modelling the impact of solar variability on climate

\PERGAMON Journal of Atmospheric and Solar!Terrestrial Physics 50 "0888# 52Ð61

S0253Ð5715:88:, ! see front matter Þ 0888 Elsevier Science Ltd[ All rights reservedPII] S 0 2 5 3 Ð 5 7 1 5 " 8 7 # 9 9 0 0 6 Ð 4

Modelling the impact of solar variability on climateJoanna D[ Haigh�

Space and Atmospheric Physics\ Blackett Laboratory\ Imperial College of Science\ Technology and Medicine\ London SW6 1BZ\U[K[

Received 2 April 0887^ received in revised form 01 October 0887^ accepted 08 October 0887

Abstract

Computer simulations of the impact on climate of solar variability generally fall into four categories[ First\ there arelower atmosphere GCM experiments\ in which enhanced solar activity is represented by changes in spectrally integratedsolar constant[ Secondly\ there are GCM studies of the dynamical response of the middle atmosphere to changes insolar ultraviolet\ mainly concentrating on the northern hemisphere winter\ and how these impact the troposphere[ Thesestudies have been instructive in providing an understanding of some of the mechanisms involved but\ because of thevery di}erent nature of the assumptions made\ give rather di}erent suggestions as to potential patterns of change[ Inparticular predicted zonal mean temperature changes in the lower stratosphere are usually of opposite sign in these twotypes of experiment[ None of these GCM studies include interactive photochemistry and the third category of modellingwork is concerned with the photochemical response of the middle atmosphere to enhanced solar ultraviolet[ Thesegenerally employ 1D models to predict changes in ozone and other gaseous species[ Recently it has been realised thatthe responses "to a variety of external forcings# of the lower and middle atmospheres are linked through both radiativeand dynamical mechanisms and should not be viewed in isolation from each other[ Thus the fourth type of modellingstudy\ which is still in its infancy\ attempts to represent solar variability by realistic changes in both irradiance andozone concentrations[ In this paper these various modelling studies are reviewed and some new results presented whichcon_rm previous theoretical suggestions that\ in the northern hemisphere winter\ the atmosphere may respond to solarchanges in a similar way as to the injection of volcanic aerosol[ The implications of the results of the model studies forthe detection of solar!induced climate change are discussed[ Þ 0888 Elsevier Science Ltd[ All rights reserved[

0[ Introduction

For over two thousand years "see Hoyt and Schatten\0886\ for an excellent review# mankind has consideredthe possibility that changes in the Sun may a}ect theweather and climate on Earth[ Many studies have pur!ported to show correlations between solar activity andvarious meteorological parameters but historical obser!vations of solar activity were restricted to sunspot num!bers and it was not clear how these could be physicallyrelated to meteorological factors[ Furthermore\ measure!ments made from the Earth|s surface\ and thus con!taminated by atmospheric variability\ suggested thatsolar irradiance was invariant[ It was therefore con!sidered by many that any modulation of the Sun|s in~u!

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ence could only be on the very long timescales associatedwith changes in the Earth|s orbital parameters[ Thelaunch of the Nimbus!6 satellite in 0867\ however\initiated a series of measurements that have been able toreveal the variability of the total solar irradiance\ esti!mated to be currently of the order of 9[0) over the 00!year cycle[ Based on these measurements\ together withother proxy measurements of solar activity and theor!etical models of the Sun\ it has been possible "Lean et al[\0881# to reconstruct time series of total solar irradianceback to the seventeenth century[ Nevertheless\ using sim!ple concepts of radiation balance\ it is still not possible toexplain the apparent magnitude of the climate response[

One way in which we can try to understand or interpretthe observations is to use numerical models of the atmo!sphere to simulate the e}ects[ In Section 1 some of themodelling studies which have investigated the impact ofchanges in total solar irradiance are reviewed[

Other measurements have shown that the amplitude

J[D[ Hai`h:Journal of Atmospheric and Solar!Terrestrial Physics 50 "0888# 52Ð6153

of solar variability in the ultraviolet is much larger thanin the visible wavelength region which dominates the total"Lean\ 0878#[ Ultraviolet radiation is largely absorbed inthe middle atmosphere where it both heats the air andinitiates the photochemical production of ozone[ Sections2 and 3 discuss the results of modelling studies whichhave looked at the impact of increasing solar ultravioleton middles atmospheric dynamical and chemical struc!tures respectively[ Finally in Section 4 some results arediscussed from a new modelling study which incorporatesthe e}ects of both ultraviolet and ozone changes andinvestigates the e}ects of these on the climate of thetroposphere[

