REPORT
◥
PALEOCLIMATE
Regional and global sea-surface
temperatures during the
last interglaciation
Jeremy S. Hoffman,
1
*† Peter U. Clark,
1
Andrew C. Parnell,
2
Feng He
1,3
The last interglaciation (LIG, 129 to 116 thousand years ago) was the most recent time in
Earth’s history when global mean sea level was substantially higher than it is at present.
However, reconstructions of LIG global temperature remain uncertain, with estimates
ranging from no significant difference to nearly 2°C warmer than present-day
temperatures. Here we use a network of sea-surface temperature (SST) records to
reconstruct spatiotemporal variability in regional and global SSTs during the LIG. Our
results indicate that peak LIG global mean annual SSTs were 0.5 ± 0.3°C warmer than the
climatological mean from 1870 to 1889 and indistinguishable from the 1995 to 2014 mean.
LIG warming in the extratropical latitudes occurred in response to boreal insolation and the
bipolar seesaw, whereas tropical SSTs wer e slightly cooler than the 187 0 to 1889 mean in
response to reduced mean annual insolation.
T
he last interglaciation [LIG, 129 to 116 thou-
sand years ago (ka)] was one of the warmest
periods during the last 800,000 years (1),
with an associated sea-level rise of 6 to
9 m above present levels (2). As such, the
LIG provides an important target for validating
global climate models used for climate-change
projections (3, 4), as well as for understand ing
the sea-level response to a warm climate. How-
ever, the existing reconstructions of LIG global
temperature needed to assess these issues sug-
gest the possibility of no significant temperature
change relative to the late Holocene to an increase
of as much as 2°C (5–8). Moreover, these esti-
mates either average temperature over part of
or all of the LIG interval or assume that the
warmest phases were globally synchronous,
and these estimates do not account for system-
atic uncertainties in proxy calibrations and age
estimates. As a result, these reconstructions are
unable to constrain spatiotemporal variability
in the response to the LIG climate forcing sug-
gested by high-latitude compilations (9, 10).
Here we reconstruct regional and global sea-
surface temperatures (SSTs) during the LIG that
include a robust asses sment of age model and
proxy uncertainties. We compiled a near-global
database of 104 published LIG SST records from
83 marine sediment core sites (Fig. 1A and tables
S1 and S2). Of these records, 19 reflect summer
SSTs and 85 reflect mean annual SSTs (from 72
sites). The sample resolution ranges from cen-
tennial to <4000 years on their published age
models, with a median resolution of 1100 years
(fig. S1 and table S1).
We developed stratigraphically consistent chro-
nologies for records from the South Atlantic,
Indian, and Pacific basins by aligning represent-
ative high–southern latitude SST records from
each basin (here referred to as reference cores)
with the European Project for Ice Coring in Ant-
arctica (EPICA) Dome C deuterium (dD) record
on the Asian speleothem-based chronology
(Speleo-Age model) (fig. S4) (11). We similarly
aligned one North Atlantic SST record to the
Greenland synthetic ice-core record that is also
on the Speleo-Age model (fig. S4) (11). The
highest correlations for lagged cross-correlations
between model-based SSTs at the locations of
our Southern Ocean reference cores and 2-m air
temperatures over EPICA Dome C (EDC) are cen-
tered at zero lag (fig. S5), supporting this align-
ment strategy. We assigned the corresponding
age model from each reference core to its ben-
thic foraminiferal oxygen isotope (d
18
O) record
(fig. S6), which was then used for alignment with
benthic d
18
O records from other cores in each
basin (12) to account for potential interbasin
variability in benthic d
18
O(13–15). A Bayesian
age-depth modeling routine (Bchron) (16)prop-
agates the sources of age-model uncertainty for
each record (12).
To compare proxy-based LIG SSTs with mod-
ern climatology, we referenced each core site in
the database to the SST value in the nearest 1° ×
1° grid cell in the Version 1.1 of the Hadley Center
Sea Ice and Sea Surface Temperature (HadISST1.1)
1870–1889 and 1995–2014 data sets (17)withthe
1870–1889 period closely corresponding to pre-
industrial temperatures (18). We took the 5° × 5°
gridded, area-weighted mean of the SST records to
develop regional and global LIG stacks of mean
annual SSTs with 2s uncertainties (Fig. 1). As-
sessment of the sensitivity of the global stack to
the resolution, spatial averaging scheme, and
number of records used in the stack suggests
little to no influence (figs. S9 to S11). We also as-
sessed proxy-dependent differences in the stack.
