Evaluating sun–climate relationships since the Little Ice Age

\ PERGAMON

Journal of Atmospheric and Solar!Terrestrial Physics 50 "0888# 14Ð25

Evaluating sunÐclimate relationships since the Little Ice Age Judith Leana\\ David Rindb a

E[O[ Hulburt Center for Space Research\ Naval Research Laboratory\ 3444 Overlook Ave SW\ Washin`ton\ DC 19264\ U[S[A[ b Columbia University and GISS\ 1779 Broadway\ New York\ NY 09914\ U[S[A[ Received 5 November 0886^ received in revised form 01 October 0887^ accepted 08 October 0887

Abstract From the coldest period of the Little Ice Age to the present time\ the surface temperature of the Earth increased by perhaps 9[7>C[ Solar variability may account for part of this warming which\ during the past 249 years\ generally tracks ~uctuating solar activity levels[ While increases in greenhouse gas concentrations are widely assumed to be the primary cause of recent climate change\ surface temperatures nevertheless varied signi_cantly during pre!industrial periods\ under minimal levels of greenhouse gas variations[ A climate forcing of 9[2 W m−1 arising from a speculated 9[02) solar irradiance increase can account for the 9[2>C surface warming evident in the paleoclimate record from 0549 to 0689\ assuming that climate sensitivity is 0>C W−0 m−1 "which is within the IPCC range#[ The empirical SunÐclimate relationship de_ned by these pre!industrial data suggests that solar variability may have contributed 9[14>C of the 9[5>C subsequent warming from 0899 to 0889\ a scenario which time dependent GCM simulations replicate when forced with reconstructed solar irradiance[ Thus\ while solar variability likely played a dominant role in modulating climate during the Little Ice Age prior to 0749\ its in~uence since 0899 has become an increasingly less signi_cant component of climate change in the industrial epoch[ It is unlikely that SunÐclimate relationships can account for much of the warming since 0869\ not withstanding recent attempts to deduce long term solar irradiance ~uctuations from the observational data base\ which has notable occurrences of instrumental drifts[ Empirical evidence suggests that SunÐclimate relationships exist on decadal as well as centennial time scales\ but present sensitivities of the climate system are insu.cient to explain these short!term relationships[ Still to be simulated over the time scale of the Little Ice Age to the present is the combined e}ect of direct radiative forcing\ indirect forcing via solar!induced ozone changes in the atmosphere\ and speculated charged particle mechanisms whose pathways and sensitivities are not yet speci_ed[ Þ 0888 Elsevier Science Ltd[ All rights reserved[

0[ Introduction Twenty years ago\ Eddy "0865# reported convincing empirical evidence that Earth|s surface warming since the Little Ice Age "0349Ð0749# had paralleled the increase in solar activity from anomalously low levels in the Sporer "0349Ð0449#\ Maunder "0534Ð0604# and Dalton "0689Ð 0729# minima to the historically high levels in the present Modern Maximum[ However\ wide ranging speculation about solar variability and climate change that had per! sisted since the previous century was generally dismissed for lack of statistical validity "Pittock\ 0868#[ In a 0871 evaluation of SunÐclimate relationships\ a

 Corresponding author[

National Academy of Sciences report "National Research Council\ 0871# concluded that {{It is conceivable that solar variability plays a role in altering weather and cli! mate at some as yet unspeci_ed level of signi_cance||[ Since then\ the recognized potential for solar!induced climate ~uctuations to mask or exacerbate anthro! pogenically forced climate change has motivated further evaluation of SunÐclimate relationships[ Based on evi! dence from more than a decade of solar and atmospheric monitoring\ a subsequent NAS study "National Research Council\ 0883# acknowledged unequivocal evidence for the Sun|s total irradiance variability and thus for direct solar forcing of climate change[ It recognized\ as well\ observational evidence for solar!related ozone and mid! dle atmosphere variability\ and the possibility of indirect solar!forced climate change[ But like its 0871 predecessor\ the 0883 NAS report was unable to quantify the extent

S0253Ð5715:88:, ! see front matter Þ 0888 Elsevier Science Ltd[ All rights reserved PII] S 0 2 5 3 Ð 5 7 1 5 " 8 7 # 9 9 0 0 2 Ð 7