1[ Total solar irradiance

Much modelling work on the potential e}ect of solarvariability on climate has been concerned with the impactof changes in the solar {constant| "SC# i[e[ a spectrallyuniform fractional change in the Sun|s total radiativeoutput[ A pioneering study was that of Wetherald andManabe "0864# who used a simpli_ed GCM with a{swamp| ocean and ice!albedo feedback[ They concludedthat the response of the tropospheric zonal mean tem!perature to a 1) increase in SC was very similar to thatwhich they calculated for a doubling of the concentrationof CO1[ That the responses might be of the same ordercould be anticipated from consideration of the radiativeforcing associated with the two modi_cations] a doublingof CO1 causes a 3 Wm−1 forcing\ while a 1) increase inSC gives about 3[7 Wm−1[ The structural similarity ofthe results can be explained by the physical mechanismsinvolved*namely a warming of the surface and con!vective adjustment of the tropospheric temperature pro!_le towards the moist adiabat[ Because the moist adia!batic lapse rate decreases with increasing temperaturethis results in greatest warming in the tropical uppertroposphere where moist convection dominates[ Thewarming was also enhanced at low levels near the polesdue to a reduction in snow cover and thus albedo[Wetherald and Manabe noted that the main di}erencebetween the SC and CO1 experiments was in the responseof the stratosphere\ which cooled in the latter case butwarmed in the former[ Nevertheless\ their SC calculationsshow a cooling in the tropical lower stratosphere pre!sumably associated with an increase in tropopause heightdue to enhanced tropical convection[

Subsequent GCM studies of the impact of changes inSC have shown similar responses in zonal mean tem!perature although generally with a smaller high!latitudeice!albedo ampli_cation[ The models vary\ however\ intheir lower stratospheric response] Nesme!Ribes et al["0882# and Sadourney "0883#\ using the LMD AGCM\show a slight cooling at high latitudes although the modelhas very few levels in the stratosphere[ Royer et al[ "0883#\

with a version of the Arpege model having a detailedrepresentation of the stratosphere but with speci_ed seasurface temperature changes\ found signi_cant coolingabove the tropopause at all latitudes and warming athigher altitudes[ The results of the ECHAM model stud!ies of Cubasch et al[ "0886# suggest very slight coolingabove the tropopause at all latitudes[ In all cases thiscooling represents a raising of the tropopause as a resultof the warming of the troposphere\ in response to surfacetemperature increases[ At higher altitudes the predictedstratospheric warming is modest[ The lack of a properrepresentation of the changes in solar ultraviolet in theseexperiments means that stratospheric heating has beenunderestimated "Sections 2\ 3# so that whether lowerstratospheric cooling actually takes place remains uncer!tain[

All the models with interactive surface temperaturesshow an increase in global annual average surface airtemperature in response to enhanced solar input but noclear pattern emerges as to the horizontal distribution ofthis warming[ Rind and Overpeck "0882#\ using the GISSGCM\ show largest e}ects over South America and theArctic Ocean^ Nesme!Ribes et al[ "0882# present morewarming in mid! to high!latitudes while Cubasch et al["0886# show maximum e}ects over northern hemisphereland areas[ It is di.cult to compare the absolute results ofthese models\ because each of them uses di}erent surfacerepresentations and makes di}erent assumptions con!cerning the magnitude of the change in solar irradiance[However\ the fact that no clear pattern emerges in surfacetemperature change suggests either that a signal has notemerged from natural model variability or that the physi!cal representations\ in at least some of the models\ areinadequate to the task[

2[ Ultraviolet irradiance*impact on dynamics

The enhancement of shorter wavelength radiation dur!ing periods of greater solar activity means that the largestdirect radiative heating of the atmosphere is felt at higheraltitudes[ More speci_cally increased levels of ultravioletradiation in the wavelength range 064Ð219 nm heat thestratosphere and mesosphere due to absorption by ozoneand oxygen[ This has been recognised in several studieswhich have used models to assess the impact of this heat!ing on the dynamics of the middle atmosphere[ Koderaand co!workers "0889\ 0880# and Kodera "0880# used aclimate model and data analysis to study the response ofthe winter polar stratosphere to solar ultraviolet changes[They showed that anomalies in zonal mean zonal wind\introduced into the mid!latitude upper stratosphere inearly winter by changes in the latitudinal distribution ofsolar heating\ could modify the interaction with the mean~ow of vertically propagating planetary waves such thatthe zonal wind anomaly propagated downwards and

