solar irradiance variations and climate

Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 677–685www.elsevier.com/locate/jastp

Solar irradiance variations and climate

S.K. Solankia ;∗, M. FliggebaMax-Planck-Institute for Aeronomy, D-37191 Katlenburg-Lindau, Germany

bInstitute of Astronomy, ETH Zentrum, 8092 Zurich, Switzerland

Abstract

The irradiance of the Sun is observed to vary in phase with the solar cycle at an amplitude of ∼0:1% and a period of roughly11 years. There is indirect evidence that the irradiance also exhibits a larger secular variation. Direct measurements only coverthe last 2 1

2 solar cycles and a longer record is needed to study the possible coupling with climate. Therefore, it is necessaryto develop successful models of solar irradiance variations and to reconstruct it back into the past. In the present paper, thecurrent observational knowledge and the state-of-the-art of the modelling are introduced and reviewed. c© 2002 Published byElsevier Science Ltd.

Keywords: Solar irradiance measurement; Solar irradiance models; Sun-Earth relations

1. Introduction

The idea that solar variability could have an e6ect on theEarth’s climate is not new. For example, Herschel (1801)plotted the sunspot number together with wheat prices inEngland, which he used as a climate indicator: low temper-atures implying poor crops and thus high prices.

More recent correlations between indicators of solar ac-tivity and climate have been presented by Eddy (1977)and Friis-Christensen and Lassen (1991). Such correlationsalone are not su<cient to settle the issue of whether the Sunreally in=uences climate. Needed are mechanisms whichcould mediate such a connection. In the ?rst step we need toidentify variable solar quantities which could in=uence theEarth’s climate system. In the second step we need to workout how they couple to the Earth’s atmosphere.

Basically, two solar quantities have the potential of havingsuch an in=uence. One is the Sun’s interplanetary magnetic?eld and the associated solar wind, the other is the Sun’sirradiance, or brightness of the whole solar disc as measuredabove the Earth’s atmosphere.

The path to climate change involving the interplanetarymagnetic ?eld has been discussed by Friis-Christensen(2001), so that we concentrate on solar irradiance. Also,

∗ Corresponding author.E-mail address: [email protected] (S.K. Solanki).

we restrict ourselves to presenting the solar results and donot touch upon the complex processes that take place in theEarth’s atmosphere.

2. Irradiance measurements

We need to distinguish between solar spectral irradiance(often also simply called solar irradiance) and total solarirradiance. The former is the irradiance at one wavelength ora set of wavelengths, while the latter is given by the integralover all wavelengths. It thus describes the total radiativeenergy input to Earth from the Sun, which is by far the mostdominant external energy source.

Total irradiance has been observed by a group of radiome-ters such as that within the Earth radiation budget (ERB)experiment (Kyle et al., 1994) on the NIMBUS satellite,ACRIM (active cavity radiometric irradiance measurement;Willson and Hudson, 1991) I and II on the solar maximummission (SMM) and upper atmospheric research satellite(UARS), respectively, and VIRGO (variability of irradianceand gravity oscillations, FrIohlich et al., 1997a,b) on SOHO.These instruments have provided an almost uninterruptedcoverage of solar irradiance since 1978. Although the var-ious instruments disagree at the 0.2% level in the absolutevalue of the solar irradiance, the relative changes within eachdataset are accurate to better than 0.01% (repeatability of

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678 S.K. Solanki, M. Fligge / Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 677–685

Fig. 1. The total solar irradiance vs. time. Plotted is a composite of the measurements made between 1978 and 1999 by various instruments(see FrIohlich and Lean, 1998, for details). Figure kindly provided by C. FrIohlich.

daily measurements). Note that, it is the variability and notthe total irradiance which is important for climate change.

Taking into account the strengths and weaknesses of thedi6erent instruments, FrIohlich and Lean (1998) have pro-duced a composite of the irradiance from the data of all theinstruments. An updated version of it is shown in Fig. 1(FrIohlich, 2000). The 11-year solar cycle is clearly visible.Note that most of the “noise”, in particular the prominentdips in brightness lasting typically 1–2 weeks, are solar inorigin (see Section 4).