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Fig[ 0[ Amplitudes of natural and anthropogenic climate forcings from 0749 to 0889 are shown from IPCC "0884#[ Each individual forcing is expected to impact the climate system in di}erent ways depending on its latitude\ altitude and history[ However\ climate change assessments lack the complexity to account for the myriad pathways of the climate system response\ and global scale studies often assume a common climate sensitivity to the di}erent forcings[

of the climate system response to either direct or indirect solar forcing\ for which empirical relationships such as that presented by Eddy\ and subsequent wide ranging additional correlative studies\ provided extensive cir! cumstantial evidence[ The present need to specify SunÐclimate relationships is more than academic[ Policy making requires a reliable scienti_c basis for the attribution of climate change to both anthropogenic and natural processes "Hansen and Lacis\ 0889^ National Science and Technology Council\ 0886#[ Increasing greenhouse gas concentrations are thought to provide the dominant forcing of con! temporary climate\ a factor of eight more than solar forcing over the past 049 years\ according to the IPCC "0884# comparison of radiative climate forcings shown in Fig[ 0[ But a full understanding of all climate forcings\ and of the history\ geographical distributions and altitude of climate responses to these forcings\ is lacking[ Perhaps most controversial is the relationship of climate change to solar variability in the industrial epoch[ Some studies report that solar variability can explain all of Earth|s surface warming since 0759 "Friis!Christensen and Lasson\ 0880#\ others entirely dismiss solar contributions to surface warming "Visser and Molenaar\ 0884#\ and a number of studies cite a partial solar contribution "Reid\ 0880^ Schlesinger and Ramankutty\ 0881^ Kelly and Wigley\ 0881^ Lean et al[\ 0884#[ From both contemporary and paleo! perspectives\ the epoch since the Little Ice Age has unprecedented value

for investigating SunÐclimate relationships[ Purported relationships can be evaluated under both pre! and post! industrial conditions\ better elucidating the comparative roles of natural and anthropogenic climate in~uences[ Figure 1 shows that relative levels of solar activity\ the volcanic dust veil index and CO1 concentrations have changed signi_cantly since 0599[ In a broad historical sense the epoch from the Little Ice Age to the present provides a paradigm for solar forcing of climate change throughout much of the Holocene[ Over a period of ½249 years\ surface temperatures increased in the range of 9[6Ð0>C "Fig[ 1a# and solar activity increased from levels in the Sporer and Maunder Minima\ that were near the lowest of the past 7999 years\ to levels near the highest[ Figure 2 shows that the ¾0>C temperature change that may have occurred from the Little Ice Age to the present is typical of the range of climate ~uctuations recorded by Holocene d07O levels in the GISP1 ice!core[ According to the d03C archive of solar activity in tree! rings "Stuiver and Reimer\ 0882^ Grootes and Stuiver\ 0886#\ also shown in Fig[ 2\ the Sporer and Maunder Minima episodes are the most recent of more than six similar events that punctuate the record of solar activity over the past 7999 years "Wigley and Kelly\ 0889#[ 1[ Empirical SunÐclimate relationships Empirical evidence for SunÐclimate relationships has accumulated since Eddy|s "0865# report of the Sun|s

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Fig[ 1[ Compared in "a# are the Bradley and Jones "0882# reconstructed record of decadal NH surface temperature since 0599 and IPCC "0881# global instrumental record since 0749[ Both natural and anthropogenic in~uences may have contributed to observed surface warming[ Shown in "b# are annual averages of the concentration of CO1\ in "c# estimated solar total irradiance "Lean et al[\ 0884#\ and in "d# volcanic aerosol loading according to the global dust veil index "Lamb\ 0866^ Robock and Free\ 0884#[

Fig[ 2[ The record of d07O in the GISP1 ice!core "upper panel# provides a proxy for surface temperature ~uctuations during the past 09\999 years\ throughout the Holocene "Grootes et al[\ 0882#[ Temperature ~uctuations generally do not exceed 0>C[ The record of d03C archived in tree rings "Stuiver and Reimer\ 0882# provides a proxy indicator of solar activity over the same time period[ Events similar to the Maunder and Sporer Minima that are associated with the lowest temperatures of the Little Ice Age occur regular[

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J[ Lean\ D[ Rind:Journal of Atmospheric and Solar!Terrestrial Physics 50 "0888# 14Ð25

Table 0 Correlations of Sun\ volcanic dust and greenhouse gases with surface temperature anomalies Parameter