J[D[ Hai`h:Journal of Atmospheric and Solar!Terrestrial Physics 50 "0888# 52Ð61 54

polewards through the winter[ They also modelled theextra!tropical response to the quasi!biennial oscillation"QBO# of tropical lower stratospheric winds and pro!posed that the QBO could modulate the solar e}ect suchthat the downward and poleward propagation of a west!erly wind anomaly would be more likely to take placeduring the east phase of the QBO[ This is consistent withthe observations of Labitzke and van Loon "0886#[ InKodera|s studies\ the westerly wind anomaly penetratesdown to the troposphere so that his work may explainsome of the observations of apparent climatic responseto solar variability in the lower atmosphere in winter[ Hedid not discuss the e}ects in the summer hemispherewhere van Loon and Labitzke "0887\ vLL# show greatestcorrelations between solar and atmospheric parameters"although they do not derive the actual magnitudes ofthese signals#[

Balachandran and Rind "0884# also used a GCM tostudy the e}ects of solar ultraviolet variability and ofthe QBO on the middle atmosphere[ Their analysis alsoconcentrated on the winter hemisphere and similarlyfound that increased UV forcing resulted in strongerextra!tropical West winds which allowed enhanced ver!tical propagation of planetary waves and thus modi!_cations to energy convergences in the middle atmo!sphere[ However\ in their experiments they used valuesfor UV change much larger than observed[ Furthermore\the atmospheric response to UV increases of 09 and 49)were not everywhere qualitatively consistent so it is notclear to what extent these results can be extrapolated tolower "more realistic# UV changes[ In addition\ strato!spheric ozone concentrations were not allowed torespond to the enhanced UV and this may further impactthe direct radiative e}ect "see Section 3#[

Rind and Balachandran "0884# analysed these samemodel experiments concentrating on the e}ects in thetroposphere[ They found that in the winter extra!tropicswarming of the upper troposphere!lower stratosphere"UTLS# by enhanced UV caused a reduction in tro!pospheric eddy energy leading to cooling in winter mid!latitudes[ In low latitudes warming of the UTLS causedan increase in static stability and thus a weakening of thetropical Hadley circulation[ The winter hemisphere in theexperiment with 09) UV increase during the westerlyQBO phase showed an opposite response to the otherthree experiments^ this tends to cast doubts on the robust!ness of the simulations although not on the conclusionsconcerning potential mechanisms[

3[ Ultraviolet irradiance*impact on stratospheric

ozone

It was noted in the previous section that the spectralcomposition of solar irradiance variations determines thevertical distribution of their impact on atmospheric heat!

ing rates[ Thus changes in UV are felt in the middleatmosphere\ enhancements in visible radiation largelyreach the surface while any change in near!infrared radi!ation would have most e}ect in the troposphere[However\ variations in irradiance can also impact thechemical composition of the atmosphere through modi!_cation of photo!dissociation rates and it was _rst sug!gested by Frederick "0866# that variations in solar UVmight a}ect stratospheric ozone concentrations[ Fur!thermore\ the magnitude of solar heating in the middleatmosphere is determined by the ozone concentrations aswell as by the solar irradiance[ Thus if the e}ects ofchanges in solar UV on the thermal structure of themiddle atmosphere are to be calculated accurately it isalso necessary to know how ozone concentrations area}ected[

