Global and Planetary Change 54 (2006) 211 – 215
www.elsevier.com/locate/gloplacha
Abrupt climate change revisited
Wallace S. Broecker ⁎
Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W/PO Box 1000, Palisades, NY 10964-8000 USA
Received 21 August 2005; accepted 15 June 2006
Available online 24 October 2006
Abstract
Taken together, evidence from east Greenland's mountain moraines and results from atmospheric models appear to provide the
answer to a question which has long dogged abrupt climate change research: namely, how were impacts of the Younger Dryas (YD),
Dansgaard–Oeschger (D–O) and Heinrich (H) events transmitted so quickly and efficiently throughout the northern hemisphere and
tropics? The answer appears to lie in extensive winter sea ice formation which created Siberian-like conditions in the regions
surrounding the northern Atlantic. Not only would this account for the ultra cold conditions in the north, but, as suggested by models,
it would have pushed the tropical rain belt southward and weakened the monsoons. The requisite abrupt changes in the extent of sea
ice cover are of course best explained by the turning on and turning off of the Atlantic's conveyor circulation.
© 2006 Elsevier B.V. All rights reserved.
Keywords: conveyor; Younger Dryas; thermohaline; abrupt change; ice cores; radiocarbon
1. Introduction
During the past couple of years one of the key mysteries surrounding the abrupt changes in climate which
punctuated glacial time appears to have been solved. The
mystery has to do with the teleconnection which allowed
the impacts of these events to be so quickly propagated
throughout the northern hemisphere and the tropics. The
breakthrough came about as the result of the combination
of a field study and a model simulation.
2. The Younger Dryas
The field study was conducted in the mountainous
region surrounding Greenland's Scoresby Sound. The
participants (Denton, Alley, Comer) focused their
attention on a set of moraines first studied by Funder
⁎ Tel.: +1 845 365 8413; fax: +1 845 365 8169.
E-mail address: [email protected].
0921-8181/$ - see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.gloplacha.2006.06.019
(1989). Pending additional radiometric dating, Denton
et al. (2005) accepts Funder's conclusion that these
moraines are Younger Dryas (YD) in age. Based on the
highest elevation of lateral moraines, the Denton team
was able to estimate the extent of the associated
snowline lowering and hence the magnitude of the YD
cooling. At first they were puzzled by how small (4° to
6 °C) it was. It was clearly at odds with the 16 °C YD
cooling obtained by Severinghaus et al. (1998) based on
isotopic measurements on N2 and Ar in air bubbles
trapped ice from the Greenland's Summit station. But
Denton et al. (2005) soon realized that while the
Severinghaus et al. estimate yielded the mean annual
temperature, theirs reflected primarily summer conditions. During YD time ablation of mountain glaciers
must have been confined to the warmest months.
Further, as suggested by the oxygen isotopic record
preserved in the Summit ice, little snow fell during YD
winters. Assuming that the 4 to 6 °C cooling reflected
summer conditions, Denton et al. (2005) concluded that
212
W.S. Broecker / Global and Planetary Change 54 (2006) 211–215
Fig. 1. Measurements by Jeff Severinghaus of the isotopic composition of the nitrogen and argon trapped in Summit Greenland ice provide a firm
estimate that the mean annual air temperature was 16 °C colder than today's during the Younger Dryas. In contrast, the lowering of the snowlines in the
mountains surrounding Scoresby Sund suggests only about a 4 °C cooling. The explanation for this large difference appears to be that winters during the
Younger Dryas were so cold that little snow fell in the mountains of eastern Greenland. Therefore the snowline lowering records only summer conditions.
Together, this 4 °C summer cooling and a hypothesized 28 °C winter cooling would yield the mean annual cooling documented by Severinghaus. The red
dots represent the location of the Summit cores and of Scoresby Sund. The ultra cold winter conditions require a climate akin to that in Siberia. This could
only have been possible if the northern Atlantic was totally ice bound during winter months. During glacial time, a shutdown of the Atlantic's conveyor
circulation would have starved the northern Atlantic of ocean-borne heat and permitted sea ice to form. In the presence of sea ice, no heat could escape
from the ocean to the atmosphere. Not shown on the Younger Dryas map are either the glacial ice which covered much of Canada and Scandinavia, or the
expansion of sea ice in the northern Pacific. The purpose of this omission is to focus the readers' attention on the northern Atlantic. The map showing the
boundaries of Arctic sea ice at the end of the winter of 2004 was provided by Gunnar Spreen (Kaleschke et al., 2001).
