QUATERNARY RESEARCH 12, 135-149 (1979) \C ^

On Time Scales and Causes of Abrupt Paleoclimatic Events HERMANN FLOHN Meteorologisches Institut der Universität Bonn, Universität Bonn, Auf dem Hügel 20, 53 Bonn l, West Germany Received May 24, 1978

During the last 7 x 105 the occurrence of abrupt climatic variations, of an intensity probably reaching 5°C/50 yr and with a duration of the order of several centuries can be demon- strated; their frequency is of the order 10~4 (sometimes even 10^3) per . Most impressive examples are sudden coolings in earlier ; in some periods the variability of past climates was obviously much greater than now. Due to the effective spatial coherence of the atmospheric and oceanic circulation their extension, not necessarily of similar intensity, is probably hemispheric or even global. They are modified by feedback mechanisms within the geophysical climatic ; orbital changes play a selective role leading either to suppression or to growth. Any physical Interpretation of such abrupt paleociimatic events remain äs yet speculative. One of the most attractive models is the occurrence of clusters of major volcanic eruptions which is more frequent than expected in random . This is similar to the clustering of severe earthquakes in recent years; both events are probably interrelated responses to the (apparently discontinuous) motions of tectonic plates. INTRODUCTION (Müller, 1965, 1974; Dansgaarde?«/., 1972; Many paleoclimatologists, especially Hecky and Degens, 1973; Osborne, 1974; nonmeteorologists, tend to be satisfied with Wijmstra, 1975; Coope, 1977); two different an Interpretation of past climatic fluctua- models for a geophysical interpretation of tions of the Late Cenozoic in terms of Or- such events have been proposed (Flohn, bital changes, i.e., in the Milankovich time 1974b, 1978). In this study it is intended to scale of 20,000-100,000 yr. Very long pa- interpret additional evidence (Woillard, leociimatic records, such äs investigated 1975, 1978 a,b; Müller, 1978), to obtain a by Shackleton and Opdyke (1973, 1976) more detailed time scale from continental from the equatorial Pacific, äs well äs the records and to discuss further possible loess profiles from Lower Austria and the causal mechanisms. In this context it is southern CSSR (Kukla, 1975; Fink and useful to distinguish between external and Kukla, 1977), indicate indeed a dominance internal causes: These terms are designed of fluctuations in this time scale during the from the viewpoint of the climatic System last 2 my. Based on the evaluation of two (GARP, 1975). The "almost-intransitive" cores from the southern Indian Ocean, behavior of this system (see E. Lorenz in Hays et al. (1976) have confirmed the role GARP, 1975, p. 132) presents one of the of orbital elements; this paper has, with its most challenging problems to scientists; critical attitude and its cautious analytical many geophysical sciences are more or less methods, convinced even skeptics. The involved. dominance of a nearly 100,000-yr cycle, which presents an enigma to climatologists, FLUCTUATIONS AT THE END OF THE has been differently (and more correctly) LAST GLACIATION interpreted by Wigley (1976) and Berger The peak of the last Ice Age (Würm- (1977, 1978). Wisconsin) occurred about 18,000 yr B.P. On the other hand much evidence for The melting of the continental ice domes of \arge-sca\ paleociimatic events of a shorter the northern hemisphere was finished about time scale (102-103 yr) has been presented 8000 yr B.P. (Fennoscandian) and finally

135 0033-5894/79/040135-15S02.00/0 Copyright © 1979 by the Universtty of Washington. All rights of reproduction in any form reserved. 136 HERMANN FLOHN about 4500 yr B.P. (Laurentide). Disappear- and lacking in the southern hemisphere. ance of the Fennoscandian ice was nearly Three readvances occurred after about contemporaneous with the disintegration of 13,000 yr B.P., about 11,900 yr B.P., and the Laurentide Ice Sheet into separate re- the last () after about 10,900 siduals, including a catastrophic incursion of yr B.P. They were separated by two warm the sea into Hudson Bay (Ives et al., 1975). interstadials: Bölling (around 12,500 yr After 6500 yr B.P. the climatic optimum B.P.) and Alleröd (around 11,400 yr B.P.). (Atlanticum, Hypsithermal) developed; The füll oscillations of the "Alleröd se- evidence for a eustatic rise of sea level of quence" together encompassed not more about 2-3 m above the present level is still than 2000 yr and the annual temperature controversial. During this period, sea ice in Variation reached, in southern Germany, the Canadian arctic archipelago disap- probably 5-6 C. Both warm interstadials peared, including the northern coast of were accompanied, in the Colombian Cor- Greenland and Ellesmere Island, with simi- dillera (5°N; 2850 m) by humid phases (van lar traces in the European and Siberian Arc- Geel and van der Hammen, 1973), in con- tic (Vasarief al., 1972). trast to the Wurm peak and the Younger The retreat of the Continental ice was by Dryas, which were both arid. Of special no means uniform. Several glacial read- interest are the high (400 m!) lake-level vances occurred nearly simultaneously fluctuations (Hecky and Degens, 1973) of (Mörner, 1973); evidence for their role in Lake Kivu (Fig. 1). the tropics is until now scanty (e.g., Fig. 1) The catastrophic readvance of the ice after the Alleröd destroyed, "wholesale," years B.P Lake Level full-grown forests. This readvance, to- -300 *• *100 11000 gether with marked cooling at Camp Cen- tury, Greenland (76°N) and at Byrd Sta- tion, West Antarctica (80°S) (Johnson et 1)500 - al., 1972), along with desiccation in Colum- bia occurred, from all indications, in a time