Correlation coe.cients surface temperature anomalies "09!yr means# Year

Sun¦greenhouse¦dust Sun¦greenhouse Greenhouse] CO1 Dust] global dust veil index Sun] reconstructed solar total radiative forcing Sun] GCM global temperature response to solar forcing

apparent impact on the severity of winters in London and Paris[ There are numerous statistical similarities* common cycles and correlated ~uctuations*among vari! ous solar activity and climate datasets on multiple tem! poral and spatial scales[ Diverse climate parameters exhi! bit\ at times\ cycles near 00\ 11\ 79 and 109 years that are prominent also in solar activity time series[ Included are land and ocean surface temperatures\ U[S[ drought\ rain! fall\ forest _res\ cyclones\ cloud cover\ tropospheric tem! peratures\ ice!core d07O and dust layer thickness\ and tropical corals[ In some epochs\ climate ~uctuations track solar activity for hundreds of years "see Burroughs\ 0881^ Hoyt and Schatten\ 0886^ Lean and Rind\ 0887 for ref! erences and details#[ Perhaps the most compelling SunÐclimate relationship from the Little Ice Age to the present is the high cor! relation of surface temperatures with solar activity over the entire period "Reid\ 0880^ Lean et al[\ 0884#[ The correlation is evident globally and separately\ as well\ in each of the three di}erent ocean basins "Reid\ 0886#[ Table 0 lists the correlation coe.cients of the recon! structed surface temperature in Fig[ 1a with recon! structed total solar irradiance\ CO1 concentrations and the dust veil index\ also shown in Fig[ 1[ The correlations are determined during the pre!industrial 06th and 07th centuries\ the industrial 08th and 19th centuries\ and over the entire four centuries[ Solar variability accounts for 63\ 48 and 55) of the variance in the decadal mean surface temperature data in these three respective periods[ If the correlation calculations use solar activity proxies based on the length of the 00!year cycle "rather than those based on cycle amplitudes\ as shown in Fig[ 1 and used in Table 0# then solar variability can account for more than 79) of the long term surface temperature variance since 0759 "Friis!Christensen and Lassen\ 0881^ Hoyt and Schatten\ 0882#[ Depending on the choice of index\ SunÐ

0509Ð0799

0509Ð1999

0799Ð1999

9[8 9[78 9[69 −9[00 9[75 9[61

9[77 9[75 9[70 −9[3 9[70 9[70

9[78 9[75 9[75 −9[46 9[66 9[73

"08 points#

"28 points#

"19 points#

climate correlations\ in general\ account for 29Ð44) of the variance in the climate record since the Little Ice Age "Crowley and Kim\ 0884#[ Yet the attribution of climate change and the nature of SunÐclimate relationships in the industrial epoch remains ambiguous[ Especially controversial is the inference from correlation studies such as in Table 0 that SunÐclimate relationships can account for a portion of recent climate change[ During the past 049 years in which instrumental surface temperature records increased almost 9[5>C\ the climate forcing of 9[2 W m−1 associated with speculated total radiative output changes "Fig[ 0# is eight times smal! ler than the 1[3 W m−1 forcing from observed increases in greenhouse gas concentrations[ Many studies assume that climate responds with similar sensitivity to di}erent forcings and cite climate change attribution proportional to forcings| relative amplitudes[ These studies tend to assume\ as well\ that the forcing known with highest certainty "i[e[\ CO1 concentration changes# is de facto the most signi_cant[ Kelly and Wigley "0881#\ for example\ conclude that greenhouse gas forcing must be the cause of climate change in the industrial era since {{it is illogical to neglect given the well!established case for its exis! tence||[ In these scenarios\ the variable Sun is considered to have at most a minimal in~uence on recent climate change\ contrary to the empirical evidence[ Compared with the industrial period of climate change since 0749\ a di}erent combination of solar and green! house gas forcings prevailed in the pre!industrial epoch[ From 0549 to 0689 surface temperatures increased ½9[2>C "an amount equal to half of the post industrial warming from 0749 to 0889# even though the greenhouse gas forcing was only 4) of that since 0749[ Recon! structed surface temperatures from 0599 to 0799 "Bradley and Jones\ 0882# correlate better with solar variability than with either greenhouse gas concentrations or with

