Total Solar Irradiance Variations Research Articles

DIMMING OF THE 17TH CENTURY SUN

Reconstructions of total solar irradiance (TSI) rely mainly on linear relations between TSI variation and indices of facular area. When these are extrapolated to the prolonged 15th - 17th century Sp\"orer and Maunder solar activity minima, the estimated solar dimming is insufficient to explain the mid- millennial climate cooling of the Little Ice Age. We draw attention here to evidence that the relation departs from linearity at the lowest activity levels. Imaging photometry and radiometry indicate an increased TSI contribution per unit area from small network faculae by a factor of 2-4 compared to larger faculae in and around active regions. Even partial removal of this more TSI - effective network at prolonged minima could enable climatically significant solar dimming, yet be consistent with the weakened but persistent 11- yr cycle observed in Be 10 during the Maunder Minimum. The mechanism we suggest would not alter previous findings that increased solar radiative forcing is insufficient to account for 20th century global warming.

Periodicity of Total Solar Irradiance

We investigate the periodicity in the PMOD composite of the daily total solar irradiance (TSI) from 21 September 1978 to 9 June 2009. Besides the Schwabe cycle period (10.32 years), the quasi-rotation period is found to be statistically significant in TSI, whose value is about 32 days, longer than that in sunspot activity (27 days), and it intermittently appears around the sunspot maximum times. The quasi-rotation period in TSI is inferred to be mainly caused by sunspot activity, but to be modulated by bright features as well. It was previously found that variations of TSI over a Schwabe solar cycle mainly come from the combination of the sunspots’ blocking and the intensification due to bright faculae, plages, and network elements, with a slight dominance of the bright-feature effect during the maximum of the Schwabe cycle. For the sunspot-blocking and the bright-feature effect to contribute to TSI over a Schwabe solar cycle, the former is inferred to lead the latter by 29 days at least.

Comment on “A homogeneous database of sunspot areas covering more than 130 years” by L. A. Balmaceda et al.

[1] In their recent paper, Balmaceda et al. [2009] make a number of statements that require correction. [2] For one, the conclusion presented in their abstract [Balmaceda et al., 2009, paragraph 1] is that “There is no basis for the claim [ascribed by the authors to me] that UV irradiance variations have a much smaller influence on climate than total solar irradiance variations.” Over 25% of the main body of the paper is devoted to disproving this claim. I have never made such a claim. [3] Foukal [2002] (cited by Balmaceda et al.) shows that 20th century global temperature variation correlates better with variation of total solar irradiance (TSI) than with UV flux variations. But it is stressed by Foukal [2002, paragraph 1] that “the [total] irradiance variation amplitude is insufficient to influence global warming in present‐day climate models.” I am not aware of any way to compare the likelihood of climate driving by a mechanism that lacks power relative to another that lacks correlation, and I have never made such a comparison. Despite their lengthy protestations, Balmaceda et al. are still unable to cite any instance where I made such a claim. In fact, my paper deals evenly with both TSI and UV flux as possible mechanisms of climate driving. [4] Second, Balmaceda et al. claim that our 2002 reconstruction of TSI was “incorrect” because it was based on “uncritical” use of sunspot area data. Actually, the spot‐ blocking function, provided by J. Lean, was adopted after critical consideration of the analysis by Fligge and Solanki [1997]. They, one of whom is a coauthor of the Balmaceda et al. paper, reported a correction factor between Royal Greenwich Observatory (RGO) and U.S. Air Force (USAF) spot areas of 1.2, thus less than half the size now claimed by Balmaceda et al. This correction would not have changed the finding that TSI and UV flux variations differ, given the very uncertain magnitude and time variability of any such correction, discussed below. [5] The correction factor is estimated to be 1.49 ± 0.19 via the Russian data used by Balmaceda et al. But if we estimate it using the RGO versus Rome and Rome versus USAF overlaps described by Fligge and Solanki [1997], we obtain (using the ratios for these overlaps from Balmaceda et al. [2009, Table 2]) the lower factor 1.30. It is puzzling why these lower values, 1.2–1.3, are not even mentioned by Balmaceda et al., although these estimates are based on almost twice the length of overlaps (between RGO and Rome and Rome versus USAF) as those available in the comparison via RGO versus Russia. [6] The formal error estimates [from Balmaceda et al., 2009, Table 2] applied to this lower correction of 1.2–1.3 would be ±0.16 if the ratio behaved even roughly according to Gaussian statistics. But the behavior is often far from Gaussian. For instance, Figure 3 of Fligge and Solanki [1997] shows that for 10 of the 17 years of overlap between RGO and Rome, the agreement was essentially perfect, so the factor would have been 1.0. Over most of the remainder it would have been ∼1.15. Almost never did the factor assume its “average” value. It is then more accurate to describe the uncertainty as a range of a bimodal distribution and to combine the ranges linearly, rather than quadratically. In that case the range would be about ±0.22. [7] In summary, the evidence supports a correction factor between approximately unity and 1.7, with about equal likelihood. It is questionable whether a TSI reconstruction based on such an uncertain and highly time‐varying correction is better than no correction at all. It also calls into question whether the correction by Balmaceda et al. results in the “homogeneous” time series they are claiming. [8] Third, the reader’s confidence in the uncertain statistics might be strengthened if the authors were able to explain the reason for higher RGO (and Russian) areas. Balmaceda et al. [2009, paragraph 24] state that “This is mainly due to the significant difference in the minimum value of the counted sunspots (1 ppm...for Russian versus 10 ppm...for SOON observations)” (reiterated in their conclusions in paragraph 41). This explanation is not correct. Examination of the RGO spot group records shows that the contribution of spots of area <10 ppm is negligible at times of appreciable activity; it does not begin to explain the 40%–50% correction claimed by Balmaceda et al. [9] The additional suggestion that differences between areas measured from photographs and drawings might contribute also does not match the evidence. The Hathaway et al. [2002] correction of 1.4 that Balmaceda et al. mention as agreeing with theirs is based on comparison of (umbral) areas measured at Mount Wilson and by RGO. Both are photographic records. Heliophysics, Inc., Nahant, Massachusetts, USA.

