ArticlePDF Available

A Reconstruction of Regional and Global Temperature for the Past 11,300 Years

Article

A Reconstruction of Regional and Global Temperature for the Past 11,300 Years

Abstract and Figures

Surface temperature reconstructions of the past 1500 years suggest that recent warming is unprecedented in that time. Here we provide a broader perspective by reconstructing regional and global temperature anomalies for the past 11,300 years from 73 globally distributed records. Early Holocene (10,000 to 5000 years ago) warmth is followed by ~0.7°C cooling through the middle to late Holocene (<5000 years ago), culminating in the coolest temperatures of the Holocene during the Little Ice Age, about 200 years ago. This cooling is largely associated with ~2°C change in the North Atlantic. Current global temperatures of the past decade have not yet exceeded peak interglacial values but are warmer than during ~75% of the Holocene temperature history. Intergovernmental Panel on Climate Change model projections for 2100 exceed the full distribution of Holocene temperature under all plausible greenhouse gas emission scenarios.
Comparison of different methods and reconstructions of global and hemispheric temperature anomalies. (A and B) Globally stacked temperature anomalies for the 5° × 5° area-weighted mean calculation (purple line) with its 1s uncertainty (blue band) and Mann et al.'s global CRUEIV composite mean temperature (dark gray line) with their uncertainty (light gray band). (C and D) Global temperature anomalies stacked using several methods (Standard and Standard 5x5Grid; 30x30Grid; 10-lat : Arithmeticmeancalculation, area-weighted with a 5° × 5° grid, area-weighted with a 30° × 30° grid, and area-weighted using 10° latitude bins, respectively; RegEM and RegEM 5x5Grid : Regularized expectation maximization algorithminfilled arithmetic mean and 5° × 5° area-weighted). The gray shading [50% Jackknife (Jack 50 )] represents the 1s envelope when randomly leaving 50% of the records out during each Monte Carlo mean calculation. Uncertainties shown are 1s for each of the methods. (E and F) Published temperature anomaly reconstructions that have been smoothed with a 100-year centered running mean, Mann08 Global (2), Mann08 NH (2), Moberg05 (3), WA07 (8), Huange04 (36), and plotted with our global temperature stacks [blue band as in (A)]. The temperature anomalies for all the records are referenced to the 1961-1990 instrumental mean. (G and H) Number of records used to construct the Holocene global temperature stack through time (orange line) and Mann et al.'s (2) reconstruction (gold vertical bars). Note the y axis break at 100. The latitudinal distribution of Holocene records (gray horizontal bars) through time is shown. (I and J) Number of age control points (e.g., 14 C dates) that constrain the time series through time.
… 
Content may be subject to copyright.
A Reconstruction of Regional
and Global Temperature for
the Past 11,300 Years
Shaun A. Marcott,
1
Jeremy D. Shakun,
2
Peter U. Clark,
1
Alan C. Mix
1
Surface temperature reconstructions of the past 1500 years suggest that recent warming is
unprecedented in that time. Here we provide a broader perspective by reconstructing regional
and global temperature anomalies for the past 11,300 years from 73 globally distributed
records. Early Holocene (10,000 to 5000 years ago) warmth is followed by ~0.7°C cooling
through the middle to late Holocene (<5000 years ago), culminating in the coolest temperatures
of the Holocene during the Little Ice Age, about 200 years ago. This cooling is largely
associated with ~2°C change in the North Atlantic. Current global temperatures of the past
decade have not yet exceeded peak interglacial values but are warmer than during ~75% of
the Holocene temperature history. Intergovernmental Panel on Climate Change model projections
for 2100 exceed the full distribution of Holocene temperature under all plausible greenhouse
gas emission scenarios.
Placing present climate into a historical per-
spective beyond the instrumental record is
important for distinguishing anthropogenic
influences on climate from natural variability (1).
Proxy-based temperature reconstructions of the
past 1500 years suggest that the warming of
the past few decades is unusual relative to pre-
anthropogenic variations (2,3), but whether re-
cent warming is anomalous relative to variability
over the entirety of the Holocene interglaciation
(the past 11,500 years) (4) has yet to be established.
The 73 globally distributed temperature re-
cords used in our analysis are based on a variety
of paleotemperature proxies and have sampling
resolutions ranging from 20 to 500 years, with a
median resolution of 120 years (5). We account
for chronologic and proxy calibration uncertain-
ties with a Monte Carlobased randomization
scheme (6). Our data set exhibits several impor-
tant strengths, as well as limitations, as compared
to global and hemispheric reconstructions of the
past 1500 years (2,3,7,8). For example, whereas
reconstructions of the past millennium rapidly
lose data coverage with age, our coverage in-
creases with age (Fig. 1, G and H). Published re-
constructions of the past millennium are largely
based on tree rings and may underestimate low-
frequency (multicentury-to-millennial) variability
because of uncertainty in detrending (9) [although
progress is being made on this front (10)], whereas
our lower-resolution records are well suited for
reconstructing longer-term changes. Terrestrial re-
cords dominate reconstructions of the past mil-
lennium, whereas our stack is largely derived
from marine archives (~80%). Unlike the recon-
structions of the past millennium, our proxy data
are converted quantitatively to temperature before
stacking, using independent core-top or laboratory-
culture calibrations with no post-hoc adjustments
in variability.
We took the 5° × 5° area-weighted mean of the
73 records to develop a global temperature stack
for the Holocene (referred to as the Standard
5×5
reconstruction) (Fig. 1, A and B). To compare our
Standard
5×5
reconstruction with modern clima-
tology, we aligned the stacks mean for the in-
terval 510 to 1450 yr B.P. (where yr B.P. is years
before 1950 CE) with the same intervals mean of
the global Climate Research Unit error-in-variables
(CRU-EIV) composite temperature record (2),
which is, in turn, referenced to the 19611990
CE instrumental mean (Fig. 1A). We then as-
sessed the sensitivity of the temperature recon-
struction to several averaging schemes, including
an arithmetic mean of the data sets, a 30° × 30°
area-weighted mean, a 10° latitudinal weighted
mean, and a calculation of 1000 jackknifed stacks
that randomly exclude 50% of the records in each
realization (Fig. 1, C and D, and fig. S4). Although
some differences exist at the centennial scale
among the various methods (Fig. 1, C and D),
they are small (<0.2°C) for most of the recon-
structions, well within the uncertainties of our
Standard
5x5
reconstruction, and do not affect the
long-term trend in the reconstruction.
In addition to the previously mentioned av-
eraging schemes, we also implemented the RegEM
algorithm (11) to statistically infill data gaps in
records not spanning the entire Holocene, which
is particularly important over the past several cen-
turies (Fig. 1G). Without filling data gaps, our
Standard
5×5
reconstruction (Fig. 1A) exhibits
0.6°C greater warming over the past ~60 yr B.P.