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Fig[ 3[ Compared are decadally averaged values of solar total irradiance reconstructed by Lean et al[ "0884# and NH summer temperature anomalies from 0509 to the present[ The solid line is the Bradley and Jones "0882# NH summer surface temperature reconstruction from paleoclimate data "primarily tree rings#\ scaled to match the NH instrumental data "IPCC 0881# "dashed line# during the overlap period[

the dust veil index proxy of volcanic activity "Table 0#[ The SunÐclimate relationship deduced from linear regression in the 199!year pre!industrial period is DT  −057[791¦S×9[012315\ where DT is surface tem! perature anomaly ">C# and S is solar total irradiance at the top of the Earth|s atmosphere "W m−1#[ The extra! polation of this relationship to the present\ shown in Fig[ 3\ suggests that a surface warming of about 9[4>C is associated with the increase in solar activity from the Maunder Minimum to the present\ i[e[\ since the Little Ice Age[ Solar forcing associated with this increase in activity may have contributed 9[14>C to the 9[5>C surface warming since 0899\ but less than one third of the warm! ing since 0869[ The empirical SunÐclimate relationship deduced from the data in Fig[ 1 prior to 0799 "DT  −057[791 ¦S×9[012315# implies a 9[05>C surface temperature increase per 9[0) irradiance increase[ This relationship di}ers from that of DT  −150[726¦S×9[080423 deduced for the 199 years after 0799\ which implies a 9[14>C surface temperature increase per 9[0) irradiance increase[ Both relationships assume that solar forcing alone caused the observed DT but the relationships are not the same[ That a single empirical relationship is not applicable for both the pre! and post!0799 portions of solar and climate time series since the Little Ice Age is signi_cant for the evaluation of SunÐclimate relation! ships\ and for the attribution of recent climate change[ Since it is unlikely that the climate system became 45) more sensitive to solar forcing after 0799\ an additional climate forcing "such as by anthropogenic in~uences# has likely contributed signi_cantly to the observed DT in the two most recent centuries\ causing the surface tem! perature to increase by more than would be expected from solar forcing alone\ based on the pre!industrial

relationship[ Independent paleoclimate evidence also favors the SunÐclimate relationship from 0599 to 0799\ rather than that from 0799 to 1999[ Surface temperature changes are apparent in the recent Holocene portion of the GISP1 d07O ice!core in conjunction with solar activity ~uctuations in d03C similar to those associated with the Maunder Minimum "Stuiver et al[\ 0886#[ SunÐclimate relationships since the Little Ice Age are apparent not only on the decadal!centennial time scales of Fig[ 3 but on sub!decadal time scales as well[ A decadal component of global sea surface temperatures is shown in Fig[ 4 to track reconstructed solar irradiance since the 0849s "correlation of 9[82#\ during the past few 00!year solar cycles "White et al[\ 0886# which have been near the strongest in the last 399 years[ Global\ hemispheric surface temperatures reported by the IPCC have also tracked the three large 00!year solar cycles since the 0849s "Lau and Weng\ 0884#[ Like the centennial scale SunÐ climate relationships\ the correlations are evident in each of the three main ocean basins\ and decrease to negligible values as ocean depth increases throughout the mixed layer[ Pronounced 00!year cycles are also evident in atmospheric parameters[ Signi_cant tropospheric pres! sure and temperature ~uctuations in both the Northern and Southern hemispheres have tracked solar activity for at least the past four solar cycles "Labitzke and vanLoon\ 0886^ vanLoon and Labitzke\ 0886#[

2[ Amplitudes of solar radiative forcing To quantify the physical mechanisms responsible for the observed empirical SunÐclimate relationships\ reliable knowledge of solar forcing amplitudes is needed at all wavelengths\ over a wide range of time scales\ and

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J[ Lean\ D[ Rind:Journal of Atmospheric and Solar!Terrestrial Physics 50 "0888# 14Ð25

Fig[ 4[ White et al[ "0886# identi_ed in global averaged sea surface temperature anomalies compiled from bathythermographs "BT#\ shown in "a#\ signi_cant annual and interannual variability as well as a decadal component that tracks solar irradiance reconstructed by Lean et al[ "0884#\ shown in "b#[

in comparison with other climate forcings[ Figure 0 com! pares estimates of these forcings since 0749 but actual space!based monitoring of solar radiative forcing exists only since the late 0869s[ Solar radiometers measured a change in total irradiance of 0[2 W m−1 "9[0)# at the top of the Earth|s atmosphere during the 0875 minimum of the 00!year solar activity cycle\ relative to activity max! ima in 0879 and 0889 "Lean\ 0886#[ This change equals a radiative climate forcing of 9[11 W m−1[ Because of a lack of observations\ the spectral distribution of solar radiative forcing remains essentially unknown except in the UV spectrum where changes of 0[0 and 9[14)\ respec! tively\ in the 199Ð299 and 299Ð399 nm wavelength bands are estimated to contribute 02 and 07) "a total of 20)# of solar cycle total irradiance variations "Lean et al[\ 0886#[ Visible and near infrared radiation contributes the remaining 58) of the total irradiance variability[ Solar models estimate variability in the range 9[96Ð9[00) for the spectrum from 399 to 0999 nm\ which is comparable to the variability of total irradiance "Solanki and Unruh\ 0887^ Fligge et al[\ 0887#[ According to the models\ radi! ation at wavelengths near 0[5 mm is least variable\ with an estimated solar cycle amplitude as small as 9[91)*a factor of _ve smaller than that of total irradiance[ The epoch of direct measurements of solar irradiance from space is too short to permit unambiguous detection of solar forcing changes on climatically relevant decadal and centennial time scales[ Claimed detection of long term irradiance ~uctuations in the space!based obser! vations\ which cover less than two 00!year cycles\ is highly controversial because of inaccuracies and sensitivity drifts in the solar radiometers[ Such ~uctuations\ pos! tulated to be occurring in addition to the 00!year cycle "Hoyt and Schatten\ 0882^ Lean et al[\ 0884#\ may be