Reply to comment by P. Foukal on “A homogeneous database of sunspot areas covering more than 130 years”

Fil: Balmaceda, Laura Antonia. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico San Juan. Instituto de Ciencias Astronomicas de la Tierra y del Espacio; Argentina

Stellar activity, differential rotation, and exoplanets

AbstractThe photospheric spot activity of some of the stars with transiting planets discovered by the CoRoT space experiment is reviewed. Their out-of-transit light modulations are fitted by a spot model previously tested with the total solar irradiance variations. This approach allows us to study the longitude distribution of the spotted area and its variations versus time during the five months of a typical CoRoT time series. The migration of the spots in longitude provides a lower limit for the surface differential rotation, while the variation of the total spotted area can be used to search for short-term cycles akin the solar Rieger cycles. The possible impact of a close-in giant planet on stellar activity is also discussed.

Natural forcing of climate during the last millennium: fingerprint of solar variability

The variability of the climate during the last millennium is partly forced by changes in total solar irradiance (TSI). Nevertheless, the amplitude of these TSI changes is very small so that recent reconstruction data suggest that low frequency variations in the North Atlantic Oscillation (NAO) and in the thermohaline circulation may have amplified, in the North Atlantic sector and mostly in winter, the radiative changes due to TSI variations. In this study we use a state-of-the-art climate model to simulate the last millennium. We find that modelled variations of surface temperature in the Northern Hemisphere are coherent with existing reconstructions. Moreover, in the model, the low frequency variability of this mean hemispheric temperature is found to be correlated at 0.74 with the solar forcing for the period 1001–1860. Then, we focus on the regional climatic fingerprint of solar forcing in winter and find a significant relationship between the low frequency TSI forcing and the NAO with a time lag of more than 40 years for the response of the NAO. Such a lag is larger than the around 20-year lag suggested in other studies. We argue that this lag is due, in the model, to a northward shift of the tropical atmospheric convection in the Pacific Ocean, which is maximum more than four decades after the solar forcing increase. This shift then forces a positive NAO through an atmospheric wave connection related to the jet-stream wave guide. The shift of the tropical convection is due to the persistence of anomalous warm SST forcing the anomalous precipitation, associated with the advection of warm SST by the North Pacific subtropical gyre in a few decades. Finally, we analyse the response of the Atlantic meridional overturning circulation to solar forcing and find that the former is weakened when the latter increases. Changes in wind stress, notably due to the NAO, modify the barotropic streamfunction in the Atlantic 50 years after solar variations. This implies a wind-driven modification of the oceanic circulation in the Atlantic sector in response to changes in solar forcing, in addition to the variations of the thermohaline circulation.