12000 - span of 200-350 yr. Similar time spans are reported from many cores; the variations of the Vegetation between temperate forest and forest—tundra in a peat bog north of 12500 - Beifort (southeast France) (Woillard, 1975) is shown in Figure 2. It should be realized, however, that Vegetation immigration needs more time than the true climatic variations; in contrast to this, an abrupt climatic 13000 - change may kill a nonadapted Vegetation in a few years or decades. An immediate response to the climate is demonstrat- ed by the abrupt sequence of two differ- ent beetle faunas from near Birmingham,

n,Fe)C03 England. Here a typical arctic fauna was Facies found at a layer dated 10,025 ± 100 yr B.P., AI2 03 Facies 13700 while no such fauna survived 10 cm higher 102 ICH1Ö7' IOW —>H1GH where an age of 9970 ± 110 yr (Osborne, Manganese (ppm) Thermocli ne FIG. 1. Lake-level fluctuations and bottom chemis- 1974) indicates a time most probably at the try of Lake Kivu (2°S), 13,700-11,000 yr. B.P. (after end of Younger Dryas cold period. Taking Hecky and Degens, 1973). the data literally, this "very rapid" warm- ABRUPT PALEOCHMATIC EVENTS 137

BOREAL

- 8600 -10200 Y. DRYAS

ALLERÖD -10800 H 1700 M. DRYAS BÖLLING ^12400 13200 U. DRYAS

C—r- —*• W FIG. 2. Vegetation History (forest versus tundra) during the Late Glacial, northeast France, Dept. Hte. Saone, Grands Pr6s Core II (after Woillard, 1975); AP = tree pollen, NAP = nonarboreal pollen, C(W) = cold (warm). ing should have occurred in a time span of last glaciation (about 25,000 yr B.P.) must about 50, but not more than 150 yr. The have been "nearly instantaneous" (Lamb July temperature change during this event and Woodroffe, 1970). has been estimated (Coope, 1977) to be Since we are now living in an 8-9 C. Within this complex "Alleröd se- period without knowing its end, the most quence" the warm phases coincide with interesting events are the cooling phases high humidity in the African tropics within the last interglacials; the following (Butzer, 1976). Figure l indicates an earlier discussion shall be concentrated on these. humid phase around 13,000 yr B.P., lasting Much evidence has been presented in re- only a few centuries (Bölling ?). cent years, including the monographs pub- The Alleröd complex was accompanied lished by Frenzel (1967, 1968). This is espe- by a rise of the world's sea level, from cially true for the last interglacial, now about -50 to about -35 m (Mörner, 1973). commonly defined äs 5e (Emiliani and At -38 m the Bering Strait reopened and Shackleton 1974), lasting approximately Arctic waters again entered the Pacific. The from 127,000 until about 115,000 yr ago latter had never been äs cold äs the North (Appendix). Convincing evidence from , where 90% of the meltwater from Barbados (Matthews, 1973) and New the continental ice domes formed a cool, Guinea (Bloom et al., 1974) has been pre- low-saline surface layer. During and after sented for the occurrence of two significant the peak of the glacial phases, Atlantic cool periods (5b, 5d) between three warm surface water was less saline than the phases (5a, 5c, 5e), with eustatic lowering Pacific, not äs a cause (Weyl, 1968), but äs a of sea level of nearly 80 m within about 5000 consequence of the glaciation of the north- yr. This is roughly equivalent to an average ern continents. annual storage of 20 cm water equivalent at a glacial area near 30 x l O6 km2. This is not EXAMPLES OF ABRUPT unrealistic, if one assumes that the annual INTERGLACIAL COOLINGS storage at the center may be äs low äs 5 At the beginning of the last glaciation, the cm/yr, while it may rise at the outer margins climatic fluctuations seem to have been äs to 50—60 cm/yr, similar to the present ac- rapid äs at its end (Appendix). During the cumulation in Antarctica. Larger estimates central part of the last glaciation several (30-45 cm/yr) have been discussed, leading important abrupt climatic shifts between to a model of an "instantaneous glacieriza- Stadiais and mterstadials have been re- tion" (Andrews and Mahaffy, 1976). The ported (Flohn, 1974b). The beginning of the earlier of these coolings ( = stage 5d, about 138 HERMANN FLOHN

