Overview and Advances in Solar Radiometry for Climate Studies
Greg Kopp, Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder
The Sun provides nearly all the energy driving Earth’s climate system. Even typical short-term variations of 0.1 percent in this incident irradiance exceed all other energy sources combined. The 33‐year space-borne total solar irradiance (TSI) measurement record, shown in Figure C.1, enables estimates of the portion of climate change due to solar variability on global and regional scales. Extensions of this modern record via proxies provide historical estimates of long-term solar variability and the corresponding climate effects. Daily spectral solar irradiance (SSI) measurements over the majority of the solar spectrum commenced a few years ago and show promise for helping researchers understand heating, circulation, and chemistry effects in Earth’s atmosphere.
To discern natural and anthropogenic effects, climate studies require long-term records of incident solar irradiances. For TSI, the net radiative energy driving Earth’s climate system, these are based on the space-borne measurements shown in Figure C.1. The offsets between these measurements, which are as large as 0.34 percent currently, are due to instrument calibration differences. Recent laboratory tests by the international teams involved in these measurements have identified the primary cause of these measurement offsets, helping improve the existing record retroactively. Such offsets, along with differing instrument drifts, must be corrected to create a composite TSI record having the accuracy and stability needed for reliable climate studies and estimates of Earth’s radiative energy balance.
I will give an overview of solar irradiance measurements and recent progress to improve this record’s accuracy. Using estimates of solar variability over long-term timescales relevant for climate studies, I will derive the record’s accuracy and stability requirements and assess the current status for achieving these requirements, comparing to recent solar minima as examples. By extending the record via proxies to paleo timescales, I will discuss estimates of climate sensitivity to solar forcing over recent and historical times. I will also present the current state of the instruments acquiring these measurements and the planned future means of continuing the TSI record as well as the newer SSI measurements.
Assessing Solar and Solar-Terrestrial Influences as a Component of Earth’s Climate Change Picture
Daniel N. Baker, Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder
Researchers have long been intrigued by questions about how solar variability and related solar-terrestrial influences can affect Earth’s middle and lower atmosphere. A goal of basic research programs has been to establish a comprehensive intellectual foundation for the investigation of the effect of solar variability on climate. It is clear that conclusive observations of cause-effect relationships (at the requisite level of confidence) are a very large challenge. Satisfactory work in this arena requires close collaboration between solar, magnetospheric, and atmospheric scientists. It is important to note that new generations of atmospheric models now are able to couple together all the layers of Earth’s extended atmosphere. Through such models, and with increasingly complete observations, we are in a steadily
FIGURE C.1 The 33-year total solar irradiance (TSI) record is the result of measurements made by several instruments. Measurement overlap helps correct for the majority of the differences between instruments, but improved instrument calibrations are needed to provide the required accuracies and stabilities in this solar climate data record for reliable climate studies. SOURCE: Courtesy of G. Kopp, University of Colorado.
improving position to understand the complex (and often subtle) ways that solar influences at high altitudes affect the lower atmosphere. This talk will discuss long-standing questions and recent progress in understanding this crucial aspect of the Sun-Earth connection.
Heliospheric Phenomena Responsible for Cosmic Ray Modulation at the Earth
Joe Giacalone, Department of Planetary Sciences, University of Arizona, Tucson
Galactic cosmic rays (GCRs) are essentially uniform in the space immediately outside Earth’s heliosphere and penetrate the solar system with very nearly equal probability from all directions. The GCR spectrum there is a smooth power law in energy over several decades. As they enter the solar system, GCRs suffer both spectral and intensity variations resulting from their interaction with the solar wind, interplanetary magnetic field, and heliosphere. The GCR intensity is known to anti-correlate with the number of sunspots on the Sun—having a higher intensity when there are few sunspots. This anti-correlation is the basis for using cosmogenic nuclei deposited in tree rings and ice cores as a proxy measure of solar activity dating back thousands of years.
In addition to the well-known 11-year cosmic-ray cycle, there is also a 22-year variation related to the polarity of the solar magnetic field. These variations can be interstood using the physics of charged-particle motion in turbulent electric and magnetic fields associated with the solar wind plasma. This talk will review the understanding of the physics of cosmic-ray transport, focusing primarily on the causes of the modulation of GCRs in the solar system. It will also discuss observations of cosmic rays from the recent long and deep sunspot minimum.
Recent Findings on Dimming of the 17th Century Sun
Peter Foukal, Heliophysics, Inc.
Sunspots and faculae modulate solar convective heat flow and total solar irradiance over the 11-year activity cycle. The amplitude of this modulation attains 0.09 percent in annual means during the largest recorded solar cycle that peaked around 1957. The rising envelope of this modulation, caused by the increase in spots and faculae observed since the beginning of regular collection of sunspot data around 1700, sets a lower limit of about 0.04 percent to the 11-year smoothed TSI increase over the intervening 2.5 centuries.
