Reprint - Fachbereich Geowissenschaften
Int J Earth Sci (Geol Rundsch) (2010) 99 (Suppl 1):S171–S189
DOI 10.1007/s00531-010-0553-y
O R I G I N A L P A P E R
Quaternary oceans and climate change: lessons for the future?
Wolfgang H. Berger • Michael Schulz •
Gerold Wefer
Received: 3 November 2009 / Accepted: 20 April 2010 / Published online: 16 May 2010
Springer-Verlag 2010
Abstract There is much interest in ice-age studies in recent decades, in the context of global warming. The relevant findings are these: large regular changes in climate occurred within the last million years, especially in the northern North
Atlantic. Extreme conditions were similar, suggesting strong negative feedback at the edges of the range of variation. The nature of the periods of climate variation suggests orbital forcing by modulation of internal oscillations involving lagged negative feedback on ice buildup. Transitions from cold to warm were rapid and they were not readily reversed, indicating that ice dynamics underlies abrupt climate change. Accelerated rates of ice decay upon warming correspond to a sea-level rise of one to two m/century.
Millennial-scale abrupt disturbances known as ‘‘Dansgaard-
Oeschger’’ and ‘‘Heinrich’’ Events occur when large ice masses are present in the northern hemisphere. They may be considered experiments on the ocean’s response to massive meltwater input. When using results from ice-age studies to project future developments, one must be aware that the future will be largely outside of experience with regard to the recent geologic past. Also, there are as yet no generally accepted explanations for striking changes in the past, such as the rapid rise of carbon dioxide during deglaciation, demonstrating a profound lack of understanding of climate dynamics. This is, so far, the lesson from ice-age studies: they say much about the deficiencies in our level of understanding, and not so much about what is ahead.
W. H. Berger
Scripps Institution of Oceanography, University of California,
San Diego, CA 92093-0244, USA
M. Schulz G. Wefer (
&
)
MARUM and Fachbereich Geowissenschaften,
Universita¨t Bremen, 28359 Bremen, Germany e-mail: gwefer@marum.de
Keywords Quaternary Climate Ice-age
Global warming Future
Introduction
Global warming has been proceeding for more than a century, and questions regarding its causes and future course engender much discussion both within the community of Earth scientists and in public discussions. To many researchers in this field, the rapid retreat of mountain glaciers across the globe is the most striking manifestation of the warming (Fig.
). Glaciers holding information from the early Holocene and beyond are now melting, signifying that the present warming is unprecedented on a time scale of thousands of years (Thompson et al.
).
The rise of sea level that corresponds to the melting of mountain glaciers (Braithwaite et al.
; Raper and
Braithwaite
) is rather modest, compared with the expansion of the water column as the ocean warms
(Levitus et al.
). It is negligible, with regard to the potential rise of sea level from the rapid decay of ice masses in Greenland and Antarctica. This problem of rapid decay (frequently referred to as ‘‘collapse’’) is central to the global warming conundrum, and it is here that geological studies can make crucial contributions. Geophysical processes involving abrupt change (such as ‘‘collapse’’ of a reservoir) are manifestations of threshold effects, which are difficult to treat from first principles. Invariably, we have to turn to experience and statistics to judge the risks; that is, we have to consider history.
The assessment of the risks of sea-level rise involves the response of polar ice caps to warming, and it involves the redistribution of heat and moisture on the planet, and the feedbacks associated with such redistribution.
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S172 Int J Earth Sci (Geol Rundsch) (2010) 99 (Suppl 1):S171–S189
Fig. 1 Rapid retreat of mountain glaciers in the Canadian Rockies: tongue of the Columbia Ice Field, Banff/Jasper National Park. Fresh morainal deposits cover much of the valley below the remnant ice tongue. They are underlain by ice in places. (Photo W.H.B. 2006.)
Fig. 2 Effects of weakening of the trade winds as seen in sea-level anomalies during the El Nin˜o event of winter 1997. (Adapted from
K }
; data from NOAA). Note the rise of sea levels along eastern boundaries ( orange ), along with a slowing of the turning of the great central gyres, especially in the northern hemisphere ( blue ).
Eastern boundary upwelling is greatly diminished
The experience that can tell us how fast sea level can rise is contained in the marine sediments that accumulated during the last million years. Without doubt, therefore, the main contribution of marine geology to the global warming discussion is to provide constraints on the rise of sea level, along with its various implications for abrupt changes in climate.
In addition, the study of Quaternary sediments has much to offer regarding changes in the ocean’s circulation, the recycling of nutrients, and the response of the ocean’s productivity to cooling and warming. Productivity patterns are part of the marine carbon cycle, which controls the partitioning of carbon dioxide between ocean and atmosphere. Since the present ocean normally holds roughly sixty times more carbon dioxide than the atmosphere
(Sundquist
), modest changes in the holding capacity of the sea have large effects on the content of greenhouse gas in the air.
These various items of vital interest—sea-level change, heat transport, carbon cycle, productivity—are all poorly understood for the present ocean. This is not owing to a shortage of data (data banks are massive and growing) but because of a lack of control on dynamics and sensitivity to disturbance. For example, while the overall patterns of a positive ENSO event are well mapped by satellite observation (Fig.
), the basic physical causes are still in doubt
(Wang and Picaut
2004 ). Apparently, trade winds weaken
and fail to pile up warm water in the western equatorial
Pacific, as suggested by Bjerknes ( 1969
) some time ago. In turn, a weakening of the trade winds leads to a weakening of upwelling regimes in eastern tropical regions. A connection between strength of trade winds and intensity of eastern boundary upwelling is also seen when reconstructing productivity patterns for glacial and interglacial periods (Berger et al.
123 correspondence is encouraging in that it supports the idea that there are large-scale patterns of ocean behavior that reappear across time scales from millennia to decades
(Fedorov et al.
). Clearly, as suggested at the very outset of paleo-oceanographic studies (Arrhenius
the strength of zonal winds is a crucial item in tracking the behavior of the ocean’s climate system.
In this essay, we attempt to summarize those results of
Quaternary research that are most relevant to the questions arising in the context of global warming. We emphasize marine geologic studies, because that field is the one we are most familiar with, from research projects pursued and conferences convened within the last two decades (Berger
; Berger et al.
,
;
Wefer et al.