110,000 yr ago) between the warm phases the time scale appears to be much less Barbados III and II, probably coincides doubtful than the intensity (amplitude) of with Melisey I (Woillard, 1975) and an early the event; further remarks shall be avoided short event in the Camp Century core since its analysis is still continuing. (Dansgaard et al., 1971), äs well äs with a In the Holstein Interglacial of Northern double cool steppe phase in Macedonia Germany, Müller (1974, 1978) investigated (Wijmstra, 1975) and with the marked an extended series of annual layers each cooling in the loess sequence of southeast- about 0.5 mm thick, verified by seasonal ern central Europe (Kukla, 1975, Fig. 2). pollen sequences, revealing several sharp The second ( = stage 5b, about 90,000 yr cooling events. In an undisturbed part of B.P.) can be identified with the famous these varves, the catastrophic destruction "Greenland Blitz" (Dansgaard et al., 1972), of a thermophilous Corylus— Taxus— Alnus Melisey II (Woillard, 1975), with a sudden forest occupied less than 100 years, after cooling in the Orgnac cave (Duplessy et al., with a subarctic Betula-Pinus forest re- 1970), and probably with the abrupt begin- covered. Figure 3 (simplified from Fig. 6 of ning of the cool steppe phase in Macedonia Müller, 1974), gives a schematic Interpreta- near 24 m (Wijmstra, 1975), accompanied tion of the (summer) temperature varia- by a sudden drop of summer temperatures tions, together with the time scale. The two of 5-8 C (Wijmstra, 1978). All these data subarctic phases lasted only 300—400 yr; indicate a time scale of the transitional the total duration of the Holstein Intergla- phases of these events which is at least one cial has been estimated äs 15,000-16,000 yr. order of magnitude smaller than the Milan- In a recent reevaluation (Müller, 1978), kovich time scale of 104 yr; an estimate of the author could obtain reliable pollen the time scale is given in the Appendix, to- counts from sections containing only 15 an- gether with a selection of additional evi- nual layers; from these data it can be con- dence from ocean cores. This shorter time cluded that the destruction of the forest oc- scale is even evidenced from the bottom of cupied most probably not more than 60 yr. the ocean, where burrowing organisms mix This event cannot be considered äs being the Sediments at a depth interval of 5 — 10 local, since similar short (varved) peaks of cm, equivalent in ocean areas with a low subarctic species (Betula, Pinus) within Sedimentation rate to a time scale of 5000 yr extended periods of a mixed thermophilous or more. Ruddiman et al. (1977b) have forest have been found at three sites in shown that the global warming at the begin- southeastern England (Müller, 1978). In ning of the wärmest stage 5e, or Eem sensu a recent comparison of land and sea stricto, in the central North Atlantic indi- chronologies, Kukla (1977) suggests the cated a temperature rise of 5.2 C/1000 yr, Holstein Interglacial of Munster-Breloh to when the polar water mass retreated from coincide with stage 11 of Emiliani and Lat. 43°N to Lat. 76°N (Kellogg, 1976) with Shackleton (1974). A short survey, without an averaged rate up to 1200 m/yr. This details and certainly incomplete, demon- warming rate, certainly much smoothed in the oceanic records, is on the same order of magnitude äs the actual changes of hemi- spheric temperatures (about 0.01 C/yr), which are, however, much more variable in sign.