Additional solar dimming might have occurred during the 17th century if the area covered by the small-diameter, bright photospheric magnetic flux tubes of the quiet Sun decreased during the extended Maunder Minimum of solar activity between about 1645 and 1715. Their contribution sets the zero level of TSI at activity minima. Evidence for their disappearance and for a resulting additional 0.2 percent dimming during the Maunder Minimum was put forward, based on stellar photometry.1 That stellar photometry evidence was retracted in 2002, but this does not necessarily mean that the dimming did not occur.
New findings from solar photometry indicate that the TSI-effectiveness per unit area of these small flux tubes increases with decreasing cross sectional area. So the progressive removal of ever-smaller flux tubes with declining solar activity during an extended minimum would dim the Sun more than expected from standard irradiance models. Such models linearly extrapolate the irradiance contribution of larger active region faculae, to the smaller flux tubes of the quiet Sun.
These findings2 make it more likely that the 17th century Sun might have dimmed by a climatologically significant (~ 0.2 percent) amount without requiring complete disappearance of photospheric magnetism. Such partial disappearance would agree better with radioisotope evidence that a weakened 11-year cycle persisted throughout the Maunder Minimum. Improved observations are under way to accurately measure the uncertain TSI contribution of the quiet Sun flux tubes down to sizes barely resolvable with the largest solar telescopes.3
Also, it remains to be seen whether 17th century photospheric magnetism weakened below the level observed during normal 11-year activity minima.4 During the extended activity minimum of 2008-2009, the main indices such as F10.7 and Mg II dipped several percent below their preceding 11-year minima. These anomalous dips during a minimum only about 1 year longer than normal, suggest that magnetism during a minimum extending for 70 years would have decayed well below quiet Sun levels. However, examination of this important conclusion using, for example, improved 10Be radioisotope evidence is desirable.
This work has been supported by NASA Living With a Star grants NNX09AP96G and NNX10AC09G.
1 J. Lean, A. Skumanich, and O. White, Estimating the Sun’s radiative output during the Maunder Minimum, Geophysical Research Letters 19(15):1591, 1992.
2 P. Foukal, A. Ortiz, and R. Schnerr, Dimming of the 17th Century Sun, The Astrophysical Journal Letters 733:L38, 2011.
3 R. Schnerr and H. Spruit, The brightness of magnetic field concentrations in the quiet Sun, Astronomy and Astrophysics 532:A136, 2011.
4 C. Schrijver, W. Livingston, T. Woods, and R. Mewaldt, The minimal solar activity in 2008-2009 and its implications for long-term climate modeling, Geophysical Research Letters 38: L06701, 2011.
The Record of Solar Forcing in Cosmogenic Isotope Data
Raimund Muscheler, Department of Earth and Ecosystem Sciences, Division of Geology, Lund University, Sweden
Cosmogenic radionuclides are the most reliable proxies for reconstructing solar activity variations thousands of years back into the past. Several characteristics of solar activity variations have been identified in these records. These range from longer-term solar cycles (e.g., the 207-year cycle), the bundling of solar minimum periods and possible longer-term changes in solar activity. Recently an increasing number of studies have attempted quantitative reconstructions of solar activity changes based on radionuclide records.5,6,7,8
Disagreements between some of the results illustrate the difficulties in isolating the solar signal from ice core 10Be and tree-ring 14C records. The challenges consist in identifying influences of weather and climate on cosmogenic radionuclide records. Failure to correctly identify a climate impact in cosmogenic radionuclide records could lead directly to an erroneous inference of a solar influence on climate. Moreover, the geomagnetic influence has to be corrected for. For quantitative estimates of absolute solar activity levels one has to correctly normalize the records and, in addition, there are different estimates of the intergalactic cosmic ray spectrum. Depending on the applied spectrum one can obtain different results.
Figure C.2 illustrates the potential and the problems of cosmogenic radionuclide-based reconstructions of solar activity changes. It shows a reconstruction of the solar modulation function based on 10Be and 14C together with the group sunspot number reconstruction. The calculations are based on the production results from Masarik and Beer,9 and the data are corrected for the geomagnetic dipole field intensity variations.10Figure C.2 shows that especially long-term changes in solar activity or absolute levels of solar activity are uncertain. Depending on the geomagnetic field correction and the interpretation of the radionuclide records one can get significant differences in the results. The same applies to the past 100 years, which is a crucial period for connecting the radionuclide records to observational data. For this period the 14C record includes the difficulty of an anthropogenic influence due to fossil fuel burning11 and nuclear weapon tests. 10Be records for the last century show differences between Greenland and Antarctica that add to the normalization uncertainty.12
Nevertheless, there is an agreement between different radionuclide records for the majority of the supposed solar minimum periods during the past 10,000 years (see 10Be can reduce the
5 R. Muscheler, F. Joos, J. Beer, S.A. Mueller, M. Vonmoos, and I. Snowball, Solar activity during the last 1000 yr inferred from radionuclide records, Quaternary Science Reviews 26(1-2):82-97, 2007.