negligible over the decade from 0875 to 0885\ or as much as 9[93) "Willson\ 0886#\ depending on the interpret! ation of solar versus instrumental components in the extant data records[ Instrumental e}ects appear to con! tribute a signi_cant part of the annual trends in the Nim! bus!6 record "Lee III et al[\ 0884^ Chapman et al[\ 0885^ Frohlich and Lean\ 0887a\ 0887b# and the Wilson result is not generally accepted "see Kerr "0886#\ who discusses the issues#[ Furthermore\ empirical models that par! ameterize the combined in~uences of magnetic sunspot and faculae features on solar irradiance\ and that can explain 78) of the variance in the composite total irradiance record\ predict similar irradiance levels during the 0875 and 0885 minima\ contrary to claims of a 9[93) change "Frohlich and Lean\ 0887b#[ Cosmogenic isotope and geomagnetic activity records both point to past levels of solar!terrestrial magnetic coupling that were reduced relative to levels during con! temporary solar minima\ indicating that solar activity has varied more in the past than it has during the two most recent 00!year cycles for which space!based radi! ometers have monitored the radiative output[ In particu! lar\ both the 09Be and 03C cosmogenic isotopes indicate periods of anomalously low solar activity in the Sporer\ Maunder and Dalton minima "Eddy\ 0865# relative to the present Modern Maximum[ The range of Ca radi! ation seen in an ensemble of stars of mass and age similar to the Sun is larger than the ~uctuation in Ca radiation from the minimum to maximum of the contemporary Sun[ Ca emission in the Sun tracks closely the brightness component of total irradiance variability "i[e[\ the residual time series obtained by removal of the in~uences of dark sunspots from the measured total irradiance# "Lean et al[\ 0881#[ That Ca levels in the present!day Sun

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appear to be at high overall levels relative to these Sun! like stars "White et al[\ 0881^ Lockwood et al[\ 0881# implies the plausibility of future decreases to levels pre! sent in other Sun!like stars\ accompanied by a reduction in the amplitude of the activity cycle[ This supports the evidence from cosmogenic isotopes that contemporary solar activity is at high levels\ and that past and future decreases are more probable than are increases[ Lacking direct observational evidence\ the amplitudes of irradiance variability on time scales longer than the 00!year activity cycle have been speculated in a number of ways\ including from the extension of variability models developed from the contemporary irradiance database[ One such scenario that adopts only those magnetic mech! anisms clearly identi_ed in the contemporary data pre! dicts long term irradiance changes con_ned to 00!year cycles alone "Foukal and Lean\ 0889#[ Another scenario "Lean et al[\ 0884# adopts\ as well\ an additional\ variable background component "as yet undetected by obser! vations# in recognition of the larger variabilities that a variety of circumstantial evidence implies for the Sun[ Such evidence\ from historical solar proxy data\ cosmog! enic isotopes and ~uctuations in Sun!like stars\ points to overall solar activity levels "and by inference total radi! ative output# that is higher in the past few decades than at any time since the Maunder Minimum[ While displays of magnetism on the solar disk*the presence of sunspots\ radiative output ~uctuations\ the occurrence of ~ares and coronal mass ejections*are evident today even during contemporary solar minima\ they were absent entirely from the solar disk for a number of years during the Maunder Minimum[ The historical solar irradiance reconstruction in Fig[ 1 assumes the existence of a long term irradiance variability mechanism such as a pos! tulated reduction in the disk coverage of bright network emission below that seen even during contemporary solar minima "White et al[\ 0881#[ This irradiance recon! struction is the basis for the 9[2 W m−1 solar forcing since 0749 shown in Fig[ 0\ which would be reduced to less than 9[0 W m−1 "the lower limit of the uncertainty bar# if solar irradiance variations actually arise only from 00! year cycles[ Estimates of the total irradiance increase from the Maunder Minimum to the present range from 9[0 to 9[5) "Hoyt and Schatten\ 0882^ Nesme!Ribes et al[\ 0882^ Lean et al[\ 0884^ Zhang et al[\ 0883^ Reid\ 0886#[ The irradiance reconstruction in Fig[ 1 lies midway among these estimates\ and agrees well with the 09Be cosmogenic isotope record from polar ice cores "Lean et al[\ 0884#[ Figure 5 compares the solar irradiance reconstruction in Fig[ 1 with other reconstructions during the past 399 years[ The temporal footprint of solar forcing change since the Little Ice Age is uncertain because various solar activity surrogates exhibit somewhat di}erent temporal ~uctuations[ For example\ using the length of the activity cycle as an irradiance proxy predicts forcing ~uctuations