(1890 to 1950 CE) than our equivalent infilled
5° × 5° area-weighted mean stack (Fig. 1, C and
D). However, considering the temporal resolution
of our data set and the small number of records
that cover this interval (Fig. 1G), this difference is
probably not robust. Before this interval, the gap-
filled and unfilled methods of calculating the
stacks are nearly identical (Fig. 1D).
Because the relatively low resolution and time-
uncertainty of our data sets should generally sup-
press higher-frequency temperature variability, an
important question is whether the Holocene stack
adequately represents centennial- or millennial-
scale variability. We evaluated this question in
two ways. First, we generated a single mean zero,
unit variance white-noise time series and used it
in place of our 73 records. The white-noise re-
cords were then perturbed through Monte Carlo
simulations using the resolution and chronolog-
ical uncertainty specific to each proxy record as
well as a common 1°C proxy uncertainty. We
composited a Standard
5x5
global stack from these
synthetic records and calculated the ratio between
the variances of the stack and the input white
noise as a function of frequency to derive a gain
function. The results suggest that at longer pe-
riods, more variability is preserved, with essen-
tially no variability preserved at periods shorter
than 300 years, ~50% preserved at 1000-year pe-
riods, and nearly all of the variability preserved
for periods longer than 2000 years (figs. S17 and
S18). Second, spectral analysis indicates that the
variance of the Holocene proxy stack approaches
that of the global CRU-EIV reconstruction of the
past 1500 years (2) at millennial time scales and
longer (figs. S20 and S23).
Our global temperature reconstruction for the
past 1500 years is indistinguishable within uncer-
tainty from the Mann et al.(2) reconstruction;
both reconstructions document a cooling trend
from a warm interval (~1500 to 1000 yr B.P.) to a
cold interval (~500 to 100 yr B.P.), which is ap-
proximately equivalent to the Little Ice Age (Fig.
1A). This similarity confirms that published tem-
perature reconstructions of the past two millennia
capture long-term variability, despite their short
time span (3,12,13). Our median estimate of this
long-term cooling trend is somewhat smaller than
in Mann et al.(2) though, which may reflect our
bias toward marine and lower-latitude records.
The Standard
5x5
reconstruction exhibits ~0.6°C
of warming from the early Holocene (11,300 yr
B.P.) to a temperature plateau extending from
9500 to 5500 yr B.P.. This warm interval is fol-
lowed by a long-term 0.7°C cooling from 5500 to
~100 yr B.P. (Fig. 1B). Extratropical Northern
Hemisphere sites (30° to 90°N), in particular from
the North Atlantic sector, contribute most of the
variance to the global signal; temperatures in this
region decrease by ~2°C from 7000 yr B.P. to
~100 yr B.P. (Fig. 2H). By comparison, the low
latitudes (30°N to 30°S) exhibit a slight warming
of ~ 0.4°C from 11,000 to 5000 yr B.P., with tem-
perature leveling off thereafter (Fig. 2I), whereas
the extratropical Southern Hemisphere (30°S
to 90°S) cooled ~0.4°C from about 11,000 to
7000 yr B.P., followed by relatively constant
temperatures except for some possible strong mul-
ticentennial variability in the past 2500 years (Fig.
2J). The Southern Hemisphere is represented by
fewer data sets (n= 11) than the equatorial (n=33)
1
College of Earth, Ocean, and Atmospheric Sciences, Oregon
State University, Corvallis, OR 97331, USA.
2
Department of
Earth and Planetary Sciences, Harvard University, Cambridge,
MA 02138, USA.
*To whom correspondence should be addressed. E-mail:
marcotts@science.oregonstate.edu
8 MARCH 2013 VOL 339 SCIENCE www.sciencemag.org1198
REPORTS
on March 10, 2013www.sciencemag.orgDownloaded from
and Northern Hemisphere (n= 29) regions, pro-
viding fewer constraints on characterizing the var-
iability in our reconstruction for this region.
Trends in regional temperature reconstructions
show strong similarities with high-resolution pre-
cipitation records, consistently associating greater
warmth with greater wetness (Fig. 2, H to J). For
example, extratropical Northern Hemisphere mid-
to-highlatitude temperature correlates well with
records of Asian monsoon intensity (14,15)and
the position of the Atlantic intertropical convergence
zone (16) (Fig. 2H), tropical temperatures track
precipitation proxies from speleothems in Borneo
(17) and Indonesia (18) (Fig. 2I), and extratrop-
ical Southern Hemisphere temperatures parallel
Fig. 1. Comparison of dif-
ferent methods and re-
constructions of global
and hemispheric temper-
ature anomalies. (Aand
B) Globally stacked tem-
perature anomalies for the
5° × 5° area-weighted
mean calculation (purple
line) with its 1suncer-
tainty (blue band) and
Mann et al.sglobalCRU-
EIV composite mean tem-
perature (dark gray line)
with their uncertainty (light
gray band). (Cand D)Glob-
al temperature anomalies
stacked using several meth-
ods (Standard and Stan-
dard
5x5Grid; 30x30Grid; 10-lat
:
Arithmetic mean calculation,
area-weighted with a 5° ×
5° grid, area-weighted
with a 30° × 30° grid, and
area-weighted using 10°
latitude bins, respectively;
RegEM and RegEM
5x5Grid
:
Regularized expectation
maximization algorithm-
infilled arithmetic mean
and 5° × 5° area-weighted).
The gray shading [50%
Jackknife (Jack
50
)] repre-
sents the 1senvelope
when randomly leaving
50% of the records out
during each Monte
Carlo mean calculation.
Uncertainties shown are
1sfor each of the meth-
ods. (Eand F) Published
temperature anomaly re-
constructions that have
been smoothed with a
100-year centered running
mean, Mann08
Global
(2),
Mann08
NH
(2), Moberg05
(3), WA07 (8), Huange04
(36), and plotted with
our global temperature
stacks [blue band as in (A)].
The temperature anom-
alies for all the records
are referenced to the
19611990 instrumen-
tal mean. (Gand H)Num-
ber of records used to
construct the Holocene
global temperature stack
through time (orange line) and Ma nn et al.s(2) reconstruction (gold vertical bars). Note the yaxis break at 100. The latitudinal distribution of Holocene records
(gray horizontal bars) through time is shown. (Iand J) Number of age control points (e.g.,
14
Cdates)thatconstrainthetimeseriesthroughtime.
www.sciencemag.org SCIENCE VOL 339 8 MARCH 2013 1199
REPORTS
on March 10, 2013www.sciencemag.orgDownloaded from
speleothem proxies of precipitation and temper-
ature from South Africa (19) and South Ame rica
(20) that are independent of our reconstruction.
The general pattern of high-latitude cooling in
both hemispheres opposed by warming at low lat-
itudes is consistent with local mean annual insolation
forcing associated with decreasing orbital obliqui ty
since 9000 years ago (Fig. 2C). The especially pro-
nounced cooling of the Northern Hemisphere ex-
tratropics, however, suggests an important role for
summer insolation in this region, perhaps through
snow-ice albedo and vegetation feedbacks (21,22).