6 S.K. Solanki, I.G. Usoskin, B. Kromer, M. Schüssler, and J. Beer, Unusual activity of the Sun during recent decades compared to the previous 11,000 years, Nature 431:1084-1087, 2004.
7 F. Steinhilber, J.A. Abreu, and J. Beer, Solar modulation during the Holocene, Astrophysics Space Science Transactions, 4:1-6, 2008.
8 M. Vonmoos, J. Beer, and R. Muscheler, Large variations in Holocene solar activity: Constraints from 10Be in the Greenland Ice Core Project ice core, Journal of Geophysical Research 111:A10105, 2006.
9 J. Masarik, J. Beer, Simulation of particle fluxes and cosmogenic nuclide production in the Earth’s atmosphere, Journal of Geophysical Research 104(D10):12099-12111, 1999.
10 S. Yang, H. Odah, and J. Shaw, Variations in the geomagnetic dipole moment over the last 12000 years, Geophysical Journal International 140:158-162, 2000.
11 H.E. Suess, Natural radiocarbon and the rate of exchange of carbon dioxide between the atmosphere and the sea pp. 52-56 in Proceedings of the Conference on Nuclear Processes in Geological Settings, (National Research Council Committee on Nuclear Science, ed.), University of Chicago Press, Chicago, Ill., 1953.
12 G.M. Raisbeck, and F. Yiou, Comment on “Millennium scale sunspot number reconstruction: Evidence for an unusually active Sun since the 1940s”, Physical Review Letters 92(19):199001, 2004.
uncertainties in radionuclide-based solar activity reconstructions. With new high-resolution 10Be and 14C records there is the potential to track solar activity variations including the solar 11-year cycle more than 10,000 years back in time. In addition, there is the prospect that one can reliably reconstruct sustained levels of high or low solar activity. Cyclic variations in cosmogenic radionuclide records might allow researchers to estimate likely levels of future solar activity. For example, the suggestions of the dawn of a new Maunder Minimum-type solar minimum are based largely on extrapolation of the cyclic behavior visible in radionuclide records. However, without a better understanding of the solar dynamo these predictions seem rather speculative at the moment.
FIGURE C.2 Two estimates of variations in the solar modulation function inferred from an ice core 10Be record (Vonmoos et al., 2006) and a tree-ring-based 14C production record (Muscheler et al., 2004) together with the group sunspot number record for the past 400 years (Hoyt and Schatten, 1998). The so-called Maunder Minimum, which is characterized by an almost complete lack of sunspots, is highlighted with the grey shading. Both radionuclide-based records are low-pass filtered with a cut-off frequency of 1/100 yr-1. See M. Vonmoos, J. Beer, and R. Muscheler, Large variations in Holocene solar activity: Constraints from 10Be in the Greenland Ice Core Project ice core, Journal of Geophysical Research 111:A10105, 2006; R. Muscheler, J. Beer, G. Wagner, C. Laj, C. Kissel, G.M. Raisbeck, F. Yiou, and P.W. Kubik, Changes in the carbon cycle during the last deglaciation as indicated by the comparison of 10Be and 14C records, Earth Planet Science Letters 219(3-4):325-340, 2004; and D.V. Hoyt, and K.H. Schatten, Group sunspot numbers: a new solar activity reconstruction, Solar Physics 179(1):189-219, 1998. SOURCE: Raimund Muscheler, Lund University, “The Record of Solar Forcing in Cosmogenic Isotope Data,” presentation to the Committee on the Effects of Solar Variability on Earth’s Climate, September 8, 2011.
Issues in Climate Science Underlying Sun/Climate Research
Isaac Held, National Oceanic and Atmospheric Administration Geophysical Fluid Dynamics Laboratory
In this talk I will discuss some aspects of climate research that provide a wider context for analyses of the climatic effects of solar variability. I divide this discussion into three parts: bottom-up effects related to the energy balance of the troposphere; top-down effects related to effects on the troposphere of the state of the stratosphere; and the possibility of solar-modulated cosmic ray-induced nucleation on cloud cover.
I will first discuss the proposition that the troposphere acts as a strongly coupled system as it responds to changes in its energy balance, insensitive to the details of the spatial distribution of the forcing—especially in the vertical, and to a more limited extent, horizontally. This coupling underlies the utility of the concept of “radiative forcing.” I’ll discuss the standard conversion factor (doubling CO2 ⇔ 2 percent change in insolation) that is a simple consequence of CO2 radiative forcing estimates, and the classic experiments at the dawn of research on climate change with global climate models that showed very similar tropospheric responses to changes in the total solar irradiance and to changes in CO2. The importance of the frequency-dependence of climate responses in relating greenhouse gas, volcanic, and solar cycle responses will be mentioned. Hydrological responses provide the most likely route through which to break away from a universal relationship between radiative forcing and response, and some geoengineering literature will be introduced in this context.
