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Comment on "Atlantic and Pacific multidecadal oscillations and Northern Hemisphere temperatures"

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Abstract

Steinman et al. (Reports, 27 February 2015, p. 988) argue that appropriately rescaled multimodel ensemble-mean time series provide an unbiased estimate of the forced climate response in individual model simulations. However, their procedure for demonstrating the validity of this assertion is flawed, and the residual intrinsic variability so defined is in fact dominated by the actual forced response of individual models.
TECHNICAL COMMENT
CLIMATE CHANGE
Comment on Atlantic and Pacific
multidecadal oscillations and
Northern Hemisphere temperatures
S. Kravtsov,
1
*M. G. Wyatt,
2
J. A. Curry,
3
A. A. Tsonis
1
Steinman et al. (Reports, 27 February 2015, p. 988) argue that appropriately rescaled
multimodel ensemble-mean time series provide an unbiased estimate of the forced climate
response in individual model simulations. However, their procedure for demonstrating the
validity of this assertion is flawed, and the residual intrinsic variability so defined is in fact
dominated by the actual forced response of individual models.
The central result of Steinman et al.sanal-
ysis (1) is the demonstration of an apparent
consistency among the responses of differ-
ent models to variable forcing in the 20th-
century climate simulations. In particular,
they claim that regional multimodel ensemble-
mean time series defines the universal forced
signal, which can be linearly rescaled to provide
unbiased estimates of the regional forced re-
sponses for individual models. Such a consisten-
cy is surprising because the models have different
physical parameterizations and the simulations
may use different forcing subsets. If their claim
were true, it would add much confidence to the
authorssemi-empirical attribution of the ob-
served multidecadal climate variability to the
forced and intrinsic sources. However, the implied
uniqueness of the forced signal defined by their
regional regression method is an artifact of their
analysis procedure, and the actual uncertainty of
the semi-empirical estimates of the observed multi-
decadal intrinsic variability is much larger than
these authors have inferred.
Consider Mtime series of length T,corre-
sponding to Mdifferent climate simulations:
xðtÞ
m;m¼1;;M;t¼1;;T. Let the bar denote
averaging across the time dimension (t)and
square brackets denote averaging across the
ensemble member dimension (m). For example,
thetimemeanofeachensemblememberxmand
theensembleaveragetimeseries½xðtÞare de-
fined as follows
xm¼1
TX
T
t¼1
xðtÞ
mð1Þ
xðtÞ¼1
MX
M
m¼1
xðtÞ
mð2Þ
Consider a decomposition of xðtÞ
minto the forced
signal fðtÞ
mand residual intrinsic variability DðtÞ
m
xðtÞ
m¼fðtÞ
mþDðtÞ
mð3Þ
Without loss of generality, we can assume
xm¼fm¼0, hence Dm¼0. If the estimated
forced signal fðtÞ
mis unbiased, then the time series
DðtÞ
m1and DðtÞ
m2of residual intrinsic variability in any
pair of simulations m
1
and m
2
must be uncorre-
lated (independent). Furthermore, if the distri-
bution of DðtÞ
mhas mean 0 and variance s2,the
ensemble mean residual time series ½DðtÞwill have
thedistribution withmean 0 and variance s2=M.
Hence, one can quantitatively assess the statis-
tical independence of different realizations of sim-
ulated intrinsic variability by comparing the actual
dispersion ½D2of the ensemble mean time series
½DðtÞwith its theoretical prediction D2=M,where
we estimated s2D2. Large values of ½D2would
indicate that assumption of statistical indepen-
dence between different realizations of intrinsic
variability DðtÞ
mis violated due to biases in the esti-
mated forced signal fðtÞ
m,sothatatleastaportion
of the common true forced signal manifests in
the estimated intrinsicresiduals DðtÞ
m.
Steinman et al. considered, among others, the
following two methods for estimating the forced
signal, both based on the multimodel ensemble
mean time series
fðtÞ
m¼xðtÞð4AÞ
fðtÞ
m¼amxðtÞð4BÞ
The differencing method (Eqs. 3 and 4A) simply
identifies the forced signal with the multimodel
ensemble mean ½xðtÞ.Theregressionmethod(Eqs.
3 and 4B) rescales the first-guess forced signal
½xðtÞfor a given simulation by finding amvia
least squares to minimize D2
min Eq. 3.
