. Solomon et al., Technical Summary in Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, 2007. 24 National Research Council, America’s Climate Choices, 2011. 25 National Research Council, Advancing the Science of Climate Change, 2010. 8