Download Evaluating sun–climate relationships since the Little Ice Age

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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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J[ Lean\ D[ Rind:Journal of Atmospheric and Solar!Terrestrial Physics 50 "0888# 14Ð25
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
J[ Lean\ D[ Rind:Journal of Atmospheric and Solar!Terrestrial Physics 50 "0888# 14Ð25
16
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[
17
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
J[ Lean\ D[ Rind:Journal of Atmospheric and Solar!Terrestrial Physics 50 "0888# 14Ð25
18
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
29
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
J[ Lean\ D[ Rind:Journal of Atmospheric and Solar!Terrestrial Physics 50 "0888# 14Ð25
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
21
J[ Lean\ D[ Rind:Journal of Atmospheric and Solar!Terrestrial Physics 50 "0888# 14Ð25
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
J[ Lean\ D[ Rind:Journal of Atmospheric and Solar!Terrestrial Physics 50 "0888# 14Ð25
22
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
23
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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