Towards a long-term record of solar total and spectral irradiance

The variation of total solar irradiance (TSI) has been measured since 1978 and that of the spectral irradiance for an even shorter amount of time. Semi-empirical models are now available that reproduce over 80% of the measured irradiance variations. An extension of these models into the more distant past is needed in order to serve as input to climate simulations. Here we review our most recent efforts to model solar total and spectral irradiance on time scales from days to centuries and even longer. Solar spectral irradiance has been reconstructed since 1947. Reconstruction of solar total irradiance goes back to 1610 and suggests a value of about 1 – 1.5 W / m 2 for the increase in the cycle-averaged TSI since the end of the Maunder minimum, which is significantly lower than previously assumed but agrees with other modern models. First steps have also been made towards reconstructions of solar total and spectral irradiance on time scales of millennia.

Modeling Total Solar Irradiance Variations Using Automated Classification Software on Mount Wilson Data

We present results using the AutoClass analysis application available at NASA/Ames Intelligent Systems Div. (2002) which is a Bayesian, finite mixture model classification system developed by Cheeseman and Stutz (1996). We apply this system to Mount Wilson Solar Observatory (MWO) intensity and magnetogram images and classify individual pixels on the solar surface to calculate daily indices that are then correlated with total solar irradiance (TSI) to yield a set of regression coefficients. This approach allows us to model the TSI with a correlation of better than 0.96 for the period 1996 to 2007. These regression coefficients applied to classified pixels on the observed solar surface allow the construction of images of the Sun as it would be seen by TSI measuring instruments like the Solar Bolometric Imager recently flown by Foukal et al., (2004). As a consequence of the very high correlation we achieve in reproducing the TSI record, our approach holds out the possibility of creating an on-going, accurate, independent estimate of TSI variations fromground-based observations which could be used to compare, and identify the sources of disagreement among, TSI observations from the various satellite instruments and to fill in gaps in the satellite record. Further, our spatially-resolved images should assist in characterizing the particular solar surface regions associated with TSI variations. Also, since the particular set of MWO data on which this analysis is based is available on a daily basis back to at least 1985, and on an intermittent basis before then, it will be possible to estimate the TSI emission due to identified solar surface features at several solar minima to constrain the role surface magnetic effects have on long-term trends in solar energy output.

Mechanisms for total and spectral solar irradiance variations

AbstractThe total and spectral irradiance varies over short time scales, i.e. from days to months, and longer time scales from years to decades, centuries, and beyond. In this talk we review the current understanding of irradiance changes from days to decades. We present the current status of observations and discuss proposed reconstruction approaches to understand these variations. The main question that ultimately needs to be answered is what are the physical processes that could explain the enhanced heating of the photosphere, chromosphere, transition region, and corona, leading to a change in the solar radiative output at various wavelengths. As semi-empirical models allow us to reproduce the solar spectrum over a broad wavelength range, they offer a powerful tool to determine the energy necessary to heat certain layers and at the same time balance the radiative losses.

A homogeneous database of sunspot areas covering more than 130 years

The historical record of sunspot areas is a valuable and widely used proxy of solar activity and variability. The Royal Greenwich Observatory regularly measured this and other parameters between 1874 and 1976. After that time records from a number of different observatories are available. These, however, show systematic differences and often have significant gaps. Our goal is to obtain a uniform and complete sunspot area time series by combining different data sets. A homogeneous composite of sunspot areas is essential for different applications in solar physics, among others for irradiance reconstructions. Data recorded simultaneously at different observatories are statistically compared in order to determine the intercalibration factors. Using these data we compile a complete and cross‐calibrated time series. The Greenwich data set is used as a basis until 1976, the Russian data (a compilation of observations made at stations in the former USSR) are used between 1977 and 1985, and data compiled by the USAF network are used since 1986. Other data sets (Rome, Yunnan, and Catania) are used to fill up the remaining gaps. Using the final sunspot areas record the Photometric Sunspot Index is calculated. We also show that the use of uncalibrated sunspot areas data sets can seriously affect the estimate of irradiance variations. Our analysis implies that there is no basis for the claim that UV irradiance variations have a much smaller influence on climate than total solar irradiance variations.