Such a mechanism that mediates seasonal insola-
tion is plausible at these latitudes, where the frac-
tion of continental landmasses relative to the ocean
is high (~50% land from 30° to 90°N; 25% land
from 30°N to 30°S; 15% land from 30° to 90°S).
We cannot fully exclude the possibility of a
seasonal proxy bias in our temperature recon-
structions (23), but a sensitivity experiment with
an intermediate-complexity model (fig. S8) sug-
gests that the effects of such a bias would prob-
ably be modest in the global reconstruction. The
dominance of the northern signal in our global
stack is consistent with Milankovitch theory, in
which summer insolation would drive the planet
toward eventual future glacial inception in the
Northern Hemisphere (24), excluding any anthro-
pogenic influence. Models support our finding of
a global mean cooling in response to an obliquity
decrease, though of lesser magnitude (25), and
also support the idea about the sensitivity of the
northern high latitudes to summer insolation (21).
Additional effects probably further influenced
the evolution of climate through the Holocene.
In the early-to-middle Holocene, the deglaciating
Northern Hemisphere ice sheets would have mod-
ulated warming of the northern high latitudes rel-
ative to peak seasonal insolation (26,27). Radiative
forcing by greenhouse gases (primarily CO
2
)rose
0.5 W/m
2
during the mid-to-late Holocene (Fig. 2D),
which would be expected to yield ~ 0.4°C warm-
ing for a mid-range climate sensitivity (28). Re-
sponse to such forcing may have been offset by
opposing orbital insolation forcing that was greater
than greenhouse gas forcing by up to one (annual)
to two (seasonal) orders of magnitude over the
course of the Holocene (Fig. 2, A to C). North-
ward heat transport in the Atlantic basin by the
meridional overturning circulation (MOC) may have
weakened since the middle Holocene ( 29), con-
tributingtothestrongcoolingintheNorthAtlan-
tic while dampening cooling in the mid-to-high
latitude Southern Hemisphere due to the bipolar
seesaw (30). Insofar as winter conditions influence
the sources and strength of deepwater formation, a
weakening MOC may partly reflect the increase
in high northern-latitude winter insolation over the
Holocene (Fig. 2A). Total solar irradiance recon-
structed from cosmogenic isotopes (31) also varied
by0.5to1W/m
2
, and volcanic eruptions occurred
throughout the duration of the Holocene (32,33),
although most of this variance is at higher frequen-
cies than those resolved by our stacked temperature
records, and the scaling of both is poorly constrained.
Although our temperature stack does not fully
resolve variability at periods shorter than 2000 yea rs,
such high-frequency changes would only mod-
estly broaden the statistical distribution of Hol-
ocene temperatures (Fig. 3 and fig. S22). Moreover,
we suggest that accounting for any spatial or sea-
sonal biases in the stack would tend to reduce its
Fig. 2. Holocene climate forcings and paleoclimate records. Contour plots of (A)
December, (B)June,and(C) annual mean latitudinal insolation anomalies relative to
presentforthepast11,500years(36). (D) Calculated radiative forcing (28)derived
from ice-core greenhouse gases (GHG) (CO
2
+N
2
O+CH
4
). (E) Total solar irradiance
anomalies (DTSI) relative to 19441988 CE derived from cosmogenic isotopes (31).
(Fand G) Proxies for the strength of the Atlantic meridional overturning circulation
(37,38). (H) Volcanic sulfate flux (in kg/km
2
)fromAntarctica(32)andvolcanic
sulfate concentration (in parts per billion) from Greenland (33)in100-yearbins.Both
records are normalized relative to the volcanic sulfate flux/concentration associated
with the Krakatoa eruption. (Ito K) Zonal mean temperature reconstructions for 60°
latitude bands from this study compared to speleothem (14,15,1720)andTidata
(16), which are proxies for precipitation and localtemperature.Speleothemdatasets
were smoothed with a seven-point running mean for clarity. ITCZ, Intertropical Convergence Zone; EASM, East Asian Summer Monsoon; AISM, Australian-
Indonesian Summer Monsoon.
8 MARCH 2013 VOL 339 SCIENCE www.sciencemag.org
1200
REPORTS
on March 10, 2013www.sciencemag.orgDownloaded from
variability because of the cancellation of noise in
a large-scale mean and the opposing nature of
seasonal insolation forcing over the Holocene, caus-
ing the Holocene temperature distribution to contract.
Our results indicate that global mean temper-
ature for the decade 20002009 (34) has not yet
exceeded the warmest temperatures of the early
Holocene (5000 to 10,000 yr B.P.). These tem-
peratures are, however, warmer than 82% of
the Holocene distribution as represented by the
Standard
5×5
stack, or 72% after making plausible
corrections for inherent smoothing of the high
frequencies in the stack (6) (Fig. 3). In contrast,
the decadal mean global temperature of the ear-
ly 20th century (19001909) was cooler than
>95% of the Holocene distribution under both the
Standard
5×5
and high-frequency corrected sce-
narios. Global temperature, therefore, has risen
from near the coldest to the warmest levels of the
Holocene within the past century, reversing the
long-term cooling trend that began ~5000 yr B.P.
Climate models project that temperatures are likely
to exceed the full distribution of Holocene warmth
by 2100 for all versions of the temperature stack
(35) (Fig. 3), regardless of the greenhouse gas
emission scenario considered (excluding the year
2000 constant composition scenario, which has
already been exceeded). By 2100, global average
temperatures will probably be 5 to 12 standard
deviations above the Holocene temperature mean
for the A1B scenario (35) based on our Standard
5×5
plus high-frequency addition stack (Fig. 3).
Strategies to better resolve the full range of
global temperature variability during the Holo-
cene, particularly with regard to decadal to cen-
tennial time scales, will require better chronologic
constraints through increased dating control. Higher-
resolution sampling and improvements in proxy
calibration also play an important role, but our
analysis (fig. S18) suggests that improvements in
chronology are most important. Better constraints
on regional patterns will require more data sets
from terrestrial archives and both marine and ter-
restrial records representing the mid-latitudes of
the Southern Hemisphere and central Pacific.
References and Notes
1. E. Jan sen et al., in Climate Change 2007: The
Physical Science Basis. Contribution of Working
Group I to the Fourth Assessment Report of the
Intergovernmental Panel on Climate Change,
S. Solomon et al., Eds. (Cambridge Univ. Press,
Cambridge, 2007), pp. 433497.
2. M. E. Mann et al., Proc. Natl. Acad. Sci. U.S.A. 105,
13252 (2008).
3. A. Moberg, D. M. Sonechkin, K. Holmgren,
N. M. Datsenko, W. Karlén, Nature 433, 613 (2005).
4. H. Wanner et al., Quat. Sci. Rev. 27, 1791 (2008).
5. The majority of the data sets can be found at NOAA National
Climate Data Center (www.ncdc.noaa.gov/paleo/paleo.html)
and the PANGAEA data repository (www.pangaea.de/).
Sources for all data sets are available online (6).
6. Materials, methods, and supporting data are available as
supplementary materials on Science Online.