,
,
,
). We proceed from a defense of the historical approach, to the nature of ice-age fluctuations, to lessons contained therein with respect to global warming.
History matters
Geologically based opinions rest on three claims: (1) What happened can be reconstructed, (2) what happened must be possible, and (3) the response to forcing observed is typical; that is, it repeats for similar events. The first claim rests on the quality of proxy reconstructions. The second acknowledges that models tuned to modern observations may not be able to capture the full range of reconstructed climate variations. The third relies on repetition, that is, on the application of statistics.
The lack of repetition that characterizes the instrumental period of climate research (roughly the last 150–100 years) is a major problem. It means that the range of variability
Int J Earth Sci (Geol Rundsch) (2010) 99 (Suppl 1):S171–S189 S173 measured by instruments may not be relevant to the mode of operation of the system that is being explored. The result is that much uncertainty attaches to the output of relevant models that create scenarios beyond known variability. By adjusting the models to deal with history, their performance can be greatly improved (see examples cited in IPCC
).
The systematic study of climate change began with geological insights, and notably with contemplating the enormous climate changes associated with the ice ages
(Croll
; Penck and Brueckner 1901–1909; Ko¨ppen and
Wegener
; Brooks
1926 ; see summary in Schwarzbach
, p. 6). The concept of ‘‘climate,’’ in meteorology, was originally meant to denote a mean state of weather fluctuations around seasonal and interannual variability. What geologists showed is that the ‘‘mean state’’ is a function of the time span considered.
An important result of this pioneering work spanning many decades is the map showing the great expanse of ice that dominated northern regions some 20,000 years ago
(Fig.
3 ; for more recent reconstructions see Clark et al.
). Enormous climate changes are implied. The very existence of ice sheets built over tens of thousands of years indicates self-stabilization through feedback from the ice caps and from a lowered sea level, while their relatively rapid disappearance points to positive feedback in decay. The repeated buildup and decay emphasizes the great sensitivity of high northern latitudes to climate change (for background see Jackson and Broccoli
2003 ). That this high sensitivity
holds for short time scales, as well, has been amply verified by observation of temperature trends in the last century
(Jones et al.
) and by modeling (Cubash et al.
The key feedbacks that amplify change in the region are the reflectivity of the ground and the moisture in the air, factors that were discussed more than a 100 years ago by the geologist James Croll and the naturalist Alfred Russell
Wallace. Wallace, for example, wrote as follows (
p. 157): … the increased heat of summer could not be in any way stored up, but would be largely prevented from producing any effect, by reflection from the surface of the snow and by the intervention of clouds and fog …
Reflectivity (albedo) is now generally recognized as the dominant feedback factor. The net contributions of clouds and fog, although clearly important, are less obvious and are difficult to quantify (cf. Soden and Held
The systematic reconstruction of ice-age climates based on microfossils in deep-sea sediments goes back to the pioneering work of Schott (
Meteor Expedition
(1925–1927) and especially to the scientists working on the long cores collected by the Albatross during the Swedish
Deep Sea Expedition (1947–1948) (Arrhenius
;
Phleger et al.
; Olausson
) (Fig.
The piston coring device used in the Albatross Expedition, invented by the oceanographer and engineer Bo¨rje
Kullenberg, turned out to be the time machine needed to penetrate deeply into the detailed history of the Quaternary.
The temperature preferences for planktonic foraminifers became central to the deciphering of the climate record, as did the isotopic analysis of foraminifers introduced by
Emiliani (
1955 ) and first applied by him to cores from the
Albatross expedition, as well as to cores from Lamont
Geological Observatory, collected in the 1950s. Like all fossils, foraminifers are not measurement devices but rather ‘‘biased reporters’’ (Wefer and Berger
their biases are recognized, however, they give extremely useful information.
The 1970s saw the first serious attempt to quantify global ice-age conditions on the basis of microfossils
(Imbrie and Kipp
This opened a path to the possibility of testing the skill of emerging climate models against real-world situations whose patterns were not included already within the models. The approach has proved very valuable, especially as the improvement in the reconstructions provided increasingly reliable targets (COHMAP Members
;
GLAMAP 2000; Sarnthein et al.
; MARGO Project members
).
Fundamental concepts of ice-age narratives include the observed range of conditions, and the rates at which they change. Especially in the context of the ongoing climate change, it is clear that the transitions from one state to another provide the focus of interest, and not equilibrium or quasi-equilibrium states.
All such questions imply the use of a detailed time frame, and detailed correlation between different records.
The necessary stratigraphic control relies heavily on the oxygen isotope analysis of foraminifers (Fig.
). In addition, the various aspects of past environments are extracted from a host of proxies, including many types of microfossils, and physical and chemical properties of sediments
(see Fischer and Wefer
Pursuing a climate framework relevant to human affairs demands detailed time resolution (tree rings, lake sediments, ice cores, varved sediments in the oxygen minimum zone, for example). Strikingly high rates of change in temperature and atmospheric CO
2 have been discovered in ice cores from Greenland (e.g. Dansgaard et al.
;
Grootes et al.
; Steffensen et al.
Antarctica (Petit et al.
1999 ; EPICA Community Members
;
spheric carbon dioxide concentration during terminations is one such much discussed change (Fig.
posed by the discovery of rapid change in climate and in atmospheric composition have not been met, so far, by generally accepted explanations. Global explanations, of
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Fig. 3 Approximate land and sea ice cover in the northern hemisphere, during glacial time.
Int J Earth Sci (Geol Rundsch) (2010) 99 (Suppl 1):S171–S189 course, will have to be based on evidence from marine sediments, in addition to that from ice cores. Also, an important role will remain for studies on sediments with intermediate resolution, which are widespread (in contrast to high-resolution records).
Perhaps the most serious drawback, when using insights from Quaternary geology in building projections for the future, is that the future has no analog in the past. Both the concentration of greenhouse gases and their rates of increase are unprecedented in the Quaternary, as far as one can tell from the record. One has to go back more than 20 million years in Earth history to encounter atmospheric
CO
2 reconstructions as great as concentrations expected in the next centuries (Zachos et al.
geography (land–sea distribution, elevation of mountain ranges) differed substantially from the modern pattern.
This renders the interpretation of potential analogs quite difficult.