The "Greenland Blitz" has been inter- b= forest boundary preted by Dansgaard et al. (1972) äs an al- FIG. 3. Temperature trend (July) and time scale most instantaneous change (i.e., within during the Holstein interglacial (after Müller, 1974). about 100 yr) from a climate warmer than Arrow = increasing time, a,b = estimated July tem- now into füll glacial severity. In this case perature levels. ABRUPT PALEOCLIMATIC EVENTS 139 strates a double or multiple time structure such "rare" events appears to be on the (with intermediate coolings) in the famous order of 1-2 x l O4 yr. Certainly each piece Philippen bog in Macedonia (Lekanis com- of evidence merits a critical discussion; in plex; cf. Wijmstra, 1978), in core KS-09 the case of stage 5, the global occurrence (Cita et al., 1977) in the central Mediterra- (Appendix) of the sequence of five different nean, in core K 708-7 (Ruddiman et al., stages (5a—e) seems to be convincing evi- 1977a) in the central North Atlantic, in sev- dence. From a paleoclimatic viewpoint the eral Caribbean cores, in core V 28-238 time scale of the climatic (not faunal or flo- (Shackleton and Opdyke, 1973) in the ral) transitions needs further clarification. equatorial Pacific, and in core E 49-18 Many fluctuations resemble a (Hays et al., 1976) in the southern Indian flip-flop mechanism, with abrupt changes Ocean (46°S, 90°E). The shortness of the between two opposite climatic states. A füll signal at Munster—Breloh (Fig. 3) seems to Continental glaciation needs, for building indicate that in central Europe a world-wide up äs well äs for decay, not more than event was only represented by a quite short 5000-8000 yr separately. (some centuries) but intense ramification. The possibility of "abortive" glaciations In an earlier publication, Müller (1965) (Flohn, 1974b), i.e., rapid widespread investigated another core (Bilshausen near coolings of several °C lasting some cen- Göttingen) with more than 25,000 annual turies, but not long enough to initiate a füll varves (each about 0.5 mm thick) from an glaciation, merits further discussion. The earlier interglacial and indicated a short most recent, but only weak example is the "cooling" of about 400 yr (15 cm, three "Little Ice Age" with its peak around sections, distinct undisturbed varves) be- 1600-1700 A.D., characterized by thin ice tween the two wärmest intervals. Here a sheets and perennial snowbanks at the warm-temperate oak forest was suddenly plateau of Baffin Island (Ives et al., 1975; replaced by a cool-temperate (or even sub- Andrews and Mahaffy, 1976). The incoher- arctic) forest with 50—60% Pinus and Be- ency between the time scale of these tula, while after 400 yr an even more ther- events and the orbital time scale suggests mophilous mixed oak forest developed, the assumption of a selective role of orbital representing the climatic Optimum of this elements, either leading, with a positive interglacial. The summer temperature may feedback, to a füll glaciation, or, in con- be tentatively estimated äs about 3 — 5 C trast, reducing a similar event to a cold lower than before and after. The Bilshausen episode or an "abortive" glaciation. core is now (Müller, 1978, Fig. 5) well dated; its basic part coincides with the POSSIBLE CAUSES OF ABRUPT Matuyama—Brunnes reversal at 700,000 yr CLIMATIC EVENTS ago and thus with stage 19 of Emiliani and The climatic System consists of several Shackleton (Kukla, 1977). The beginning Subsystems, atmosphere, ocean, snow, ice, and end of this interglacial are both and soil, with different response times (Fig. characterized by a sequence of two or three 4) and many degrees of freedom. Even driv- sudden coolings and warmings, with a time en by a constant energy source, such a scale near 1000 yr quite similar to the well- System must operate with much interannual known sequence of the last Late Glacial variability, like any other self-regulatory (14,000-10,000 yr B.P.). system. As indicated by Frenzel (1967, 1968) such Internal feedback processes with a stor- sudden "coolings" in the more recent (and age time of weeks or months (e.g., an early better-known) interglacials are apparently a snow cover or upwelling of surface waters) regulär feature. Smaller events of the same and covering areas in the order of l O5-l O6 type occur only in records with a sufficient km2, can significantly alter the regional heat time resolution; the recurrence interval of budget. Processes of this kind, together 140 HERMANN FLOHN

km mblh Pa) 50 0 Str atos,phere 100 -500 d 12 200 $ ^ ^ TROPOP.AUSE Continental Ice Troposphere 4-8°

i i / 1000 " j F orest Drift Ice 1-5 yrs 60 yrs km '' \ l bar(105Pa) )l„„VJ$&,~ 0 b> c* 0 m .05 UpperMixed Layer 60-200ci Soit 5-20d .40 PYCNOCUNE • n -t 1 *^^^j%^^* THERMOCUNE * Runoff Groundwater 10-104 years Deep Ocean -1500 years 100

T 7 /.n n n

-> Weak Interaction => Strong FIG. 4. Climatic System with time scale of Subsystems and their interaction.

with positive (or negative) feedback, con- solar rotation rate (Eddy, 1976). Computa- trol the tropospheric circulation and the tions with comprehensive climate models tracks of traveling cyclones and anticy- (Wetherald and Manabe, 1975) demonstrate clones for periods of several months. Vary- strong effects and should provide a Stimulus ing frequency of such anomalies charac- towards regulär surveillance of the solar terizes climatic fluctuations with a time radiation by satellites. No convincing evi- scale of 101—l O2 yr; longer time series usu- dence, however, has been given äs yet on ally produce a spectrum with indefinite and abrupt significant changes of solar radiation inconstant peaks (Kutzbach and Bryson, in its visible, most intense part; at present, 1974). Is climate completely random? Or their role in climatic history can neither be what external sources may add or subtract confirmed nor be refuted (Lamb, 1977). energy to or from the climatic system, thus Frequency of explosive volcanic erup- causing abrupt climatic "changes" with a tions. The possible role of volcanic activity lifetime of 102-104 yr? Three groups of ex- has been discussed on the basis of Lamb's ternal causes shall be considered: (1970) catalog. Volcanic particles (mostly (i) solar events, 0.1-1 yum) in the stratosphere absorb and (ii) frequency of explosive volcanic backscatter part of the sun's radiation. Ab- eruptions, sorbed radiation heats stratospheric layers (üi) glacial surges of very large ice with but little effect on the ground. The loss sheets (Antarctica). of backscattered radiation thus produces surface cooling (Pollack et al., 1976; Hunt, Solar events. Variations of the solar 1977; Oliver, 1976; Mass and Schneider, "constant" with a time scale of l O2 yr and 1977). Convincing evidence for a simulta- above are unknown. Remarkable evidence neous occurrence of volcanic activity and for strong variations of solar activity during glacial events has recently been presented the "Little Ice Age" has recently been (Bray, 1974). giyen, together with small changes of the From Lamb's catalog (period 1500-1968) ABRUPT PALEOCLIMATIC EVENTS 141 the frequency of volcanic eruptions ex- simultaneous eruptions is shorter (about ceeding a given "dust veil index" (DVI) is one half) than predicted. In recent years the derived; to this the eruption of Fuego (Nov. frequency of severe earthquakes has in- 1974) has been added. For eruptions with creased substantially, while no similar peak DVI 3= 1000 the average recurrence time is of volcanic eruptions has been observed 21.5 yr (Table 1). Obviously the nearly si- until now (August 1978); a nearly simulta- multaneous occurrence of several eruptions neous occurrence of seismic and volcanic may produce strong climatic effects. As- activity has been predicted on the basis of suming a Poisson distribution of indepen- plate tectonics. There is little doubt that dent rare events, one can estimate the fre- tectonic processes manifest themselves quency with which eruptions cluster in a discontinually; the repeated sudden occur- period of 10 or 25 yr. Table l shows the rence of fissures of the Atlantic Rift s'ystem recurrence time of clustering volcanic in northern Iceland (near Lake Myvatn) eruptions for these periods and for different during 1977 was an impressive example for DVI values. the casual visitor. Checking the assumption of a Poisson The residence time of volcanogenic par- distribution against observations (Fig. 5), it ticles in the stratosphere is probably not is obvious that the real frequency distribu- constant, but depends on latitude. Apart tion is not random (Lamb, 1970); small and from the tropical Hadley cell (Reiter, 1975), large frequencies occur more frequently only isentropic exchanges in the area of than in a Poisson distribution. Apparently a tropopause gaps within jet streams are in- kind of "infectious" process exists; nearly strumental in removing particles from the simultaneous eruptions alternate with stratosphere. These processes are rare and longer periods of weak volcanic activity. inefficient in polar regions north of about The Poisson-predicted frequencies (Table Lat. 75°. In most seasons a stratospheric 1) of clustering volcanic activity are defi- circulation is developed above the poles nitely too low, and the recurrence time of (Hesstvedt, 1964) (Fig. 6). Impressed by