Analysis of ground!based "Zerefos et al[\ 0886# andsatellite "McCormack et al[\ 0886# ozone measurementssuggest a variation of about 1) in global total ozonebetween periods of minimum and maximum activity overthe Sun|s 00!year cycle[ Two!dimensional pho!tochemical!transport models "Brasseur\ 0882^ Haigh\0883^ Fleming et al[\ 0884# also predict a variation ofaround 1) change in global total ozone in response tochanges in ultraviolet spectral irradiance measured bythe SBUV and SME satellite instruments[ However\ themodels do not reproduce the latitudinal structure\ in par!ticular the peaks in response seen around 29> in bothhemispheres\ found by McCormack et al[ "0886# in ananalysis of TOMS and SBUV:1 ozone column data[ Fur!thermore\ analysis of the vertical structure of SBUV1ozone data "McCormack and Hood\ 0885# suggests thatthe models tend to overestimate the response in the mid!dle stratosphere and underestimate it in the lower strato!sphere[ The 1D models cannot satisfactorily representchanges in three!dimensional planetary!scale wave trans!port of ozone and this may be the cause of the discrep!ancies[ However\ it is di.cult to explain how ozone pro!duced in the upper stratosphere can be transported tothe lower stratosphere through a region of minimumconcentration[ The satellite data cover less than two solarcycles and it is possible that other e}ects "perhaps het!erogeneous ozone chemistry in the lower stratosphere#have been aliased onto the solar signal[ Furthermorethe analysis may have been a}ected by the eruptionsof volcanoes El Chichon in 0871 and Pinatubo in 0880"Solomon et al[\ 0885#[

Thus\ although it seems to be important in determiningthe climatic impact of solar variability "see Section 4#\ thelatitude!height response of ozone to solar UV changes isnot well established[

4[ Combined irradiance:ozone experiments

From the discussion above it seems clear that to obtaina realistic simulation of the impact of solar variability on


climate it is necessary to include the e}ects both of thespectrum of the change in irradiance and of the changein ozone concentrations[ A _rst attempt to do this wasdescribed by Haigh "0885\ H85# in which GCM experi!ments were carried out with speci_cations of solarirradiance and ozone representing periods of maximumand minimum activity during the Sun|s 00!year activitycycle[ It was suggested that heating of the lower strato!sphere could result in signi_cant changes in troposphericclimate\ particularly in the summer hemisphere\ due tobroadening of the tropical Hadley cells and polewardmovement of the sub!tropical jets and mid!latitude stormtracks[ Haigh "0886\ H86# showed further that the tro!pospheric response depended on the structure of theassumed changes in ozone[

It is interesting to compare the response of the Hadleycells in the di}erent types of model experiments describedabove[ The results of Rind and Overpeck "0882# suggestthat increasing the SC will strengthen the Hadley cir!culation because of enhanced tropical sea surface tem!peratures[ Sadourney "0883#\ on the other hand\ shows aweakened Hadley circulation which can be explained bythe greater warming of surface temperatures at mid!lati!tudes than in the tropics[ In their increased UV experi!ment Rind and Balachandran "0884# _nd a weaker Had!ley circulation because the warming of the lowerstratosphere results in increased tropospheric static stab!ility[ H85 and H86 suggest a weakening of the Hadleycirculation because of a latitudinal redistribution of radi!ative forcing at the tropopause[ These di}erences do notrepresent fundamental contradictions but merely re~ectthe varying speci_cations of the model experiments[Nevertheless\ they do suggest that\ while potential mech!anisms for solar forcing of climate are being investigated\a full understanding is some way from being achieved[

H85 and H86 described experiments carried out withthe UGAMP GCM "Slingo et al[\ 0883# run in perpetualJanuary mode at T31 horizontal resolution "equivalentto approximately 1[7> longitude by 1[7> latitude# and 08levels with the top level at 09 hPa[ This formulation waschosen so that the e}ects of changing lower stratospherictemperatures on tropospheric climate could be studiedwith su.cient horizontal resolution to satisfactorilyresolve eddy features such as the mid!latitude stormtracks whilst not demanding excessive computationalrequirements[ The cut!o} in the middle stratosphere pre!vents simulation of the dynamical responses of the middleatmosphere\ and the tropospheric responses to these\such as discussed by Kodera et al[ "0880# and Bal!achandran and Rind "0884#[ However the combinedapplication of realistic changes in both ultravioletirradiance and ozone concentrations does produce areasonable simulation of lower stratospheric heating[

Two di}erent _elds for ozone fractional change wereused in H86\ the _rst based on the results of a two!dimensional interactive transport!radiation!chemistry

model "Haigh\ 0883# and the second based on an analysisof TOMS:SBUV ozone column data by Hood "personalcommunication#[ For the latter a constant value withheight was assumed in the stratosphere^ this was notsuggested by Hood|s analysis but used _rstly because ofa lack of information on the true pro_le "see Section 3#and secondly as a means of testing model sensitivity totwo very di}erent structures[ The results of the H86 studyare now discussed further[