Sources of Solar Total Irradiance Variations

The daily images and magnetograms acquired by MDI are a rich source of information about the contributions of different types of solar regions to variations in the total solar irradiance (TSI). These data have been used to determine the temporal variation of the MDI irradiance, the mean intensity of the solar disk in the continuum at 676.8 nm. The short-term (days to weeks) variations of the MDI irradiance and TSI are in excellent agreement with rms differences of 0.011%. This indicates that MDI irradiance is an excellent proxy for short-term variations of TSI from the competing irradiance contributions of regions causing irradiance increases, such as plages and bright network, and regions causing irradiance decreases, such as sunspots. However, the long-term or solar cycle variation of the MDI proxy and TSI differ over the 11-year period studied. The results indicate that the primary sources of the long-term (several months or more) variations in TSI are regions with magnetic fields between about 80 and 600 G. The results also suggest that the difference in the long-term variations of the MDI proxy and TSI is due to a component of TSI associated with sectors of the solar spectrum where the contrast in intensity between plages and the quiet Sun is enhanced (e.g., the UV) compared to the MDI proxy. This is evidence that the long-term variation of TSI is due primarily to solar cycle variations of the irradiance from these portions of solar spectrum, a finding consistent with modeling calculations indicating that approximately 60% of the change in TSI between solar minimum and maximum is produced by the UV part of the spectrum shortward of 400 nm (Solanki and Krivova, Space Sci. Rev. 125, 53, 2006).

Solar variability does not explain late-20th-century warming

The hypothesis of Nicola Scafetta and Bruce West (see their Opinion piece, PHYSICS TODAY, March 2008, page 50), that most of the observed global warming trend since 1950 is due to variations in total solar irradiance (TSI), is at odds with observations and theory. They argue that near-surface temperatures are strongly influenced by both the well-known 11-year cycle in TSI and shorter-time-scale TSI variations. As for the 11-year cycle in TSI, satellite measurements of TSI since 1979 show variations that are very small (about 0.1%) and cyclical (see figure 1). If Earth’s surface temperature were highly sensitive to those variations, the 11-year cycle in TSI would produce an 11-year cycle in observed surface temperatures. The filtered and smoothed temperature record presented by Scafetta and West appears to show a significant 11-year cycle. However, an exhaustive analysis of the latter half of the 20th century by Gerald North and his collaborators found a temperature cycle with an amplitude of no more than a few hundredths of a degree Celsius, with a significance level of only 25%.1 Even that small signal, which is roughly consistent with the expected signal that North and colleagues estimated using an energy-balance model, may be in part an artifact of fortuitous spacing of major volcanic eruptions (see the letter by Peter Foukal, PHYSICS TODAY, October 2008, page 10). Hence, the historical surface temperature record does not support the hypothesis of strong temperature sensi tivity to solar irradiance changes. Additionally, that hypothesis predicts greater-than-observed temperature variations during prehistoric times (see the letter by Diedrich Schmidt, PHYSICS TODAY, October 2008, page 10). For example, strong sensitivity to solar variations would have resulted in a much colder climate during the Maunder Minimum (a period of reduced solar activity during the 17th century), when the TSI is thought to have been less than today by anywhere from 0.2% 2 to 1.2%.3 In any case, as is evident from figure 1, TSI variations since 1979 are cyclical, with no discernible multidecadal trend. Hence, the variations cannot explain the observed warming trend since 1979, which has continued over more than two 11-year cycles (see figure 2). That conclusion is supported by thorough analyses by Foukal and his collaborators4 and by others. Is it possible that TSI variations, although having little direct effect on Earth’s energy balance, nonetheless control climate by somehow regulating more energetic processes? (An analogy would be a transistor.) The “complexity matching” argument of Scafetta and West is along those lines. Their argument seems unlikely to be correct, however, since observations by Sydney Levitus and his collaborators show that the climate system as a whole, particularly the ocean, has warmed along with the atmosphere; 5 the energy content of the climate system increased between about 1955 and about 1995 by about 2 × 1023 J, equivalent to an energy imbalance at the top of the atmosphere of 0.3 W/m2. Because incoming solar energy—that is, TSI—has not changed, the imbalance must result from increased absorption of outgoing energy, such as by increased greenhouse gases. Indeed, satellite measurements by John Harries and coworkers confirm that absorption of outgoing terrestrial radiation by several greenhouse gases became stronger 6 between 1970 and 1997. An additional failing of Scafetta and West’s hypothesis is that it requires greenhouse gases to have very little ef1376