W.S. Broecker / Global and Planetary Change 54 (2006) 211–215
the YD winters must have been 26 to 28 °C colder than
today (see Fig. 1). In this way, taken together with the
moraine-based summer cooling, the Severinghaus et al.
16 °C mean-annual cooling could be explained. Based
on a review of the literature, Denton showed that while
pollen, beetle and snowline evidence suggested a 4 to
6 °C YD summer cooling in northern Europe,
periglacial features (such as ice polygons) suggested
ultra cold winter conditions. Hence, the idea that the YD
winter cooling must have been far greater than that in the
summer is not new.
There is only one way that winters in Greenland and
northern Europe could have gotten so cold during the
Younger Dryas, namely sea ice must have covered the
entire Norwegian Sea extending perhaps to the southern
end of the British Isles (Fig. 1). This ice cover would
have choked off any heat release from the underlying
ocean and created winter conditions akin to those in
today's Siberia.
This scenario fits well with the idea that the Younger
Dryas was triggered by the catastrophic release to the
northern Atlantic of meltwater stored in proglacial Lake
Agassiz (Kennett and Shackleton, 1975; Broecker et al.,
1989). The resulting dilution of the salt content of the
surface waters in the northern Atlantic would bring to a
halt deep water formation (Rooth, 1982). The combination of a low-salinity cap and a large reduction in the
delivery of ocean heat allowed sea ice to form.
The other half of this breakthrough comes from an
atmospheric modeling study conducted by Chiang et al.
(2003) who showed that, if sea ice was introduced into
the northern Atlantic, the model's tropical rain belt was
shifted southward. This nicely explains the records in
the Atlantic sector of the tropics (Chiang and Koutavas,
2004). Modeling studies by Barnett et al. (1988) suggest
that more extensive and longer lasting snow cover in
213
Eurasia would weaken the monsoons. This would nicely
explain the results from the Arabian Sea sediments
(Schulz et al., 1998; Altabet et al., 2002) and from
Chinese stalagmites (Wang et al., 2001).
Not only do these modeling studies suggest a means
of exporting a signal from the northern Atlantic to the
tropics, but they account for the prompt response to that
signal. Once the delivery of conveyor heat was shut
down by the meltwater flood, the northern Atlantic
could freeze over during the following winter.
3. Heinrich events
As was the case for the Younger Dryas, the sudden
injection of meltwater to the northern Atlantic appears to
have produced a shutdown of the conveyor circulation
after each Heinrich event (Hemming, 2004). The difference is that, in the case of the Heinrich events, the fresh
water was generated in place by the melting of armadas
of icebergs launched into the northern Atlantic from
eastern Canada.
The Chiang et al. (2003) suggestion that freeze-overs
of the northern Atlantic push the tropical rain belt to the
south clarifies something that has puzzled me: namely,
how to account for the difference between the
distribution of Heinrich (H) impacts on one hand and
Dansgaard–Oeschger (D–O) impacts on the other.
Records in eastern Brazil and central Florida record
only H impacts. By contrast, the records in Greenland
ice and that in Cariaco Basin sediment are dominated by
D–O impacts. Falling in between are records from the
Arabian Sea, western Mediterranean sediments and
Chinese stalagmites in which both sets of impacts
appear, with those associated with the H events being
stronger than those associated with the D–O events
(see Table 1).
Table 1
Information regarding the 12 locales shown in Fig. 2
Map No.
1
2
3
4
5
6
7
8
9
10
11
12
Location
Summit, Greenland
Santa Barbara basin
Santa Barbara basin
Alboran Sea, W. Med.
Lake Tulane, Florida
Cariaco basin
Brazil margin
S.E. Brazil Cave
Arabian Sea
Arabian Sea
Hulu Cave, China
South Island, N.Z.
Off Chile
Latitude
71°N
34°N
34°N
36°N
28°N
11°N
4°S
10°S
18°N
23°N
32°N
43°S
42°S
Longitude
40° W
120° W
120° W
3° W
82° W
65° W
38° W
40° W
58° E
66° E
119° E
170° E
76° E
Medium
Proxy
Driver
Reference
Ice
Sediment
Sediment
Sediment
Sediment
Sediment
Sediment
Calcite
Sediment
Sediment
Calcite
Moraine
Sediment
18
Temperature
Temperature
Circulation
Temperature
Rain
Rain
Rain
Rain
Monsoon
Monsoon
Monsoon
Temperature
Temperature
Stuiver and Grootes (2000)
Hendy and Kennett (2000)
Behl and Kennett (1996)
Cacho et al. (1999)
Grimm et al. (1993)
Peterson et al. (2000)
Arz et al. (1998)
Wang et al. (2004)
Altabet et al. (2002)
Schulz et al. (1998)
Wang et al. (2001)
Denton and Hendy (1994)
Ninnemann, pers. comm.
