Abrupt climate change: causal constraints provided by the paleoclimate record

Earth-Science Reviews 51 Ž2000. 137–154 www.elsevier.comrlocaterearscirev

Abrupt climate change: causal constraints provided by the paleoclimate record W.S. Broecker ) Lamont-Doherty Earth ObserÕatory of Columbia UniÕersity, Palisades, NY 10964, USA Received 15 March 2000; accepted 23 May 2000

Abstract Important aspects of the physics involved in the climate changes of late Quaternary time elude us. The paleoclimatic record documents in detail that these changes were vast and in many cases happened abruptly. Every element of the global climate system was involved. While a case can be made that these reorganizations of the ocean’s thermohaline circulation acted as the trigger for these jumps, no one has as yet been able to articulate exactly how it is that these reorganizations are capable of inducing such large changes in the way in which the atmosphere operates. In this review, I point out what to me are the key constraints placed by the records kept in ice and in sediments. The timing of these changes suggests that they have been paced by seasonality changes related to periodicities in the Earth’s orbital elements and by a curious 1500-year cycle imprinted on sediments in the northern Atlantic. But as these pacemakers are inherently weak, in order for them to generate what we see in the record, there must exist powerful and likely nonlinear feedbacks in the system. It is my hope that by bringing together this information, I will provide an impetus for creative thinking with regard to physical scenarios capable of illuminating these feedbacks. Not only is this the crucial step in understanding our past, but it is also a necessary step if we are to properly evaluate the possible consequences of the ongoing greenhouse gas buildup. q 2000 Elsevier Science B.V. All rights reserved. Keywords: climate change; causal constraints; paleoclimate

1. Introduction Studies of ice recovered from borings through the Greenland ice cap reveal a hitherto unknown characteristic of the Earth’s climate system, namely, its ability to make abrupt jumps from one state of operation to another. These jumps, which occurred

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Tel.: q1-914-65-8413; fax: q1-914-365-8169. E-mail address: [email protected] ŽW.S. Broecker..

roughly once each millennium during portions of the last glaciation, were accomplished in a few decades and led to large changes in global climate. While climate appears to have remained in a single state during periods of peak interglaciation, LamontDoherty’s Gerard Bond has demonstrated the existence of a 1500-year cycle, which is operative during both periods of glaciation and periods of interglaciation ŽBond et al., 1997.. This cycle not only appears to have paced the large millennial duration jumps in climate during glacial time, but also the smaller changes in climate characterizing interglacials. The

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most recent of these interglacial cycles was the Medieval Warm to Little Ice Age swing. While impressive documentation exists regarding the geographic patterns, timing and magnitude of the abrupt climate jumps, which punctuated the last glaciation, relatively little progress has been made in understanding the physics behind them. Many of us are convinced that the transitions from one climatic state to another were triggered by reorganizations of the ocean’s thermohaline circulation, but it is not at all clear how these reorganizations manifested themselves in global-scale changes in the operation of the atmosphere. So difficult has it been to come up with conceptual models, which generate the temperature, precipitation and dustiness changes associated with

these jumps that some scientists look instead to the tropical Pacific as the trigger point. They argue that as the changes in climate are symmetrical about the equator they must involve the tropical heat engine. If so, why not place the trigger in the tropics instead of in the polar oceans? Clearly, the challenge is to come up with conceptual models capable of explaining the changes recorded in ice and sediment and then to test these scenarios through the use of climate models. There is an urgency associated with this endeavor for when forced by increasing greenhouse gas inventories, the thermohaline circulation in most of the existing joint ocean–atmosphere models slows ŽManabe and Stouffer, 1993; Weaver and Hughes, 1994; Stocker and Schmittner, 1997.. This slowing is

Table 1 Summary of the major findings with regard to the conditions that prevailed during peak glacial timea Referencea Expanded ice coÕer Mountains Continents Sea Lower temperatures Deep sea Ice caps Temperate lands Tropics Pole to equ. grad.

Snowlines down ; 920 meters Ice sheets around N. Atlantic but not in E. Siberia or Alaska Extended sea ice northern Atlantic and circum Antarctic

Rind and Peteet, 1985 Denton et al., 1981

; 38C colder; 18 O in benthics and pore waters Much colder; Greenland DT s y15 " 38C DT s 58C; Noble gas thermometry DT s y3 " 18C; Mountain snowline lowering Larger

Schrag et al., 1996 Cuffey et al., 1995; Cuffey and Clow, 1997 Stute et al., 1995 Betts and Ridgway, 1992; Pierrehumbert, 1999 Boyle, 1997

CLIMAP, 1976, 1981

Differing precipitation pattern World’s deserts More, at least for the US Great Basin and Dead Sea Basin Tropics Less, at least for E. Africa and the Red Sea

Broecker et al., 1998a,b

Lowered greenhouse gases CO 2 Down 30% CH 4 Down 50%

Petit et al., 1999 Chappellaz et al., 1993

Greater dustiness Temperate regions Polar ice caps

Loess deposition 10-fold increase

Kukla, 1987 De Angelis et al., 1987; Mayewski et al., 1994

Ocean circulation Atlantic

Weakened conveyor

Southern ocean Thermocline

Lower O 2 in deep water Better oxygenated

Duplessy et al., 1984; Boyle, 1986; Lynch-Stieglitz et al., 1999 Kumar et al., 1995 Behl and Kennett, 1996; Schulz et al., 1998

a

Benson et al., 1990

Rather than list all the references pertaining to each item, I have listed papers that summarize what is known and also list auxiliary references.

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the result of a progressive warming and wetting of the polar regions. So if indeed it is reorganizations of ocean circulation that trigger jumps from one state of atmospheric operation to another, then by burning vast quantities of fossil fuels, we may be tampering with a very sensitive element in the Earth’s climate system. Hence, if we are to properly evaluate the consequences of the greenhouse buildup, it is obligatory that we come to grips with the physics driving this phenomenon. In this review, I attempt to point out what I consider to be the important constraints provided by the paleoclimatic record. The hope is that this information will help to spawn novel concepts designed to explain how it is that the Earth’s climate system accomplishes these large swings in climate.

