BALIUNAS Et Al. - Evidence on the Climate Impact of Solar Variations

7/28/2019 BALIUNAS Et Al. - Evidence on the Climate Impact of Solar Variations

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1286 SALLIE BALIUNAS and ROBERT JASTROW

10.0

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3 10.5L:

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3

ct;

11.5

g-0.2 et

. SOLAR ACTIVITY 3

Px TEMPERATURE F

-0.4-*

12.0 1 1660 1660 1900 1920 1940 1960 1960 2000

YEAR

Fig. 1. Comparison between Northern Hemisphere temperature anomalies (x) (Ref. 2, Jones etal) and solar surface magnetic activity, measured by the length of the solar cycle(+) (Ref. 2,

Friis-Christensen and Lassen). The cycle length is inversely correlated with level of surfacemagnetic activity. (Ref. 23.)

No evident physical mechanism would seem to connect changes in the earths cli-mate with changes in the length of the sunspot cycle or other aspects of the solar activitycycle. The absence of a physical explanation has led to the suggestion that although strik-ing, the correlation between temperature and solar activity may be fortuitous. This inter-pretation is supported by the fact that the total radiative output associated with surfacemagnetic activity - involving mainly ultraviolet emission, x-rays and gamma rays - isvery small compared to total the solar energy flux in the visible and infrared, and cannotdirectly affect climate to a significant degree.

Correlation Between Solar Magnetic Activity and Solar h-radiance

The correlation becomes physically meaningful, however, if the variations in thelevel of solar magnetic activity are only the surface manifestation of some deep-seatedchange in the solar interior which affects both the suns magnetic activity and its total ener-gy output. This suggestion finds support in theoretical studies of the effect of changes inthe configuration of the magnetic field within the solar convective zone. The theoreticalestimates indicate that these changes, by influencing the efficiency of convective transportof energy to the suns surface, can produce changes of the order of (O.l-l.O)% in solar bright-ness.3 Observations of changes in the solar diameter are compatible with long-termvariations of as much 10% in solar brightness.4 A solar change of the order of 1% wouldcorrespond to a radiative forcing of 2.4 watts/m2 and would be comparable to the radiativeeffect of all the greenhouse gases expected to accumulate in the atmosphere in the next 40years.5

These theoretical estimates of changes in solar brightness are difficult to quantifyand can only be regarded as suggestive. Firmer support for a connection between magneticactivity variations and brightness changes is found in observations of the sun and solar-


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Climate impact of solar variations 1287

type stars.V Satellite observations of the sun from 1978 to 1989 during Solar Cycle 21,combined with observations of solar surface magnetic activity, reveal that solar brightnessand surface magnetic activity change are positively correlated (Fig. 2). Observations of

solar-type stars, described in more detail below, indicate that this relationship betweenbrightness and surface magnetic activity is common in stars similar to the sun.

g Sun0.10

El29 0.05

3h.d* 0.00320 -0.05

3:2 -0.10 (a)

E0.20 . . . . - . . . . . - . . .

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25 0.00

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4 -0.10 =

:- -0.20[ I

I. . . . I . . . , . . . , . . . . lb)

1975 1980 1985 1990 1995

Year

Fig. 2 (a) Measurement of changes in the suns surface magnetic activity between 1974 and1992, indicated by the CaII K emission lines. (Ref. 20, Livingston and White 1992) (b) ACRIM- Solar Maximum Mission satellite measurements of the changes in the solar total irradiancebetween 1978 and 1989. (Ref. 6, Willson and Hudson 1991.)

The change in the suns total irradiance during Solar Cycle 21 was only O.l%, whichis too small in amplitude and duration to have a significant climate impact. The 0.1% vari-

ation within one solar cycle has been explained as the result of a change in the relative con-tribution of bright (faculae) and dark magnetic areas (sunspots) on the suns surface.8However, no theoretical or observational evidence precludes the possibility of larger vari-ations at other times or over longer periods than one cycle. 53 Observations of a relativelysmall sample of solar-type stars have already revealed brightness changes as large as 0.6%over an activity cycle.7

Before examining the evidence for climatically significant changes in solar bright-ness, it may be useful to place the suggestion of a sun-climate connection in the context ofearlier work.