Steinman et al. further claimed that both of
these methods provided independent realizations
of residual intrinsic variability in climate-model
simulations, based on the fact that the resulting
variance ½D2of the ensemble mean residual time
series was much smaller than the theoretical value
of D2=M. However, it is easy to show that, due
to the choice of forcing derived using either Eq.
4A or Eq. 4B, this ensemble mean residual time
series is identically zero
½DðtÞ¼0; t¼1;;Tð5Þ
and so is its variance ½D2¼0. Hence, the ex-
treme smallness of the dispersion of ensemble
average intrinsic variability attributed in (1)to
the statistical independence of its different real-
izations is actually an artifact of the algebraic
constraint (Eq. 5) [see (25)]. This flaw does
not mean that the residuals are necessarily cor-
related (not independent), but a different test is
required to determine that.
We now show directly that the regional regres-
sion approach (1) of defining the forced signal
leads to the correlated samples of residual in-
trinsic variability in the individual-model ensem-
bles (subensembles of simulations using a single
model with fixed physics package and an iden-
tical forcing history). For these subensembles, it
is the expression (Eq. 4A) that naturally gives an
unbiased estimate of the forced variability. We
considered 18 such subensembles from the Cou-
pled Model Intercomparison Project Phase 5
(CMIP5)modelswithfourormore20th-century
simulations (6), totaling 116 individual simula-
tions out of the 170 available simulations. The
multimodel ensemble mean based on these 116
simulationsisnearlyidenticaltotheonecom-
puted using all of the available 170 simulations.
We defined two alternative sets of the model-
simulated intrinsic variability. In method A, we
formed realizations of intrinsic variability by
subtracting the 5-year low-pass-filtered ensem-
ble mean of each model from this modelsindi-
vidual simulations (i.e., Eq. 4A applied separately
to individual model ensembles). The second set
(method B) defined the residual intrinsic varia-
bility using the forced signal estimated from re-
gional multimodel regression (1) (i.e., Eq. 4B
appliedtothewholeensembleof116simulations).
To quantify independence of different realiza-
tions of intrinsic variability in the individual-
model ensembles, we introduced an ensemble
correlation measure Cby summing positive cor-
relations among all possible pairs of an individ-
ual modelsMensemble members
C¼2
MðM1ÞX
m>l
Cml HðCml Þð6Þ
where HðxÞis the Heaviside step function (7);
the quantity Cranges from 0 (no positive cor-
relations between individual ensemble mem-
bers) to 1 (all ensemble members are perfectly
correlated). The correlation measure (Eq. 6) was
computed for raw and low-pass-filtered intrin-
sic variability defined using methods A and B
[Fig. 1, A to C shows results for the Geophysical
RESEARCH
SCIENCE sciencemag.org 11 DECEMBER 2015 VOL 350 ISSUE 6266 1326-b
1
Department of Mathematical Sciences, Atmospheric Science
group, University of Wisconsin-Milwaukee, Post Office Box
413, Milwaukee, WI 53201, USA.
2
Department of Geological
Sciences, University of Colorado, Boulder, CO, USA.
3
School
of Earth and Atmospheric Sciences, Georgia Institute of
Technology, Atlanta, GA, USA.
*Corresponding author: kravtsov@uwm.edu
Fluid Dynamics Laboratory (GFDL) CM3 model ;
see (8)]. Method A produces intrinsic variability
with Cvalues well within the range expected from
random uncorrelated red-noise samples generated
using an autoregressive model of order 3 (AR-3) (9).
In contrast, Steinman et al.smethodBresultsin
samples that are significantly correlated due to their
systematic difference from the true forced signal.
We then used 18 versions of the forced signal,
estimated by the unbiased method A, to isolate
intrinsic variability in observed surface temper-
atures via Eq. 3 and Eq. 4B (Fig. 1, D to F). The
spread among the 18 estimates of intrinsic var-
iability in observations is much larger than the
tight bootstrap-based error bounds on the semi-
empirical estimates of the observed intrinsic
variability in figure 3 in (1). Hence, the actual
uncertainty of the semi-empirical attribution
by SMM is also much larger (10), thereby pre-
venting any clear inferences about the cause of
the false pausein the global warming (11,12).
REFERENCES AND NOTES
1. B. A. Steinman, M. E. Mann, S. K. Miller, Science 347,988991
(2015).
2. The standard deviation of intrinsic variability computed in
Steinman et al. (1) is small, but not exactly zero because of
their using a data adaptive low-pass filter before averaging
intrinsic variability among different simulations.