In what follows, we discuss major elements of the changing climate system, for the last million years, with a view toward their relevance for the ongoing discussions about global warming. We begin with topics in geophysics
(nature of ice-age cycles, patterns of sea-level change, heat transport, abrupt change) and then move to aspects of the carbon cycle, which is intimately linked to a number of other geochemical cycles, through the various controls on productivity of the ocean.
General aspects of the ice-age record
Concerning climate change of the last million years, two iconic images stand out: (1) The maximum extent of ice cover in the northern hemisphere (Flint
; Imbrie and
Imbrie
) and (2) the sequence of oxygen isotopes in foraminifers from deep-sea sediments (Emiliani
;
Shackleton and Opdyke
et al.
; Berger and Wefer
; Mix et al.
et al.
). The sequences contain periodicities related to changes in astronomically driven insolation patterns, as emphasized by Emiliani
), in accord with expectations based on arguments
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Fig. 5 The planktonic foraminifer Globigerinoides sacculifer (adult form with a special final chamber). This is one of the best known of all species used in marine Quaternary research, as the main carrier of oxygen isotope information. (SEM image S.I.O.; diameter ca.
400 l m)
Fig. 4 The Swedish research vessel Albatross on its famous coring expedition (1947–1948). Adapted from Pettersson (
) presented by Ko¨ppen and Wegener (
(
).
As mentioned in the introduction, the extent of northern hemisphere ice sheets during glacial times was enormous
(Fig.
3 ). In fact, these ice masses of approximately
20,000 years ago, now vanished, exceeded the ice masses of present-day Antarctica, whose ice volume represents a potential sea-level change of 70 m. Antarctica remained largely glaciated throughout the ice-age cycles owing to its being centered on the South Pole. The general cooling started 50 million years ago. Three million years ago it reached the appropriate threshold for allowing ice buildup in the northernmost land areas, well away from the pole.
(For background and references on planetary cooling, see
Zachos et al.
2001 , 2008 ). That the northern ice sheets
could grow at all, and maintain themselves over tens of millennia, is a function of positive feedback from albedo, greenhouse effect, and dust in the atmosphere, within the general framework of a cooling planet.
When dark ground is covered by snow and ice, there is great amplification of the change toward cold conditions, and the same is true for the sea, which is dark when open, and reflects light when covered by sea ice. Also, cold air holds little water vapor, which is the most important of the greenhouse gases. The ice cover, then, makes its own climate, which tends to stabilize the ice.
The high sensitivity of northern circumpolar regions to climate change is the basic reason that Milankovitch forcing, when calculated for summer insolation in high northern latitudes, correlates well with major patterns in the ice-age record. When Wladimir Ko¨ppen proposed 65 8 N as the most sensitive latitude to insolation forcing (Milankovitch
p. A138), he had the subarctic feedbacks in mind. Ko¨ppen’s assessment is supported by modern observations. Recent global warming patterns at the surface suggest enhanced sensitivity to current CO
2 forcing at latitudes near 60 N (see
Figure. 3.5 in IPCC
2007 ). The present rapid decrease in
sea ice cover in the Arctic (Stroeve et al.
) attests to the fact that this subarctic feedback extends farther north, as well.
The high sensitivity of the subarctic climate system provides the amplification to outside forcing that reverberates throughout the global climate. Other highly sensitive elements of the system exist as well, of course. Thus, the albedo of large land areas in the subtropics changes substantially, depending on the balance of precipitation and evaporation. Desert surfaces tend to be highly reflective
(and also provide dust in air that reflects the sunlight), while vegetation cover is dark. Deserts invaded forested areas during glacials (e.g., Dupont
123
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Fig. 6 Antarctic temperature evolution relative to modern values and atmospheric greenhouse-gas concentrations over the past 800,000 years.
Data from ice cores drilled in
Antarctica (after Lu¨thi et al.
240
220
200
180
300
0
280
260
100 200
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300 400 500 600 700 800
3
0
-3
-6
-9
-12 800
700
600
500
400
300
0 100 200 300 400 500
Age [1000 years before present]
600 700 800 ground albedo and dust load. Regarding the change from plant cover to no cover in desert regions, it is thought that it can be very rapid, due to positive feedback from the trapping of moisture by the vegetation or the lack thereof
(Claussen
In addition to increased albedo and dust load during glacial periods, there is the diminished greenhouse effect from a decrease in carbon dioxide and water vapor in the atmosphere. Together, these three types of feedback— albedo, greenhouse, dust—lock the planet into its glacial state, until the mountains of ice become unstable and decay, in response to favorable orbital conditions, giving way to the interglacial regime.
The fact that the CO
2 content of the glacial atmosphere is greatly reduced relative to warm conditions still awaits a succinct explanation. In the meantime, there is no shortage of hypotheses (see Archer et al.
). In any case, the large size of the carbon reservoir of the ocean together with changes in the storage capacity of the sea and the exchange with the atmosphere will eventually provide the answer, as initially suggested by
Broecker (
1982 ). However, it is unlikely that a single cause
will strongly dominate: the marine carbon cycle is highly complex (see Archer et al.
).
The single most important type of marine record offering evidence for rates of change and for the amplitudes of past climate excursions is provided by oxygen isotope analysis of planktonic and benthic foraminifers. In addition, this type of record is the chief tool for correlation and for age assignments in deep-sea sediments, based on signal matching. A number of ‘‘standard’’ isotopic curves are available for the purpose, starting with the SPECMAP
scale of Imbrie et al. ( 1984 ), which is valid for the last
650 kyr. For the time before that, the readjustments to the
Quaternary time scale of Shackleton et al. (
be considered.
The isotopic record shown in Fig.
is based on the compilation of Zachos et al. (
year intervals (Berger
2009 ). Two distinct periods are
readily apparent within this record: the early Pleistocene, beginning near 1,800 thousand years ago and lasting to about 900,000 years ago, and the late Pleistocene, from
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900,000 thousand years ago to the Holocene. Near
900,000 years ago, there is a shift to a regime with higher average index values, from a mean of 3.8 to one of 4.1. The
‘‘heavier’’ isotopes indicate colder conditions and greater ice mass. The change in mean is accompanied by a shift to larger amplitudes and longer waves in the climate fluctuations. The event is referred to as the ‘‘Mid-Pleistocene
Climate Shift’’ (Clark et al.