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Historical Evidence Relating to Solar Control over Global Temperatures

Historical climate records contain evidence suggesting that major climate changesare associated with changes in the suns magnetic activity over century timescales. Ananalysis of climate records indicates that during the last millennium, protracted periods ofcold weather occurred roughly every 200 years - in the 13th, 15th, 17th and 19th cen-turies.10 The best known of these cold periods was the Little Ice Age of the 17th century.Temperatures in the Little Ice Age are estimated to be (0.4-0.6)% below the average overthe last three centuriesll

The climate changes in the last thousand years can be compared with the record ofsolar activity over the same interval, inferred from the abundance of radioactive isotopes,such as carbon-14, in trees. Carbon-14 is produced by cosmic rays in the earths upperatmosphere. During periods of high solar activity, magnetic fields carried into the solar

system by the solar wind tend to deflect cosmic rays from the inner part of the solar system,and the production rate of Carbon-14 decreases. During periods of weak solar activity, thecosmic ray intensity increases and the production rate of Carbon-14 rises. A fraction of theCarbon-14 atoms reaches the earths surface and is incorporated into the bodies of trees.Thus, the radiocarbon record, dated by dendrochronology, yields information on changesin solar activity over time.12

Figure 3 shows the Carbon-14 record during the last thousand years.13 The figureindicates surprisingly regular cycles of solar activity, with periods of low activity at approxi-mately 200-year intervals - midway between the 10th and 11th centuries, and in the 13th,15th, 17th and 19th centuries. These periods of low solar activity roughly match the cold

spells in the Lamb climate record for the last millennium.

14C Production Rate

- 40

-30

-20

-10

0

10

20

30

40 I I I I I I__-- _--- _ _--1000 1200 1400 1600 1800 2000

Year

Fig. 3. Changes in carbon-14 production rates (parts per mil, arbitrary scale) associated withchanges in solar activity. The negative of the production rate is plotted so that periods of lowsolar activity appear as low points on the chart. Dates are corrected for the -5O-year resi-dence time of carbon-14 in the atmosphere (Ref. 13.)


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The carbon-14 record of solar activity extends only to the beginning of the 19th cen-tury, because the interpretation of the carbon-14 production rates becomes less reliable afterca. 1800.14 However, sunspot records confirm relatively low average solar activity in the19th century, followed by a rise in the 20th century to the current high level of magneticactivity.

Wigley and Kelly have compiled additional evidence for the recurrence of coldperiods every few centuries going back 10,000 years. 11 Their climate history is drawn fromglobal records compiled by Riithlisberger on the advance and retreat of glaciers. The pat-tern of recurring cold periods reported by Wigley and Kelly on the basis of the glacialrecords is not as regular as Lambs records. However, the glacial data have greater validitybecause they are global, while the Lamb climate record is regional and may be valid onlyfor northern Europe. Comparing the Carbon-14 record to their climate history, Wigley andKelly found that six out of the seven most severe decreases in solar activity correspond

closely to cold spells in the climate record. One of these coincidences was the MaunderMinimum, the period of very low values of sunspot numbers in the 17th Century whichcoincided with the Little Ice Age.16

Evidence on Recent Changes in Solar Brightness

Samuel Langley of the Smithsonian Institution initiated, and Charles Greely Abbotcontinued, a program of measurements of the solar constant that spanned 60 years (1902-1962) and relied on several measuring stations around the globe. However, the method-ology used by Abbot was not adequate to reveal small changes of the order of 1% or less.Inferring changes in solar irradiance of this magnitude from the ground requires that anextremely accurate account be taken of changes in atmospheric transparency with time.The atmospheric transparency in the Abbot measurements was determined from solarspectra recorded on photographic plates, which could not have yielded sufficient accuracyto remove satisfactorily the effects of atmospheric extinction. In addition, a number ofundocumented adjustments were made to the data.17

The Smithsonian data have been reanalyzed by Hoyt.18 After examining the datafor trends in solar brightness over the 30-year period covered by the Smithsonian obser-vations, he concluded that definite values were questionable, and the data could only beused to set an upper limit on brightness changes. Hoyt determined the upper limit to be is

0.3% over the 30-year period of observation.