Figure 0 shows the di}erence in zonal mean tem!perature between solar minimum and solar maximum forexperiments with a change in irradiance and "a# no changein ozone "subsequently referred to as run NO2#\ "b# ozonechanges from the 1!D model "MO2# and "c# ozone chan!ges based on satellite observations of O2 column "SO2#[Note that in the lower stratosphere of the tropics and ofthe summer hemisphere the warming is strongly depen!dent on the ozone speci_cation[ Run NO2\ with no ozonechange\ shows only very slight warming while the warm!ing in run MO2 is much larger than that of run SO2except in summer high latitudes and in a small regionbetween 29Ð49 hPa and 19Ð14>N[ Note that the coolingin the low stratosphere seen in GCM experiments withincreased SC\ as described in Section 1\ is not seen exceptperhaps at summer high latitudes in run NO2[ However\as _xed sea surface temperatures are used in these experi!ments\ there is little scope for warming the entire tropo!sphere through surface heating and convective adjust!ment[ The banding structure in the tropospherictemperature changes\ as has been mentioned above\ isdue to horizontal expansion of the Hadley cells and pole!wards shifts of the sub!tropical jets and mid!latitude wavepatterns[

Figure 1 shows the di}erence in zonal mean geo!potential height of the 29 hPa surface for the three experi!ments[ Very little e}ect is seen in run NO2 "dotted line#but runs MO2 "dashed line# and SO2 "dash!dotted line#show positive values at low latitudes and across the sum!mer hemisphere[ In run MO2 the shape of the curve\ withlargest values in summer mid!latitudes and a smaller peakin the winter sub!tropics\ corresponds well with the cor!relation pattern found by vLL in studies usingNCEP:NCAR Reanalysis data[ Because vLL do notderive seasonal amplitudes the model and observationalresults cannot be directly compared[ However\ vLL doshow a solar cycle variation of 29Ð59 m in annual meanzonal mean 29 hPa geopotential height in the 19Ð39>Nlatitude band which is considerably larger than the ½09m seen in the model results for this region for January[Nevertheless\ the model results are very sensitive to theozone change speci_cation "see Haigh\ 0887 for furtherexperiments# and vary with season so that further analysisis required to fully test the model simulations[

In Figure 2 are shown the changes in the geopotentialheight of the 499 hPa surface for runs MO2 and SO2[In the summer hemisphere the two runs have di}erent


Fig[ 0[ Di}erence between solar maximum and solar minimum in zonal mean temperature "K# for runs] "a# NO2^ "b# MO2^ "c# SO2[


Fig[ 1[ Di}erence between solar maximum and solar minimum in zonal mean geopotential height of the 29 hPa surface "m# for runs]NO2 "dotted line#^ MO2 "dashed line#^ SO2 "dash!dotted line#[ The solid line shows the value of one standard error of the model|svariability taking 29!day sections of output as independent samples[

impacts but in the winter hemisphere they show a verysimilar pattern except in the far North Paci_c[ This pat!tern is much like that found\ in satellite data studies\ tobe the result of the in~uence of volcanic eruptions on thenorthern hemisphere winter by Graf et al[ "0882# andKodera "0883#[ Similarly\ the impacts on lower tro!pospheric temperature\ shown in Fig[ 3\ are similar bothto each other and to the volcanic in~uence found byRobock and Mao "0881# and Graf et al[ "0882#[ Mod!elling studies of the e}ect of heating by volcanic aerosol"Graf et al[\ 0882\ Robock and Liu\ 0883# show a similarresponse[ These patterns were found by Perlwitz and Graf"0884# to be part of a dominant mode of response of thenorthern hemisphere winter and they concluded that thismode was enhanced by heating of the low latitude lowerstratosphere due to absorption of solar radiation by vol!canic aerosol[

Kodera "0884# suggested that\ as well as volcanic aero!sol\ other sources of anomalous heating in the strato!sphere\ such as due to solar in~uences or the QBO\ couldtrigger a particular mode of response in the northernhemisphere winter[ He showed that such heating couldcause anomalies in meridional temperature gradients\and thus zonal wind structure\ which could propagatedownwards to the troposphere[ Although the model used

here cannot simulate the e}ects in the upper stratospherethe results\ showing a response in the troposphere andlower stratosphere to solar in~uences very similar tothose found in other models due to volcanic in~uences\provide further evidence to support Kodera|s analysis[