Changes in the Sun’s spectral irradiance, or the Sun’sspectrum are harder to measure. Until recently reliable mea-surements were only possible in the UV. Such measure-ments have been made during parts of two solar cyclesby the SUSIM (solar ultraviolet spectral irradiance moni-tor, Vanhoosier et al., 1988; Brueckner et al., 1993) andSOLSTICE (solar-stellar irradiance comparison experiment,Rottman, 1988; Rottman et al., 1994), which =ew repeat-edly on the space shuttle (SUSIM) and on UARS (SUSIM,SOLSTICE). These instruments reveal a rapid increase inirradiance variability towards shorter wavelengths, so that

the irradiance changes by roughly 10% at � ≈ 200 nm overthe solar cycle. The estimated relative di6erence in the UVspectrum of the Sun between activity maximum and mini-mum is plotted in Fig. 2 (Lean, 1991).

The irradiance within three 5 nm wide ?lter bands in thevisible has been followed since 1996 by VIRGO using threephotometers. Due to the limited long-term stability of thephotometers they most reliably track the short-term changes,i.e. those on the solar rotation time scale. In Fig. 3 we showthe irradiance measured by VIRGO between mid-1996 andmid-1998. In the top frame the irradiance around 862 nm, i.e.in the near-IR, is plotted followed by the irradiance around500 nm (middle frame) and 402 nm (bottom frame). Notethe increase in RMS variation from near-IR (top frame) tonear-UV (bottom frame).

3. Sources of solar irradiance variations

The main source of solar irradiance variations on timescales up to the solar cycle is the magnetic ?eld at the solar

S.K. Solanki, M. Fligge / Journal of Atmospheric and Solar-Terrestrial Physics 64 (2002) 677–685 679

Fig. 2. The relative change in the UV spectral irradiance between solar maximum and minimum (see Lean, 1991, for details). Observedvalues are restricted to wavelengths below 400 nm. The line at longer wavelengths is a simple extrapolation. Data for the ?gure kindlyprovided by J. Lean.

Fig. 3. Time series of, from top to bottom, irradiance in the VIRGO red, green and blue channels. In the individual colour channels thelong-term trend, is less reliably measured than the short-term variations (data kindly provided by C. FrIohlich).

surface (Foukal and Lean, 1986, 1988; Fligge et al., 1998,2000a,b; Solanki and Fligge, 2002). The ?eld is mainly or-dered into =ux tubes, which may be described roughly as

bundles of tightly packed ?eld lines. Flux tubes of di6er-ent cross-sectional sizes are present on the Sun. The largestare the dark sunspots, the smallest are magnetic elements,


Fig. 4. A trace of the total solar irradiance recorded by VIRGO in November and December 1996 (thick solid curve). Above it MDI(Michelson Doppler imager) full-disc continuum images for 5 days within the period of the marked dip in irradiance are plotted. Note thepassage of the two small sunspots across the solar disc in this time frame.

Fig. 5. Time series of total irradiance (top) and sunspot number (bottom). Plotted are daily values. Note the good long-term correlationbetween the two curves. Short-term variations display an anti-correlation.

groups of which form the bright network and active regionfaculae (see, e.g., Solanki, 1993).

On the solar rotation time scale, irradiance variations aredominated by the passage of sunspots across the solar discdue to solar rotation, as is illustrated by Fig. 4. Sunspots

on the disc produce a distinct decrease of the total solarirradiance. On the longer activity cycle time scale, how-ever, the correlation reverses, with the Sun being on averagebrighter when the number of sunspots on its disc is larger,as can be seen from Fig. 5, where daily averages of solar


Fig. 6. Time series of total and spectral irradiance. Plotted are both the observed (dashed curves) and modelled (solid curves) values (fromFligge et al., 2000a,b).

irradiance and sunspot number are plotted. Obviously, thein=uence of the increased brightness of faculae and the net-work outweighs the darkening due to sunspots.

