reflect a sensitivity to anthropogenic forcing
( 19 , 20 , 33 ), a finding supported by select cli-
mate models simulations of future climate
change ( 34 , 35 ). Both modeling and data-
based assessments of climate change im-
pacts on ENSO must overcome the high degree
of intrinsic variability, a detection threshold
that is not met in the present study with re-
spect to volcanic forcing. Whereas models can
rely on ensembles to elevate signal-to-noise
ratios, in the paleoclimate record, as in re-
ality, we are limited to one realization. This
ultimately limits our ability to detect changes
in ENSO derived from model-based targets.
However, the sign, structure, and magnitude
differ between volcanic versus greenhouse
forcing, ultimately limiting the relevance of
the volcanic-forcing response of ENSO to po-
tential responses of ENSO to greenhouse gas
forcing.
Finally, this work highlights a role for high-
resolution paleoclimate reconstructions and
model simulations of LM volcanism in the as-
sessment of potential geoengineering schemes
designed to offset greenhouse warming in
coming decades. There are few results from
natural experiments available that facilitate
investigations of the climate system response
to sulfate aerosol loading. The volcanic re-
sponse of major climate modes such as ENSO
is key to assessing regional climate impacts
under solar radiation management scenar-
ios. As such, our work suggests that contin-
ued efforts toward data-model comparisons
of the effects of volcanic eruptions on re-
gional climate over the LM are critical to a
robust assessment of the climatic effects
of sulfate aerosol–induced geoengineering
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ACKNOWLEDGMENTS
We thank M. Toohey for assistance with the evolv2k dataset,
A. LeGrande and J. Russell for valuable insights on this
work, and P. Grothe for assistance with coral chronological
assignments. Funding: This research was supported by the
Peter Voss Postdoctoral Fellowship in the Institute at Brown
for Environment and Society, Brown University, as well
as the University of Texas at Austin, Institute for Geophysics
Postdoctoral Fellowship, both awarded to S.D.; by NSF Marine
Geology and Geophysics awards 0752091, 1502832, and
1836645 to K.C.; NSF OCE award 0752585 to C.D.C. and K.C.;
and NOAA grant NA18OAR4310426 to J.E.G. Author
contributions: S.D., K.C., J.E.G., and T.A. formulated research
questions. K.C. and C.D.C. collected, sampled, and analyzed
oxygen isotopes in the coral data with assistance from L.E. and
H.C. S.D. performed analysis on coral data, generated figures,
and extracted model output with guidance from K.C., J.E.G., and
T.A. All authors contributed to the writing of the manuscript.
Competing interests: The authors declare no competing
interests. Data and materials availability: All data an d code
are available in the supplementary materials and are publicly
available on the National Climatic Data Centers website at
https://www.ncdc.noaa.gov/paleo/study/27490.SUPPLEMENTARY MATERIALS
science. /content/367/6485/1477/suppl/DC1 Section S1:
Materials and Methods
Section S2: Data and Matlab Code
Tables S1 to S5
Figs. S1 to S9
References ( 38 – 48 )1 March 2019; accepted 2 March 2020
10.1126/science.aax2000Pacific (Fig. 4). The Palmyra data, which show
no significant change in the frequency, occur-
rence, or magnitude of El Niño events nor a
significant change in central tropical Pacific
d^18 O, thus highlight an important data-model
discrepancy surrounding the sensitivity of the
tropical Pacific to external forcing. The broad
range of model responses shown in Fig. 4 un-
derscores structural uncertainties in model
forcing and/or physics. Directly addressing
uncertainties surrounding the size and par-
titioning between tropospheric and strato-
spheric aerosols during volcanic events, new
high-resolution sulfur isotope measurements
that differentiate between sulfate aerosols
derived from stratospheric intrusions are
poised to refine forcing estimates from ice
cores spanning the LM ( 24 ). Additionally,
mounting evidence suggests that volcanic
forcing used to drive current-generation cli-
mate models is up to 67% too large because
of structural uncertainties in stratospheric
aerosol physics ( 30 ). This results in overes-
timated responses to volcanism in terms of
intensity and longevity ( 31 ). Forthcoming
improvements to the volcanic forcing applied
in models may result in a weaker climatic
response to a given aerosol loading ( 30 ).
Furthermore, recent work has shown that
eruption season may prove as important as
eruption magnitude in shaping the response
of tropical Pacific climate to volcanic forc-
ing ( 10 ), complicating data-model compar-
ison. Many model simulations assume that
all eruptions occur in a single month [e.g.,
January ( 21 , 22 ) or April ( 25 )], which in-
troduces uncertainties in the response with
respect to proxy signatures of real-world
eruptions. As highlighted in ( 10 ), multiproxy
tropical temperature reconstructions lend
some support for a warming response in the
year after a tropical eruption, but data from
moisture-sensitive trees in ENSO-teleconnected
regions yield evidence of tropical Pacific cool-
ing. In assessing these lines of evidence, we
note that the volcanic response of extra-
tropical tree ring width reflects two or more
confounding sources: (i) local changes in pre-
cipitation caused by the direct radiative cool-
ing from volcanic events ( 32 ) and (ii) changes
in ENSO state, whether they be endogenous to
the tropical Pacific or induced by volcanism.
Moreover, a modeling study suggests that
these two responses bear a close resemblance
to each other ( 2 ). Such studies highlight the
importance of using proximal records of trop-
ical Pacific state, such as that presented in this
work, to assess the relationship between vol-
canism and ENSO.
Climate model predictions of the climate sys-
tem’s response to continued greenhouse emis-
sions depend crucially on accurate simulation
of ENSO’s response to external forcing. Recent
work suggests that recent ENSO properties
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