3. Steinman et al. also used weighted ensemble means to define a
version of their model-based forced signal. In this case, the con-
straint (Eq. 5) would not be exact but would still be approximately
valid, because the weighted and nonweighted estimates of the
forced signal are in fact very close (not shown here).
4. Comment (3) above also applies to a possible variation of the
regression method (Eq. 4B) in which, instead of scaling each
individual simulation by its own factor am, one would estimate
and use the single scaling factor for all simulations of each
model; this scaling factor can be defined, for example, as the
ensemble mean of amestimates computed for individual
simulations of a given model.
5. One way to try to alleviate constraint (Eq. 5) would be to
estimate the forced signal for a given subset of models using
the ensemble mean time series of the complement subset of
models. However, this would only be effective if the sizes of
these two subsets are comparable. Otherwise, the multimodel
averaging over the much larger complement subset of models
would also be very close to the all-model ensemble mean, and
the algebraic constraint (Eq. 5) would still approximately hold.
6. There are 13 models with four or more 20th-century
simulations in the CMIP5 data set, but considering separately
the ensembles of the Goddard Institute for Space Studies
(GISS) models with different physics packages makes up 18
independent ensembles.
7. The Heaviside step function is used here merely to streamline
the mathematical notations in the multiple correlation measure
(Eq. 6) by zeroing out negative terms in the sum of
correlations, leaving positive terms unchanged.
8. Other models exhibit a similar behavior; see the corresponding
images at https://pantherfile.uwm.edu/kravtsov/www/downloads/
KWCT2015/TIFF_FILES.
9. If one does not divide the large ensembles of the GISS models
into the subensembles with different physics (6), the
correlation-measure diagnosis does identify the dependency
between the model realizations, because the true forced
responses in these versions of the model are different from the
grand ensemble mean response, and similar long-term biases
across the same-physics model simulations ensue.
10. The bootstrap resampling used in (1) is equival ent to co nsider ing
subensembles of about two-thirds of independent models (or
simulations), thus effectively averaging out the intramodel un-
certainty of the forced response emphasized in our Fig. 1, D to F.
11. This is exacerbated further by the unfortunate linear extrap-
olation of the CMIP5 runs from 2005 to 2012 used in (1)to
estimate recent intrinsic trends.
12. B. Rajaratnam, J. Romano, M. Tsiang, N. S. Diffenbaugh,
Clim. Change 133, 129140 (2015).
ACKNO WLE DGME NTS
We thank Steinman et al. for making their data and analysis code
publicly available. This research was supported by NSF grants
OCE-1243158 (S.K.) and AGS-1408897 (S.K. and A.A.T.). All data
and MATLAB (MathWorks, Natick, MA) scripts for this paper are
available for downloading from http://pantherfile.uwm.edu/
kravtsov/www/downloads/KWCT2015.
15 April 2015; accepted 6 November 2015
10.1126/science.aab3570
1326-b 11 DECEMBER 2015 VOL 350 ISSUE 6266 sciencemag.org SCIENCE
Fig. 1. Intrinsic variability in the 20th-century model simulations with four or more ensemble
members identified using two different methods for estimating the forced signal: the classical
subtraction of the individual-model ensemble mean (method A) and the multimodel regional
regression method (1)(methodB).(Ato C) The correlation measure (Eq. 6) of statistical indepen-
dence between multiple realizations of the GFDL CM3 model (five realizations) for (A) Atlantic Multi-
decadal Oscillation (AMO), (B) Pacific Multidecadal Oscillation (PMO), and (C) Northern Hemisphere
Multidecadal Oscillation (HMO) indices; these correlations were computed for running-mean low-pass-
filtered residual time series (which characterize intrinsic variability) and are plotted here against the
averaging window size. Low correlation measure indicates statistical independence of intrinsic residuals.
Dashed lines show the 99th percentile of the correlation measure based on the 1000 simulations of the
corresponding AR-3 red-noise model. (Dto F) Estimates of the observed multidecadal intrinsic
variability for (D) AMO, (E) PMO, and (F) HMO. The semi-empirical estimates (thin black lines) were
computed as in (1) based on the forced signals obtained using method A for each of the 18 model
ensembles considered, with the heavy red line indicating the average over these individual estimates.
Additional heavy lines (see legend) are for results based on linear detrending.The distance between the
black dashed lines in each plot shows the 95th percentile of the standard deviations for multidecadal
intrinsic variability estimated using method A for each of 116 simulations considered.
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