‘‘Milankovitch Chron’’ comprises the last third of the
Quaternary. It is characterized by the familiar ca. 100-kyr periods, which are commonly referred to the changing eccentricity of the Earth’s orbit. The chron bearing the 100kyr cycles happens to be virtually congruent with the time span originally studied by Milankovitch when proposing the hypothesis that ice-age fluctuations are linked to the changing summer insolation in high northern latitudes
(Milankovitch
).
The cause for the Mid-Pleistocene climate shift is not known. Since the shift is permanent, it probably has to do with changing the framework conditions on the planet, not with the orbital forcing per se or with feedbacks from circulation and other reversible factors. A general threshold important for ice buildup was crossed (for example, in the uplift of mountains) during a time of cool northern summers (beginning about 950 kyr ago; von Dobeneck and
Schmieder
, p. 618) and in consequence the ice mass grew by roughly 20 percent. After that, the climate system never returned to its former mode of operation.
Fig. 7 Ice-age cycles of the last two million years, shown in the d
18
O values of benthic foraminifers from deep-sea sediments, as compiled by
Zachos et al. (
) and resampled for 1-kyr resolution.
a Time series. Numbers are marine isotope stages ( MIS ).
The means differ between early and late Pleistocene; a shift took place near 900,000 years ago.
b Nature of the Mid-Pleistocene climate shift regarding the periodicity of climate fluctuations, based on Fourier analysis. A cycle near
41,000 years is present in both early and late section, but a
100,000-year cycle strongly dominates in the last third of the
Pleistocene
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In general, the fluctuations in the early Pleistocene are dominated by a cycle near 41 kyr, which persists. But after the shift, a 100-kyr cycle becomes dominant (Ruddiman et al.
,
The shift indicates that the apparent size of the sun
(governed by the changing Earth-Sun distance, hence eccentricity) becomes important when the northern ice sheets are large. Presumably, obliquity alone can no longer do the job of melting the ice and that makes the precession effect increasingly important. A host of mechanisms have been put forward, some quite complex, to explain the shift in periodicity between early and late Pleistocene. Basically, it seems to us, longer periods of fluctuation win out over shorter ones because larger ice sheets have more inertia.
The shift appears because the available periods that interact with the oscillating system are few and are fixed. Thus, as the potential for ice buildup grows, the system adjusts its main oscillation to the most compatible of the available periodicities, moving from 40 to * 80 to 100 kyr (Mudelsee and Schulz
The message of the Great Shift in the central Quaternary is that 900,000 years ago the system crossed a threshold and changed its response to forcing, in ways that would have been difficult or impossible to predict from previous experience.
The topic of unexpected events brings us to that aspect of ice-age fluctuations that is at the same time among the most vexing and the most important issues: the rapid decay of large ice fields. The importance of these events has been recognized for some time (Broecker and van Donk
While the timing of these ‘‘terminations’’ is somewhat predictable owing to a link to orbital forcing (Paul and Berger
1997 ; Raymo 1997 ; Lisiecki and Raymo
the mechanisms and rates involved are as yet poorly understood.
The 100-kyr riddle enigma (which includes the termination riddle) has two aspects. The first is that eccentricity alone has very little power within the Milankovitch paradigm. Eccentricity becomes effective solely through the modulation of seasonal contrast through precession. The second is that the smallest compatible multiple for the precessional periods (23.7 and 19.0) is 95 (factors of 4 and
5, respectively), so 95 is the period we should expect to see in the record if precessional forcing is important. The problem was recognized and investigated by Muller and
MacDonald (
1997 ), who insisted that eccentricity cannot
explain a 100-kyr cycle, and proposed that it should be replaced with a different orbital parameter.
Before taking such a step, it would be well to gain a more complete understanding of the ice dynamics involved in making the 100-kyr cycles. Ice dynamics, we think, are central to the question of the nature of the terminations that define the 100-kyr cycle. Much has been written about the
123 last termination, which includes a cold spell at the halfway point, when ice ceased to melt for about a 1,000 years. The event is well recorded in deep-sea sediments (Jansen and
Veum
). The pause in deglaciation is known as the
‘‘Younger Dryas’’ period (YD), after periglacial deposits in
Denmark, which contain remains of the Arctic flower
Dryas octopetala . The onset and end of the Younger Dryas period are clearly defined in the oxygen isotope stratigraphy of ice cores on Greenland (see Fig.
).
The most striking and significant aspect of the pattern discovered by studying ice core records from Greenland is the fact that the climate is highly unstable during glacial conditions, but is quite stable during the Holocene (Alley et al.
2003 ). The contrast signifies that the northern ice
masses were themselves a source of instability when large
(Schulz et al.
). We do not know what made them so; perhaps it had to do with ice masses grounded well below sea level. Such masses exist today in West Antarctica, and it has been suggested that they are vulnerable to warming and sea-level rise. The ‘‘Heinrich’’ events (named for
Heinrich’s
documentation of layers of ice-rafted debris in North Atlantic deep-sea sediments) are thought to indicate sporadic invasions of enormous fleets of icebergs into the North Atlantic (Bond et al.
Oeschger events are sudden and sustained warm spells on
Greenland, which apparently followed the release of ice and meltwater to the sea. If so, the question arises why such release should lead to warming, since presumably it interferes with deepwater production, which helps drive heat import into the Nordic realm (e.g. Sarnthein et al.
).
The warm period immediately preceding the YD
(Fig.
) is the first step of deglaciation; it initiates the transition into the present interglacial, the Holocene.
Explaining the cold spell that followed this initial largescale melting has been a major challenge. One often-
Fig. 8 Nature of the transition between the last glacial and the
Holocene as seen in the Greenland ice record. YD, Younger Dryas cold spell. Note the instability of glacial conditions (Dansgaard-
Oeschger events, Heinrich events), compared with the stability of the
Holocene. Adapted from Ganopolski and Rahmstorf (
Int J Earth Sci (Geol Rundsch) (2010) 99 (Suppl 1):S171–S189 S179 invoked scenario is a slowdown of the production of North
Atlantic Deep Water (McManus et al.
), based on the concept that NADW production is a vital link in the heat transport from low latitudes to high ones in the North
Atlantic (Broecker and Denton
). At some point, this slowdown scenario became the basis for warnings regarding the risk of interfering with the northward transport of heat in the North Atlantic as the planet warms (Broecker
).