7. J. Esper, E. R. Cook, F. H. Schweingruber, Science 295,
2250 (2002).
8. C. M. Ammann, E. R. Wahl, Clim. Change 85,71
(2007).
9. J. Esper, D. C. Frank, R. J. S. Wilson, EOS 85, 113, 120
(2004).
10. J. Esper et al., Nat. Clim. Change 10.1038/nclimate1589
(2012).
11. T. Schneider, J. Clim. 14, 853 (2001).
12. E.R.Wahl,C.M.Ammann,Clim. Change 85,33
(2007).
13. E. R. Wahl, D. M. Ritson, C. M. Ammann, Science 312,
529, author reply 529 (2006).
14. C. A. Dykoski et al., Earth Planet. Sci. Lett. 233,71
(2005).
15. Y. Wang et al., Science 308, 854 (2005).
16. G. H. Haug, K. A. Hughen, D. M. Sigman, L. C. Peterson,
U. Röhl, Science 293, 1304 (2001).
17. J. W. Partin, K. M. Cobb, J. F. Adkins, B. Clark,
D. P. Fernandez, Nature 449, 452 (2007).
18. M. L. Griffiths et al., Nat. Geosci. 2, 636 (2009).
19. K. Holmgren et al., Quat. Sci. Rev. 22, 2311 (2003).
20. F. W. Cruz Jr. et al., Nature 434, 63 (2005).
21. H. Renssen et al., Clim. Dyn. 24, 23 (2005).
22. A. Ganopolski, C. Kubatzki, M. Claussen, V. Brovkin,
V. Petoukhov, Science 280, 1916 (1998).
23. G. Leduc, R. Schneider, J.-H. Kim, G. Lohmann,
Quat. Sci. Rev. 29, 989 (2010).
24. P. C. Tzedakis, J. E. T. Channell, D. A. Hodell, H. F. Kleiven,
L. C. Skinner, Nat. Geosci. 10.1038/ngeo1358 (2012).
25. D. F. Mantsis, A. C. Clement, A. J. Broccoli, M. P. Erb,
J. Clim. 24, 2830 (2011).
26. D. S. Kaufman et al., Quat. Sci. Rev. 23, 529 (2004).
27. F. C. Ljungqvist, Geografia 116, 91 (2011).
28. V. Ramaswamy et al., Eds., Radiative Forcing Of
Climate Change. Climate Change 2001: The Scientific
Basis. Contribution of Working Group I to the Third
Assessment Report of the Intergovernmental Panel on
Climate Change (Cambridge Univ. Press, New York,
2001).
29. B. A. A. Hoogakker et al., Paleoceanography 26, PA4214
(2011).
30. W. S. Broecker, Paleoceanography 13, 119 (1998).
31. F. Steinhilber et al., Proc. Natl. Acad. Sci. U.S.A. 109,
5967 (2012).
32. E. Castellano et al., J. Geophys. Res. 110, D06114
(2005).
33. G. A. Zielinski, P. A. Mayewski, L. D. Meeker, S. Whitlow,
M. S. Twickler, Quat. Res. 45, 109 (1996).
34. P. Brohan, J. J. Kennedy, I. Harris, S. F. B. Tett,
P. D. Jones, J. Geophys. Res. 111, D12106 (2006).
35. G. A. Meehl et al., in Climate Change 2007: The
Physical Science Basis. Contribution of Working
Group I to the Fourth Assessment Report of the
Intergovernmental Panel on Climate Change, S. Solomon
et al., Eds. (Cambridge Univ. Press, Cambridge,
2007), pp. 747845.
36. S. Huang, Geophys. Res. Lett. 31, L13205 (2004).
37. A. Berger, M.-F. Loutre, Quat.Sci.Rev.10, 297
(1991).
38. J. F. McManus, R. Francois, J.-M. Gherardi, L. D. Keigwin,
S. Brown-Leger, Nature 428, 834 (2004).
Acknowledgments: We thank J. Alder, T. Bauska, E. Brook,
C. Buizert, V. Ersek, S. Hostetler, P. Huybers, N. Pisias, J. Rosen,
G. Schmidt, A. Schmittner, M. Tingley, and our reviewers for
invaluable insight and helpful discussion. The data sets provided
by T. Barrows, J.-H. Kim, Y. Kubota, I. Larocque, G. Leduc,
M. McGlone, T. Rodrigues, C. Rühlemann, J. Sachs, R. Schneider,
H. Seppä, J. Tierney, M. Yamamoto, B. Vinther, and C. Waelbrock,
as well as the data sets compiled from the National Oceanic and
Atmospheric Administration Climate Data Center and PANGAEA
databases, made this research possible. Funding for this work
was provided by the NSF Paleoclimate Program for the
Paleovar Project.
Supplementary Materials
www.sciencemag.org/cgi/content/full/339/6124/1198/DC1
Supplementary Text
Figs. S1 to S26
References
27 July 2012; accepted 4 January 2013
10.1126/science.1228026
Standard5x5 +
high frequency
Standard5x5
Standard
Standard30x30
Standard10-lat
RegEM5x5
no N Atlantic
0.5NH + 0.5SH
Jack50
Mg/Ca
UK'37
Other
0
0.02
0.04
0.06
0.08
0.1
Frequency
-101234567
Temperature Anomaly (oC from 1961-1990 CE)
Year 2000 constant composition
B1
A1T
B2
A1B
A2
A1FI
2000-2009
1900-1909
Holocene
Fig. 3. Holocene temperature distribution compared to modern temperature and future projections.
Shown are relative frequency plots of Holocene temperature anomalies in 0.05°C bins using multiple data
subsets and reconstructions (colored lines), instrumental means for 19001909 and 20002009 CE
(vertical black lines), 2100 CE projectionsbasedonvariousemissionsscenarios(35) (black squares and
graybarsgivethebestestimateand66%confidenceinterval),andtheHolocenemedianand66%range
from Standard
5×5
+ high-frequency stack (black square and blue bar). Projections in (35) were referenced
to 19801999 CE, whereas we reference them to 19611990 CE here. Data sets are divided by proxy
type: UK37, Mg/Ca, and the remainder (Other); method: arithmetic mean (Standard) and RegEM; weighting:
equal Northern and Southern Hemisphere weighting (0.5NH + 0.5SH), 5° × 5°grid, and 30° × 30°grid;
exclusionofdatasets: noNorthAtlanticand Jack
50
; and high-frequency addition: red noise with the same
power spectrum as Mann et al.(2) added to the global stack (supplementary materials).
www.sciencemag.org SCIENCE VOL 339 8 MARCH 2013 1201
REPORTS
on March 10, 2013www.sciencemag.orgDownloaded from
... The annual warming trend from the early to mid-Holocene accords with another pollen-based terrestrial stack for North America and Europe 9 (MA18), and NH multiproxy reconstructions 3,4 (KA20), while another multiproxy stack 2 (MC13) exhibits an HTM phase (Fig. 1a). The long-term cooling trend after~7 ka BP has been widely detected in syntheses of proxy records [2][3][4][24][25][26] , in global/NH ocean temperature records 2,3,8 , and accords with the general pattern of NH glacier advances 27 , but it is not well shown in the previous pollen-based reconstruction for North America and Europe 9 (MA18, Fig. 1a), and in a model-data assimilation output of global surface temperature constrained by marine proxy records 8 . ...