Stratospheric influences on the troposphere have been discussed in the context of research on the response to volcanoes, solar cycle ultraviolet forcing, the quasi-biennial oscillation, and, especially, the ozone hole—as well as in the long-range weather prediction context. I will describe an emerging picture of the mechanisms responsible for this coupling, especially in regard to coupling to the tropospheric annular modes and the central role played by meridional temperature gradients at the tropopause/lower stratosphere. I will argue that the ozone hole plus observed trends in Southern Hemisphere winds provides a useful quantitative check on models of this coupling, indicating in particular how large the perturbation to lower stratospheric temperature gradients needs to be to generate an observable effect.
Finally, I will briefly mention research on the indirect aerosol effects through which particles affect climate not by direct alternation of solar fluxes but by modifying cloud condensation nuclei. Observational and modeling studies of the susceptibility of cloud radiative properties to changes in cloud condensation nuclei are especially relevant as background for any discussions of conceivable solar activity-modulated cosmic ray ionization effects on cloud condensation nuclei and climate.
Indirect Climate Effects of the Sun Through Modulation of the Mean Circulation Structure
Caspar Ammann, National Center for Atmospheric Research
Solar irradiance changes have now been monitored from space for several decades. Based on the collected data, there are no clear indications that the total irradiance changes might have varied substantially more than the range directly observed. In fact, climate reconstructions would suggest that larger irradiance changes are not necessary to explain the mean temperature fluctuations of past centuries and millennia. There is good confidence in this interpretation because the global/hemispheric mean temperature of the globe is tightly linked to the radiative balance of the planet, actually remarkably so, despite the often stated low level of understanding of solar radiative forcing. Therefore, direct irradiance changes most likely have left relatively small, albeit discernible, imprints in large-scale mean temperatures.
However, two other issues can be raised that, in combination, might offer a suggestion for an indirect pathway for the Sun to affect the climate system.
Most recently, the observations by the Solar Radiation and Climate Experiment (SORCE) spectral irradiance monitor have highlighted large variability in the higher-frequency component of the solar spectrum, even higher than previously acknowledged. Independent of how exactly the variations are
distributed across the wavelength, a strong shift in its distribution will have effects on the vertical temperature profile of Earth’s atmosphere, and therefore on various important properties, such as the chemical composition, temperature gradients, and steering of waves through refraction or reflection. Strong spectral changes in solar output might therefore be a possible cause of mean circulation changes. The strongest effects would be expected above the lower atmosphere, but because of vertical coupling, some effects down toward the surface are possible.
This is where the second factor potentially could be helping to substantiate such a solar modulation of the climate system. Paleoclimate records have long been used to suggest a solar influence on climate. The problem generally has been that a unifying framework was lacking as to how to interpret that influence. Very often, the interpretation of a solar influence in the paleoclimate archives was simply based on statistical analyses, most commonly through quasi-identification of certain frequencies of variability close to “known solar bands.” Criticism of picking and choosing of records, as well as the lack of process understanding that would have helped explain mechanistically a solar influence on the records, separated the paleo community on the Sun-climate connection question. However, as researchers gain more insight into the various paleoclimate records, it becomes possible to interpret the various time series within a quantitative geophysical framework, a framework held together by dynamical processes, not mere correlations. With this approach, new ways to reconstruct multivariate climate fields have emerged. These approaches allow for a more flexible and comprehensive inclusion of different climate signals that include seasonally dependent temperature, and moisture, as well as links to regional or large-scale dynamics such as atmospheric wave structure, coastal upwelling, and even ocean overturning.
Based on such a system-level interpretation of past climate, it becomes possible to analyze both temporal and spatial changes in light of different climate drivers. This topic offers a fruitful environment for scientific investigations across the solar physics, climate dynamics, paleoclimate, and climate modeling disciplines. Although not conclusive, a solar influence on climate can be postulated more robustly in the arena of indirect effects on large-scale circulation rather than through direct irradiance alone. At the same time, such a multiscale approach might offer an important evaluation of climate models in their ability to reproduce changes in variability that are ultimately going to be responsible for regional climate, just as they have in the past.
Climate Response to the Solar Cycle as Observed in the Stratosphere
Lon L. Hood, Lunar and Planetary Laboratory, University of Arizona
Multiple linear regression analyses of satellite-derived stratospheric ozone and temperature records indicate the existence of significant responses to 11-year solar forcing primarily at tropical and subtropical latitudes. The observed 11-year variation of ozone and temperature in the tropical upper stratosphere is attributable to direct photochemical and radiative forcing by solar irradiance at ultraviolet wavelengths, which is mainly responsible for the production of ozone in the stratosphere. In addition, a significant 11-year variation of ozone and temperature is observed in the tropical and subtropical lower stratosphere that has a dominantly dynamical origin and is currently not well understood. The lower stratospheric ozone variation is the principal contributor to the solar-cycle variation of total (column) ozone. At higher latitudes in the polar upper stratosphere and lower mesosphere, solar and magnetospheric energetic particle precipitation produces detectable interannual and decadal changes in ozone, especially in the Southern Hemisphere. Finally, in the polar lower stratosphere, a nonlinear response to 11-year solar forcing of temperature and geopotential height is observed with a sign that depends on the phase of the equatorial quasi-biennial wind oscillation (QBO).