We propose that the problem is not to explain why there was a cold spell at a time when enormous ice sheets still covered Canada and Scandinavia. Cold weather is the expectation for such conditions. The problem is to explain how the Boelling/Alleroed—the warm period preceding the Younger Dryas—started in the first place, and how it maintained itself for a 1,000 years during a period of rapid sea-level rise and freshwater input, which presumably did impact circulation in the manner postulated by Broecker and Denton (
1989 ) along the lines of the shutdown
hypothesis.
Apparently, contrary to the shutdown concept, melting of ice and concomitant freshwater input set in motion positive feedback mechanisms that favored additional melting. If so, one has to explain what stopped the melting, not what caused the cooling after it stopped. One possibility is that the system ran out of vulnerable ice that could be melted given the comparatively weak orbital forcing available to do the job (Berger and Jansen
). Other
possibilities exist. Thus, Schulz et al. ( 2002
) suggested that the Boelling/Alleroed is in fact nothing but a Dansgaard-
Oeschger event that was instigated by the declining ice volume during the last deglaciation. A corollary of this hypothesis is that Younger Dryas type events should have occurred in earlier deglaciations. While there is some evidence for such occurrence in the penultimate deglaciation (Sarnthein and Tiedemann
; Lototskaya and
Ganssen
; Siddall et al.
), some studies suggest the opposite (Carlson
). This question is unresolved.
The main point in the story concerning the transition from glacial to postglacial conditions is that there is unstable ice and stable ice, and that we cannot readily tell the difference before the unstable ice shows its true nature by rapidly wasting. The uncertainty is relevant in the context of the future response of the West Antarctic ice sheet to global warming (Oppenheimer and Alley
vary somewhat. Fifteen years ago, in a report sponsored by the U.S. National Academy of Sciences, Revelle et al.
) concluded (p. 4) the ‘‘one hundred years from now it is likely that sea level will be 0.5 to 1 m higher than it is at present.’’ The data we have from the Quaternary record identify this assessment as being realistic to conservative.
To obtain estimates on possible rates of sea-level rise from the wasting of polar ice, we analyzed the oxygen isotope record shown in Fig.
years as compatible with the present workings of the climate system. Linking the oxygen isotopic record to sealevel change is a difficult undertaking and fraught with quite large error bars (e.g., Matthews
For calibration, we use the usually considered 120 m of sea-level rise since the last glacial maximum (Shepard
; Curray
; Alley et al.
) and relate it linearly to the observed range in the oxygen isotope record, over the same time span (Berger
) (see
Fig.
). The assumption is that the change in the isotope values is largely driven by changes in the volume of continental ice, and that the residual that is not so driven runs parallel to the main driving factor. The error is thought to be modest (less than ten percent; Sima et al.
).
The range of isotopes over the relevant interval may be taken as 1.5 permil (Fig.
7 ). Thus, when compared with a
sea-level rise of 120 m, a change of isotope values of
0.125 permil corresponds to a change in sea level of 10 m.
What is of interest in the present context are historical conditions similar to the Holocene, that is, roughly the top
5 percent of the 1000 isotope values defining the last million years in 1-kyr steps. In the data set at hand, we use a cutoff of \ 3.4 permil to obtain the relevant subset. For this set of interglacial values, we compile the rates of change (in permil per kyr) between neighboring points to get the pattern of abundance distributions (Fig.
Rates of change of sea level
There is no question that sea level is rising, a fact that is well appreciated by geologists studying the flooding of
Venice, in a region that is sinking (see e.g. Seibold and
Berger
1996 , p. 152). Estimates regarding the global effect
Fig. 9 Abundance distribution of rates of change of oxygen isotope values in Holocene conditions during the last million years. Fit of histogram ( gray area ) by eye. Data are those of Zachos et al. (
resampled (Fig.
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S180 Int J Earth Sci (Geol Rundsch) (2010) 99 (Suppl 1):S171–S189
The rates of change associated with a rise in sea level
(marked positive) have some large values, with (20% of rise values showing a rise of more than 1 m per century. In accordance with Rohling et al. (
suggests that a rise of 1 m/century would hold no surprise.
This provides support for assessments calling on at least
* 1 m of sea-level rise by the end of the present century
(Rahmstorf
; Grinsted et al.
somewhat higher than the estimate of Kopp et al. (
who, based on an analysis of past sea-level changes, consider a 56–92 cm/century range for sea-level rise as likely.
The asymmetry in the patterns of rising and falling sea level reflects the fact that large ice masses involved have a component of instability, as mentioned above. According to the isotope record, this instability is potentially greatest during those times of deglaciation when one-third of the vulnerable ice mass is left to melt (that is, when sea level is near 40 m).
The cause of instability is fairly obvious: (1) ice masses contain gravitational energy, which turns into heat when ice flows downhill, (2) heat transfer into the interior of an ice sheet, by meltwater from the surface percolating into cracks where it can refreeze, is a one-way street (downward only), and (3) meltwater can soften the ground below the ice or form puddles and lakes, which will decrease friction at the base and favor downhill gliding. The point is, together with isostatic sinking under the ice load (which brings the base and rims of ice sheets into vulnerable positions below sea level), there are several mechanisms available to accumulate instability through time, until a threshold is reached for rapid deglaciation.
Detailed quantitative treatment of the various factors involved is complicated, and a matter of much discussion
(for background, see Denton and Hughes
; Oerlemans and van der Veen
; Van der Veen
; Alley et al.
). Nevertheless, some general patterns of ice growth and decay, within the Milankovitch chron, readily emerge from the abundance distribution of sea-level positions.
The main feature of the histogram describing the distributions (Fig.
10 ) is that 90 percent of the range is
between sea-level positions of 10 and 115 m. Between these border values, the likelihood of positions seems to stay roughly the same; there is no indication of a central tendency. This suggests that positive feedback enhances movement away from the center. Near the edges of this quasi-rectangular distribution, negative feedback starts to exceed positive feedback, creating a threshold condition.
Occasionally, the threshold is exceeded, perhaps aided by stochastic resonance in times of unusually warm summers
(cf. Benzi and Sutera
1985 ). Apparently it is possible, once
a threshold is crossed, for the sea level to move into an extreme position (marked ‘‘x-warm’’ in Fig.
), with its own quasi-rectangular distribution. The pattern, if read
123
Fig. 10 Histogram of sea-level positions, based on the oxygen isotope data of Zachos et al. (
), transposed by assuming a typical glacial-postglacial range of 120 m. Note the extreme position of present sea level correctly, suggests that there is much reluctance of the system to move into a sea-level position higher than
20 m, but that, once the present position is reached, the probability of moving up or down is roughly the same.