... The annual warming trend from the early to mid-Holocene accords with another pollen-based terrestrial stack for North America and Europe 9 (MA18), and NH multiproxy reconstructions 3,4 (KA20), while another multiproxy stack 2 (MC13) exhibits an HTM phase (Fig. 1a). The long-term cooling trend after~7 ka BP has been widely detected in syntheses of proxy records [2][3][4][24][25][26] , in global/NH ocean temperature records 2,3,8 , and accords with the general pattern of NH glacier advances 27 , but it is not well shown in the previous pollen-based reconstruction for North America and Europe 9 (MA18, Fig. 1a), and in a model-data assimilation output of global surface temperature constrained by marine proxy records 8 . ...
... Our spatial patterns of annual and seasonal temperature variability generally agree with previous reconstructions 2,9,14 and our independent proxy compilations (see Methods), as indicated by the EOF results for the 11-7 ka BP ( Supplementary Fig. 5) and 7-0 ka BP (Supplementary Fig. 6) periods, and the regional stack curves (Supplementary Fig. 4). For the annual temperatures, we attribute the divergence of early to mid-Holocene trends between Marcott et al. 2 and our stack (Fig. 1a) mainly to their low coverage over land where the warming was widely detected, especially in eastern North America and Europe (Supplementary Figs. 4 and 5), which was also highlighted by Marsicek et al. 9 . ...
Article
Full-text available
The origin of the temperature divergence between Holocene proxy reconstructions and model simulations remains controversial, but it possibly results from potential biases in the seasonality of reconstructions or in the climate sensitivity of models. Here we present an extensive dataset of Holocene seasonal temperatures reconstructed using 1310 pollen records covering the Northern Hemisphere landmass. Our results indicate that both summer and winter temperatures warmed from the early to mid-Holocene (~11–7 ka BP) and then cooled thereafter, but with significant spatial variability. Strong early Holocene warming trend occurred mainly in Europe, eastern North America and northern Asia, which can be generally captured by model simulations and is likely associated with the retreat of continental ice sheets. The subsequent cooling trend is pervasively recorded except for northern Asia and southeastern North America, which may reflect the cross-seasonal impact of the decreasing summer insolation through climatic feedbacks, but the cooling in winter season is not well reproduced by climate models. Our results challenge the proposal that seasonal biases in proxies are the main origin of model–data discrepancies and highlight the critical impact of insolation and associated feedbacks on temperature changes, which warrant closer attention in future climate modelling. The study reconstructed Holocene seasonal temperatures using 1,310 pollen records covering the Northern Hemisphere landmass, and show that both summer and winter temperatures peaked at ~7 ka BP, but with significant spatial variability.
... concentrated, and warm freshwater can directly contact Arctic sea ice because of buoyancy. Compared to the present climate, the Arctic climate in MH summer became warmer and had less sea ice 10,18 , along with stronger solar insolation in the Northern Hemisphere [19][20][21][22] . As indicated by the modern global warming process 13,15 , a stronger thermal flux associated with discharge water by pan-Arctic rivers would have contributed to the loss of sea ice in the MH summer due to the higher-than-present solar insolation. ...
... The larger Russian pan-Arctic river heat discharge was determined by both the increased river runoff and the higher freshwater temperature during the MH. Compared to the LH, the stronger summer solar insolation in the middle and high latitudes led to higher surface air temperatures by~1-3°C during the MH [20][21][22] , resulting in intensified thawing of land snow/ice and permafrost ( Fig. 2g-i). Previous biogeochemical studies have indicated that the insolation-driven warming condition severely deepened the thawed active layer of river basin (inland Siberia) ice-rich permafrost during the MH 45,46 . ...
... h Reconstruction of Siberian permafrost thawing 45,46 . i Summaries of air temperature in mid-and high-latitude regions 21 of river freshwater discharge and the large amounts of sediment transported into the ESAS during the MH. As a result, the salinity of the ESAS surface water decreased to~13 PSU, and the Siberian Rivers discharged quantities of green algae and freshwater diatoms at this stage 49,50 (Fig. 2f). ...
Article
Full-text available
Arctic sea ice retreat is linked to extrapolar thermal energy import, while the potential impact of pan-Arctic river heat discharge on sea-ice loss has been unresolved. We reconstructed the Holocene history of Arctic sea ice and Russian pan-Arctic river heat discharge, combining ice-rafted debris records and sedimentation rates from the East Siberian Arctic Shelf with a compilation of published paleoclimate and observational data. In the mid-Holocene, the early summer (June–July) solar insolation was higher than that during the late Holocene, which led to a larger heat discharge of the Russian pan-Arctic rivers and contributed to more Arctic sea ice retreat. This intensified decline of early-summer sea ice accelerated the melting of sea ice throughout the summertime by lowering regional albedos. Our findings highlight the important impact of the larger heat discharge of pan-Arctic rivers, which can reinforce Arctic sea-ice loss in the summer in the context of global warming.
... Thus, the patterns of vegetation succession in the Daihai basin and the Gonghai area were dissimilar, reflecting different driving forces over time. (Marcott et al., 2013). (b) Holocene temperature record in China (Hou and Fang, 2011). ...
... The increase in the sedimentary black carbon concentration indicates that the fire frequency increased substantially, and therefore in addition to climatic factors, slash-and-burn cultivation began to significantly impact the vegetation (Han et al., 2012;Dong et al., 2016). Despite decreasing Northern Hemisphere temperatures (Hou and Fang, 2011;Marcott et al., 2013), human societies gradually adapted to climatic and other changes in the ecological environment. Climatic cooling led to three cultural dislocations in the Daihai basin, but each was followed by the emergence of a new and more prosperous culture-the Laohushan and Zhukaigou cultures (Tian and Tang, 2001;Xiao et al., 2019). ...
... The significant increase in the sedimentary black carbon concentration indicates an increase in the fire frequency, associated with dryland agricultural practices and the use of fire for heating and food processing (Fig. 3) (Han et al., 2012;Tan et al., 2013). The temperatures at mid-to high-latitudes of the Northern Hemisphere were not substantially different from those during the previous cultural phase, and there was a decrease in the monsoon precipitation (Hou and Fang, 2011;Marcott et al., 2013;Chen et al., 2015b). Against this climatic background, humans were able to migrate and prosper within the region, demonstrating an increase in their ability to both adapt to and transform the natural environment. ...