The origin of the observed tropical and subtropical lower stratospheric response to 11-year solar forcing is a topic of current research and has implications for understanding of solar-induced climate change in the troposphere. Two end-member mechanisms can be identified. First, it is possible that direct solar (mainly ultraviolet) forcing in the upper stratosphere perturbs stratospheric circulation in such a way as to modify planetary wave propagation and decelerate the mean meridional (Brewer-Dobson)
circulation (BDC) near solar maxima. This “top-down” mechanism would then result in an 11-year variation of the tropical upwelling rate, which would in turn advectively modulate ozone concentrations in the lower stratosphere, consistent with observations. Second, it is possible that there is a significant troposphere-ocean response to solar variability that is driven mainly by direct changes in total solar irradiance. This “bottom-up” mechanism would then reduce planetary wave amplitudes in the troposphere near solar maxima, which would also modulate the BDC, the tropical upwelling rate, and ozone concentrations in the lower stratosphere, as is observed. These mechanisms are not mutually exclusive and both may be important. For example, top-down forcing from the upper stratospheric response may, in principle, produce significant indirect effects on surface climate, which would then have dynamical feedbacks on the stratosphere. However, if it is found that top-down ultraviolet forcing mainly produces the lower stratospheric response, then support would be obtained for the view that top-down solar ultraviolet forcing is the primary driver of solar-induced tropospheric climate change. If, on the other hand, the observed lower stratospheric response is primarily a consequence of bottom-up dynamical feedbacks from a troposphere-ocean response that is driven mainly by changes in TSI, then it would follow that direct TSI forcing of near-surface climate is the main driver of solar-induced climate change.
Current work focuses mainly on investigation of the bottom-up mechanism for producing the lower stratospheric response to 11-year solar forcing at low latitudes. Specifically, we are investigating whether a statistically significant solar cycle response of the troposphere-ocean system exists that has characteristics consistent with producing the observed lower stratospheric response through a modification of planetary wave amplitudes. To characterize the troposphere-ocean response, a multiple linear regression statistical model is applied to Hadley Centre sea level pressure (SLP) and sea surface temperature (SST) data, which are available back to ~1870. In agreement with previous authors, the most statistically significant positive response is obtained for SLP in the North Pacific during northern winter, consisting of a weakening and westward shift of the Aleutian low near solar maxima relative to solar minima. This response is similar to that which occurs during the cold (La Niña) phase of the El Niño-Southern Oscillation. To test whether the response is indeed solar (rather than a consequence of aliasing from a few strong El Niño-Southern Oscillation events), the analysis is repeated for two separate time periods (1880-1945 and 1946-2009). It is found that the North Pacific SLP response to 11-year solar forcing is approximately repeatable during the two time periods, supporting the reality of the solar response. An associated response of North Pacific wintertime SST is also obtained but is less repeatable for separate time periods. In addition, a marginally significant SLP decrease over eastern Europe is obtained near solar maxima relative to solar minima.
The “La Niña-like” character of the North Pacific SLP response is in agreement with previous analyses using compositing methods13 and with some climate model studies.14 It also agrees with some paleoclimate studies, which have found evidence for La Niña-like conditions in the Pacific region during periods of prolonged solar activity increases, such as the “medieval climate anomaly.”15 Both the positive North Pacific SLP response and the negative eastern European SLP response under solar maximum conditions correspond to regions of known tropospheric precursors of anomalous stratospheric circulation changes.16 Increases in North Pacific SLP tend to weaken and shift westward the Aleutian low, while decreases in eastern European SLP tend to weaken and shift eastward the Siberian high. To first order, this weakens the wave one quasi-stationary Rossby wave forcing at northern middle to high latitudes,
13 See, for example, H. van Loon, G. Meehl, and D. Shea, Coupled air-sea response to solar forcing in the Pacific region during northern winter, Journal of Geophysical Research 112:D02108, 2007.
14 See, for example, G. Meehl, J. Arblaster, K Matthes, F. Sassi, and H. van Loon, Amplifying the Pacific Climate System response to a small 11-year solar cycle forcing, Science 325(5944):1114-1118, 2009.
15 See, for example, M. Mann, Z. Zhang, S. Rutherford, R. Bradley, M. Hughes, D. Shindell, C. Ammann, G. Faluvegi, and F. Ni, Global signatures and dynamical origins of the Little Ice Age and Medieval Climate Anomaly, Science 326(5957):1256-1260, 2009.