O ice
O Forams
Bioturbation
Alkenones
Pollen
Color
Fe/Ca
18
O
15
N
Org. C.
18
O
Wood
18
O
18
214
W.S. Broecker / Global and Planetary Change 54 (2006) 211–215
Fig. 2. Shown here are the locales of selected paleoclimate records. Those records indicated by circles are dominated by Dansgaard–Oeschger (D–O)
events; those by triangles by Heinrich (H) events. In those indicated by squares, both are present but the H events stand out as more extreme than the
D–O events. The diamonds show sites in the southern hemisphere which appear to follow the deglacial pattern recorded in Antarctic ice rather than
that recorded in Greenland ice. In the locales designated by filled symbols, the record appears to be dominated by temperature changes while at those
designated by open symbols, rainfall changes appear to dominate. Details are given in Table 1.
This geographic pattern might be explained if it is
assumed that the sea ice expansion associated with the H
events exceeded that associated with the cold phases of
D–O events (see Fig. 2). If so, the observation that H
coolings in the western Mediterranean were larger than
those associated with the D–O events could be
explained: more sea ice — greater cooling. Further,
more sea ice following H events would mean a larger
southward push of the tropical rain belt producing Hassociated wet events in currently dry eastern Brazil (see
Fig. 3). The shifts associated with D–O events, while
too small to impact the cave site in Brazil, were large
enough to impact the Caribbean's Cariaco Basin. In
records from China and from the Arabian Sea, both H
and D–O events are recorded. However, the impacts
associated with H events are larger. This could be
explained by the greater magnitude of the northern
cooling and hence larger snow extent in Asia associated
with H events.
This geographic model is also consistent with the
muted response in the south temperate region (i.e., that
part of the globe beyond the reach of the tropic rain belt).
The main impacts in the south appear to be offset from
those in the north perhaps as the result of a seesawing in
the relative strength of deep water formation in the
Southern Ocean relative to that in the northern Atlantic
(Broecker, 1998; Stocker, 1998). Ule Ninneman of the
University of Bergen has results (as yet unpublished)
which clearly demonstrate that off Chile (∼ 42°S) the
Fig. 3. Map showing the locations of the Cariaco Basin and of Wang
et al.'s (2004) eastern Brazil's stalagmite record. Also shown by the
dotted line are the present-day positions of the ITCZ's northern limit
(September) and its southern limit (March). As can be seen, even small
southward excursions of these boundaries would leave the Cariaco
Basin and the adjacent land in Venezuela outside the rain belt. By
contrast, as the stalagmite locale lies to the southeast of the southern
limit of today's ITCZ, a modest southward shift is required to bring
rainfall to the area of these caves. As shown by Wang et al. (2004), this
happens for brief periods at the time of Heinrich events #1 and #4.
W.S. Broecker / Global and Planetary Change 54 (2006) 211–215
temperature during the last deglaciation in both surface
waters and in waters at intermediate depth follows the
pattern seen in Antarctic ice cores. Also, it now appears
that the Waiho Loop glacial advance on New Zealand's
South Island reached a maximum during the latter part of
the Antarctic cold reversal rather than during early in the
YD (Denton, personal communication).
4. Discussion
In this case, as for many scientific discoveries, there is
room for debate as to whom the credit should be awarded. I have already mentioned that a number of earlier
studies suggested that changes in winter temperature
were considerably larger than those for the summer. One
of the reviewers of this paper pointed out earlier studies
that called on the northward transport of heat in the
Atlantic as a means to force a southward displacement of
the ITCZ. It is not the purpose of this paper to pass
judgment in this regard. I will say, however, that the
conjunction of the convincing field evidence presented
by Denton et al. (2005) with the model results of Chiang
et al. (2003) convinced me that a mystery about which I
had long puzzled had been solved.
References
Altabet, M.A., Higginson, M.J., Murray, D.M., 2002. The effect of
millennial-scale changes in Arabian Sea denitrification on
atmospheric CO2. Nature 415, 159–162.
Arz, H.W., Pätzold, J., Wefer, G., 1998. Correlated Millennial-scale
changes in surface hydrography and terrigenous sediment yield
inferred from last-glacial marine deposits off northeastern Brazil.
Quaternary Research 50, 157–166.
Barnett, T.P., Dümenil, L., Schlese, U., Roeckner, E., 1988. The
effect of Eurasian snow cover on global climate. Science 239,
504–507.