On average, global stacks of geochemical proxies
[ratio of magnesium to calcium (Mg/Ca) of plank-
tonic foraminifera and the alkenone unsaturation
index (U
K′
37
)] yield warmer SSTs than those of
microfossil proxies (figs. S12 and S13). How-
ever, within uncertainties, there is good agree-
ment between proxies within the same core for
12 existing comparisons (fig. S14). Similarly,
proxy reconstructions within the same region
are mostly in agreement and identify region-
ally coherent SST patterns (Fig. 2), suggesting
that the difference in proxy-specific global stacks
reflects spatial sampling bias. The one exception
is the U
K′
37
SST proxy that suggests a tendency
for warmer tropical SSTs than either Mg/Ca or
microfossil proxies (Fig. 2 and fig. S19). This may
reflect a spring-summer dependence of U
K′
37
and
a bias toward higher unsaturation ratios (and
thus warmer SSTs) in areas of dynamic oceano-
graphic settings (19).
The global SST stack shows that the global
ocean was already similar to the HadISST1.1
1870–1889 mean within uncertainties (0.1° ± 0.3°C)
at the onset of the LIG (129 ka) (Fig. 1E). There
is a continuation of a warming trend from the
penultimate deglaciation until 125 ka, when SSTs
reached 0.5° ± 0.3°C relative to the 1870–1889
mean and were indistinguishable (0.1° ± 0.3°C)
from the 1995–2014 mean (Fig. 1E). This warm
LIG SST anomaly was followed by a cooling trend
through the remainder of the LIG, reaching the
HadISST1.1 1870–1889 mean by 120 ka as global
cli ma te approached the last glacial inception.
We estimate the thermal expansion of the
ocean during the peak LIG warm interval from
10,000-year integrationswithacoupledclimate
model (20). These model estimates yield an equi-
librium sea-level change in the range of 0.42 to
0.64 m °C
−1
, suggesting a thermosteric contri-
bution to the LIG sea-level high stand of 0.08
to 0.51 m. These estimates do not account for
uncertainty in the different spatial distribution
of the warming in models and the dependence
of the expansion on local temperature and salinity.
Regional stacks and time-slice global maps
reveal considerable regional differences in LIG
SST anomaly timing, amplitude, and duration.
The tropical (between 23.5°N and 23.5°S) SST
stack has a structure similar to that of the global
stack, but SSTs remained slightly below the
HadISST1.1 1870–1889 mean throughout the LIG
(Figs. 1G and 2), which is lower than in previous
reconstructions (7, 8). If there is a warm bias in
the U
K′
37
proxy estimates (fig. S19), then tropical
SSTs would be lower than suggested by our stack.
LIG SSTs in the extratropical regions of the
southern and northern hemispheres (SH and
NH, respectively) are significantly higher than
RESEARCH
Hoffman et al., Science 355, 276-279 (2017) 20 Januar y 2017 1of4
1
College of Earth, Ocean, and Atmospheric Sciences, Oregon
State University, Corvallis, OR 97331, USA.
2
School of
Mathematics and Statistics, University College Dublin, Dublin
4, Ireland.
3
Center for Climatic Research, Nelson Institute for
Environmental Studies, University of Wisconsin–Madison,
Madison, WI 53706, USA.
*Present address: Science Museum of Virginia, 2500 West Broad
Street, Richmond, VA 23220-2057, USA. †Corresponding author.
Email: jhoffman@smv.org
on November 13, 2019 http://science.sciencemag.org/Downloaded from
the tropics (Figs. 1F, 2, and 3). However , SH and
NH extratropical SSTs experienced distinctly dif-
ferent SST trajectories early in the LIG (Figs. 1F
and 2A), in agreement with previous high-latitude
reconstructions (10). In the SH extratropics, LIG
SSTs that were 1.1° ± 0.6°C greater than the
1870–1889 mean were already reached by 129 ka.
SSTs remained near this level until 120 ka, when
they then experienced a cooling trend that con-
tinued through the remainder of the LIG. NH
extratropical SSTs show a similar LIG warming
of 1.3° ± 0.5°C relative to the 1870–1889 mean,
but this warming was not reached until ~125 ka
and was preceded by 1.3° ± 0.9°C of warming
since the start of the LIG. Subsequent cooling
through the remainder of the LIG occurred at a
similar rate in both high-latitude hemispheres.
The differences in these extratropical SST his-
tories during the early LIG were even more pro-
nouncedintheAtlanticbasin(Figs.1H,2,and3).