Article
Full-text available
Beginning in the middle to late Holocene, anthropogenic land cover change has had a profound impact on both regional and continental environments. Hence, assessing temporal and spatial differences in the intensity of human activity in different regions and geomorphologic contexts has become a focus of current global change research. Here we use two representative pollen records from different geomorphologic units in North China (Lake Gonghai in the mountains, and Lake Daihai in a large basin) together with a novel methodology to quantitatively reconstruct Holocene land cover changes. The results indicate diverse vegetation succession patterns in different regions and geomorphologic contexts. In the Daihai basin, the vegetation cover changed relatively little, maintaining values of 45-50%, and only increased during the interval of 8-5.1 ka, when it attained a maximum of 67%. In the Gonghai area, the vegetation cover remained at a higher level at 70-80% for an extended interval, before decreasing substantially to 58% after 1.4 ka. We propose that changes in the intensity of human activities was a major cause of the observed regional disparities in vegetation succession. Comparison of the results with records of prehistoric human activity shows that, prior to 5.1 ka, land cover change (especially of the vegetation composition) in the Daihai basin evolved naturally, under the influence of climate. Then, during ~ 5.1-2.8 ka, a transitional stage occurred, driven by both climate change and human activities. Finally, from ~ 2.8 ka to the present, human activities dominated the pattern of vegetation change. In contrast, land cover change in the alpine Gonghai area was controlled by natural processes until 1.4-1.3 ka, when human activities exceeded the influence of natural variability and became the dominant factor controlling local vegetation development. In larger basin/plain areas, the favorable climatic conditions of the mid-Holocene promoted increased human activity; while later, population pressure, the increased demand for resources, and political factors may have triggered the diffusion of human populations from basins to the mountains or previously undeveloped areas, with resulting effects on their vegetation succession. The anthropogenic impacts have dominated the natural environment of mountainous areas in north-central China for at least the last 1,300 years.
... Past warm climate periods, such as the Holocene thermal maximum (HTM, about 10,000 to 6,000 years BP; Kaufman, McKay, Routson, Erb, Dätwyler, et al., 2020;Marcott et al., 2013;Renssen et al., 2009), show warm conditions due to the much stronger Northern Hemisphere summer solar insolation than during the pre-industrial period (PI) (Berger, 1978). Proxy-based information reveals that this thermal maximum was most pronounced at high latitudes, including Greenland, Western Arctic, and Northern Europe (Gajewski, 2015;Kaufman et al., 2004;Kaufman, McKay, Routson, Erb, Davis, et al., 2020). ...
Article
Full-text available
Plain Language Summary Vegetation‐climate feedback is an essential process between the terrestrial biosphere and the atmosphere. Greening of the Arctic can be invoked by increased solar radiation, for example, during the mid‐Holocene (6,000 years ago), or increased greenhouse gas, for example, ongoing global warming. Here we perform simulations of vegetation and climate interactions during the mid‐Holocene and demonstrate the impact of vegetation‐climate feedback on the Arctic climate. We show that compared to the pre‐industrial period (1850 CE), the Arctic region experiences an expansion of vegetation under a warm climate due to increased solar radiation during the mid‐Holocene. The greening leads to reduced land albedo and enhanced evapotranspiration. These changes and resultant sea ice loss, in combination with associated atmospheric circulation changes, further amplify the Arctic warming. These changes alone are responsible for 0.33°C increase in surface air temperature and 0.35 × 10⁶ km² increase in sea ice loss. Given the facts of observed ongoing Arctic greening in recent decades, our modeling results suggest that the effect of the vegetation‐climate feedback will amplify the future Arctic warming and sea ice loss even more.
... A humid and semi-humid climate due to low evaporation made northeastern China rich in natural resources, providing the Jurchen with the basis for mixed livelihoods based on fishing, hunting, pastoralism, and agriculture (Han, 1999(Han, , 2000. However, a significant cooling in the 11th-12th century in Northern China can be observed in the phenological records and the reconstructed temperature established by paleoclimate proxy records (Zhu, 1973;Yang et al., 2002;Ge et al., 2003;Marcott et al., 2013). For example, the two snow disasters in 1078 CE and 1126 CE were listed as the most serious strong cold wave disasters in the Northern Song Dynasty (Wen and Ding, 2008). ...
... The pre-industrial temperature baseline is expected to be located somewhere between the natural warm and cold anomalies of the MCA and LIA. Modern Andean glaciers' retreat commenced after the peak of the LIA, which (besides the 8.2 k yr event) represents the coldest phase in the global climate evolution of the past 10,000 years (Chambers et al., 2014;Marcott et al., 2013;PAGES 2k Consortium, 2013). ...
Article
Andean glaciers have been shrinking due to long-term climatic warming during the past 100 years. Stuart-Smith et al. (2021) used observations and numerical models to evaluate the anthropogenic contribution to the centennial retreat of the Palcaraju Glacier in the Peruvian Cordillera Blanca. According to their central estimate, the glacier retreat is thought to be entirely the result of the observed 1 ◦C warming since 1880 in this region, of which they consider 85–105% as human-induced warming. However, this attribution must be questioned because the numerical models used by the authors fail to replicate the well-documented Andean temperature and glacier history of the Common Era. In a recent literature synthesis we have demonstrated that Andean glaciers retreated significantly during the Medieval Climate Anomaly (MCA, 1000–1200 CE) when the vast majority of all South American land sites experienced a warm phase, recorded as a near-global natural event, that is not linked with human activity (Lüning et al., 2019a). The MCA was followed by the Little Ice Age (LIA, 1300–1850 CE) when many Andean glaciers advanced significantly, some of them even reaching their maximum Holocene downvalley extension. In contrast, the “hindcast” of Stuart-Smith et al. (2021) erroneously suggests hardly any glacier length fluctuations for pre-industrial times. Given the unsuccessful “hindcast”, we do not consider the attribution results of the study as robust.
... A humid and semi-humid climate due to low evaporation made northeastern China rich in natural resources, providing the Jurchen with the basis for mixed livelihoods based on fishing, hunting, pastoralism, and agriculture (Han, 1999(Han, , 2000. However, a significant cooling in the 11th-12th century in Northern China can be observed in the phenological records and the reconstructed temperature established by paleoclimate proxy records (Zhu, 1973;Yang et al., 2002;Ge et al., 2003;Marcott et al., 2013). For example, the two snow disasters in 1078 CE and 1126 CE were listed as the most serious strong cold wave disasters in the Northern Song Dynasty (Wen and Ding, 2008). ...