The only available direct measurements of solar brightness change that have auseful degree of precision are satellite data, for example, from ACRIM-SMM and Nimbus-7starting in Solar Cycle 21, shown in Fig. 2ta1.6 As already noted, the change in the sunsbrightness over that 1Zyear period was only O.l%, but larger changes are possible overintervals of decades to centuries.

Changes in Magnetic Activity in Solar-Type Stars

Solar-type stars - stars similar to the sun in mass and age - display a cyclic mag-netic activity similar to the ll-year solar cycle. 19 Changes in surface magnetic activity canbe recorded for these stars (whose surface detail is unresolved because of their distance) byspectroscopic methods. On the sun, a close correspondence is observed between magneticfield strength and the strength of the singly ionized emission lines of calcium, the H (396.7nm) and K (393.4 nm) lines.20 A substantial portion of the emission of the H and K linesresults from nonthermal heating by magnetic fields. The H-K emission fluxes observed onstars provide a measure of the intensity of surface magnetic activity averaged over the diskof the star. The changes in the H-K fluxes trace the changes in surface magnetism, e.g.,starspot cycles.

EGY 18: 12-H


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1290 SALLIE BALILJNAS and ROBERT JASTROW

Observations f the H-K fluxes of approximately 100 nearby stars have been made atMount Wilson Observatory since 1966.19 The solar-type stars observed by this methodappear to be either in a cyclic state, in a state of low and flat magnetic activity resembling

the Maunder Minimum, or in a transition between the cyclic and Maunder MinimumstateS~9

Examples of long-term variations in the surface magnetic activity of solar-type starsare shown in Fig. 4. HD 10476 top panel) is in a cyclic state. HD 10700 bottom panel) is inthe low and relatively flat state of magnetic activity resembling the Maunder Minimum.HD 3651 appears to have been captured in a transition from a cyclic state to a MaunderMinimum state during the 25-year period of observation. Note that the apparent transi-tion to the Maunder minimum is rapid and without warning.

Starspot Cycles0.30 , , , , I 1 I I I I I I I I # 1 I 1 I I , I

ENTERING MAUNDER MINIMUM1

0.30 , , , , I 8 8 I I I 1 , I 1 I , 1 1 I 1 I

0.25 - IN THE MAUNDER MINIMUM -

0.20 -

Fig. 4. Surface magnetic activity (Ca II H and K emission lines) averaged on a monthly basisin solar-type stars (Ref 19.) The star (HD 10476) at top is in a cyclic state resembling the ll-year solar cycle. The middle star (HD 3651) appears to be exiting from a cyclic state into aMaunder Minimum state. Apparently the transition can be rapid - only 10 to 15 years in thiscase. The bottom star (HD 10700) is in a magnetically flat and low-level state resembling theMaunder Minimum episode on the sun. The yearly gaps in each record are times when the staris above the horizon in daytime, and therefore unobservable. The seasonal spread in thedata is caused by the passage of starspots and other magnetic features across the disk of thestar during its rotation.


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Changes in Brightness in Solar-Type Stars

Photometric observations of brightness changes in solar-type stars have been made

at Lowell Observatory in parallel with the magnetic activity measurements over the eight-year interval 19841991. Brightness and surface magnetic activity vary in phase in thesestars, as they do in the sun. However, the changes in the brightness of this group of solar-type stars can be considerably greater than those observed for the sun itself in the 19781989period.7

Figure 5 shows the changes in magnetic activity and brightness for HD 10476, a starsimilar to the sun and displaying a cyclic pattern of variations in surface magnetic activitysimilar to the 11-year sunspot cycle. As in the case of the sun, the brightness of this starvaries in phase with the changes in its level of magnetic activity. However, over theperiod of observation the magnitude of the brightness change is 0.6%, compared to 0.1%

observed for the sun in the magnetic activity cycle of the 1980s.