2. Climate during peak glacial time Putting aside for the moment the abruptness of climate transitions, it is instructive to consider the sheer magnitude of the difference between peak glacial and peak interglacial conditions Žsee Table 1 for summary.. During periods of glaciation, temperatures were lower everywhere on the planet with the largest cooling occurring in polar regions. The colder atmosphere allowed mountain snowlines to descend by about 1 km everywhere on the planet Žsee Fig. 1.

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and sea-ice margins in the polar regions to move equatorward. Of course, this led to large continental ice sheets in the north and correspondingly lower sea levels. The tropical oceans were 3 " 18C colder than now ŽPierrehumbert, 1999.. More rain than now appears to have fallen in the world’s deserts and less in the tropics. The concentrations of greenhouse gases CO 2 and CH 4 were substantially lower and dust transport through the atmosphere was an order of magnitude greater. Finally, the Atlantic’s conveyor circulation was weaker and the ventilation of regions in the oceanic thermocline currently depleted in O 2 was considerably more intense. The aspect of these differences that stands out in my mind is that the atmosphere’s 08C isotherm descended in elevation by about 1 km. This allowed continental ice sheets to form in Scandinavia and Canada and sea ice to expand throughout the Norwegian Sea. This expansion greatly reduced air temperatures over portions of the high-latitude oceans by replacing water Žnever colder than y1.88C. with ice over which winter air temperatures could plummet to several tens of degrees below the freezing point. The impact of this change is clearly recorded in Greenland’s ice where bore-hole thermometry indicates that during glacial time air temperatures averaged about 158C colder than now ŽCuffey and Clow, 1997.. In my estimation, such a cooling can only be explained by the expansion of northern Atlantic sea

Fig. 1. Contrast between the elevation of today’s snowline along the American cordillera Žsolid line. with those reconstructed from geomorphic features created by those of glacial time Ždashed line.. Reproduced from Broecker and Denton Ž1990..

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ice eliminating open water from the vicinity of Greenland. Furthermore, these expanded polar conditions steepened the latitudinal thermal gradient thereby promoting storminess and in turn dust loading of the atmosphere. A further consequence of the sea-ice expansion was that the deep waters produced in the northern Atlantic were close to the freezing point Žy1.88C. in contrast to today’s q28C to q48C temperatures. Deep waters formed in the Southern Ocean must, then as now, have been close to the freezing point. In this way, the 38C glacial cooling of the deep ocean demanded by 18 O measurements ŽSchrag et al., 1996. can be explained Žsee Table 2.. Although a strong case has been made that on the time scale of tens of thousands of years, the Earth’s climate is being paced by the so-called Milankovitch cycles Ži.e., changes in the seasonal distribution of solar heating related to periodicities in the Earth’s orbital elements., because they lack the required powerful non-linear feedbacks, no general circulation model forced by orbital periodicities is capable of reproducing the observed glacial–interglacial climate swings. Those models, which purport to do so, fall into one or the other of two categories, i.e., general circulation models in which important aspects of the glacial world Ži.e., ice cover, surface ocean temperature, atmospheric carbon dioxide. are prescribed such that the rest of the world is forced into a glacial-like condition, or AtoyB models whose parameters are set to achieve a sensitivity capable of reproducing very large responses to the very small Milankovitch forcing. We must therefore face up to the reality that we simply do not understand how it is that the Earth’s climate is capable of achieving its glacial state.

Table 2 Peak glacial to peak interglacial 18 O to 16 O ratio difference as recorded in ben-thic foraminifera from throughout the world ocean. As demonstrated by deconvolutions of pore water 18 O profiles, the contribution of excess continental ice constitutes about 57 percent of this total ŽSchrag et al., 1996.. The remainder is attributable to a glacial cooling of the deep sea of about 38C D d18 O benthic foram Ice volume contribution

1.75"0.05‰ 1.00"0.10‰

Residual attributable to deep sea cooling Equivalent cooling

0.75"0.12‰ 3.0"0.58C

3. Terminations Although broadly speaking, the major glacial–interglacial cycles were in phase across the entire planet, it is now clear that this synchroneity does not apply during the rapid transitions. For example, there is evidence that the sequence of events associated with the terminations, which brought to a close each of the 100,000-year duration major climate cycles began in the Southern Hemisphere. This sequence is beautifully displayed in the Vostok Antarctica ice core record. The onset of the terminations is heralded by a waning of the intense glacial dust rain onto the Antarctic ice cap. Only at the toe of this dust demise do the rises in three properties Ži.e., the DrH ratio in the snow falling on Antarctica, the CO 2 content of the air trapped in the encapsulated bubbles, and the methane content of these same bubbles. commence. The isotope change reflects a warming of the air over Antarctica; the CO 2 rise reflects a change in the sea’s biochemical cycles, and the methane rise reflects a wetting and warming of tropical wetlands. These climbs continued in parallel for about 7000 years before the peak interglacial values were achieved. Only at this point did the 18 O to 16 O ratio in the O 2 trapped in ice bubbles begin to fall. The most obvious explanation for this fall is that it mirrors the 18 O to 16 O ratio decrease in sea water resulting from the meltback of the large Northern Hemisphere ice sheets. As the O 2 molecules in the Earth’s atmosphere are replaced with new photosynthetic oxygen about once each millennium and as all the water involved in the manufacture of this O 2 comes ultimately from the sea, the change in oceanic 18 O to 16 O ratio in atmospheric O 2 should track, with only a small lag, the 18 O to 16 O ratio in sea water. Further, changes in the 13 C to 12 C ratio in benthic foraminifera parallel those in 18 O to 16 O. As the latter are proxies for ice volume and the former are proxies for thermohaline circulation changes, a case can be made that the oceanic circulation changes associated with terminations did not take place until the warming of Antarctica and the increases in atmosphere’s CO 2 and CH 4 inventories were nearly complete. Finally, the demise of the large glacial rain of dust onto Greenland did not occur until the meltdown of the northern ice sheets was well underway. A diagramatic representation of the happenings asso-