16 See, for example, C. Garfinkel, D. Hartmann, and F. Sassi, Tropospheric precursors of anomalous Northern Hemisphere stratospheric polar vortices, Journal of Climate 23:3282-3299, 2010.
which allows a strengthening of the polar vortex and a deceleration of the BDC. The observed SLP response is therefore most consistent with a bottom-up mechanism for driving the tropical lower stratospheric response. A simplified analytic model suggests that much of the observed tropical lower stratospheric response, including the solar cycle variation of total ozone, can be explained by this mechanism.
However, these results are preliminary, and more work is needed to establish the relative importance of the bottom-up and top-down mechanisms for producing the lower stratospheric response. More detailed comparisons with global climate model simulations with and without a coupled troposphere and ocean, with and without a simulated QBO, with TSI changes alone, and with various possible solar spectral irradiance changes would assist in identifying the dominant physical mechanisms.
Amplifying the Pacific Climate System Response to a Small 11-Year Solar Cycle Forcing
One of the mysteries regarding Earth’s climate system response to variations in solar output is how the relatively small fluctuations of the 11-year solar cycle can produce the magnitude of the observed climate signals in the tropical Pacific associated with such solar fluctuations. These observations include, for peaks in the 11-year sunspot cycle, below-normal sea surface temperatures in the equatorial eastern Pacific, enhanced precipitation in the Pacific intertropical convergence zone and South Pacific convergence zone, and above-normal sea level pressure in the midlatitude North and South Pacific. To investigate what could be producing these signals in observations, two mechanisms, the top-down stratospheric response of ozone to fluctuations of shortwave solar forcing, and the bottom-up coupled ocean-atmosphere surface response, are included in versions of three global climate models with either mechanism acting alone or both together. We show that the two mechanisms act together to enhance the climatological off-equatorial tropical precipitation maxima in the Pacific, lower the eastern equatorial Pacific sea surface temperatures during peaks in the 11-year solar cycle, and reduce low-latitude clouds to amplify the solar forcing at the surface.
Detection of the Solar Signal in Climate from Paleorecords
Raymond S. Bradley, Climate System Research Center, University of Massachusetts
All paleoclimatic studies of solar forcing rely on the record of 14C or 10Be anomalies from tree rings or ice cores as a proxy for changes in irradiance, even though the relationship between variations in these cosmogenic isotopes and total (or ultraviolet) solar irradiance remains enigmatic. Furthermore, the record of anomalies that is generally used is derived by first removing the (large) geomagnetic signal in some way, adding uncertainty to the resulting anomaly series. But this is generally the starting point for paleoclimatic studies that involve solar forcing. Although many paleoclimate studies claim that there is a record of solar forcing in proxy records, very few of these demonstrate a convincing, statistically significant relationship. Often, the argument rests on nothing more than a crude similarity between a time series of the proxy and the cosmogenic isotope anomaly series. In other cases, the claim may be based on spectral power in the proxy record at the approximate frequencies known to be present in the cosmogenic isotope series. Proxies may be temperature-related, or hydrological indicators. In short, although there is a lot of literature on this topic, very little of it stands up to scrutiny. Nevertheless, rather surprisingly, if the individual series (with their inherent limitations) are accepted, and the implied relationships are mapped out, a fairly coherent pattern emerges, providing an intriguing hint that some of the recognized climate changes during the Holocene may indeed have been driven by solar activity changes. Notably, periods of low solar activity are generally associated with lower temperatures at mid- to high-latitude sites, and with weaker monsoon activity in the tropics. However, most published paleoclimate studies do
not recognize any link with solar forcing, and so there is a danger of the “reinforcement syndrome,” whereby only a very small number of well-publicized studies establish a paradigm that may not be supported by most records.
General circulation models (with a well-developed stratospheric chemistry component) could be used to assess the possible effects of solar irradiance changes (total and ultraviolet) on global and regional climate and thereby guide a paleoclimate research strategy. Current models suggest that climate effects are possible, even with relatively small changes in total irradiance (though most models have used larger changes in TSI than current research suggests is likely). Importantly, they indicate that there may have been distinct changes in atmospheric circulation, resulting in regional patterns of climate change, rather than simply overall warming or cooling.
This field cries out for a more systematic and rigorous approach to determine whether solar forcing has played an important role in past climate changes. A well-designed and statistically rigorous strategy, using a network of well-dated high-resolution proxies, in association with general circulation modeling studies, is needed.