Article
Full-text available
Human livelihoods provided a crucial economic foundation for social development in ancient times and were influenced by various factors including environmental change, agricultural origin and intensification, as well as long-distance exchange and culinary tradition. The effect of geopolitical change on human subsistence, especially the shifts between agricultural and nomadic regimes, has not been well understood due to the absence of detailed historical records and archaeological evidence. During the 12th century, the control of the Zhengding area in Hebei Province of north-central China changed from the Northern Song (960–1127 CE) to the Jurchen Jin Dynasty (Jin Dynasty; 1115–1234 CE). Recent excavation of the Zhengding Kaiyuan Temple South (ZKS) site in the area provides a rare opportunity to study human livelihood transformation in relation to geopolitical change. In total, 21,588 charred crop caryopses including foxtail millet, wheat, broomcorn millet, hulled barley, and rice, and other carbonized remains including 55.15 g of boiled foxtail millet and 353.5 g of foxtail millet caryopses were identified, and nine AMS 14C dates of crop remains were obtained from the Northern Song and Jin layers at the ZKS site. This revealed that the dominant plant subsistence transformed from wheat to foxtail millet during the change from the Northern Song to the Jin Dynasties in Zhengding area. By comparing with historical documents and paleoclimate records, we propose that this abnormal shift of primary staple food from the relatively high-yield wheat to low-yield foxtail millet was induced by the traditional dietary preference for foxtail millet in the nomadic Jin society. The Jin government levied foxtail millet as taxation and promoted massive immigration from northeastern China to north-central China to consolidate their rule, which resulted in the adoption of foxtail millet as the most important crop in Zhengding area. The advantage for the cultivation of this frost-sensitive crop in north-central China over northeast China was probably enhanced by notable cold events during the 12th century, while the primary influencing factor for the transformation of human livelihoods in north-central China during that period was geopolitics rather than climate change.
... From about 7000 years ago to the twentieth century, global mean annual temperatures were comparatively stable (figure 2), even if temperatures evolved differently at local to regional scales and for specific seasons. Reconstructions of the Holocene global temperature changes showed a 'Holocene Thermal Maximum' at around 6-7000 BP (Marcott et al 2013, Marsicek et al 2018, Kaufman et al 2020a. However, the recent reconstruction of Osman et al (2021) does not show this feature, and there is an ongoing debate about whether the Holocene Thermal Maximum is an artifact of spatial bias (Osman et al 2021) or reflects a seasonal (summer) signal (Bova et al 2021, Wanner 2021. ...
Article
Full-text available
Recent decades have seen the rapid expansion of scholarship that identifies societal responses to past climatic fluctuations. This fast-changing scholarship, which was recently synthesized as the History of Climate and Society (HCS), is today undertaken primary by archaeologists, economists, geneticists, geographers, and paleoclimatologists. This review is the first to consider how all scholars in all of these disciplines approach HCS studies. It begins by explaining how climatic changes and anomalies are reconstructed by paleoclimatologists and historical climatologists. It then provides a broad overview of major changes and anomalies over the 300,000-year history of Homo sapiens, explaining both the causes and environmental consequences of these fluctuations. Next, it introduces the sources, methods, and models employed by scholars in major HCS disciplines. It continues by describing the debates, themes, and findings of HCS scholarship in its major disciplines, and then outlines the potential of transdisciplinary, “consilient” approaches to the field. It concludes by explaining how HCS scholars increasingly attempt to identify relationships between past climatic and human histories that can inform policy development and activism around anthropogenic global warming.
Preprint
Full-text available
Here we present a newly developed ice core gas-phase proxy that directly samples a component of the large-scale atmospheric circulation: synoptic-scale pressure variability. Surface pressure variability weakly disrupts gravitational isotopic settling in the firn layer, which is recorded in krypton-86 excess (86Krxs). We validate 86Krxs using late Holocene ice samples from eleven Antarctic and one Greenland ice core that collectively represent a wide range of surface pressure variability in the modern climate. We find a strong correlation (r = -0.94, p < 0.01) between site-average 86Krxs and site synoptic variability from reanalysis data. The main uncertainties in the method are the corrections for gas loss and thermal fractionation, and the relatively large scatter in the data. We show 86Krxs is linked to the position of the eddy-driven subpolar jet (SPJ), with a southern position enhancing pressure variability. We present a 86Krxs record covering the last 24 ka from the WAIS Divide ice core. West Antarctic synoptic activity is slightly below modern levels during the last glacial maximum (LGM); increases during the Heinrich Stadial 1 and Younger Dryas North Atlantic cold periods; weakens abruptly at the Holocene onset; remains low during the early and mid-Holocene, and gradually increases to its modern value. The WAIS Divide 86Krxs record resembles records of monsoon intensity thought to reflect changes in the meridional position of the intertropical convergence zone (ITCZ) on orbital and millennial timescales, such that West Antarctic storminess is weaker when the ITCZ is displaced northward, and stronger when it is displaced southward. We interpret variations in synoptic activity as reflecting movement of the South Pacific SPJ in parallel to the ITCZ migrations, which is the expected zonal-mean response of the eddy-driven jet in models and proxy data. Past changes to Pacific climate and the El Niño Southern Oscillation (ENSO) may amplify the signal of the SPJ migration. Our interpretation is broadly consistent with opal flux records from the Pacific Antarctic zone thought to reflect wind-driven upwelling. We emphasize that 86Krxs is a new proxy, and more work is called for to confirm, replicate and better understand these results; until such time, our conclusions regarding past atmospheric dynamics remain tentative. Current scientific understanding of firn air transport and trapping is insufficient to explain all the observed variations in 86Krxs.
Article
Full-text available
An understanding of climate history prior to industrialization is crucial to understanding the nature of the 20th century warming and to predicting the climate change in the near future. This study integrates the complementary information preserved in the global database of borehole temperatures [Huang et al., 2000], the 20th century meteorological record [Jones et al., 1999], and an annually resolved multi proxy model [Mann et al., 1999] for a more complete picture of the Northern Hemisphere temperature change over the past five centuries. The integrated reconstruction shows that the 20th century warming is a continuation to a long-term warming started before the onset of industrialization. However, the warming appears to have been accelerated towards the present day. Analysis of the reconstructed temperature and radiative forcing series [Crowley, 2000] offers an independent estimate of the transient climate-forcing response rate of 0.4–0.7 K per Wm−2 and predicts a temperature increase of 1.0–1.7 K in 50 years.
Article
Full-text available
LJUNGQVIST, F.C. (2011): The Spatio-Temporal Pattern of the Mid-Holocene Thermal Maximum. Geografie, 116, No. 2, pp. 91-110. This article presents a review of the spatio-temporal pattern of the mid-Holocene Thermal Maximum as it occurs in 60 different reconstructions of annual mean temperature from locations around the globe. The geographical coherency of multi-centennial periods with annual mean temperatures at least 1 degrees C and 2 degrees C above the pre-industrial (similar to 1750 AD) equivalents are presented. Although the reconstructions show a heterogeneous temperature pattern for the period c. 10-8 ka BP, a rather coherent period of temperatures exceeding the pre-industrial ones are seen for c. 8-4 ka BP. The onset of the Neoglaciation takes place 4-3 ka BP and cumulates during the Little Ice Age (c. 1300-1900 AD). Overall, our review points towards a more homogeneous mid-Holocene Thermal Maximum than hitherto reported. However, the still limited data coverage, especially for the Southern Hemisphere, restricts the possibility to draw any firm conclusion regarding the amplitude and spatio-temporal pattern of the maximum mid-Holocene warming.