The in=uence of solar surface magnetism on solar irra-diance can be modelled in numerous ways. The simplestconsists of linear regressions of chromospheric proxiesof the magnetic ?eld (e.g. Ca IIK or Mg II core-to-wingratio) to the measured irradiance (e.g. FrIohlich and Lean,1998), while more sophisticated approaches employ modelsof the atmospheric structure of sunspots and faculae to re-

construct not just the total irradiance but also the change ofthe solar spectrum (Solanki and Unruh, 1998; Fligge et al.,2000a,b).

The results of such models are compared with observedtrends in Figs. 6 and 7. In Fig. 6 we compare reconstructedtotal and spectral irradiance (solid) in the three VIRGO?lter bands between mid-August and mid-September 1996with the measured values (dashed). Fig. 7 displays the mea-sured and modelled relative change in solar spectral irradi-ance between solar activity minimum and maximum for the


Fig. 7. Same as Fig. 2, but now with the results of a model (solidcurve) overplotted. At �¿ 400 nm the dotted curve is only anextrapolative estimate (from Unruh et al., 1999).

wavelength range of 200–1000 nm. In both ?gures the corre-spondence between model and data is gratifying. In additionto the plotted quantities, these models also satisfy a numberof other observational constraints. Thus they reproduce thevariation of the line-blanketing and the facular=sunspot area

Fig. 8. Evolution of the open magnetic =ux at the solar surface since the end of the Maunder minimum in 1700 as predicted by a model ofthe surface evolution of the Sun’s magnetic ?eld (upper panel, dark solid curve). For comparison, the =ux of the interplanetary magnetic?eld (Lockwood and Stamper, 1999) reconstructed from the geomagnetic aa-index (light solid curve) and the 10Be concentration in icecores (Beer et al., 1990) (dotted curve and left-hand, inverted scale) are also plotted. The interplanetary =ux values have been multiplied bya factor of 2 in order to obtain the total unsigned =ux. The 10Be record has been plotted without any smoothing or ?ltering. For comparison,the lower panel shows the corresponding time sequence of the sunspot number, R (see Solanki et al., 2000, for details).

ratio over the solar cycle (Fligge et al., 1998; Unruh et al.,1999).

Hence, we can be reasonably con?dent that we have iden-ti?ed the main causes of irradiance variations, at least ontime scales up to the solar cycle, although we cannot as yetrule out a minor contribution (at below the 20% level) fromsources unrelated to the solar surface magnetic ?eld.

4. Reconstructing past solar irradiance

On longer time scales the possibility that the Sun exhibitsa secular variation needs to be considered. There is evidencefrom di6erent sources suggesting that this is indeed the caseand that the secular variations actually outstrip the cyclicvariations in magnitude. However, much of this evidenceis indirect and is based on observations of Sun-like stars.These suggest that the magnetic activity, as measured bythe Ca K =ux, of numerous stars is far below that of thepresent-day Sun (Baliunas and Jastrow, 1990). These starsare thought to be in a non-cycling Maunder-minimum-likephase—similar to the Sun between 1630 and 1700. Sincethe Ca K =ux scales roughly with the facular brightness,


Fig. 9. Left-hand scale: irradiance reconstructions, Srec, between 1880 and 1989. The dashed and solid curves result from models based ondi6erent assumptions. The hatched area indicates the range within which Srec may lie. Right-hand scale: temperature records. Plotted are theglobal land and sea temperature (solid curve, TG) and the northern hemisphere land and sea temperature (dashed curve, TNH) relative toepoch 1950. The temperature data are provided by the World Data Center (WDC), Boulder, CO. Global temperatures are as in the reportof the International Panel on Climate Change (IPCC, 1992). All curves have been subjected to an 11-year running mean (from Solanki andFligge, 1998).

these stars should also be less bright than the correspondingstars in a cycling phase. For the Sun the observations implythat during the Maunder minimum it was 2.5–8 W=m2 lessbright than during recent activity minima (White et al., 1992;Lean et al., 1992, 1995; Zhang et al., 1994).