Behl, R.A., Kennett, J.P., 1996. Brief interstadial events in the Santa
Barbara basin, NE Pacific, during the past 60 kyr. Nature 379,
243–246.
Broecker, W.S., 1998. Paleocean circulation during the last deglaciation: a bipolar seesaw? Paleoceanography 13, 119–121.
Broecker, W.S., Kennett, J.P., Flower, B.P., Teller, J.T., Trumbore, S.,
Bonani, G., Wolfli, W., 1989. Routing of meltwater from the
Laurentide Ice Sheet during the Younger Dryas cold episode.
Nature 341, 318–321.
Cacho, I., Grimalt, J.O., Pelejero, C., Canals, M., Sierro, F.J., Flores,
J.A., Shackleton, N., 1999. Dansgaard–Oeschger and Heinrich
event imprints in the Alboran Sea paleotemperatures. Paleoceanography 14, 698–705.
215
Chiang, J.C.H., Koutavas, A., 2004. Tropical flip-flop connections.
Nature 432, 684–685.
Chiang, J.C.H., Biasutti, M., Battisti, D.S., 2003. Sensitivity of the
Atlantic inter tropical convergence zone to last glacial maximum
boundary conditions. Paleoceanography 18 (4), 1094, doi:10.1029/
2003PA000916.
Denton, G.H., Hendy, C.H., 1994. Younger Dryas age advance of
Franz Josef Glacier in the Southern Alps of New Zealand. Science
264, 1434–1437.
Denton, G.H., Alley, R.B., Comer, G.C., Broecker, W.S., 2005. The
role of seasonality in abrupt climate change. Quaternary Science
Reviews 24, 1159–1182.
Funder, S., 1989. Quaternary geology of East Greenland, Chapter 13.
In: Fulton, R.J. (Ed.), Quaternary Geology of Canada and
Greenland, Geological Survey of Canada, Geology of Canada,
vol. 1, pp. 743–822.
Grimm, E.C., Jacobson Jr., G.L., Watts, W.A., Hansen, B.C.S.,
Maasch, K.A., 1993. A 50,000-year record of climate oscillations
from Florida and its temporal correlation with the Heinrich events.
Science 261, 198–200.
Hemming, S.R., 2004. Heinrich events: massive Late Pleistocene
detritus layers of the North Atlantic and their global climate
imprint. Review of Geophysics 42, 1–43 RG1005.
Hendy, I.L., Kennett, J.P., 2000. Dansgaard/Oeschger cycles and the
California current system: planktonic foraminiferal response to
rapid climate change in Santa Barbara Basin, ODP hole 893A.
Paleoceanography 15, 30–42.
Kaleschke, L., Lüpkes, C., Vihma, T., Haarpaintner, J., Bochert, A.,
Hartmann, J., Heygster, G., 2001. SSM/I sea ice remote sensing for
mesoscale ocean-atmosphere interaction analysis. Canadian Journal of Remote Sensing 27, 526–537.
Kennett, J.P., Shackleton, N.J., 1975. Laurentide Ice Sheet meltwater
recorded in Gulf of Mexico deep-sea cores. Science 188, 147–150.
Peterson, L.C., Haug, G.H., Hughen, K.A., Röhl, U., 2000. Rapid
changes in the hydrologic cycle of the tropical Atlantic during the
last glacial. Science 290, 1947–1951.
Rooth, C., 1982. Hydrology and ocean circulation. Progress in
Oceanography 11, 131–149.
Schulz, H., von Rad, U., Erlenkeuser, H., 1998. Correlation between
Arabian Sea and Greenland climate oscillations of the past
110,000 years. Nature 393, 54–57.
Severinghaus, J.P., Sowers, T., Brook, E.J., Alley, R.B., Bender, M.L.,
1998. Timing of abrupt climate change at the end of the Younger
Dryas interval from thermally fractionated gases in polar ice.
Nature 391, 141–146.
Stocker, T.F., 1998. The seesaw effect. Science 282, 61–62.
Stuiver, M., Grootes, P.M., 2000. GISP2 oxygen isotope ratios.
Quaternary Research 53, 277–284.
Wang, Y.J., Cheng, H., Edwards, R.L., An, Z.S., Wu, J.Y., Shen, C.-C.,
Dorale, J.A., 2001. A high-resolution absolute-dated Late
Pleistocene monsoon record from Hulu Cave, China. Science
294, 2345–2348.
Wang, X., Auler, A.S., Edwards, R.L., Cheng, H., Cristalli, P.S.,
Smart, P.L., Richards, D.A., Shen, C.-C., 2004. Wet periods in
northeastern Brazil over the past 210 kyr linked to distant climate
anomalies. Nature 432, 740–743.