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ciated with terminations in Fig. 2 serves as a guide to this sequence of events. The existence of this lag was one of the most important findings by the SPECMAP program ŽImbrie et al., 1992, 1993.. One possible pitfall in the above logic must be mentioned. The 18 O to 16 O ratio in atmospheric O 2 is offset from that in sea water by about 24‰. This offset reflects mainly the fact that light O 2 molecules Ž16 O– 16 O. are consumed in slight preference to heavy ones Ž18 O– 16 O. during respiration. The problem is that the magnitude of this offset has not remained exactly constant. We know this because the 18 O to 16 O ratio in O 2 has a stronger 20,000-year periodicity than does the 18 O to 16 O ratio in benthic foraminifera. Hence, it is possible that the delay in the rise of the 18 O to 16 O in ice core O 2 during terminations reflects an anomaly in the magnitude of

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the respiration offset rather than a lag in Northern Hemisphere ice sheet melting. However, evidence exists that this is not the case. Records based on planktonic elements in a number of cores from the Southern Ocean suggest that the warming of surface water associated with the terminations precede the change in 18 O in coexisting benthic foraminifera Žsee Broecker and Henderson, 1998.. Putting aside this caveat, it appears that the melting of the Northern Hemisphere ice caps and the reorganization of thermohaline circulation, which brought to an end each major glacial cycle were preceded by a warming of the Antarctic continent, a warming and wetting of the tropical wetlands and a weakening of the ocean’s biological pump. If correct, this observation has important consequences.

4. Carbon dioxide

Fig. 2. Sequence of events associated with the terminations of each 100,000-year duration climate cycle. As recorded in Antarctic ice, the sequence begins with the demise of the rain of dust. At the toe of this demise, a 7000-year duration climb in air temperature over Antarctica and in the CO 2 and CH 4 contents of the global atmosphere commences. Based on the assumption that the 18 O to 16 O ratio in the O 2 trapped in air bubbles follows closely on the heels of meltback of the Northern Hemisphere ice sheets, this retreat did not begin until after the atmosphere’s CO 2 and CH 4 contents had reached close to their interglacial values. Only for the last termination can the demise of north polar dust rain be tied to the melt pack of the ice sheets. This drop occurred rather abruptly at the onset of the BA warm.

As recorded in Antarctic ice during the course of the last four 100,000-year cycles, the CO 2 content of the atmosphere has swung back and forth between a low of 190 ppm during peak glacials and a high of 280 ppm during peak interglaciations. While the consequent difference in greenhouse capacity can account for no more than half of the 58C global cooling of glacial time, it cannot be put aside as unimportant in maintaining the Earth’s cold state. Despite many attempts, no entirely satisfactory explanation has been provided as to the exact cause of this CO 2 change. For example, if as suggested by the record kept in Antarctic ice, the CO 2 rise at the end of each 100,000-year cycle was largely completed before either the onset of the meltback of the Northern Hemisphere ice sheets or the reorganization of the ocean’s thermohaline circulation, then the only feasible scenarios are those involving the ocean’s biological pump. In particular, the fact that the demise in the dust rain onto Antarctica began before the onset of the CO 2 rise points the finger at the iron released from the dust to the upper ocean as responsible for the increased strength of the ocean’s biologic CO 2 pumping action. As first realized by the late John Martin Ž1990., the current dearth of iron limits plant productivity in areas of today’s ocean where strong upwelling occurs causing a portion of

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the available NO 3 and PO4 to go unutilized. This idea has given rise to three related scenarios for increased CO 2 drawdown in the ocean’s surface waters: Ž1. Iron fertilization of the Southern Ocean greatly increases the utilization efficiency of NO 3 and PO4 . Two problems face this explanation. First, since the residence time of iron in the sea is extremely short, the fact that the demise of dust input was largely complete before the onset of the CO 2 rise would require that the enhancement of productivity attributable to iron input saturates at a value way below the maximum rate of glacial dust input to the Southern Ocean. Ž2. The impact of iron is indirect, its role being to enhance the fixation of nitrogen by marine bacteria. As these organisms have a tenfold higher Fe demand than do marine algae, perhaps they respond more vigorously to iron fertilization. This scenario has the advantage that as fixed nitrogen has a marine residence time of three or so thousand years ŽGruber and Sarmiento, 1997., it could account for the delay between the dust demise and the CO 2 rise. But it is not at all clear that additional NO 3 can by itself enhance marine productivity. Most marine chemists and biologists would concur that without excess PO4 such enhancement would not occur. Ž3. Another possibility is that the availability of the excess iron released from dust triggers production by marine biota of the ligands responsible for holding this element in solution in the deep sea. As it is the upwelling of this ligand-bound iron, which meets much of the iron demand by surface dwelling organisms, the larger the ligand inventory, the smaller the ocean’s iron shortage. The fact that the dissolved iron concentration in the deep Atlantic water is similar to that in the deep Pacific water ŽJohnson et al., 1997. implies that the residence time of the ligands in the sea exceeds one ocean mixing time Ži.e., ) 1000 years.. If the ligand residence time were to turn out to be several thousands years, then, as for fixed nitrogen, this scenario would be able to account for the delay between the dust demise and the CO 2 rise. So we come up against the dust dilemma. What maintained its very high glacial rain rate? Why was the demise of dust rain in Antarctica forerunner to the series of events characterizing glacial termina-

tions? If, as suggested below, dust delivery to the atmosphere reflects storminess and storminess, in turn, the steepness of the latitudinal temperature gradient, then the demise of the dust fall is likely a reflection of the retreat of sea ice surrounding the Antarctic continent.