TABLE l RECURRENCE TIME OF n VOLCANIC EVENTS (Poisson Distribution) m (years) °

10-yr period 25- yr period

n DVI &1000 &500 3=300 &1000 &300

1 213 9.01 5.46 3 925 133 61 300 150 4 7,870 480 133 1,025 130 5 83,700 2,200 365 4,360 145 6 1.05 my 11,630 1,195 22,300 190 7 15.9 my 73,500 4,570 0.13 my 290 8 0.53 my 20,000 0.89 my 505 9 4.35 my 98,000 6.2 my 1,000 10 — — 0.54 my . — . 2,170 11 — — — — 5,210 12 — — — . — 13,660 13 — — — — 38,640 14 — . — . . — — 0.12 my 15 — — — — 0.38 my " m = average recurrence time, data from Lamb (1970). 142 HERMANN FLOHN

scene. It may also have been to the author's

Observed surprise, that 4 yr later one of his basic as- / ^ ^^\X d i s i r i b u t i o nV sumptions, melting processes at the bottom _ _ X —l _ of the ice, had been confirmed by the first deep Antarctic ice core (Gow et al., 1969). - While in East Antarctica only small, widely dispersed meltwater lakes have been ob- 1 >iI 01 2345678OOÜ 9 10 served (Oswald and de Robin, 1973), the ice Number of eruptions per 25-year-period ^ cover of West Antarctica (based below sea FIG. 5. Frequency of volcanic eruptions per 25-yr period (Lamb's catalogue 1500-1974 (DVI ^ 300)) and level) is perhaps in an unstable state Poisson distribution. (Hughes, 1975). A possible collapse of the West Antarctic ice could be related to a re- repeated visual observations of a strong treat of the grounding line of the ROSS Ice dust layer well above the tropopause in the Shelf. It may be suggested that the actual interior Arctic (notably after eruptions of world-wide sea-level rise of 1.2 mm/yr, in- Agung and Fuego), the author has proposed consistent with a positive mass budget of (Flohn, 1974a) a longer residence time in Antarctica, can be partly attributed to this the polar stratosphere (perhaps 3—5 yr) retreat. Assuming a retreat of 400 m/yr, an compared to the global average of l -1.5 yr. average thickness and width of shelf ice at A near coincidence of major eruptions, this line of 800 m and 800 km, respectively, such äs in the period 1810-1835, can thus yields an annual increase of floating shelf produce a prolonged and intensified ice of 0.32 x 800 ~250 knrYyr equivalent to stratospheric dust layer above the polar a global sea-level rise of 0.7 mm/yr. cap, with significant consequences for The author has speculated (Flohn, 1974b, thickness and extension of the Arctic ice Fig. 4) how an Antarctic surge in the Wed- and for a cooling of the polar vortex during dell Sea—Atlantic section could eventually summer. produce, by ice —ocean-atmosphere in- Glacial surges. The idea of an instabüity teractions, a quasipermanent cold low of the Antarctic ice dorne (Wilson, 1964) above the Gulfof Mexico. This could act äs arrived äs a real surprise on the scientific an anchor to a tropospheric summer trough