Using the oceans as a calorimeter to quantify the solar radiative forcing

Over the 11‐year solar cycle, small changes in the total solar irradiance (TSI) give rise to small variations in the global energy budget. It was suggested, however, that different mechanisms could amplify solar activity variations to give large climatic effects, a possibility which is still a subject of debate. With this in mind, we use the oceans as a calorimeter to measure the radiative forcing variations associated with the solar cycle. This is achieved through the study of three independent records, the net heat flux into the oceans over 5 decades, the sea‐level change rate based on tide gauge records over the 20th century, and the sea‐surface temperature variations. Each of the records can be used to consistently derive the same oceanic heat flux. We find that the total radiative forcing associated with solar cycles variations is about 5 to 7 times larger than just those associated with the TSI variations, thus implying the necessary existence of an amplification mechanism, although without pointing to which one.

Variations on Sun’s role in climate change

In their Opinion piece “Is Climate Sensitive to Solar Variability?” (Physics Today, Physics Today 0031-9228 61 3 2008 50 https://doi.org/10.1063/1.2897951. March 2008, page 50 ), Nicola Scafetta and Bruce West draw attention to a “phenomenological solar signature” they find in preindustrial records of global temperature. Correlations between temperature proxies and solar activity have been reported since the 18th century; see, for example, reference 11. J. Eddy, Science 192, 1189 (1976). https://doi.org/10.1126/science.192.4245.1189 . However, their significance cannot be judged without considering volcanism, which may account for much of the apparent correlation with solar activity over the past millennium. 2 2. G. Hegerl et al. , J. Climate 20, 650 (2007). https://doi.org/10.1175/JCLI4011.1 Scafetta and West assume that the only secular influence on preindustrial climate is the Sun.Scafetta and West also state that anthropogenic influence on global warming is overestimated if climate is as sensitive to solar activity as their studies suggest. But in their referenced work, they admit that such a high sensitivity requires more powerful couplings between solar activity and climate than have been identified so far. That is hardly news; for more than a decade, the big question has been whether such powerful Sun–climate coupling mechanisms exist. For reasons we reviewed recently, 3 3. P. Foukal et al. , Nature 443, 161 (2006). https://doi.org/10.1038/nature05072 variation in total solar irradiance (TSI) seems to lack the required power, and the solar UV flux variation—another candidate mentioned by the authors—seems to account for less than 10% of the variance in the 20th-century global temperature. Solar modulation of cosmic rays and their possible effect on cloud cover is the most complex candidate and remains hardest to test.Additionally, Scafetta and West use solar flare statistics as a proxy for TSI variations to derive a similarity in the power law indices of solar fluctuations and Earth temperature variations. Their claim is puzzling since solar flares make a negligible contribution to TSI variation. Why not just analyze the widely available TSI record itself?In summary, a solar effect on global temperature may well exist, and incisive correlation studies can play a role in its investigation. However, it is increasingly clear that deployment of improved radiometric and photometric instruments will be required to discriminate between suggested Sun–climate coupling mechanisms.REFERENCESSection:ChooseTop of pageREFERENCES <<CITING ARTICLES1. J. Eddy, Science 192, 1189 (1976). https://doi.org/10.1126/science.192.4245.1189 , Google ScholarCrossref, ISI2. G. Hegerl et al. , J. Climate 20, 650 (2007). https://doi.org/10.1175/JCLI4011.1 , Google ScholarCrossref, ISI3. P. Foukal et al. , Nature 443, 161 (2006). https://doi.org/10.1038/nature05072 , Google ScholarCrossref, ISI© 2008 American Institute of Physics.