5[ Conclusions

Modelling studies of the climatic impact of solar varia!bility have historically been restricted to consideration ofchanges in the amplitude of the solar constant[ Such workhas been able to indicate the magnitude of the potentialresponse in surface temperature\ and how the tropo!sphere might adjust to this[ The geographical distributionof the response in surface temperature\ however\ remainsvery uncertain[ Furthermore\ because the spectrum of thechange in solar irradiance is not taken into account inthese studies\ the pro_le of heating rate anomalies is notproperly represented[ In particular temperature changespredicted for the lower stratosphere are likely to beunderestimated\ or even of the wrong sign[ This wouldbe problematical if model patterns of the vertical struc!ture of temperature change were to be used in climatesignal detection:attribution studies "Tett et al[\ 0885#[


Fig[ 2[ Di}erence between solar maximum and solar minimum in geopotential height of 499 hPa surface "m# for runs] "a# MO2^ "b#SO2[

Simulations in which solar ultraviolet radiation isenhanced are better able to represent the potentialresponse of the middle atmosphere[ Such experimentshave con_rmed theoretical suggestions that changes inthe temperature and zonal wind structure can a}ect theupward propagation of planetary Rossby waves in thenorthern hemisphere winter and\ through wave!mean~ow interaction\ cause zonal wind anomalies to propa!

gate downwards from the stratosphere to the tropo!sphere[ However\ in order to produce unambiguousresults most of these simulations have utilised enhance!ments in ultraviolet much larger than are feasible^ theyalso do not consider summer hemisphere e}ects[

As solar ultraviolet increases so does ozone productionin the upper stratosphere[ The ozone thus produced istransported downwards and polewards and thus


Fig[ 3[ Di}erence between solar maximum and solar minimum in temperature at 749 hPa "K# for runs] "a# MO2^ "b# SO2[

increases the concentrations in the lower stratosphere[The extra ozone absorbs longer wavelength UV and thusstratospheric heating is more than might be anticipatedfrom the initial solar UV changes alone[ A warmer strato!sphere increases its emission of longwave radiation intothe troposphere and thus enhances the radiative forcingof climate[ First attempts at model simulations of this

e}ect suggest that small\ but potentially detectable\ chan!ges in tropospheric temperatures can result[ The stat!istical signi_cance of the tropospheric temperature chan!ges is greatest in the summer hemisphere[ However\ newresults presented in this paper suggest that solar forcingcan excite the same mode of variability in the northernhemisphere winter troposphere:stratosphere as is excited


by the injection of volcanic aerosol into the lower strato!sphere[

To what extent\ then\ might it be possible to separatethe solar and volcanic climate signals< In both cases thelongwave and shortwave transmittances of the strato!sphere are decreased\ and in both cases the lower strato!sphere warms[ However\ in the solar case the amount ofsolar radiation reaching the troposphere increases "exceptpossibly in winter mid! to high latitudes^ see Haigh\ 0883#while in the volcanic case it decreases[ In the winter hemi!sphere the absolute amount of solar radiation is smalland\ from the discussion above\ it seems likely that thedominant response is an enhancement of the samedynamical mode due to heating of the lower stratosphere[Thus in the winter it may not be easy to separate the twocases\ except possibly by magnitude or timing[ In thesummer hemisphere\ however\ it seems probable that thetroposphere cools in the volcanic case and warms in thesolar case\ although the exact response will depend onthe latitudinal distributions of the volcanic aerosol andsolar!induced ozone changes[ Further modelling of theresponse of the atmosphere to these two types of forcingmay reveal particular patterns of change that could byapplied to observational data in detection:attributionstudies[

Experiments with simpli_ed models are valuable interms of identifying and testing speci_c mechanismsinvolved in the climatic impact of solar variability[ How!ever\ it seems clear that\ for realistic simulations\ futuremodelling studies should include not only interactive sur!face temperatures but also a more detailed representationof the stratosphere[ In particular the e}ects of changesin the spectrum of solar irradiance\ both in calculatingradiative heating and in stratospheric ozone photo!chemistry\ must be included as well as transport of theozone and its subsequent impact on the radiation balance[

Acknowledgements

This work and the UGAMP programme are fundedby the U[K[ Natural Environment Research Council[SCOSTEP provided travel funds to enable this paper tobe presented at the IAGA meeting in Uppsala\ 0886[

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modelling the impact of solar variability on climate

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