F1.0 , 1, , 1, , (I ( , , , , , , ( , , , , , , , , , I,

(a)1066 1970 1975 1980 1990

! 1

fl i i ,i:&

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Year

Fig. 5. (a) Changes in brightness observed for the solar-type star, HD 10476, show a 0.6%drop over seven years. The dispersion of the photometric data within a years observationsis caused by starspots carried across the disk of the star by rotation. (b) As in the case of the

sun, the brightness varies in phase with simultaneous changes in surface magnetic activityover the same interval, shown by the dashed-line box (Ref. 21 .I

Relation Between Magnetic Activity Change & Brightness Change

Figure 6 shows a more general relation between brightness changes and surfacemagnetic activity changes, based on observations of the sun and ten solar-type stars veryclose to the sun in mass and age: Surface magnetic activity in this chart is measured by thequantity RHK, which includes corrections designed to minimize the color (and hence


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1292 SALLIE BALIUNAS and ROBERT ASTROW

mass-) dependence of observed surface magnetic activity.19 The fit to the slope from thedata in Fig. 6 yields

A(Brightness) (%) - (8 f 4) x lo4 A(R)&(1)

This relationship allows an estimate of the brightness changes accompanying changes insurface magnetic activity, using the strength of the Ca II H and K lines as a proxy.

0.30

0.0

Variation of Ca II H and K Emission (AR&lO)

Fig. 6. Relation between changes in brightness and changes in surface magnetic activity

(measured by changes in RHK) for a group of solar-type stars. The line is the linearregression fit to the observations (Ref. 21.)

The indeterminacy in the slope, which is defined by the 95% confidence levels ofthe individual slopes of the solar-type stars, reflects the scatter in Fig. 6. Zhang et al statethat this scatter cannot be attributed entirely to measurement error and may be due partlyto time dependence.21 According to that interpretation, the slope for the sun may be largeror smaller over time than it is at present.

Application to the Determination of Solar Brightness Changes Since the MaunderMinimum

We assume that the physical processes in the convective zones of the sun and solar-type stars, which lead to the relationship in Fig. 6 or Eq. (1) between changes in brightnessand magnetic activity, are also valid over century timescales going back as far as theMaunder Minimum of the 17th century. This assumption is based on the fact that both in-tervals of time -the eight years of observations that went into Fig. 6 and the 300-odd yearsgoing back to the Maunder Minimum - are extremely short compared to the thermalrelaxation time of 10s years for perturbations within the convective zone.22


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On the basis of this assumption, we apply Eq. (1) to the solar changes in RHK overthe last three centuries, to obtain information on the total change in solar brightness overthat interval. While direct measurements of RHK are only available for the last 20-odd

years, the solar value of RHR during the Maunder Minimum can be estimated from solar-type stars in the non-cyclic phase.21

Inserting the current (decade of 1980s) and Maunder-Minimum values of RHR intoEq. (11, we find that from the Maunder Minimum to the present the suns brightness in-creased by roughly 0.4%. Uncertainties in the estimate, mainly resulting from the indcterminacy in the slope in Fig. 6 or Eq. (11, yield a range of brightness changes ranging from0.2% to 0.7%. This result is consistent with the estimate by Wigley and Kelly that a drop of(0.22-0.55)% in solar irradiance from the 17th century to the present can explain the coldclimate of the Little Ice Age. 11 In addition, the timing of solar brightness changes over thelast 350 years can be obtained from the transformation of the sunspot record to RHK

values.