5. Millennial duration cycles The discovery of millennial duration oscillations in Greenland ice cores provided a powerful clue as to how the Earth’s climate may have accomplished the large swings described above. The Greenland record suggests that Earth climate does not undergo a linear response to Milankovitch forcing but rather that it jumps among discrete states ŽBroecker and Denton, 1990.. Which of the available states is favored appears to depend on the global pattern of seasonal isolation Ži.e., Milankovitch cycles.. Thus, one might say that Milankovitch cycles somehow orchestrate the Earth’s climate states. While many of the important features of the climate system are capable of undergoing rapid change Ži.e., sea ice, atmospheric water vapor, cloudiness, vegetation cover, storminess, thermohaline circulation, . . . ., those aspects associated with continental glaciation are more sluggish Ži.e., ice-sheet extent and thickness, sea level, isostatic response to ice loading, water storage in and release from ice caps.. So also apparent are those processes governing the atmosphere’s CO 2 content. The interplay between those aspects of the system capable of rapid response and those aspects of the system capable only of sluggish response is likely responsible for much of the complexity associated with the Earth’s Quaternary climate history. I would be remiss were I not to mention a drawback to the suggestion that the large difference between conditions characterizing full glacials and full interglacials might have been accomplished by jumps among the climate systems available states. While the millennial duration events appear to have been antiphased between Antarctica and Greenland, broadly speaking, conditions characterizing the two polar regions were in phase during the course of each 100,000-year cycle. Both polar regions experienced their cold conditions during the same time

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Fig. 3. Oxygen isotope record for one of the two Summit Greenland ice cores ŽGRIP, Dansgaard et al., 1993. showing the millennial duration Dansgaard–Oeschger events. Following Bond et al. Ž1993., the launch times for the six Heinrich iceberg armadas into the northern Atlantic are placed on this record. Also shown are the BA warm, the Younger Dryas cold, and the cooling event at 8200 BP Žpresumably a brief reduction in the strength of the Atlantic’s conveyor circulation resulting from a sudden release of stored meltwater, Barber et al., 1999..

span and both enjoyed their warmest conditions during the same time span. Only during the transitions between the cold and warm extremes is there clear evidence that the sequence of events in the north lagged those in the south.

6. Dansgaard–Oeschger events The record contained in Greenland ice reveals that during much of the last glacial interval the 18 O to 16 O ratio in the snow falling there underwent rectangular cycles Žsee Fig. 3.. The low 18 O portions of the

record correspond to millennial duration periods of intense cold and the higher 18 O portions to millennial duration intervals of moderate cold. Although a convincing means of establishing the magnitude of the air temperature shifts between these states has yet to be found, the DT likely lies in the range 68C to 108C. Keeping in mind that the average cooling for glacial time was 158C Žbased on bore-hole thermometry, Cuffey and Clow, 1997., this means that during the intervals of intense cold the air temperature was 188C to 208C colder than now. Cuffey et al. Ž1995. conclude that these intense colds were 218C to 238C colder than now ŽTable 3..

Table 3 Summary of records that display part or all of the millennial duration Dansgaard–Oeschger eventsa Property

Magnitude of change

Air temperature over Greenland Surf ocean temperature, Bermuda Surf ocean temperature, Santa Barbara O 2 content thermocline waters off California, in the Arabian Sea and Cariaco Trench Dust delivery to Greenland Sea salt delivery to Greenland Ice rafting supply to N. Atlantic Atmospheric CH 4 inventory Southern Ocean–Antarctic temperatures

"68C to 108C "48C to 58C "38C to 58C Better ventilation during episodes of extreme cold

a

Threefold higher during episodes of extreme cold More during episodes of extreme cold More during episodes of extreme cold "15% Appear to be antiphased with respect to rest of planet

Referencea Sachs and Lehman, 1999 Hendy and Kennett, 2000 Behl and Kennett, 1996; Schulz et al., 1998 Mayewski et al., 1994 Mayewski et al., 1994 Bond et al., 1997 Chappellaz et al., 1993 Bender et al., 1994

Rather than list all the references pertaining to each item, I have listed papers, which summarize what is known and also list auxiliary references.

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The records of calcium and methane in Greenland ice reveal changes synchronous with those of 18 O. Calcium Ždelivered as CaCO 3 . is several-fold higher during the intense cold periods ŽAlley et al., 1995; Alley, 2000. suggesting increased storminess in the region supplying dust to Greenland Ži.e., the great Asian deserts, Biscaye et al., 1997.. Methane Žcontained in air bubbles. is 10% to 20% lower during intervals of intense cold suggesting a drying and perhaps cooling of the tropical wetlands ŽChappellaz et al., 1993.. Highly detailed electrical conductivity measurements Žsee Fig. 4. show that the ratio of CaCO 3 input to acid input underwent abrupt shifts marking the transitions between states ŽTaylor et al., 1993a,b.. During the intense cold intervals, CaCO 3 was present in sufficient quantities to neutralize the acid byproducts of compounds manufactured by marine and terrestrial plants and thereby to squelch the electrical conductivity. Furthermore, during each of these several decade duration transitions, the conduc-

tivity flickered. Severinghaus et al. Ž1998. and Severinghaus and Brook Ž1999. have used measurements of the content of CH 4 and of 15 N to 14 N in N2 trapped in the air bubbles contained in Greenland ice to show that the onset of the rise in the atmospheric methane during the abrupt warmings lagged Greenland’s warming by only a few decades or less. The bottom line from studies of the record kept in Greenland ice is that the transitions between the two glacial states were abrupt Žoccurring in 20 to 40 years.. Not only did air temperatures over Greenland’s ice cap undergo large changes at the times of these jumps, but so also did the storminess over the Asian deserts and likely the water budget for tropical wetlands. Greenland ice is not the only archive containing evidence regarding Dansgaard–Oeschger events. Studies of sediment from rapidly accumulating AdriftsB in the northern Atlantic Ocean reveal that pulses of ice-rafted debris corresponded to the

Fig. 4. Electrical conductivity record for that section of the Summit Greenland GISP II ice core ŽTaylor et al., 1993a,b. representing the last deglaciation. During cold intervals such as the late glacial and YD, the very large influx of CaCO 3 -bearing dust completely neutralized the incoming acids thereby squelching the conductivity. In contrast, during the warm intervals such as the BA and early Holocene, the acid completely dissolved the CaCO 3 , and the presence of acid protons allowed an electric current to flow in response to the electrodes as they were scraped along the clean cut ice surface. In the blow up of the abrupt transition between the YD and Holocene, it can be seen that the conductivity flickered before it was locked into the Holocene state. Even so, the transition was completed in less than four decades Ž11,668 to 11,623 BP.. The time scale is based on annual layer counts.