South Atlantic extratropical SSTs were 2.0° ±
0.9°C higher than the 1870–1889 mean at 129 ka
and cooled by ~1.0°C through the remainder of
the LIG. In contrast, North Atlantic extratropical
SSTs were –1.2° ± 0.9°C cooler than the 1870–1889
mean at 129 ka, increased during the early LIG
to reach 0.6° ± 0.5°C relative to the 1870–1889
mean at 125 ka, and were followed by a cooling
trend that began at ~120 ka. Summer-specific
proxies similarly suggest cooler summer SSTs
in the North Atlantic extratropics early in the
LIG followed by warmer SSTs at 125 ka (fig. S20).
Our results confirm that LIG global mean an-
nual surface temperatures simulated with most
global climate models forced with LIG boundary
conditions (insolation and greenhouse gas con-
centrations) are too low (3, 4), with a multi-
model estimate of 0.0° ± 0.5°C at 125 ka (21)as
compared to 0.5° ± 0.3°C in our SST recon-
struction. One recent simulation for 125 ka
found global mean warming of 0.5°C with a
spatial pattern similar to our reconstruction
(warmer extratopics and slightly cooler tropics
relative to the preindustrial era) (22). This agree-
ment may reflect the higher resolution (T159) of
the atmospheric model used in that study than
for any of those used in the multimodel ensemble
(highest resolution of T85) (3). Most of these
models do successfully simulate the weak cool-
ing found in our tropical SST stack in response
to reduced mean annual insolation at those
latitudes (Fig . 1C), but fail to reproduce the
evolution of early LIG extratropical SSTs, with
model responses to insolation forcing at 130 ka
resulting in modest NH warming and no change
in the SH extratropics (3, 4), which is opposite to
that seen in our reconstructions (Figs. 1 to 3).
The relatively cool-NH versus warm-SH signal
during the early LIG was first inferred from
speleothem and ice-core records (23)andfurther
documented from SST records (10), including
those used in our compilation, and has been
attributed to the bipolar seesaw mechanism
(23, 24). In particular, a global climate model simu-
lation for 130 ka reproduced this asymmetric
temperature signal by perturbing the Atlantic
meridional overturning circulation (AMOC) with
Hoffman et al., Science 355, 276-279 (2017) 20 Januar y 2017 2of4
Fig. 1. Last interglacial proxy-based sea-surface temperature stacks, site locations, and cli-
mate context. (A) Site locations and types of pro xy SST estimates included in this study. Symbols
correspond to microfossil transfer functions (red triangles ), Mg/Ca of planktonic foraminif era (green
squares), and U
K′
37
(blue circles). (B) Histogram relating the number of LIG proxy recor ds to 15° latitude
bins. (C) Contour plot of annual mean latitudinal insolation anomalies between 130 ka and 115 ka,
relative to insolation at 115 ka. (D) Ice-core records of atmospheric CO
2
concentrations on Antarctic Ice
Core Chronology 2012 (AICC2012) time scale, with one standard deviation on measurement. Symbols
indicate recor ds from EPICA Dome C (EDC) ice cor e (gre en diamonds) (30), Talos Dome ice core (red
circles ) (31), and EPICA Dronning Maud Land (EDML) ice core (blue squares) (31). (E) The global LIG 5° ×
5° gridded, area-weighted mean annual SST stack relative to the HadISST1.1 1870–1889 mean (black line)
and the HadISS T1.1 1995–2014 mean (green line). (F) Extratropical (>23.5°N-S) mean annual SST stacks
relative to the HadISST1.1 1870–1889 data. Dark blue, NH extratropics; light blue, SH extratropics.
(G) The tropical (23.5°N-S) mean annual SST stack relative to the HadISST1.1 1870–1889 data. (H)The
North Atlantic (NATL) and South Atlantic (SATL) extratropical mean annual SS T stacks (sites >23.5°N-S
within the Atlantic basin) r elative to the HadISST1.1 18 7 0–1889 data. The proxy-based SST stacks and
their uncertainty are the 5° × 5° gridded, area-weighted mean and 2s uncertainties of 1000 realizations
used to construct the stacks.
RESEARCH | REPORT
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Hoffman et al., Science 355, 276-279 (2017) 20 Januar y 2017 3of4
Fig. 2. Maps of proxy-based sea-surface temperature anomalies for
three different times during the last interglaciation. (A) Distribution
and pro xy-based mean annual SST anomalies relative to HadISS T1.1 1870–1889
data at 129 ka, with the mean pro xy-ba sed SS T anomaly uncertai nty shown in
the bottom left corner. Symbols correspond to microfossil transfer functions
(inver ted triangles), Mg/Ca of planktonic forami nifer a (squares) , and U
K′
37
(circles). (B)Sameasin(A),butfor125ka.(C)Sameasin(A),butfor120ka.