Variations on Sun’s role in climate change

Rest assured that the Opinion piece on solar contributions to climate change will find its way hastily into the policy—or should I say political—community and will be misused to stall efforts to limit greenhouse gas emissions. And on what scientific grounds? The work by Nicola Scafetta and Bruce West ignores decades of fundamental physical research and is roundly criticized on technical grounds. 1,2 1. J. L. Lean, Geophys. Res. Lett. 33, L15701 (2006). https://doi.org/10.1029/2005GL025342 2. R. E. Benestad, “A Phenomenological Sequel,” RealClimate.org, http://www.realclimate.org/index.php/archives/2007/11/a-phenomenological-sequel. More important, their basic approach to the question of how the Sun influences climate defies sound scientific logic.Despite their sophisticated statistical treatments, the authors commit a fallacy by ignoring an established physical forcing (greenhouse gases) while trying to assess the contribution of a separate forcing (solar irradiance); both push the climate in the same direction, if one assumes that the questionable ACRIM satellite time series on solar irradiance is accurate. With IR-trapping gases omitted, the analysis by Scafetta and West must overestimate the contribution of total solar irradiance variations to surface warming. Is the contribution overestimated slightly or dramatically? The authors’ work offers no insights.Even if Scafetta and West take issue with the statistical treatments done by the Intergovernmental Panel on Climate Change, they should nonetheless appreciate the indispensable requirement to account for all relevant forcings, as the IPCC does in its analyses. If they hope to make an authentic contribution to our understanding of the Sun’s role in climate change, they must build on an existing body of knowledge; ignoring more than a century of physical science will not help.The policy community relies on professional scientific publications to provide sound information on relevant topics. When Physics Today publishes opinions that are physically unsound and defy basic scientific logic, the policy community is misled. In my experience, once fundamental misconceptions about science are introduced to the policy community, they are difficult to correct. Moreover, confusion and embarrassment produced by the process of rooting out misconceptions can tarnish a policymaker’s image of science.REFERENCESSection:ChooseTop of pageREFERENCES <<1. J. L. Lean, Geophys. Res. Lett. 33, L15701 (2006). https://doi.org/10.1029/2005GL025342 , Google ScholarCrossref, ISI2. R. E. Benestad, “A Phenomenological Sequel,” RealClimate.org, http://www.realclimate.org/index.php/archives/2007/11/a-phenomenological-sequel. Google Scholar© 2008 American Institute of Physics.

Do Photospheric Brightness Structures Outside Magnetic Flux Tubes Contribute to Solar Luminosity Variation?

Variations in total solar irradiance (TSI) correlate well with changes in projected area of photospheric magnetic flux tubes associated with dark sunspots and bright faculae in active regions and network. This correlation does not, however, rule out possible TSI contributions from photospheric brightness inhomogeneities located outside flux tubes and spatially correlated with them. Previous reconstructions of TSI report agreement with radiometry that seems to rule out significant “extra-flux-tube” contributions. We show that these reconstructions are more sensitive to the facular contrasts used than has been generally recognized. Measurements with the Solar Bolometric Imager (SBI) provide the first reliable support for the relatively high, wide-band, disk-center contrasts required to produce 10% rms agreement. Longer term bolometric imaging will be required to determine whether the small but systematic TSI residuals we see here are caused by remaining errors in spot and facular areas and contrasts or by extra-flux-tube brightness structures such as bright rings around sunspots or “convective stirring” around active regions.