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episodes of intense Greenland cold ŽBond et al., 1997.. Alkenone thermometry on deep-sea sediments from the Bermuda area Žsee Fig. 5. has replicated a major portion of the Greenland record demonstrating that sea surface temperature differences between the intervals of the intense cold and those of moderate cold were 48C to 58C ŽSachs and Lehman, 1999.. Nor is this evidence restricted to the northern Atlantic basin. Sediment cores taken beneath O 2 poor regions of the main oceanic thermocline Ži.e., from the Santa Barbara Basin off California ŽBehl and Kennett, 1996., the Arabian Gulf off Pakistan ŽSchulz et al., 1998., and the Cariaco Trench off Venezuela. reveal that the episodes of intense Greenland cold were characterized by major increases in the O 2 content of thermocline waters. Finally, studies of high accumulation rate sediment from the southern Atlantic suggest that climate changes in the far south were antiphased with respect to those elsewhere on the planet ŽCharles et al., 1996.. A similar antiphasing appears to be present in the isotope record for the

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interior of the Antarctic ice cap ŽBlunier et al., 1998..

(BA) –Younger Dryas (TD) 7. The Bolling–Allerod ¨ oscillation The cold climates of glacial time in the northern Atlantic basin were abruptly terminated about 15,500 calendar years ago Ž12,600 14 C years ago. — ushering in conditions not too different than those that we enjoy today. These warm conditions persisted for about 2000 years. Then, rather suddenly, the northern Atlantic and its surrounding lands were plunged back into conditions approaching those characterizing glacial time. This cold snap lasted for about 1200 years before giving rise to the abrupt warming, which heralded the onset of the Holocene. The interval of warmth is known as the BA and the subsequent cold snap as the YD. As the magnitude and duration of this climate oscillation is similar to those

Fig. 5. Comparison of the temperature records for the Greenland ice cap Žexpressed as the 18 O to 16 O ratio in the ice. and for surface waters on the Bermuda Rise Žbased on alkenone ratios as published by Sachs and Lehman, 1999.. Because of widely varying input of sediment to this site, the alkenone record has been squeezed and stretched in order to align the dramatic warmings with those in the Greenland record. In addition, the scales for 18 O and alkenone temperature are chosen in such a way that the best amplitude match is achieved. Finally, the time scale is that for the Greenland ice cores obtained by counting annual layers.

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characterizing much of glacial time, the BA–YD oscillation is often referred to as the last of the Dansgaard–Oeschger cycles. I single it out here because much more is known about its global extent ŽTable 4.. A surprising aspect of this oscillation is that the climate of Antarctica underwent antiphased changes ŽBender et al., 1994; Jouzel et al., 1995; Blunier et al., 1998.. The warming of Antarctica, as recorded in numerous long ice cores, began about 5000 years before the onset of BA warm in the northern Atlantic region. This warming plateaued during the BA and then, during the YD, it not only resumed but the rate of warming was greater than that during the period prior to the onset of the BA plateau Žsee Fig. 6.. The

boundary separating that portion of the globe whose climate followed the northern Atlantic pattern and that portion whose climate followed the Antarctic pattern appears to lie in the Southern Ocean Žsee Alley and Clark, 1999 for discussion.. We know this because of YD age advances in mountain glaciers in the New Zealand Alps ŽDenton and Hendy, 1994., as well as in the European Alps ŽIvy-Ochs et al., 1996., the North American Rockies ŽGosse et al., 1995., and the tropical Andes ŽClapperton et al., 1997. have been shown to be synchronous. One of these, the Franz Josef Glacier, located in New Zealand at 438S, places a limit on the location of the boundary suggesting that it lies somewhere in the Southern Ocean. As discussed below, this antiphasing of climate re-

Table 4 Summary of records for the time interval encompassing the YD a Magnitude of change

Referencea

Prominent cooling Modest cooling Prominent warming

Dansgaard et al., 1989 Thompson et al., 1998 Jouzel et al., 1995

O r16 O CaCO3 European lakes Atm. CH 4

Prominent cooling 30% drop

Eicher and Siegenthaler, 1981; Goslar et al., 1995 Chappellaz et al., 1993

Dust loading Greenland ice cap Andean glaciers Antarctic ice cap

Prominent increase No increase No increase

Mayewski et al., 1994 Thompson et al., 1995 De Angelis et al., 1987

Mountain glaciers European Alps Western USA Tropical Andes New Zealand Alps

Prominent advance Moderate advance Prominent advance Prominent advance

Ivy-Ochs et al., 1996 Gosse et al., 1995 Clapperton et al., 1997 Denton and Hendy, 1994

Ocean Thermocline Conveyor North American Drainage

Ventilation event Shutdown Through St. Lawrence

Hughen et al., 1996; Behl and Kennett, 1996 Hughen et al., 1998; Boyle, 1986 Teller, 1995

Property 18

16

O r O ice Greenland Andes Antarctica

18

Pollen assemblages Northern Europe Central Europe Southern Europe Maritime Canada Eastern USA Coastal Alaska a

Walker et al., 1994 Ammann et al., 1994 de Beaulieu, 1994 Mott, 1994 Peteet et al., 1994 Peteet and Mann, 1994

Rather than list all the references pertaining to each item, I have listed papers, which summarize what is known and also list auxiliary references.

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does so via this route, such a shutdown would cause newly produced radiocarbon atoms to be backlogged in the atmosphere and upper ocean. The decline of 14 C to C ratio during the remainder of the YD requires then that the supply to the deep sea was reestablished. Had this reestablishment involved a rejuvenation of deep water formation in the northern Atlantic, it would be expected that the intense cold conditions in the northern Atlantic basin would have abated. They did not. Hence, Broecker Ž1998. called on a new style of deep water formation in the Southern Ocean to carry radiocarbon to the deep sea. A consequence of this circulation mode was heat release to the atmosphere over the Southern Ocean and, hence, the antiphasing of the millennial duration events in Antarctica.