A Relationship Between Magnetic Activity Change and Changes in Cycle Length

Figure 7(a), derived from sunspot records for the last 250 years, shows the amplitudeof the sunspot cycle versus the length of the cycle. The length of the magnetic cycle periodis seen to be inversely related to the amplitude of surface magnetic activity changes withina cycle; the shorter the cycle, the greater the amplitude of surface magnetic activity. Figure7(a) confirms that the length of the sunspot cycle is related to solar surface magneticactivity.

F_60 -

& Sun

1750 to 1990

60 -

40 -

-,

6.0 , , , Sunspot Cycle Length (years)I v I 1 I I , I I , 1 I I I

Solar-type Stars1966 to 1991

0.01 (1I3 6 10 12 14 16 16

Stellar Activity Cycle Length P, (years)

(a)

Fig. 7. (a) Amplitude of the sunspot cycle vs. length of the cycle, for the period 1750 - 1990(Ref. 23.) fb) Relation between magnetic cycle amplitude and length of cycle for a group ofsolar-type stars (Ref. 23.) The lines are a linear regression fit to the observations.

The relationship exhibited in Fig. 7(a), combined with the observations connectingchanges in the magnetic activity of the sun with changes in its brightness, leads to the con-


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elusion that short solar cycle periods are associated with larger amplitudes of solar bright-ness change.23 This result suggests that the observed correlation between changes in thesuns cycle period and changes in global temperature observed by Friis-Christensen andLassen is a significant physical relationship rather than a fortuitous correspondence.

A similar relationship between brightness and length of cycle period can be derivedfrom observations of these quantities for solar-type stars [Fig. 7(b)l.a The trend in Fig. 7(b)is qualitatively similar to the trend obtained for the sun [Fig. 7(a)]: When the length of themagnetic activity cycle decreases, the amplitude of the cycle increases, and from Eq. (11, thestars brightness also increases.

Independent Determination of Solar Brightness Changes Since the MaunderMinimum

The relationships indicated by Fig. 7 provide a second means of estimating pastsolar brightness changes over the 250-year interval for which records on sunspots and sun-spot cycle lengths are available. The estimate may be made using either the data from thesun or the data from the group of solar-type stars. First, combining the relationship be-tween cycle period and cycle amplitude for the sun in Fig. 7(a) with the relationshipbetween cycle amplitude and brightness change in Fig. 6, we find an increase in solarbrightness of zGO.l% rom ca. 1750 to 1990. Second, using the relationship between cycleperiod and cycle amplitude for solar-type stars [Fig. 7(b) instead of Fig. 7(a)], we find anincrease in brightness of 50.2%.

These results, added to the previous range of estimates of as small as 0.2 to as largeas 0.7% for the increase in the suns brightness since the Maunder Minimum, lead to anextended range of 0.1% to 0.7% for the estimated increase in brightness since that period.As noted earlier, the low end of this range would not have an appreciable climate impact.The midpoint of the range, 0.496, agrees well with climate-model estimates of the increasein solar irradiance requird to account for the recovery from the Little Ice Age.

The indeterminacy in the results obtained by the three methods of estimatingchanges in solar brightness suggests that it is premature to accept a definitive determina-tion of solar brightness change based on changes in the suns magnetic activity. However,the demonstration of a qualitative correlation between cycle period and brightness change

for the sun and solar-type stars illuminates the result obtained by Friis-Christensen andLassen, and is potentially of considerable interest to the climate change problem.

Acknowledgments

We would like to thank the Electric Power Research Institute, the RichardLounsbery Foundation, the Mobil Foundation, and the Langley-Abbot Scholarly Studiesand James Arthur Funds of the Smithsonian Institution for the support which made thisresearch possible. We are also grateful to E.N. Parker, W. Soon and Q. Zhang for severalilluminating conversations regarding the theoretical aspects of the relation connectingsolar magnetic activity and brightness changes in the sun. We are indebted to O.R. White,W. Livingston, H. Hudson and R. Willson for solar data. The observations at MountWilson Observatory were made possible as a result of a collaborative agreement betweenthe Carnegie Institution of Washington and the Mount Wilson Institute.

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BALIUNAS Et Al. - Evidence on the Climate Impact of Solar Variations

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