Obviously, this is not a state of affairs that favors stability of sea level, upon strong warming.
A more welcome message from this pattern is that it seems to be very difficult indeed to move sea level upward beyond a position 10 m above the present (Berger
There remains, however, a risk that if future greenhousegas concentrations stay high for a sustained period of time that the existing ice sheets may become unstable (Hansen et al.
For times of extreme cold (sea level at 120 m and lower) we do not see the type of quasi-rectangular distribution seen in the x-warm range. Apparently, building up additional ice, once the sea level is below 115 m, is quite difficult. The issue presumably is one of suitable space; that is, of areas with cool summers and at an elevation high enough to bear snow and ice.
The record (Figs.
,
) suggests that the usual discussions about the relative importance of factors involved in the recent modest sea-level rise (summarized in Warrick and Oerlemans
2001 ) will turn out to be irrelevant to the
problem of major sea-level rise. For high rates one must assume that the dynamics of ice sheet collapse and meltwater effects in the sea will dominate the situation. It is clear that studies of the role of expansion of seawater or of the input from melting mountain glaciers can teach us much about the workings of the present climate system and about the physics of glacier response to warming. However, once serious melting starts such knowledge will be of limited use.
The overall message from this analysis of the available deep-sea data is that, seen against the ice-age background, negative feedback dominates the climate system near the
Int J Earth Sci (Geol Rundsch) (2010) 99 (Suppl 1):S171–S189 S181 boundary conditions (starting near 20 m, in the present context). This helps keep the system stable. However, in the peculiar Holocene-type condition (extra warm, 10 to
?
10 m), there is no preference attached to any one sealevel position: the sea level can go up as easily as down, by several meters. Does a rapid sea-level rise cause additional rise? Apparently this is so, during deglaciation. However, the Holocene is inherently stable, within narrow limits. To address the question whether a rise in Holocene-type conditions stimulates further rise, we check how many of the rise events during Holocene-type conditions (Fig.
were followed by additional rise. The answer is 18 out of
41 pairs, slightly less than a 50/50 expectation. Thus, the answer, on the thousand-year scale, is ‘‘not necessarily.’’
But to answer the question on the century scale, which is relevant to human affairs, we would have to have much more detailed information on the sea-level fluctuations of the past. Encouraging steps have been taken in that direction (Siddall et al.
; Rohling et al.
et al.
; Kopp et al.
North Atlantic heat piracy
Fig. 11 Surface currents in the Atlantic and associated temperature anomalies (W, warm; C, cold) as drawn by Dietrich et al. (
Note the cross-equatorial flow of warm surface water and the role of
Gulf Stream transport in bringing warm water from the subtropics to high latitudes
It has been suggested that global warming might lead to a decrease in the heat supply to the northern North Atlantic, because of a slowdown or shutdown of North Atlantic
Deep Water (NADW) production (e.g., Broecker
When considering deglaciation, however, we found no evidence in the record to support this idea. There is no question that heat transport to the northern North Atlantic is important, of course. That heat imported from far to the south warms northern Europe is well appreciated for more than a century (Croll
; Wallace
) and has been an integral part of introductory oceanography for many decades (Sverdrup et al.
; Dietrich et al.
). The possibility that climate change impacts such transport of heat definitely is a concern.
The topics of changes in thermohaline circulation and implications for heat transport have long intrigued physical oceanographers (Stommel
,
raphers (Duplessy et al.
; Crowley
; McManus et al.
; Gherardi et al.
recently, these topics have generated much interest in modeling, as computing power has grown (e.g. Marotzke
; Rahmstorf
; Paul and Scha¨fer-Neth
Traditionally, the Gulf Stream is cited as a crucial element in the heat transport system, especially with a view to subarctic regions in the North Atlantic, and this connection is deeply engrained in popular thinking about climate (e.g., in describing the warming of the coast of Norway). This is not entirely wrong (Fig.
11 ), although the nature of the
Gulf Stream as part of a wind-driven gyre implies that most of the water masses involved soon return south, rather than moving northward (Stommel
water masses, however, does move north to replenish the waters used in NADW production.
The basic element of North Atlantic heat piracy is the transport across the equator, by both winds and currents, and the general transport northward of heat from the tropics and subtropics within the North Atlantic. Zonal temperature distributions within the Atlantic (see Sverdrup et al.
, p. 127) readily show the overall pattern, with the
South Atlantic distinctly colder than the North Atlantic at all latitudes (Fig.
In recent modeling experiments on heat transport by ocean and atmosphere, the transport of latent heat by winds has been identified as the dominant mechanism changing sea-surface temperature in the North Atlantic, on a decadal time scale (Seager et al.
). These results suggest that winds may be more important than ocean currents in moving heat and thus in producing the enormous temperature anomaly of the northern North Atlantic.
However, there is no question that the heat content of the mixed layer in the sea introduces an element of stability in the positioning of high and low pressure regions, so that winds tend to stay with the paths mapped out by currents.
The feeding of heat to the Iceland Low, through moist winds from the Gulf region, and aided by the Gulf Stream, plays a special role in the dynamics of the present climate.
123
S182
Fig. 12 Zonal averages of surface temperatures in the
Atlantic Ocean. Based on
Table 31 in Sverdrup et al.
(
). Note that the maximum difference to southern latitude zones is at 60–70 N. The northward heat transport features contributions from the agulhas current (Ag), the benguela current (Ben), the south equatorial current (C Eq), and the general motion of warm water and moist warm winds toward the intertropical convergence zone (ITCZ).
Within the North Atlantic, the
Gulf stream (Gulf), the westerly winds and currents in temperate latitudes (West) and the Iceland low are involved in heat transport
Int J Earth Sci (Geol Rundsch) (2010) 99 (Suppl 1):S171–S189
The Iceland Low acts as a turntable moving heat into northwestern Europe. The strength of this transport varies on a decadal scale, producing the ‘‘North Atlantic Oscillation.’’ The mechanisms causing the oscillation are not known, nor have the periods expressed therein received much attention (as evidenced in the symposium edited by
Hurrell et al.
). As concerns the conditions during the last glacial maximum, we can be confident that the Iceland
Low was greatly weakened by the surrounding sea ice, by the expansion of the Greenland High, and the general southward movement of the polar front (Fig.