Detecting the Solar Cycle via Temperature Proxies Back to the Maunder Minimum
Gerald R. North, Department of Atmospheric Sciences, Texas A&M University
This talk focuses on the very faint (a few hundredths of a degree centigrade) temperature signature associated with the 11-year solar cycle. If solar TSI changes were the only cause of the response, a simple climate model could be used to map the amplitude and phase lag of the response over the planet. One can use regression/detection methods to estimate the strength of the signal simultaneously with the volcanic, greenhouse, and aerosol signals. The signal strength is consistent with this kind of forcing and response. Data from ice cores can also be used to detect the 11-year response through the 18O isotope record. Such a peak in the spectra is indeed present in several cores from Greenland and Antarctica. Data from the core taken from Taylor Dome in Antarctica have a high enough signal-to-noise ratio that one can reconstruct the time series in a narrow band about the 11-year peak by band-pass filtering. The reconstruction clearly shows the Maunder Minimum at its correct time. The Dye-3 core from southern Greenland shows some indications of the same, but the signal-to-noise ratio is less favorable. This research represents an independent indicator, based on temperature response, of past solar influences in forcing climate change at the decadal timescale.
Climate Response at Earth’s Surface to Cyclic and Secular Solar Forcing
Ka-Kit Tung, University of Washington
I will review recent results on responses at the surface to the 11-year solar cycle and to the longer-term secular trend in the longest global temperature dataset. Finally I will discuss some new results on analyzing the 350-year Central England temperature record back to the Maunder Minimum, to see if there is a larger solar signature.
Solar Effects Transmitted by Stratosphere-Troposphere Coupling
Joanna Haigh, Imperial College, London
Data from satellite-borne radiometers indicate that total solar irradiance is greater when the Sun is more active, by about 0.1 percent at the maximum relative to the minimum of the 11-year cycle. Based on simple energy balance arguments, and a standard estimate of climate sensitivity to radiative forcing, this translates into a variation in the global mean surface temperature of around 0.1 K over the solar cycle. Analysis of observational records concurs with this, but the distribution of the solar signal is decidedly
non-uniform. Within the troposphere the largest response occurs in midlatitudes with bands of warming (of approximately 0.5 K) extending from the surface to the tropopause. Above this, in the lower stratosphere, greatest warming appears in the tropics. Zonal winds in the lower atmosphere show a solar cycle response in which the midlatitude jet-streams (and associated storm tracks) move slightly poleward when the Sun is more active.
The observed patterns in zonal mean temperature and wind can be reproduced qualitatively in experiments with climate models in which solar ultraviolet is increased but with surface temperatures fixed. The amplitude of the signal is found to be enhanced if ozone concentrations in the stratosphere are allowed to respond to the increased solar ultraviolet. The magnitude of the modeled response, however, is smaller than the observed response. From this we conclude that ultraviolet heating of the stratosphere may make a contribution to the solar effect on surface climate, and that the magnitude of the ultraviolet change and, importantly, its effect on ozone, are significant in determining the magnitude.
Experiments with simplified global climate models have provided indications of the mechanisms whereby changes in the thermal structure of the lower stratosphere may influence the atmosphere below. The deposition of zonal momentum near the tropopause by upward-propagating synoptic-scale waves is affected by the change in local temperature structure producing zonal accelerations and changes to the mean meridional circulation of the troposphere. These affect the zonal wind at lower levels and thus the background flow upon which subsequent wave propagation takes place. This provides a feedback between the waves and mean flow anomalies that serves to reinforce the initial changes. These results have a wider application in understanding the climate effects of other stratospheric perturbations (such as chemical ozone depletion or the injection of volcanic aerosol) and could be important in terms of assessing the role of human activity in past and future climate, as well as providing a good testbed for current understanding of atmospheric dynamics.
Over the past few years the Sun has been in a state of very low activity, and measurements from the SORCE satellite are suggesting that the solar spectrum has been behaving in an unexpected fashion. In particular, daily measurements by the Spectral Irradiance Monitor (SIM) show a much larger (factor of four to six) decay at near- ultraviolet wavelengths over the latter part of the most recent solar cycle than previously understood. If, as suggested above, ultraviolet heating of the stratosphere makes a contribution to the solar effect on surface climate, then the larger ultraviolet changes shown by SIM would imply a larger role for the stratosphere in determining the tropospheric response to solar variability.
The Impact of Energetic Particle Precipitation on the Atmosphere
Charles H. Jackman, National Aeronautics and Space Agency Goddard Space Flight Center
Energetic precipitating particles (EPPs) include both solar particles and galactic cosmic rays, which can influence the atmosphere. Solar particles cause impacts in the polar middle atmosphere, and galactic cosmic rays create impacts in the lower stratosphere and troposphere.
The solar particles can cause significant constituent changes in the polar mesosphere and stratosphere (middle atmosphere) during certain periods. Both solar protons and electrons can influence the polar middle atmosphere through ionization and dissociation processes. Solar EPPs can enhance HOx (H, OH, HO2) through the formation of positive ions followed by complex ion chemistry and NOx (N, NO, NO2) through the dissociation of molecular nitrogen.