Autumn Spring

Öl— S 60° 30° 0° 30° 60° N FIG. 6. Meridional circulation and transport (transitional seasons), tropopauses, and Junge Layer of volcanic dust. ABRUPT PALEOCLIMATIC EVENTS 143 along the east coast of North America, äs a nous, but diachronous, spread over several necessary prerequisite for a permanent centuries or millennia (see also Pisias et snow cover at Labrador—Ungava, and äs a al., 1975). Since the sea-surface temperature nucleus for a North American ice dorne. distribution is one of the most powerful However, the estimated magnitude of an "forcing functions" for many atmospheric Antarctic surge (Flohn, 1974b), is appar- circulation models, this fact is, from the ently too high. If we assume instead a surge view point of a climätologist, one of the of the order of l X 106 km3, 80% of which most important discoveries made during the could break up into tabular icebergs cover- CLIMAP program. Another example is the ing 4 x 106 km2 with a thickness of 220 m, Start of the Altithermal during the and 20% into small ice floes, covering more äs early äs 9000 yr B.P. in the Subantarctic than 20 x 106 km2, the climatic. effect (Hays, 1977), where the thin seasonal sea should not be too different. Such a surge ice can easily retreat during the last radia- would produce a sea-level rise of 2.5 m. tion peak, compared with 6500 yr B.P. in Hughes (1975) and HoUin (1976) have col- the northern hemisphere with its long- lected evidence for such an event happen- lasting retreat of Continental ice sheets. The ing about 110,000 yr ago, ciosing the un- regionally different history of Continental usually warm Eem-Sangamon interglacial ice sheets (Starkel, 1977) is another reason period at the beginning of stage 5d. The Ant- for lack of synchrony. The different storage arctic Ice Sheet and the subantarctic drift time ("memory") of the climatic subsys- ice, 18 x 106 km2 of which is freezing and tems (Fig. 4) is responsible for the dia- melting seasonally, should be permanently chronism, which limits also the "almost- monitored by satenites äs a warning System intransitivity" of the climatic system. against initiation of an abrupt (and possibly disastrous) climatic event. 1NQUIRY INTO THE INITIATION OF A In all three cases the tacit assumption of a GLACIATION nearly simultaneous occürrence of climatic Most probably the nucleus of a new change, coherent with the "almost intran- glaciation of the northern continents should sitive" behavior of the climatic system, develop at the same location where the last should probably be revised. Ruddiman et decaying remnants of the preceding glacia- al. (1977a, 1977b) have recently shown that tion have been observed. However, this as- rapid climatic changes, äs indicated above, sumption may be challenged with the indi- at the North Atlantic Ocean occur äs a cation that during the development the ice time-dependent migration of a sharp baro- center may have migrated or split. Without clinic zone, i.e., the "polar front" of furiher arguments (Ives et al., 1975), we oceanographers, with a steep temperature may discuss a model with the first assump- gradient. This may cause, at a fixed point, a tion. temperature change on the order of 12 The initiation of a new glaciation must C/103 yr, but a migration speed of the start with a sufficiently large (synoptic- "polar front" over distances of 800—3000 scale) snow cover surviving one summer. km is only of the order 1 — 1.5 km/yr (or In this case, the next winter adds to its less). Certainly this figure has to be under- thickness, and the probability of persisting stood äs a result of strong smoothing, but rises from year to year. In a forest, the diameter of the oceanic eddies disturb- an effective permanent snow cover could ing the polar front is with 250 km much less be built up in a few decades (Flohn, 1974b). than that of atmospheric vortices, and their It is quite difficult to imagine how the pow- life time is much longer. This means that erful feedback process between rising snow in the human time scale these apparently cover, high albedo, and cold summer tem- abrupt climatic changes are not synchro- peratures could be effectively interrupted 144 HERMANN FLOHN and reversed; once started, it will produce, season feature interpreted äs a quasistation- within a few millennia, a large-scale ice ary Rossby wave downwind of the East dorne until an equilibrium state is reached. American trough. The evolution of such a nucleus on the vast Many of the earlier conclusions (Flohn, tundra plateaus of Baffin Island would 1974b) remain unaltered when the (still- probably need the same time. Here an ini- uncertain) assumption of a large Antarctic tial cooling of 2°C, lasting say, 50 yr, would surge is replaced by a cluster of volcanic be sufficient without increase of precipita- events or by a sudden decrease of the solar tion, since the plateaus of north-central constant. In such case äs the initiation of a Baffin Island are only 100-200 m below the new glaciation, an event which has oc- present snow line. Such a permanent curred at least 10-12 times during the last snowbed acts äs another anchor of a million years, imagination has to be con- quasistationary trough along the eastern trolled by physical reasoning and, finally, coast of North America during summer, mathematical modeling. i.e., during that season where it actually CONCLUSIONS disappears. This Situation leads immedi- From the viewpoint of a climatologist, ately to cooling and increasing snowfall the most important result of these investi- over the Ungava Plateau (53-60°N Lat.), gations is the fact, that within the "human" where a similar snow cover could form. time scale of about 100 yr or less, our cli- A mathematical model has been designed mate is (or can be in some periods) much by Andrews and Mahaffy (1976), assuming more variable than hitherto assumed. Es- a net balance of snowfall increasing by a pecially important, and indeed disquieting, factor 3. In two experiments with this is the evidence of abrupt coolings within model the ice volume and area increase warm (interglacial) periods, apparently äs only