A Comparison of Feature Classification Methods for Modeling Solar Irradiance Variation

Physical understanding of total and spectral solar irradiance variation depends upon establishing a connection between the temporal variability of spatially resolved solar structures and spacecraft observations of irradiance. One difficulty in comparing models derived from different data sets is that the many ways for identifying solar features such as faculae, sunspots, quiet Sun, and various types of “network” are not necessarily consistent. To learn more about classification differences and how they affect irradiance models, feature “masks” are compared as derived from five current methods: multidimensional histogram analysis of NASA/National Solar Observatory/Kitt Peak spectromagnetograph data, statistical pattern recognition applied to SOHO/Michelson Doppler Imager photograms and magnetograms, threshold masks allowing for influence of spatial surroundings applied to NSO magnetograms, and “one-trigger” and “three-trigger” algorithms applied to California State University at Northridge Cartesian Full Disk Telescope intensity observations. In general all of the methods point to the same areas of the Sun for labeling sunspots and active-region faculae, and available time series of area measurements from the methods correlate well with each other and with solar irradiance. However, some methods include larger label sets, and there are important differences in detail, with measurements of sunspot area differing by as much as a factor of two. The methods differ substantially regarding inclusion of fine spatial scale in the feature definitions. The implications of these differences for modeling solar irradiance variation are discussed.

The Sun and the Earth's Climate

Variations in solar activity, at least as observed in numbers of sunspots, have been apparent since ancient times but to what extent solar variability may affect global climate has been far more controversial. The subject had been in and out of fashion for at least two centuries but the current need to distinguish between natural and anthropogenic causes of climate change has brought it again to the forefront of meteorological research. The absolute radiometers carried by satellites since the late 1970s have produced indisputable evidence that total solar irradiance varies systematically over the 11-year sunspot cycle, relegating to history the term “solar constant”, but it is difficult to explain how the apparent response to the Sun, seen in many climate records, can be brought about by these rather small changes in radiation. This article reviews some of the evidence for a solar influence on the lower atmosphere and discusses some of the mechanisms whereby the Sun may produce more significant impacts than might be surmised from a consideration only of variations in total solar irradiance.

Aerosol nucleation over oceans and the role of galactic cosmic rays

Abstract. We investigate formation of sulfate aerosol in the marine troposphere from neutral and charged nucleation of H2SO4 and H2O. A box model of neutral and charged aerosol processes is run on a grid covering the oceans. Input data are taken from a model of galactic cosmic rays in the atmosphere, and from global chemistry and transport models. We find a weak aerosol production over the tropical oceans in the lower and middle troposphere, and a stronger production at higher latitudes, most notably downwind of industrial regions. The strongest aerosol production however occurs in the upper troposphere over areas with frequent convective activity, in particular in the tropics. This finding supports the proposition by which non-sea salt marine boundary layer aerosol in tropical regions does not form in situ, but nucleates in the upper troposphere from convectively lifted and cloud processed boundary layer air rich in aerosol precursor gases, from where it descends in subsiding air masses compensating convection. Convection of boundary layer air also appears to drive the formation of condensation nuclei in the tropical upper troposphere which maintains the stratospheric aerosol layer in the absence of volcanic activity. Neutral nucleation contributes only marginally to aerosol production in our simulations. This highlights the importance of other mechanisms, including charged binary and ternary, and neutral ternary nucleation for aerosol formation. Our analysis indicates that the variation of ionization by galactic cosmic rays over the decadal solar cycle does not entail a response in aerosol production and cloud cover via the second indirect aerosol effect that would explain observed variations in global cloud cover. We estimate that the variation in radiative forcing resulting from a response of clouds to the change in galactic cosmic ray ionization and subsequent aerosol production over the decadal solar cycle is smaller than the concurrent variation of total solar irradiance.

Coupled climate model simulation of Holocene cooling events: oceanic feedback amplifies solar forcing

Abstract. The coupled global atmosphere-ocean-vegetation model ECBilt-CLIO-VECODE is used to perform transient simulations of the last 9000 years, forced by variations in orbital parameters, atmospheric greenhouse gas concentrations and total solar irradiance (TSI). The objective is to study the impact of decadal-to-centennial scale TSI variations on Holocene climate variability. The simulations show that negative TSI anomalies increase the probability of temporary relocations of the site with deepwater formation in the Nordic Seas, causing an expansion of sea ice that produces additional cooling. The consequence is a characteristic climatic anomaly pattern with cooling over most of the North Atlantic region that is consistent with proxy evidence for Holocene cold phases. Our results thus suggest that the ocean is able to play an important role in amplifying centennial-scale climate variability.