Fig. 3. Proxy-based sea-surface temperature anomalies by latitude for three different times during the last interglaciation. (A)Globalproxy-
based mean annual SST anomalies with their uncertainties relative to HadISS T1.1 187 0–1889 data at 129 ka plotted against core-site latitude. Symbols
correspond to microfossil transfer functions (red triangles), Mg/Ca of planktonic foraminifera (green squares) , and U
K′
37
(blue circles). Yellow lines and sh ading
are the fit of a 2nd-order polynomial to the data and its 95% simultaneous functional bounds. (B)Sameasin(A),butfor125ka.(C)Sameasin(A),butfor120ka.
(D) Same as in (A), but only those recor ds within the Atlantic basin are plotted. (E)Sameasin(D),butfor125ka.(F)Sameasin(D),butfor120ka.
RESEARCH | REPORT
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freshwater forcing from remnant Northern Hem-
isphere ice sheets (25), which offset NH boreal
insolation forcing (4) to sustain cold North
Atlantic SSTs while causing warmer SH extra-
tropical SSTs through reduced ocean heat trans-
port. We suggest that the subsequent increase
of the AMOC (26) combined with boreal sum-
mer insolation forcing (4) to induce NH extra-
tropical warming by 125 ka, particularly in the
North Atlantic region. At the same time, the ther-
mal memory of the seesaw response in the SH
extratropics, likely associated with sea-ice retreat
(27, 28), combined with CO
2
forcing (Fig. 1D) to
sustain warm SSTs there, thus resulting in the
symmetrical high-latitude warming seen in our
reconstruction (Figs. 2B and 3), as well as the
warmer global SSTs than those simulated in global
climate models that do not include the sea-ice
feedback (3, 4). Subsequent high-latitude cooling
through the remainder of the LIG (Fig. 1) then likely
occurred in response to the dominant obliquity
forcing (Fig. 1C) and associated feedbacks (4, 29).
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AC KN OW LE D GM E NT S
Research was supported by an NSF Graduate Research Fellowship
to J.S.H. under NSF grant 1314 109-DGE and by NSF grants 1335197
and 1503032 to P.U.C. F.H. was supported by NSF grant AGS-
1502990 and by the National Oceanic and Atmospheric
Administration's Climate and Global Change Postdoctoral
Fellowship program, administered by the University Corporation for
Atmospheric Research. We acknowledge high-performance
computing support from Yellowstone (ark:/85065/d7wd3xhc)
provided by the National Center for Atmospheric Research’s
Computational and Information Systems Laboratory, sponsored by
the NSF. This research used resources of the Oak Ridge
Leadership Computing Facility at the Oak Ridge National
Laboratory, which is supported by the Office of Science of the U.S.
Department of Energy under contract no. DE-AC05-00OR22725.
We thank the members of the Past Global Changes–Paleoclimate
Modelling Intercomparison Project (PAGES-PMIP) Working Group
on Quaternary Interglacials, P. Bakker, H. A. Bauch, A. E. Carlson,
M. S. Hoecker-Martinez, S. A. Marcott, H. L. O. McClelland,
N. McKay, A. C. Mix, N. G. Pisias, and J. D. Shakun for helpful
discussions. The authors report no conflicts of interest. J.S.H. and
P.U.C. designed the study and wrote the manuscript with help from
A.C.P. and F.H. We acknowledge insightful comments from two
anonymous reviewers. The data reported in this paper are tabulated
in the supplementary materials.
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Materials and Methods
Figs. S1 to S43
Tables S1 and S2
References (32–118)
Data File S1
19 August 2016; accepted 21 December 2016
10.1126/science.aai8464
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Regional and global sea-surface temperatures during the last interglaciation
Jeremy S. Hoffman, Peter U. Clark, Andrew C. Parnell and Feng He
DOI: 10.1126/science.aai8464
(6322), 276-279.355Science
, this issue p. 276Science
the last interglacial period were 6 to 9 m higher than they are now.
2014 mean. This is a sobering point, because sea levels during−were 150 years ago and indistinguishable from the 1995
0.5°C warmer than they∼which lasted from about 129,000 to 116,000 years ago. The global mean annual values were
compiled estimates of sea surface temperatures during the last interglacial period,et al.affect it in the future. Hoffman
Understanding how warm intervals affected sea level in the past is vital for projecting how human activities will
Sea surface temperatures of the past
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