In an important but poorly known review, the meteorologist Flohn (
1985 ) has enumerated many implications of
the great planetary climatologic asymmetries linked to northern heat piracy. In his view, the buildup of northern ice caps should lead to greater symmetry in heat distribution, as the strengthened zonal winds north of the equator push the ITCZ toward the equator. It is difficult to tell, from reconstruction of the conditions of the last glacial maximum (LGM), whether or not there was considerable heat transport across the equator (Otto-Bliesner et al.
).
According to the reconstructions of Scha¨fer-Neth and
), their Fig.
), it appears that the temperature anomaly of the North Atlantic vanished beyond 40 N (as expected from the southward advance of the polar front) but that heat transfer from south to north persisted in the tropical regions (aided by strong southeasterly winds off the
Amazon; see Paul and Scha¨fer-Neth
balance, Flohn’s concept, that northward heat transport should be greatly decreased during glacials by centering the
ITCZ closer to the equator, does not seem to be strongly supported by the available data in the Atlantic (Paul and
Scha¨fer-Neth
, p. 568, their Figs.
and
Fig. 13 Reconstruction of the glacial Atlantic, according to CLIMAP
1984 (as given in Wefer et al.
). The arrow (here added) shows the path of heat transport suggested by the temperature contours (W, warm; C, cold). The eastern South Atlantic is seen as a heat collector for the western tropical Atlantic, similar to the present situation
If cross-equatorial transport of heat in the Atlantic is not highly sensitive to the presence or absence of northern ice sheets (Fig.
13 ), there is little reason to believe that it
would be highly sensitive to the ongoing global warming.
123
Int J Earth Sci (Geol Rundsch) (2010) 99 (Suppl 1):S171–S189 S183
Nevertheless, the difference in patterns (though small) suggests a slight increase in heat piracy of the North
Atlantic, as warming proceeds. This change can be rationalized by considering that warming is stronger on the northern hemisphere with its large land area, and that this excess warming, on the whole, should strengthen monsoon-type import of heat-bearing winds from the south.
For the northern North Atlantic, projections about heat transport are commonly tied to expectations about the response of NADW production to rapid warming and supply of freshwater. The comparison of the last glacial maximum with Holocene conditions, while interesting, would seem to be of only modest relevance to this problem. To a lesser extent, the same is true for the response of NADW production to sustained and rapid meltwater influx during deglaciation. There is no question that massive freshwater input at high and sustained rates must interfere with deep vertical mixing and hence with thermohaline circulation (Olausson
; Berger et al.
; Broecker and Denton
; Manabe and Stouffer
; Stocker and Marchal
;
Rahmstorf
; Tarasov and Peltier
). The question is at what rate and over what period of time does the system enter the state at which stratification wins out over convection in high latitudes. The answer is not clear: it depends on the behavior of a complex set of nonlinear feedbacks.
When assessing the lessons from Quaternary studies for future heat flow patterns, it is well to recall that until very recently computations based on fairly large data sets for the modern ocean ( massive when compared with geologic data) yielded but tentative results for meridional heat flow in the South Atlantic (Schlitzer
). Much progress has been made since, especially with regard to incorporating geologic information (e.g., Paul and Scha¨fer-Neth
but error bars are still substantial.
Carbon cycle and productivity
How the ocean will react to the increasing partial pressure of CO
2 in the atmosphere is clearly a major issue. It is linked to the temperature increase because both an increase in temperature of upper waters, and a decrease in vertical mixing fostered by the warming, will, on the whole, decrease the proportion of carbon dioxide that enters the sea. The chemistry itself—reaction of the carbon dioxide with the water and with the carbonate ion—also leads to a lessening of uptake (as recognized half a century ago by
Revelle and Suess
). Calculations are made difficult not so much from a lack of understanding the basic elements of the chemistry involved, but from the intricacies of vertical circulation and gas exchange at the surface (which determine physical carbon fluxes) and the large number of communicating reservoirs in the biological carbon cycle.
The necessary data to assess the rates of export of carbon from surface waters to deeper layers, in response to climate change, are still rather sparse, although much has been
) reviewed the pertinent problems
(Wefer
; Fischer and Wefer
; Usbeck et al.
The problem of projecting the response of the ocean to
CO
2 values well above present is greatly complicated by the fact that there is no past analog for the present carbon dioxide content of the atmosphere, as far as that can be determined, for the last 800,000 years (see Petit et al.
;
EPICA,
2004 ). Since the atmosphere is a relatively small
reservoir and is intimately linked to a large one—the ocean—the uncertainties concerning the expected atmospheric level, for different conditions postulated for the ocean, are rather large. The rise from lower carbon dioxide values in glacial periods (around 190 ppm) to higher values in interglacials (around 290 ppm) is remarkably rapid and follows closely on the temperature changes seen in the ice
(Fig.
14 ). These, in turn, are highly correlated with the
Fig. 14 Deuterium and carbon dioxide values from terminations in the Vostok ice core (Petit et al.
) mapped on Termination I to show the similarities and differences between these events. During warming the rise of CO
2 follows a regular pattern. (Exceptional value circled ) (From Berger
123
S184 Int J Earth Sci (Geol Rundsch) (2010) 99 (Suppl 1):S171–S189 oxygen isotope record in deep-sea sediments (Berger et al.
). It has proved very difficult indeed, so far, to account for the change of 100 ppm in the carbon dioxide content of the atmosphere, across the ice-age cycles. In addition, one must account for another 40 ppm of change that is hidden in the growth of forests and associated soil carbon pools, which change with the climate cycles.
It is unlikely that the mix of mechanisms responsible for increasing carbon dioxide values in atmosphere, at the time of deglaciation, will soon be reconstructed with great confidence (see, e.g., Sigman and Boyle
; Monnin et al.
). However, the message from the record is clear: The system reacts to warming and to a rise in sea level with an increase in atmospheric carbon dioxide.
Another lesson is that equilibrium thinking is unlikely to contribute very much to an appreciation of what the future might bring. The action is with rapid change and its consequences.
Concerning the response of the ocean’s productivity to global warming, the same uncertainties arise as for the carbon cycle, as the two topics are intimately linked.