Additional indirect evidence comes from reconstructionsof the heliospheric interplanetary magnetic ?eld strengthfrom the geomagnetic aa-index (Lockwood et al., 1999).The interplanetary magnetic ?eld exhibits a signi?cant secu-lar trend indicating that it has doubled in strength over∼100years. The variations of concentrations of cosmogenic iso-topes such as 14C and 10Be (Beer et al., 1994; Beer, 2000)display secular trends that agree with the heliospheric ?eld.These concentrations are driven by the cosmic ray density,which in turn is determined by the heliospheric magnetic?eld and the solar wind, so that this parallel behaviour is notunexpected. Fig. 8 shows the strength of the heliospheric?eld as reconstructed from the aa-index, as well as that re-sulting from a model of =ux emergence and evolution onthe solar surface. Also plotted is the 10Be concentration (onan inverted scale).

Although the heliospheric ?eld strength is well repro-duced by a model of the ?eld’s evolution, only a smallfraction of the surface =ux (¡10%) ?nds its way into theheliosphere (Solanki et al., 2000). Thus, the heliospheric?eld does re=ect changes of the solar surface magnetic ?eld,

but these are minor compared to the changes needed for alarge secular irradiance trend.

Based on this evidence (and in spite of the caveats thatapply) various reconstructions of the irradiance have in-cluded a secular increase of 2.5–5 W=m2 since the Maunderminimum, or correspondingly less over a shorter time (e.g.Hoyt and Schatten, 1993; Lean et al., 1995; Solanki andFligge, 1998, 1999; Lockwood and Stamper, 1999; Fliggeand Solanki, 2000).

The reconstructed irradiances are most reliable since1874, due to the improved solar data available from thattime onward (the Royal Greenwich Observatory started ob-servations frommultiple sites in that year). As an illustrationof the results of these reconstructions we plot in Fig. 9 the11-year smoothed solar irradiance and global air tempera-tures on Earth since 1880 (Solanki and Fligge, 1998). Eachquantity is represented by two curves and the shaded regionbetween the two curves is a measure (although probably anoptimistic one) of the uncertainty in the relevant quantity.

In the case of the irradiance the two curves result fromdi6erent assumptions regarding the exact time-dependenceof the secular variation of the irradiance. In one case, it isassumed to follow the strength of the solar cycle (dashed—as proposed by Lean et al., 1995), in the other case the cy-cle length (solid—e.g. Friis-Christensen and Lassen, 1991;Fligge et al., 1999; cf. Hoyt and Schatten, 1993). The two


climate curves display the evolution of temperature over thewhole Earth and over its northern hemisphere, respectively.

Prior to ∼1980 the temperature and irradiance curvesare consistent with a causal relationship between the ir-radiance and climate. Thus the main trends are ?rst seenin irradiance. The dip starting around 1880 in the Earth’stemperature may be in=uenced by the major volcanic erup-tions of 1878 (Ghaie, Bismarck-Achipelage) and 1883(Krakatau, Indonesia). This dip corresponds to a peak in thevolcanic eruption frequency (Lamb, 1988). Another peak ispresent between 1900 and 1920 and in the early 1960s (towhich the eruption of Agung on Bali in 1963 was a majorcontributor).

Since 1980, however, the Earth’s temperature has con-tinued to increase at a rapid pace, but no corresponding in-crease in the irradiance is seen. This suggests that at leastafter this date another source for the change in the Earth’sclimate, such as forcing by man-made greenhouse gases,becomes dominant.

5. Conclusions

A brief introduction has been given to solar irradiancevariations, with observations and modelling of solar irradi-ance as well as the reconstruction of past irradiance vari-ations being discussed. In spite of the signi?cant progressmade in this ?eld in the past years numerous uncertain-ties and unanswered questions remain. To some of thesequestions, however, solutions appear to be in sight andwe expect considerable advances in the next couple ofyears.

References

Baliunas, S., Jastrow, R., 1990. Evidence for long-term brightnesschanges of solar-type stars. Nature 348, 520–523.

Beer, J., 2000. Long term indirect indices of solar variability.In: Friis-Christensen, E., FrIohlich, C., Haigh, J., SchIussler, M.,von Steiger, R. (Eds.), Solar Variability and Climate. KluwerAcademic Publishers, Dordrecht, pp. 53–66.