20

that are at times out of phase by 19 years with a proxy based on cycle amplitude[ The present lack of under! standing of the mechanisms for long term solar irradiance variability inhibits improved de_nition of historical solar forcing changes[ While the historical surrogates pertain to various aspects of solar magnetism that are related empirically to changes in contemporary solar forcing\ their explicit physical connection with solar irradiance remains to be physically speci_ed[ As well\ speculation about solar forcing of climate change deduced from stel! lar monitoring is controversial because of the lack of understanding of how the relative contributions of dark sunspots and bright faculae change with overall stellar activity levels "Foukal\ 0883#[

3[ Climate sensitivity and climate change Climate forcing amplitudes\ alone\ are insu.cient to de_ne the physical mechanisms responsible for empirical SunÐclimate relationships\ and climate change[ Reliable knowledge of the sensitivity of the climate system to the various forcings is equally crucial[ Many di}erent feedbacks compose this sensitivity\ and each has an indi! vidual spatial\ temporal and altitudinal dependence[ IPCC "0884# estimates climate sensitivity to be such that a global temperature change in the range of 9[2Ð0[9>C occurs per W m−1 radiative forcing occurs[ Achieving the new equilibrium temperature requires su.cient time for full realization of all climate feedbacks\ which involve the atmosphere and ocean\ and occur on a range of time scales[ For this reason\ only a fraction of the equilibrium temperature response is assumed to be manifest in the global temperature on time scales much less than a decade[ The Sun provides the bulk of its energy to the Earth in the form of radiation\ and a change in this radiation is therefore considered the most likely mechanism for SunÐ climate relationships[ The equilibrium surface tem! perature change corresponding to a solar radiative forc! ing increase of 9[5 W m−1 "9[14)#\ estimated since the Maunder Minimum "Fig[ 1#\ is in the range 9[1Ð9[5>C[ Consistent with this\ a simulation by the GISS general circulation model "GCM# predicts an equilibrium surface temperature increase of 9[36>C in response to such a forcing "Rind and Overpeck\ 0882#[ Also consistent are time dependent GCM simulations which show in Fig[ 6 a 9[34>C surface temperature increase in response to reconstructed solar irradiance changes from the Maunder Minimum to the present[ The magnitudes of solar forcing "9[14)#\ climate sensitivity "0>C W−0 m−1# and surface temperature changes "9[7>C# are all compatible with pre! sent understanding of these values[ The GCM calculates feedbacks that involve water vapor\ high and low cloud cover and sea ice^ additional climate mechanisms are not required[ Furthermore\ the simulations agree well with

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Fig[ 5[ There is speculation that the level of the Sun|s total irradiance increased during the past 249 years as solar activity increased from the Maunder Minimum to the present[ But the magnitude of the increase\ and its temporal structure remain poorly de_ned[ Compared are some speculations derived from various circumstantial evidence based on solar and stellar variability[ Long term variability estimates of 9[14Ð9[24) compare with the 9[97) amplitude of annual mean data during the 00!year solar cycle[