Article
No glacial inception is projected to occur at the current atmospheric CO 2 concentrations of 390 ppmv (ref.). Indeed, model experiments suggest that in the current orbital configuration-which is characterized by a weak minimum in summer insolation-glacial inception would require CO 2 concentrations below preindustrial levels of 280 ppmv (refs 2-4). However, the precise CO 2 threshold 4-6 as well as the timing of the hypothetical next glaciation 7 remain unclear. Past interglacials can be used to draw analogies with the present, provided their duration is known. Here we propose that the minimum age of a glacial inception is constrained by the onset of bipolar-seesaw climate variability, which requires ice-sheets large enough to produce iceberg discharges that disrupt the ocean circulation. We identify the bipolar seesaw in ice-core and North Atlantic marine records by the appearance of a distinct phasing of interhemispheric climate and hydrographic changes and ice-rafted debris. The glacial inception during Marine Isotope sub-Stage 19c, a close analogue for the present interglacial, occurred near the summer insolation minimum, suggesting that the interglacial was not prolonged by subdued radiative forcing. 7 Assuming that ice growth mainly responds to insolation and CO 2 forcing, this analogy suggests that the end of the current interglacial would occur within the next 1500 years, if atmospheric CO 2 concentrations did not exceed 240 ±5ppmv.
Article
High resolution flow speed reconstructions of two core sites located on Gardar Drift in the northeast Atlantic Basin and Orphan Knoll in the northwest Atlantic Basin reveal a long-term decrease in flow speed of Northeast Atlantic Deep Water (NEADW) after 6,500 years. Benthic foraminiferal oxygen isotopes of sites currently bathed in NEADW show a 0.2‰ depletion after 6,500 years, shortly after the start of the development of a carbon isotope gradient between NEADW and Norwegian Sea Deep Water. We consider these changes in near-bottom flow vigor and benthic foraminiferal isotope records to mark a significant reorganization of the Holocene deep ocean circulation, and attribute the changes to a weakening of NEADW flow during the mid to late Holocene that allowed the shoaling of Lower Deep Water and deeper eastward advection of Labrador Sea Water into the northeast Atlantic Basin.
Article
A detailed history of Holocene volcanism was reconstructed using the sulfate record of the European Project for Ice Coring in Antarctica Dome C (EDC96) ice core. This first complete Holocene volcanic record from an Antarctic core provides a reliable database to compare with long records from Antarctic and Greenland ice cores. A threshold method based on statistical treatment of the lognormal sulfate flux distribution was used to differentiate volcanic sulfate spikes from sulfate background concentrations. Ninety-six eruptions were identified in the EDC96 ice core during the Holocene, with a mean of 7.9 events per millennium. The frequency distribution (events per millennium) showed that the last 2000 years were a period of enhanced volcanic activity. EDC96 volcanic signatures for the last millennium are in good agreement with those recorded in other Antarctic ice cores. For older periods, comparison is in some cases less reliable, mainly because of dating uncertainties. Sulfate depositional fluxes of individual volcanic events vary greatly among the different cores. A volcanic flux normalization (volcanic flux/Tambora flux ratio) was used to evaluate the relative intensity of the same event recorded at different sites in the last millennium. Normalized flux variability for the same event showed the highest value in the 1100-1500 AD period. This pattern could mirror changes in regional transport linked to climatic variations such as slight warming stages in the Southern Hemisphere (Southern Hemisphere Medieval Warming-like period?).
Article
Hughen et al. [1998] have documented that during the first 200 years of Younger Dryas time the 14C content of atmospheric CO2 increased by ˜50‰ and that during the remainder of this 1200-year-duration cold event it steadily declined. The initial increase in 14C/C was likely the result of a reduction in the Atlantic's conveyor circulation. However, were the subsequent radiocarbon decline due to the rejuvenation of this potent heat pump, then it is difficult to understand why the climate conditions in the northern Atlantic basin remained cold throughout the Younger Dryas. Modeling exercises by Stocker and Wright [1996], Mikolajewicz [1998], and Schiller et al. [1998] show that if the conveyor is terminated, the transfer of radiocarbon into the deep sea shifts to the Southern Ocean, thereby stabilizing the atmospheric 14C/C ratio. Paleoclimatic evidence from the Antarctic continent suggests that this model-based scenario might have been played out in the real world. While the Younger Dryas cooling has been documented in many places around the world, including New Zealand [Denton and Hendy, 1994], Sowers and Bender [1995], using their 18O in O2-based correlation between the ice core 18O in ice records for Antarctica and Greenland, have demonstrated that in Antarctica the Younger Dryas was a time of maximum warming. The point of this paper is that the steep rise in 18O rise in Antarctic ice which commenced close to the onset of the Younger Dryas might have been caused by heat released to the atmosphere in response to an increase in deep-sea ventilation in the Southern Ocean.
Article
No glacial inception is projected to occur at the current atmospheric CO2 concentrations of 390 ppmv (ref. ). Indeed, model experiments suggest that in the current orbital configuration--which is characterized by a weak minimum in summer insolation--glacial inception would require CO2 concentrations below preindustrial levels of 280 ppmv (refs , , ). However, the precise CO2 threshold as well as the timing of the hypothetical next glaciation remain unclear. Past interglacials can be used to draw analogies with the present, provided their duration is known. Here we propose that the minimum age of a glacial inception is constrained by the onset of bipolar-seesaw climate variability, which requires ice-sheets large enough to produce iceberg discharges that disrupt the ocean circulation. We identify the bipolar seesaw in ice-core and North Atlantic marine records by the appearance of a distinct phasing of interhemispheric climate and hydrographic changes and ice-rafted debris. The glacial inception during Marine Isotope sub-Stage 19c, a close analogue for the present interglacial, occurred near the summer insolation minimum, suggesting that the interglacial was not prolonged by subdued radiative forcing. Assuming that ice growth mainly responds to insolation and CO2 forcing, this analogy suggests that the end of the current interglacial would occur within the next 1500 years, if atmospheric CO2 concentrations did not exceed 240+/-5ppmv.
Article
The feedbacks involved in the response of climate to a reduction of Earth's obliquity are investigated in the GFDL Climate Model version 2.1 (CM2.1). A reduction in obliquity increases the meridional gradient of the annual mean insolation, causing a strengthening of the atmospheric and ocean circulation that transports more heat poleward. The heat transport does not balance the direct obliquity forcing completely, and additional local radiative fluxes are required to explain the change in the equilibrium energy budget. The surface temperature generally increases at low latitudes and decreases at high latitudes following the change in the insolation. However, in some areas, the sign of the temperature change is opposite of the forcing, indicating the strong influence of feedbacks. These feedbacks are also responsible for a decrease in the global mean temperature despite that the change in the global mean insolation is close to zero. The processes responsible for these changes are increases in the ice fraction at high latitudes and the global cloud fraction both of which reduce the absorbed solar radiation. A reduction in the global greenhouse trapping, due to changes in the distribution of the water vapor content of the atmosphere as well as a change in the lapse rate, has an additional cooling effect. Among these feedbacks, clouds and the lapse rate have the larger contribution, with water vapor and surface albedo having a smaller effect. The implications of the findings presented here for interpretation of obliquity cycles in the paleoclimate record are discussed.