8. Pacing

Fig. 6. Reconstruction of changes in the 14 C to C ratio in atmosphere–upper ocean carbon based on measurements on annually varved sediments in the Cariaco Trench ŽHughen et al., 1998.. The 50‰ radiocarbon increase during the first 200 years of the YD is thought to reflect a shutdown of the Atlantic’s conveyor circulation, which in today’s ocean is the major conduit for radiocarbon to the deep sea. Then during the remainder of the YD, the 14 C to C ratio was drawn back down presumably reflecting a renewal of deep sea ventilation. But as cold YD conditions in the northern Atlantic basin continued interrupted, it is more likely that the ventilation conduit was located in the Southern Ocean. If so, this switch in the site of deep water formation and the consequent switch in heat release to the atmosphere could account for the antiphasing between the Bryd–Antarctica and GISP II-Greenland temperature records Žsee lower panel..

sponse has led to the concept of a bipolar seesaw in deep water formation. Another important observation is that reconstructions of atmospheric 14 C to C ratio show that during the first 200 years of the YD this ratio climbed by 5% ŽHughen et al., 1998.. Then, during the remaining 1000 years of the YD, it slowly declined Žsee Fig. 6.. The most reasonable explanation for this change is that at the onset of the YD, the production of deep water in the northern Atlantic was squelched. As roughly 80% of the 14 C reaching today’s deep sea

Just as the 20,000-, 40,000- and 100,000-year climate periodicities found in the climate record appear to be paced by Milankovitch’s orbital cycles ŽHays et al., 1976., the millennial duration Dansgaard–Oeschger oscillations appear to have been paced by a curious 1500-year cycle. Bond et al. Ž1997. report that some of the ice-rafted quartz grains found in northern Atlantic sediments are iron stained and others are not. More importantly, they found that the percentage of quartz grains, which were stained with iron oxide, shifted back and forth from about 7% to about 25%. This composition cycle had much the same character during times of glaciation when large-scale ice rafting occurred and during times of interglaciation when ice rafting was minimal. The amazing thing is that throughout the last 150,000 years, regardless of climate, the length of the cycle averaged close to 1500 years. Further, the Dansgaard–Oeschger oscillations of glacial time appear to be geared to this 1500-year cycle. Something in the Earth system must oscillate on this time scale. As there are several sources for iron-stained quartz grains scattered around the perimeters of the Arctic and the northern Atlantic basins, it is possible to come up with a range of scenarios, which might account for this cycle. But it seems to me that the most reasonable explanation involves changes in the

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pattern of the surface ocean currents, which redistribute the icebergs entering the northern Atlantic. If indeed this is the correct explanation, then one can go a step further and call on changes in the strength of the Atlantic’s conveyor circulation associated with the bipolar seesaw. Although relatively little is known about the Holocene climatic impacts of the 1500-year cycle, we have learned some important things about the most recent of these cycles, namely, the Medieval Warm–Little Ice Age oscillation. It has long been known that this oscillation played a key role in the colonization of Greenland by the Vikings Žsee Grove, 1988.. During the Medieval Warm Žcentered at about 1000 AD., the seas around Greenland were sufficiently free of ice that navigation was possible and the climate sufficiently warm that sheep could be maintained in Greenland’s fjords. These favorable conditions persisted until the fourteenth century. Then ice extent increased and grass crops failed. The Little ice Age had begun. Sea-ice coverage records from Iceland and glacial extent records from the European Alps make clear that these cold conditions prevailed in the 17th century and persisted until the latter part of the 19th century. Then, just before the turn of the century, conditions around the northern Atlantic eased as the Little Ice Age came to a close. Further, at this time, mountain glaciers throughout the world began retreats, which have continued right up to the present. Of particular interest in this regard is the Franz Josef Glacier at 438S in New Zealand’s Alps, which began a major retreat in the late 1800s. If the 1500-year cycle involves the bipolar seesaw, then one might expect that Antarctica would have undergone antiphased temperature changes. Indeed Gary Clow of the US Geological Survey has obtained evidence that this is indeed the case. His deconvolution of the temperature profile in the Taylor Dome bore hole reveals that air temperatures at that site were 38 colder during the time corresponding to the Medieval Warm than during the time corresponding to the Little Ice Age.

9. Deep-sea ventilation So the finger points toward thermohaline circulation in the world ocean as a major player in the

1500-year cycle both during glacials and interglacials. In today’s ocean, the uniform PO4) ŽPO4 q O 2r175–1.95. concentration of 1.38 mmolrkg for deep Pacific and Indian Ocean waters falls halfway between that of 0.73 mmolrkg for deep waters presently forming in the northern Atlantic and that of 1.95 mmolrkg for deep water presently forming in the Southern Ocean. This requires on the average over the course of the last ventilation cycle Ži.e., ; 1000 years. that half of the deep water in these oceans was supplied from source regions in the northern Atlantic and about half from source regions in the Southern Ocean ŽBroecker et al., 1998a,b.. In order to balance the deep sea’s radiocarbon budget, the rate of supply from each of these sources must have averaged about 15 = 10 6 m3ryear. This is puzzling for if the bipolar seesaw is an important element in the Earth’s climate system, then why at this particular point in time does it appear that deep water production is nicely balanced between the two polar regions? My opinion is that this balance does not exist. Physical oceanographic observations in the Weddell Sea, thought by many to be the dominant Southern Ocean source region, come up way short of the required 15 = 10 6 m3rs. Rather, they indicate only 3 " 2 = 10 6 m3rs Žsee Broecker et al., 1998a,b for a summary of the evidence.. A CFC11 inventory based on recent surveys in the deep Southern Ocean suggest that averaged over the past 25 years the production of ventilated deep water in the entire Southern Ocean was in the range 4 to 8 = 10 6 m3rsec ŽOrsi et al., 1999.. My read on this is that during the 500-year duration Little Ice Age deep water formation in the Southern Ocean was considerably stronger than now and deep water formation in the northern Atlantic was correspondingly weaker ŽBroecker et al., 1999.. Then, about 120 years ago, deep circulation reorganized in such a way that deep water formation in the Southern ocean underwent a demise and that in the northern Atlantic was strengthened. I suspect that it was this reorganization that brought the LIA to a close. If my analysis is correct, then indeed deep ocean circulation is reorganizing once each 750 years in accord with the Bond et al. Ž1997. 1500-year cycle. This being the case, then the properties of the slowly ventilated deep Pacific and deep Indian Oceans do not reflect today’s mode of operation.