slowly, slower äs indicated by the ob- rare events with a recurrence time on the servations (see Bloom et al., 1974). In Ex- order of 104 yr. Apparently their intensity periment I the prescribed model input of the can surpass (with up to 5°C/50 yr) all cli- time Variation of the equilibrium-line al- matic changes during the Holocene. The titude is obviously too slow; in Experiment last comparable events occurred during the II its instantaneous initial lowering of 400 m "Alleröd complex" of the Late Glacial (equivalent to -2 to -3°C) was assumed, (14,000-10,000 yr ago), feit also in equato- but a positive-feedback effect of the grow- rial South America and Africa. Similar ing snowfields has been introduced only events were observed at the transition be- after 2000 yr. A slightly different positive- feedback effect (Kellogg, 1973) between tween stages 5a and 4 (about 73,000-65,000 yr ago) (cf. also Stuiver et al., 1978).] snowcover, albedo, temperature, and Speculations about their geophysical spreading snowfall would further accelerate mechanism have been made (above). The the processes, which are almost certainly faster than in the model. occurrence of clusters of volcanic eruptions seems to be the most appropriate model, to Similar arguments could be given for be checked by further investigations, espe- Fennoscandia (Ives et al., 1975), with a sligriüy different orography. But from a meteorological viewpoint this is unneces- 1 In this context the radical faunal discontinuity at sary: The European ice dorne was much the Cretaceous-Tertiary boundary (about 65 my ago) smaller and disappeared earlier. Most may deserve attention; according to Kent (1977), the probably its formation started some cen- overturn of planktonic foraminifera, slightly earlier turies later, following the formation of a than the extinction of dinosaurs (Butler et al., 1977,), lasted not longer than 104 yr. Until now, no evidence permanent upper trough west of the for an accompanying climate change is known (cf. Norwegian coast, actually a frequent cold- however the hypothesis proposed by McLean). ABRUPT PALEOCLIMATIC EVENTS 145 cially looking at laminated cores from ice APPENDIX: ESTIMATED TIME SCALE sheets and fossil lakes. Of special (current) OF GRAND PILE PEAT BOG interest is the onset of these events. It has Any estimate of the accumulation rate of been suggested (Mitchell, pers. comm., the Grand Pile Peat Bog (Woillard, 1975, 1978), that the destruction of living forests 1978) must be based on the differentiation may be only a response to a more gradual between gyttja accumulated during warm cooling passing through a threshold beyond phases, and clay or silty-clay sediment which survival was no longer possible. during the cold phases. Only one radiocar- This Suggestion formulates an interesting bon date exists for the last interstadial; an- question for a climate/ecology model. After other date in an older interstadial gives only looking into details of several relevant pol- a minimum age. Nevertheless the approxi- len analyses of long cores, the author feels mate dates found by analogy to well- that the simultaneous disappearance of evidenced chronologies of the last glacia- different thermophilous trees together with tion and to the world-wide chronology of a drastic increase of subarctic trees (Betula, the last interglacial with its characteristic Pinus) or even of graminae only seems to be two intermediate cold phases can be used rather inconsistent with such a conserva- äs a reasonable first approximation. tive view. Local pests could be made re- The lithological differentiation (Woillard, sponsible at an individual site; this in- 1978a, Table 1) suggests the assumption terpretation seems to be unlikely in a case that the accumulation rates of gyttja and (Holsteinian) where such apparently si- gyttja-clay (G) and of silty clay (SC) are multaneous events are observed at far- constant during the whole interval between distant localities. the beginning of the last interglacial The short duration and high intensity of (127,000 yr ago) and the end of the last gla- such events in some continental areas cial, which can be put at 14,000 yr B.P. This suggests the hypothesis of a diachronous identifies the two sediment types with more semiabortive glaciation, occurring first in or less constant environmental conditions. Baffinland—Labrador and later (with some From Table 2 of Woillard (1978a; depth in delay on the Order of 103 yr) in Fennoscan- cm) the following equations can be derived: dia, at a time when the orbital elements do not favor a major glaciation. This could ex- Last glaciation 270 G + 492.5 SC = 59,000 yr plain an appreciable increase of the global Last interglacial 518 G + 39.5 SC = 54,000 yr ice volume spread over a period of, say, To avoid entering into the present confu- 3000 yr together with a sharp but short sion of terminology, the author defmes here cooling in Europe lasting not more than a äs "last glaciation" only stages 4-2 of few centuries, accompanied by severe cli- Emiliani and Shackleton (1974), and matic anomalies in northern and tropical likewise äs "last interglacial" stages 5a-e. Africa. Such a terminology seems to represent The problem of abrupt intense coolings quite satisfactorily the time sequence of during an interglacial climate similar to the Grand Pile Bog (Woillard, 1978a) and some present climate resembles, to some extent, other European data; the author feels in- the Damocles' sword hanging high above competent to make large-scale comparison the globe and its inhabitants. Because of its (see Kukla, 1977). possible consequences for the human race, Another estimate has been based on the its study deserves a much higher priority. It assumption of two constant accumulation has been discussed in climatological circles rates representing cold and warm periods; more than once; this study can hopefully this yielded similar, but apparently less- only serve äs a trigger for a more serious reliable values for G and SC. In these treatment, including an assessment of risk. equations, the position of the beginning of 146 HERMANN FLOHN