Beer, J., Blinov, A., Bonani, G., Hofmann, H.J., Finkel, R.C., 1990.Use of Be-10 in polar ice to trace the 11-year cycle of solaractivity. Nature 347, 164–166.

Beer, J., Baumgartner, S.T., Dittrich-Hannen, B., Hauenstein, J.,Kubik, P., Lukasczyk, C., Mende, W., Stellmacher, B., Suter,M., 1994. Solar variability traced by cosmogenic isotopes. In:Pap, J.M., FrIohlich, C., Hudson, H.S., Solanki, S.K. (Eds.), IAUColloquium 143. The Sun as a Variable Star: Solar and StellarIrradiance Variations. Cambridge University Press, Cambridge,pp. 291–300.

Brueckner, G.E., Edlow, K.L., Floyd, L.E., Lean, J.L., Van Hoosier,M.E., 1993. The solar ultraviolet spectral irradiance monitor(SUSIM) experiment on board the upper atmosphere researchsatellite (UARS). Journal of Geophysical Research 98, 10,695–10,711.

Eddy, J.A., 1977. Climatic Change 1, 173–190.

Fligge, M., Solanki, S.K., 2000. The solar spectral irradiance since1700.1 Geophysical Research Letters 27, 2157–2160.

Fligge, M., Solanki, S.K., Unruh, Y.C., FrIohlich, C., Wehrli, C.,1998. A model of solar total and spectral irradiance variations.Astronomy and Astrophysics 335, 709–718.

Fligge, M., Solanki, S.K., Beer, J., 1999. Determination of solarcycle length variations using the continuous wavelet transform.Astronomy and Astrophysics 346, 313–321.

Fligge, M., Solanki, S.K., Unruh, Y.C., 2000a. Modelling irradiancevariations from the surface distribution of the solar magnetic?eld. Astronomy and Astrophysics 353, 380–388.

Fligge, M., Solanki, S.K., Meunier, N., Unruh, Y.C., 2000b.Solar surface magnetism and the increase of solar irradiancebetween activity minimum and maximum. In: Wilson, A. (Ed.),Proceedings of the First Solar & Space Weather Euroconference“The Solar Cycle and Terrestrial Climate”, ESA SP-463,Noordwijk, pp. 117–121.

Foukal, P., Lean, J., 1986. The in=uence of faculae on totalsolar irradiance and luminosity. Astrophysical Journal 302,826–835.

Foukal, P., Lean, J., 1988. Magnetic modulation of solarluminosity by photospheric activity. Astrophysical Journal 328,347–357.

Friis-Christensen, E., 2001. Journal of Atmospheric andSolar-Terrestrial Physics, accepted for publication.

Friis-Christensen, E., Lassen, K., 1991. Length of the solar cycle:an indicator of solar activity closely associated with climateScience 254, 698–700.

FrIohlich, C., 2000. Observations of irradiance variations. SpaceScience Reviews 94, 15–24.

FrIohlich, C., Lean, J., 1998. The sun’s total irradiance: cycles,trends and related climate change uncertainties since 1976Geophysical Research Letters 25, 4377–4380.

FrIohlich, C., Andersen, B., Appourchaux, T., Berthomieu, G.,Crommelynck, D.A., Domingo, V., Fichot, A., Finsterle, W.,Gomez, M.F., Gough, D., Jimenez, A., Leifsen, T., Lombaerts,M., Pap, J.M., Provost, J., Cortes, T.R., Romero, J., Roth,H., Sekii, T., Telljohann, U., Toutain, T., Wehrli, C., 1997a.First results from VIRGO, the experiment for helioseismologyand solar irradiance monitoring on SOHO. Solar Physics 170,1–25.

FrIohlich, C., Crommelynck, D.A., Wehrli, C., Anklin, M., Dewitte,S., Fichot, A., Finsterle, W., Jimenez, A., Chevalier, A., Roth,H., 1997b. In-=ight performance of the VIRGO solar irradianceinstruments on SOHO. Solar Physics 175, 267–286.