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Rather, they represent an average over a time interval spanning two swings of the seesaw. What is it then that sets the duration of the 1500-year cycle? My suspicion is that it is related to the transport of fresh water through the atmosphere. As shown by Zaucher and Broecker Ž1992., 0.25 " 0.10 = 10 6 m3rs of water move through the atmosphere from the Atlantic to the Pacific and Indian Oceans. Were this atmospheric transport not compensated by a corresponding transport of excess salt from the Atlantic to the Pacific, then the average salt content of Atlantic waters would increase at the rate of about 0.5 grl each 750 years Ži.e., in one half of a seesaw cycle.. The densification caused by such an increase would be about the same as that associated with a 28C cooling of polar surface waters. Up until recently, my view has been that this compensating transport of salt operated at steady state during the course of the Holocene. But now with Bond’s discovery that the 1500-year cycle continued during interglacials and with Clow’s finding that the MWLIA oscillation appears to have been antiphased at the Taylor Dome Antarctica site, I am not so sure. Perhaps, instead, the Atlantic undergoes an alternation between periods of salt buildup Žconveyor weak. and salt export Žconveyor strong..

10. The role of catastrophes Rooth Ž1982. first suggested that the diversion of the outflow of pro-glacial Lake Agassiz from the Mississippi to the St. Lawrence drainage supplied a knock out punch to deep water formation in the northern Atlantic and thereby initiated the YD cold event. Subsequent studies have confirmed that indeed the northward retreat of the southern margin of the Laurentide ice cap opened a new outlet for this lake, which occupied the ice cap’s marginal trough. This shift in outlets caused Agassiz’s level to drop 30 or so meters leading to a catastrophic release of fresh water to the northern Atlantic. Radiocarbon dating demonstrates that this diversion coincided with the onset of the YD. To me, this evidence is convincing. This was not the only such catastrophe connected with abrupt changes in climate. Although the radiocarbon evidence is less firm, a case has been made

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that the brief but prominent early Holocene cooling event recorded in the Greenland ice was triggered by yet another melt water release from a pro-glacial lake located to the south of what is now James Bay ŽBarber et al., 1999.. In this case, the water drilled its way northward through the remnant ice northward into Hudson Bay and from there spilled through Hudson Strait into the Labrador Sea. The presumption is that it led to a short-lived shutdown of conveyor circulation Žsee Alley et al., 1997 for discussion.. In addition to these two catastrophic melt water releases, six great armadas of icebergs punctuated the last glacial period ŽHeinrich, 1988.. The extensive deposits of ice-rafted debris recording four of these armadas have very sharp bases suggesting that they were launched suddenly. If so, upon melting, they must have released a large amount of fresh water to the northern Atlantic. While these events appear to have impacted the sequence of millennial duration events, the nature of their impact remains murky. While most records are dominated by Dansgaard–Oeschger events with little or no overprint attributable to Heinrich events, in two records, the opposite is the case. One is the Lake Tulane Florida pollen record ŽGrimm et al., 1993., which shows an alternation of intervals of oak and pine dominance. While the pine intervals have 14 C dates roughly corresponding to the Heinrich events, as the oak and pine intervals are represented by core intervals of roughly equal length, it is not clear whether these episodes precede, span, or follow the Heinrich events. What is clear, however, is that there is one pine episode for each Heinrich event. The second record is that kept in rock varnish of the US Great Basin. The manganese content of the varnish in this region is 3–10 items higher during peak glacial than during peak interglacial time. Further, during the course of the last glacial period, the manganese content alternates between intervals of high and intermediate manganese content. Although exact dating has yet to be established, there is one high manganese layer for each Heinrich event. While the so-called Heinrich ice armadas and the melt water floods likely punctuated ventilation in the northern Atlantic, there were far too few of these events to account for the many abrupt changes. Thus, a large majority of the abrupt transitions in climate

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must have been triggered in some other way. It is my opinion that they involved an oscillation in the pattern of deep water formation Ži.e., the bipolar seesaw.. While catastrophic events may have pretriggered some of the transitions, such impetuses were not required. Rather, given enough time, the transitions would have happened anyway Žsee Broecker et al., 1990.. The YD may be an exception, for it occurred at the mid-point of a glacial termination. As similar YD-like cold punctuations did not occur during terminations of the earlier 100,000-year cycles, it may be that, but for the freak release of Agassiz water, climate would not have undergone this last cold punctuation. The cooling event, which occurred during the early Holocene, also sends us an important message. Its very short duration Ž; 150 years. compared to those of the Dansgaard–Oeschger cold phases seems to be telling us that when in its interglacial state the ocean system has a preferred pattern of operation, which, if disrupted, quickly reforms.

11. A trial scenario Generating scenarios capable of explaining the magnitude, abruptness and global extent of the Dansgaard–Oeschger climate transitions proves to be a daunting challenge. As a start, we need qualitative conceptualizations of what might have happened. I present here an example of such a scenario. I start with the assumption that the jumps between states were triggered by switches in deep water production between the northern and southern polar regions Ži.e., the bipolar seesaw.. Then, in an attempt to explain how it is that these oscillations led to large climate changes during glacials and small ones during interglacials, I look to sea-ice cover in the northern Atlantic. As summarized in Fig. 7, I postulate that during the cold portions of the D–O cycles, the Atlantic’s conveyor circulation was ‘off’ and, as a consequence, the entire Norwegian and Labrador Seas were covered with sea ice. Then, when the conveyor came back ‘on’, this excess ice rapidly melted, creating open water. Not only could this explain the large D–O temperature changes recorded in Greenland ice, but, perhaps, also the temperature

Fig. 7. The hypothetical impact of the 1500-year cycle on the extent of sea-ice cover. During intervals when deep water formation in the northern Atlantic dominated, sea-ice cover was reduced in the North and was expanded in the South. The opposite was the case during the intervals when deep water production in the South dominated. The impact of the consequent changes on the steepness of the latitudinal thermal gradient was stronger during glacial periods when sea-ice cover was, on the average, considerably greater in both polar regions.