The solar EPP-created HOx increases can lead to ozone destruction in the mesosphere and upper stratosphere via several catalytic loss cycles. Such middle atmospheric HOx-caused ozone loss is rather short-lived due to the relatively short lifetime (hours) of the HOx constituents. The HOxcaused ozone depletion of greater than 30 percent has been observed during several large solar proton events (SPEs) in the past 40 years. HOx enhancements due to SPEs were confirmed by observations in the past solar cycle. A number of modeling studies have been undertaken over this time period that show predictions of enhanced HOx accompanied by decreased ozone due to energetic particles.
The solar EPP-created NOx family has a longer lifetime than the HOx family and can also lead to catalytic ozone destruction. EPP-caused enhancements of the NOx family can affect ozone promptly, if produced in the stratosphere, or subsequently, if produced in the lower thermosphere or mesosphere and transported to the stratosphere. NOx enhancements due to auroral electrons, medium- and high-energy electrons, relativistic electron precipitation events, and SPEs have been measured and/or modeled for decades. Model predictions and measurements show that certain years have significant wintertime meteorological events, which result in the transport of EPP-caused NOx enhancements in the upper mesosphere and lower thermosphere to lower altitudes.
The NOx-caused ozone depletion has also been observed during several solar proton events in the past 40 years. Model predictions indicate that the longer-lived SPE-caused polar stratospheric ozone decrease was statistically significant, but less than 5 percent, in the Northern Hemisphere for the extremely active 5-year time period average (2000-2004). Computations of total ozone do not indicate any long-term SPE total ozone impact over the 1965-2004 period.
Galactic cosmic rays also create NOx and HOx constituents, but at lower altitudes since these particles have much higher energies. The inclusion of galactic cosmic ray-created NOx constituents can increase the odd nitrogen or NOy (N, NO, NO2, NO3, N2O5, HNO3, HO2NO2, ClONO2, BrONO2) family in the lower stratosphere by up to about 20 percent, with small associated ozone decreases of <2 percent. However, the variation in the GCR-driven change in NOy from solar maximum to solar minimum is less than about 5 percent, which results in annually averaged total ozone variations of <0.06 percent.
This talk will provide an overview of several of the EPP-related important processes and their impacts on the atmosphere.
Cosmic Rays, Aerosols and Clouds
Jeffrey Pierce, Dalhousie University, Halifax, Nova Scotia
Cloud cover has been reported to correlate with the flux of galactic cosmic rays to the troposphere, although these correlations are still controversial. Because the tropospheric galactic cosmic ray flux is affected by solar activity, this GCR/cloud connection could be an important pathway for the Sun to influence climate. However, we are just beginning to understand the physical pathways connecting GCRs and clouds. The proposed pathways include (1) the ion-aerosol clear-sky hypothesis whereby GCRs ionize gases and thus may enhance aerosol nucleation rates and cloud condensation nuclei concentrations, and (2) the ion-aerosol near-cloud hypothesis whereby GCRs affect the charge distribution near clouds and thus may affect the freezing of supercooled drops, which will affect precipitation. In this talk, I will review the reported observations of GCR/aerosol/cloud correlations, discuss the proposed physical pathways of GCRs affecting clouds, and present research evaluating the strength of the ion-aerosol clear-sky hypothesis. I will conclude with thoughts on the next steps in GCR/aerosol/cloud research.
The Frequency of Solar Grand Minima Estimated from Studies of Solar-Type Stars
Dan Lubin, Scripps Institution of Oceanography, University of California, San Diego
The Maunder Minimum is a key event in climate change research (1) from the vantage point as a natural control experiment in which greenhouse gas (GHG) abundances were at a pre-industrial constant while solar forcing changed by a magnitude comparable to recent GHG increases, and (2) given recent interest and speculation that a similar grand minimum might occur later this century. To date, periodicity in solar grand minima has been difficult to detect in geophysical proxy data, and an alternative approach involves estimating the frequency of the Sun’s lifetime spent in a grand minimum state by searching for evidence of grand minima in solar-type stars. Most often this is done by measuring calcium (Ca),
hydrogen (H), and potassium (K) flux as an indicator of chromospheric activity, or by photometric observations of solar cycles on decadal timescales. Early estimates of grand minimum frequency in solar-type stars ranged from 10 to 30 percent. However, these early studies inadvertently included many stars that have evolved off the main sequence. More recently, Hipparcos parallax measurements have yielded reliable differentiation between true main sequence stars and slightly evolved stars. In addition, measurements of stellar lithium abundance, and spectroscopically derived metallicity, can provide additional constraints on age and help refine detections of grand minimum analogs in solar-type stars. At the same time, some evidence suggests that instantaneous Ca, H, and K flux measurements alone may be unsuitable for detecting grand minimum analog candidates: at least one plausible candidate has been identified in time series data including a flat activity cycle but with chromospheric activity greater than present-day solar activity. Based on the most recent studies, an estimate emerges in the range of 5 to 6 percent for the fraction of the Sun’s lifetime spent in a low-activity and reduced-luminosity state analogous to the Maunder Minimum.