Herschel, W., 1801. Philosophical Transactions of the RoyalSociety, Vols. 265 and 354.

Hoyt, D.V., Schatten, K.H., 1993. A discussion of plausiblesolar irradiance variations, 1700–1992. Journal of GeophysicalResearch 98, 18,895–18,906.

Kyle, H.L., Hoyt, D.V., Hickey, J.R., 1994. A review of theNimbus-7 ERB solar dataset. Solar Physics 152, 9–12.

Lamb, H.H., 1988. Weather, Climate and Human A6airs.Routledge, London.

Lean, J., 1991. Variations in the Sun’s radiative output. Reviewsof Geophysics 29, 505–535.

Lean, J., Skumanich, A., White, O., 1992. Estimating the Sun’sradiative output during the Maunder minimum. GeophysicalResearch Letters 19, 1595–1598.

Lean, J., Beer, J., Bradley, R., 1995. Reconstruction ofsolar irradiance since 1610: Implications for climate changeGeophysical Research Letters 22, 3195–3198.


Lockwood, M., Stamper, R., 1999. Long-term drift of the coronalsource magnetic =ux and the total solar irradiance. GeophysicalResearch Letters 26, 2461.

Lockwood, M., Stamper, R., Wild, M.N., 1999. A doubling of thesun’s coronal magnetic ?eld during the past 100 years. Nature399, 437–439.

Rottman, G.J., 1988. Observations of solar UV and EUV variability.Astronomy and Astrophysics, Advances in Space Research 8,53–66.

Rottman, G.J., Woods, T.N., White, O.R., London, J., 1994.Irradiance observations from the UARS=SOLSTICE experiment.In: Pap, J.M., FrIohlich, C., Hudson, H.S., Solanki, S.K. (Eds.),IAU Colloquium 143. The Sun as a Variable Star: Solarand Stellar Irradiance Variations. Cambridge University Press,Cambridge, pp. 73–80.

Solanki, S.K., 1993. Small scale solar magnetic ?elds—anoverview. Space Science Reviews 63, 1–188.

Solanki, S.K., Fligge, M., 1998. Solar irradiance since 1874revisited. Geophysical Research Letters 25, 341–344.

Solanki, S.K., Fligge, M., 1999. A reconstruction of totalsolar irradiance since 1700. Geophysical Research Letters 26,2465–2468.

Solanki, S.K., Fligge, M., 2002. How much of the solar irradiancevariations is caused by the magnetic ?eld at the solar surface?Advances in Space Research, in press.

Solanki, S.K., Unruh, Y.C., 1998. A model of the wavelengthdependence of solar irradiance variations. Astronomy andAstrophysics 329, 747–753.

Solanki, S.K., SchIussler, M., Fligge, M., 2000. Secular evolutionof the Sun’s large-scale magnetic ?eld since the Maunderminimum. Nature 408, 445–447.

Unruh, Y.C., Solanki, S.K., Fligge, M., 1999. The spectraldependence of facular contrast and solar irradiance variations.Astronomy and Astrophysics 345, 635–642.

Vanhoosier, M.E., Bartoe, J.-D.F., Brueckner, G.E., Prinz, D.K.,1988. Absolute solar spectral irradiance 120–400 nm (resultsfrom the solar ultraviolet spectral irradiance monitor—SUSIM—experiment on board Spacelab 2). Astrophysical LettersCommunication 27, 163–168.

White, O.R., Skumanich, A., Lean, J., Livingston, W.C., Keil,S.L., 1992. The Sun in a noncycling state. Publications of theAstronomical Society of Paci?c 104, 1139–1143.

Willson, R.C., Hudson, H.S., 1991. The Sun’s luminosity over acomplete solar cycle. Nature 351, 42–44.

Zhang, Q., Soon, W.H., Baliunas, S.L., Lockwood, G.W., Ski6,B.A., Radick, R.R., 1994. A method of determining possiblebrightness variations of the Sun in past centuries fromobservations of solar-type stars. Astrophysical Journal 427,L111–L114.

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