Changes in the wind field—a weakening of the trade winds, for example—will greatly influence the patterns of upwelling. Based on observational data, Bakun (
proposed that global warming due to greenhouse gases should increase upwelling globally. A reconstruction of upwelling intensity off northwest Africa (McGregor et al.
) supports this inference.
Changes in upwelling will affect the several interacting nutrient cycles. In turn, the changes engendered will result in changes in composition of the plankton produced. It is important to realize that large phytoplankton (diatoms) support a food web completely different from that supported by small phytoplankton (such as bacterioplankton or coccolithophores). High-energy consumers (marine mammals and birds) rely on short food chains to provide for an efficient transfer of energy from photosynthesis to consumption (Ryther
1969 ). As a result, we see marine
mammals and birds on seasonal migrations to feeding grounds where diatoms are abundant (Berger
).
Geologists studying the record of productivity have come up against a paradox, well illustrated in the upwelling sediments off Angola and Namibia: Here, glacial sediments contain more marine organic matter than interglacial ones, but show a deficiency of silica relative to warm periods.
Based on the conflicting results of Diester-Haass (
more siliceous fossils during warm periods) and Oberha¨nsli
(
); increased glacial upwelling based on foraminifers), it appears unavoidable to postulate a change in nutrient content in upwelled waters, between glacial and interglacial periods, in agreement with Hay and Brock (
general pattern of silicate deficiency during glacial times may be indicated, for the Namibia upwelling region
123
(Berger and Wefer
; ‘‘Walvis Paradox’’). The Namibia upwelling system may be considered a client of the
Southern Ocean, which ultimately delivers the nutrients for upwelling (Sarmiento et al.
). Thus, changes in the
(sub-)Antarctic silica distribution system seem to be involved.
However, similar strange patterns (increased production, less silica) were discovered in the northern Pacific (Berger and Lange
) and in the eastern equatorial Pacific
(Ganeshram and Pedersen
). This complicates things and suggests that a general shortage of silica arose, because of increased silica deposition in early glacial upwelling systems. A down-draw of atmospheric CO
2 during increased silica deposition in upwelling regions would be expected from the linking of silica and carbon cycles in the sea
(Berger
1991 ; Archer and Maier-Reimer 1994 ; Ragueneau
et al.
From these considerations, it appears that global changes in nutrient content of the thermocline are just as important as changes in the rates of upwelling, as factors controlling ocean productivity.
As concerns the likely effect of warming on ocean productivity, it is apparently closely tied to the response of the trade wind system. In the record, in any case, strong trade winds during glacial times produce increased upwelling both in coastal regions and along the equator.
On the whole, trade winds are a response to the planetary temperature gradient. As this gradient weakens in the northern hemisphere (because of the differential warming of high northern latitudes) we should expect a corresponding weakening of the northern trade winds. Predators that depend on high food density (such as marine mammals and sea birds) would then suffer from a general decrease in the focusing of ocean productivity. Fisheries depending on herring-like fish that strain upwelling waters for food also will suffer, beyond the damage done by overfishing.
We realize that this type of conclusion is embarrassingly general, given the rich background information on production history (or rather proxy history) in many regions of the ocean. However, when proxies disagree, as they do in the Walvis Paradox complex, our knowledge of nutrient cycles and ecosystem dynamics is called upon to resolve the conundrum. This knowledge is as yet deficient, as illustrated by the lively discussions regarding the role of iron in controlling the productivity of large parts of the ocean (Martin and Fitzwater
, and many since). Some have maintained that, rather than iron deficiencies, it is rapid grazing that prevents phytoplankton from depleting the surface waters. Obviously, these are not minor differences in opinion. If one can have a discussion on which is more important, nutrient control or grazing, the system is not well understood.
Int J Earth Sci (Geol Rundsch) (2010) 99 (Suppl 1):S171–S189 S185
Conclusions
We set out to assess which of the many discoveries about climate change in the Late Quaternary (roughly the last million years) would help us most in projecting the response of the climate system to the ongoing overall warming. The (preliminary and tentative) results of this survey are as follows:
• From Quaternary studies, we learned a great many things about the workings of the system, over a large range of climatic conditions, but we did not find an analog to the present or future situation.
• The most instructive periods are those dominated by rapid change, especially from cold to warm, since our own time is a transition of that kind.
• Rapid transitions are driven by strong positive feedback, which is delivered by various effects of warming, including changing albedo patterns, greenhouse-gas increase, sea-level rise, freshwater input, and changes of the wind field and corollaries.
• The last 10,000 years were unusually stable as far as climate change. The critical factor for introducing instability seems to be the presence of ice that is ready to collapse. Historically, when the sea level was in the position that it occupies at present, it could readily move up or down by a meter or two within a century, for several centuries.
• The steps of the last deglaciation have a message that once a threshold is reached, sea level rises rapidly for some time (as long as vulnerable ice is present). The role of the ocean in providing feedback is much discussed and quite poorly understood.
• The North Atlantic will likely continue to draw heat from south of the equator, and store much of it in the
Caribbean realm. In northern subtropical regions, upwelling productivity will likely suffer because of the decrease in zonal winds that drive upwelling.
• The East Antarctic ice sheet is fairly stable. Thus, the winds around the Antarctic are expected to persist, and also the high seasonal productivity that accompanies the deep mixing provided by such winds.
• Warming engenders CO
2 increase (based on ice core results), and there is a strong possibility that diatom productivity (that is, the marine silica cycle) is vitally involved in controlling CO
2 changes. However, neither the carbon cycle nor the silica cycle is well understood.
Perhaps the most obvious conclusion is that a strong link between marine geologic research and other activities (ice coring, climate modeling) can greatly enhance our understanding of the climate system. But we should not expect immediate answers from the past concerning the questions for the future. We should use the experience gathered from climate history and climate modeling to make commonsense projections of what is ahead and find ways to deal with the risks emerging, even in the face of great uncertainty.
Sea-level rise is a phenomenon subject to runaway, based on the history of deglaciation. There is no question that marine-based ice is more vulnerable to destruction than ice safely anchored on solid dry ground. As sea level slowly rises, more and more of the ice becomes marinebased. At some point, further rise, by removing support from marine-based ice, can trigger substantial surging, and also open access for seawater to the base of previously grounded ice. The task, obviously, is to avoid any threshold inviting the start of runaway dynamics.
Acknowledgments We thank William W. Hay and Christian Dullo, who read the article and made useful suggestions for improvement.
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