changes documented by Sachs and Lehman Ž1999. for the surface ocean near Bermuda. Now comes the tricky part. In order to create the far field impacts of the D–O events Ži.e., dustiness, methane, thermocline ventilation, . . . ., I call on a steepening of the latitudinal temperature gradient caused by southward expansion of the sea ice and, hence, also of frigid Arctic-like winter conditions. In particular, I postulate that this steepening intensified winter storminess promoting the injection of dust and sea salt into the upper troposphere. Further, following Yung et al. Ž1996., I call on this dust to alter some combination of the atmospheric water vapor inventory and cloud albedo in such a way so as to cool the entire Northern Hemisphere and perhaps also the remainder of the globe. Such temperature changes would have altered the wind patterns and upper ocean currents ŽAlley and Clark, 1999; Agustsdottir et al., 1999.. Somehow, this caused the ventilation of the Northern Hemisphere thermocline to become more intense, thereby more vigorously ventilating the low O 2 portions of the thermocline. Clearly, this scenario is little more than an exercise in arm waving. How large an expansion of sea-ice cover would be needed? Would it have to have occurred in the northern Pacific, as well as in the northern Atlantic? Is the dependence of storminess on the thermal gradient strong enough to create

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the observed dustiness changes? What is the influence of dust and aerosols on drop size in clouds and on the residence time of water molecules in the atmosphere? What changes in salinity are required to allow the ventilation of the thermocline of the northern Pacific from local outcrops rather than all the way from the south temperate zone as is the case today? Arm waving, yes, but a necessary step toward facing up to the challenge. Until testable conceptual models are placed on the table, progress toward the truth will remain largely stalled.

The charge to this group is to probe the physics behind the dramatic climate changes so richly recorded in the Greenland ice-core record. Further, stimulation has come from interactions with my Lamont-Doherty colleagues, in particular, with Gerard Bond, Jean Lynch-Stieglitz, Martin Stute, Bob Anderson, Julian Sachs, George Kukla, and Dorothy Peteet. LDEO contribution number 6051.

12. Conclusions

Agustsdottir, A.M., Alley, R.B., Pollard, D., Peterson, W., 1999. Ekman transport and upwelling from wind stress from GENESIS climate model experiments with variable North Atlantic heat convergence. Geophys. Res. Lett. 26, 1333–1336. Alley, R.B., Finkel, R.C., Nishiizumi, K., Anandakrishnan, S., Shuman, C.A., Mershon, G.R., Zielinski, G.A., Mayewski, P.A., 1995. Changes in continental and sea-salt atmospheric loadings in central Greenland during the most recent deglaciation. J. Glaciol. 41, 503–514. Alley, R.B., Mayewski, P.A., Sowers, T., Stuiver, M., Taylor, K.C., Clark, P.U., 1997. Holocene climatic instability: a prominent, widespread event 8200 years ago. Geology 25, 483–486. Alley, R.B., Clark, P.U., 1999. The deglaciation of the northern hemisphere: a global perspective. Ann. Rev. Earth Planet. Sci. 27, 149–182. Alley, R.B., 2000. The Younger Dryas cold interval as viewed from central Greenland. Quat. Sci. Rev. 19, 213–226. Ammann, B., Eicher, U., Gaiullard, M.-J., Haeberli, W., Lister, G., Lotter, A.F., Maisch, M., Niessen, F., Schluchter, C.H., Wohlfarth, B., 1994. The Wurmian late-glacial in lowland Switzerland. J. Quat. Sci. 9, 119–127. Barber, D.C., Dyke, A., Hillaire-Marcel, C., Jennings, A.E., Andrews, J.T., Kerwin, M.W., Bilodeau, G., McNeely, R., Southons, J., Morehead, M.D., Gagnon, J.-M., 1999. Forcing of the cold event of 8,200 years ago by catastrophic drainage of Laurentide lakes. Nature 400, 344–348. Behl, R.J., Kennett, J.P., 1996. Brief interstadial events in the Santa Barbara Basin, NE Pacific during the past 60 kyr. Nature 379, 243–246. Bender, M., Sowers, T., Dickson, M.-L., Orchardo, J., Grootes, P., Mayewski, P.A., Meese, D.A., 1994. Climate correlations between Greenland and Antarctica during the past 100,000 years. Nature 372, 663–666. Benson, L.V., Currey, D.R., Dorn, R.I., Lajoie, K.R., Oviatt, C.G., Robinson, S.W., Smith, G.I., Stine, S., 1990. Chronology of expansion and contraction of four Great Basin lake systems during the past 35,000 years. Palaeogeogr., Palaeoclimatol., Palaeoecol. 78, 241–286. Betts, A.K., Ridgway, W., 1992. Tropical boundary layer equilibrium in the last ice age. J. Geophys. Res. 97, 2529–2534. Biscaye, P.I., Grousset, F.E., Revel, M., Van der Gaast, S.,

Were I to select one aspect of the climate record on which to focus attention, it would be dust. Of all the properties of the system, the rain of dust underwent the largest change, being thirtyfold higher during times of peak glaciation than during times of peak interglaciation in both polar regions. Loess, which accumulated in immense quantities throughout the world’s temperate zones during times of peak glaciation is unknown during interglacials. The demise of the dust rain onto Antarctica heralded the onset of the termination of each 100,000-year cycle. The final demise of the dust rain in the Northern Hemisphere occurred only at the end of the YD. The iron carried to the ocean by dust is the prime candidate for the driver of the low glacial atmospheric CO 2 content. Finally, the extra dust loading in the atmosphere may hold the key to cooling the Earth. Thus, were we to gain an understanding of the role of dust in setting the strength of the ocean’s biological pump and in perturbing the albedo and cycle of water in the atmosphere and were we to figure out how it is that the dust and aerosol loading to the atmosphere underwent several-fold discontinuous changes at the boundaries of the Dansgaard– Oeschger cycles, we would have taken a major step toward solving the riddles of the ice ages.

Acknowledgements My motivation to write this review came from the stimulation provided by the get-togethers of the NOAA-sponsored panel on abrupt climate change.

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