estimates from pre!industrial empirical Sun!climate relationships\ both that in Fig[ 3\ and those from the GISP d07O and tree!ring d03C correlations of surface tem! perature changes for Maunder Minimum type events[ While the solar forcing\ climate sensitivity and surface temperature change magnitudes are each within the range of accepted values\ all have signi_cant uncertainties[ Solar forcing may be as small as 9[0 W m−1 "instead of the assumed 9[5 W m−1# if solar irradiance variability occurs only on 00!year and not on longer time scales[ Furthermore\ the climate sensitivity of the GISS GCM is higher than the average of several "Cess et al[\ 0889#[ If climate sensitivity is as small as 9[2>C W−0 m−1 and solar forcing as small as 9[0 W m−1\ then the surface temperature change since the Little Ice Age would be only 9[92>C[ On the other hand\ with solar forcing as high as 0[3 W m−1 "corresponding to an irradiance change of 9[5)# and climate sensitivity 0>C W−0 m−1\ the esti! mated surface temperature change is 0[3>C[ However\ neither of these extreme cases is consistent with the empirical SunÐclimate relationship during 0599Ð0799[ Further complicating the evaluation of SunÐclimate relationships and climate change attribution since the Little Ice Age is the poor de_nition of surface tem! peratures themselves prior to the 0759s[ The temperature reconstruction in Fig[ 1a is derived from a composite of primarily Northern Hemisphere tree ring decadal values "Bradley and Jones\ 0882#\ scaled to match the IPCC instrumental record\ with uncertainties of a few tenths

>C[ A new speci_cation of recent climate change in the Arctic generally tracks the NH reconstruction in Fig[ 1 and likewise points to a solar role for at least a part of climate change since 0599 "Overpeck et al[\ 0886#[ That regional spatial scale responses are likely super! imposed on the global response adds further complexity to the evaluation of SunÐclimate relationships[ GCM simulations indicate that dynamical patterns are set up by di}erential solar heating of the land and ocean and induce regional e}ects\ evolution of which could cause apparent ambiguities in site!speci_c climate proxy records\ and account for non!linearities in empirical SunÐ climate relationships[ Not included in the GCM simu! lations made thus far of climate change since the Little Ice Age are the possible indirect impacts of middle atmo! sphere variability forced by changes in solar UV radiation that likely have distinct regional signatures[ Simulations of the atmosphere|s response to the 00!year cycle suggest that the dynamical and chemical pathways can facilitate the relationships\ but the sensitivities of existing models are smaller than the empirical evidence suggests "Rind and Balachandran\ 0884^ Haigh\ 0885#[ Likewise\ the speculated role of energetic charged particles based on short term empirical relationships is unknown on longer time scales[ In this latter case both the pathways and their sensitivities remain to be speci_ed[ Because of the signi_cant uncertainties in solar forcing amplitudes\ climate sensitivity and the extent of climate change\ there is wide latitude to interpret climate change

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Fig[ 6[ Shown in "a# is reconstructed solar radiative forcing "Lean et al[\ 0884# and\ in "b#\ the average GISS GCM simulation of the global surface temperature response to this forcing\ compared with the observed and reconstructed surface temperature[ On the right are periodograms of the respective solar forcing and climate response time series[

since the Little Ice Age[ Other mechanisms unrelated to solar variability are speculated to have had signi_cant roles\ including volcanism and natural internal varia! bility[ Rather than solar forcing\ internal oscillation of the ocean!atmosphere system "Halliwell\ 0886^ Nak! amura et al[\ 0886# is an explanation often advanced to explain natural climate variability on decadal and cen! tennial time scales[ Large scale changes in the inter basin circulation of the ocean conveyor belt are a favored cause of centennial climate change "Broecker\ 0886^ Bond et al[\ 0886# while intra basin oscillations are identi_ed as a source of decadal climate change[ Simulations with gen! eral circulation climate models can replicate such oscil! lations[ The apparent relationship between decadal solar and global ocean surface temperature ~uctuations shown in Fig[ 4 suggests that solar forcing may also play a role in decadal climate variability\ even though the mech! anisms are not properly understood[ Climate feedbacks instigated by solar forcing are generally believed to require much longer than 00 years to achieve their full equilibrium responses[ As a result\ the SST decadal amplitudes of 9[0>C in Fig[ 4 are 2Ð4 times larger than the 9[91Ð9[92>C surface temperature changes simulated

with models that incorporate known climate feedbacks\ in response to radiative forcing during the 00!year solar cycle "Wigley and Raper\ 0889^ North and Kim\ 0884#[

4[ Discussion The record of solar variability and climate change since the Little Ice Age o}ers insight about the separation of natural and anthropogenic climate in~uences\ a task that has proven di.cult in the post!industrial epoch alone[ An empirical SunÐclimate relationship de_ned for the period from 0599 to 0799 "when greenhouse gas and sulphate aerosol forcings were reduced considerably rela! tive to their recent levels# attributes to solar in~uences much of the climate ~uctuations during the Little Ice Age but less than half of the long term increase since then[ Such a scenario is plausible in that the solar forcing amplitude\ climate sensitivity and climate change are all within presently accepted ranges[ GCM simulations rep! licate surface temperature changes consistent with this scenario in response to forcing by reconstructed solar total irradiance[ Both the empirical evidence and the

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J[ Lean\ D[ Rind:Journal of Atmospheric and Solar!Terrestrial Physics 50 "0888# 14Ð25

GCM simulation indicate that a solar!type event like the Maunder Minimum produces a surface temperature change in the range 9[3Ð9[4>C\ which agrees with evidence from the GISP ice core of a similar SunÐclimate relation! ship during the recent Holocene "Stuiver et al[\ 0886#\ and substantiates Eddy|s "0865# originally conjectured SunÐclimate relationship[ The indication that real SunÐclimate relationships have occurred in the recent past\ and have contributed to glo! bal warming since 0749\ con~icts with the favored attri! bution of industrial climate change to increasing green! house gas concentrations\ based on comparisons of various climate forcing amplitudes[ Yet the warming expected from increasing greenhouse gas concentrations exceeds the observed surface temperature increase "for the range of climate sensitivities endorsed by IPCC#\ and accounting for the instrumental temperature record requires a signi_cant cooling by some other mechanism\ presumably anthropogenic sulphate aerosols[ Any additional source of global warming during the past cen! tury*whether from SunÐclimate relationships\ reduced volcanic dust or natural internal climate oscillations* exacerbates this requirement[ Because of large uncer! tainties in the amplitudes of various climate forcings\ and in climate sensitivities to these forcings\ the attribution of climate change since the Little Ice Age remains ambiguous[ Accommodating a signi_cant SunÐclimate relationship\ such as that deduced for the pre!industrial period from 0599 to 0799\ requires cooling by sulphate aerosol forcing in the range of 0Ð1 W m−1 since 0749\ a range that is not inconsistent with IPCC "0884\ see Fig[ 0# or recent evaluations "Penner et al[\ 0884^ Schwartz and Andreae\ 0885^ Hansen et al[\ 0886# but it is near the upper limit of probable e}ects[ SunÐclimate relationships also remain controversial in general because not all climate time series exhibit com! mon cycles or correlated ~uctuations[ But this objection\ which cites that solar!related climate cycles are typically neither stationary\ nor deterministic\ nor evident in all climate parameters\ nor always global\ nor phase locked with solar forcing\ is insu.cient reason to dismiss SunÐ climate relationships since climate cycles themselves\ whatever their origins\ are neither stationary nor phase stable\ nor even necessarily linearly related to the assumed forcings[ Resolving the present ambiguities about SunÐclimate relationships since the Little Ice Age\ and the role of the variable Sun compared with anthropogenic in~uences in recent climate change requires answers to some crucial questions[ "0# Does the Sun|s radiation vary over time scales longer than the 00!year cycle< The assumption that it does\ based on a variety of circumstantial evidence\ is the basis for climate model simulations that o}er a physi! cal explanation of empirical SunÐclimate relation!

ships[ If they are occurring\ what are the amplitudes of long term solar total radiative forcing and how do the changes relate to the 09Be\ 03C proxy solar activity records< In the absence of long term total irradiance changes\ might the larger UV irradiance cycles account for the observed empirical associations\ but through di}erent physical mechanisms< How do long term solar forcing amplitudes\ whether from total or UV radiation\ compare with other climate forcing amplitudes< "1# Are common cycles and SunÐclimate correlations real or fortuitous< Accepting that there is a relationship between the Sun and climate assumes that the empiri! cal evidence\ by virtue of its ubiquity\ is real[ If\ instead\ the connection of the Little Ice Age with the Maunder Minimum is coincidental then what other mechanisms "e[g[\ internal climate system oscil! lations\ volcanic in~uences# are the causes\ and are these alternative explanations plausible< Are the mechanisms similar*or di}erent*on long and short time scales< Given that decadal and centennial cli! mate cycles exist\ why are they non!stationary in phase and amplitude< What was the true extent of climate change\ itself\ in the recent past\ especially in the Little Ice Age< "2# How do other climate forcings and the climate state "ENSO and QBO# impact SunÐclimate relationships\ in the present\ past\ and future\ and geographically over regional spatial scales< Until answers are found for these questions the dis! missal of SunÐclimate relationships is premature[

Acknowledgements Supported by the joint NOAA and DOE Climate Change Detection Program[

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J[ Lean\ D[ Rind:Journal of Atmospheric and Solar!Terrestrial Physics 50 "0888# 14Ð25

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