TABLE AI DURATION OF SHORT COLD AND WARM PHASES DURING THE TRANSITION BETWEEN STAGES 5e AND 4 AT GRAND PILE BOG (WOILLARD, 1978a) Depth (cm) Duration (yr) Cold phases: Early (Wurm) Stadial II 1245-1260 980 Early (Wurm) Stadial I 1267.5-1272.5 330 Melisey II incl. Tundra (=5b) 1358-1382.5 1600 Melisey I incl. Tundra (=5d) 1547.5-1562.5 980 Warm phases: Ognon II 1237.5-1245 740 Ognon I 1260-1267.5 740

stage 4 (=73,000 yr) is put tentatively at the the last interglacial (=stage 5a—e) may also end of St. Germain II at a depth of 1272.5 be derived beginning of 5e = 127,000 yr): cm. The solution of these equations yields the short cold phases were then situated at an estimated accumulation rate of gyttja of 100,000 yr (=stage 5d) and 83,000-82,000 10.1 cm/103 yr, of silty clay of 15.3 cm/103 yr (=stage 5b). This is at variance with the yr, with an estimated error of less than data given by Bloom et al. (1974) (ca. 10—15%. Using these values, the following 117,000 and 93,000 yr, respectively). The durations for the interesting short phases duration of the Bern Interglacial (stage 5e, can be estimated, äs shown in Table AI. including boreal taiga) can be estimated äs These phases apparently have a duration about 26,000 yr. Since the substantial vari- similar to the fluctuations at the end of ations of the accumulation rate cannot be Wurm Glaciation, around Bölling and Al- excluded, this result remains tentative (see leröd interstadials found in nearby cores also Woillard, 1978b). (Woillard, 1975, 1978b). Since these esti- In her pollen analysis of the warm phas- mates include the transition times and are es, Woillard distinguishes 144 sections with derived from the Vegetation, which re- a thickness varying between 2.5 and 25 sponds to climatic change only with some cm, presumably dependent on the varying delay, the time scale of climatic change at pollen content; this gives an average time the beginning and end of each of these resolution of about 350 yr. Such a resolu- phases cannot be much larger than 100 yr. tion is much better than that of ocean cores An approximate absolute chronology of (well above 1000 yr). If the activity of

TABLE A2 SELECTION OF OCEAN CORES REPRESENTING STAGES 5a-e INDIVIDUALLY

Core Latitude Longitude Author V 27-60 72.2° N 8.6° E T. Kellogg (1976) v 28-14 64.8° N 29.6° W T. Kellogg (1976) K 708-8 52.7° N 22.5° W Ruddiman et al. (1977a,b) Meteor 12392 23.1° N 17.7° W Shackleton(1977) V 18-357 15° N 80° W Prell et al. (1976) V 22-196 13.8° N 19° W Gardner et al (1976) V 25-59 1.4° N 33.5° W Be et al. (1976) V 180-73 0.2° N 23° W Gardener et al. (1976) V 25-60 3.6° S 35.2° W Be et al. (1976)

V 28-238 1.0° N 160.5° E Shackleton and Opdyke (1973) V 19-29 3.6° S 83.2° W Shackleton (1977) RC 11-120 43.5° S 79.4° E Hayse/a/. (1976) ABRUPT PALEOCLIMATIC EVENTS 147 bottom-dwelling animals in a freshwater Butzer, K. W. (1976). Pleistocene Climates. Geosci- lake causes similar bioturbation, the often ence and Man 13, 27-43. abrupt changes of pollen content between Cita, M. B., Vergnaud-Grazzini, C., Robert, C., Chamley, H., Ciaranfi, N., and S. d'Onofrio (1977). two adjacent sections seems to exclude sig- Paleoclimatic record of a long deep-sea core from nificant mixing above, say, 1 — 2 cm. the eastern Mediterranean. Research 8, Further investigations with more precise 205-235. methods are needed. Coope, G. R. (1977). Fossil coleoptera assemblages äs The füll sequence of stage 5 (Emiliani and sensitive indicators of climatic changes during the Devensian (Last) cold stage. Phttosophical Trans- Shackleton, 1974) with its three warm actlons of the Royal Society. London B 280, phases and two cold interruptions is repre- 313-340. sented, in addition to Greenland, Southeast Dansgaard, W., Johnson, S. J., Clausen, H. B., and France, and Macedonia, by a large number Langway, C. C., Jr. (1971). Climatic record revealed of ocean cores, mainly in the Atlantic by the Camp Century ice core. In "The Late Cenozoic Glacial Ages" (K. K. Turekian, Ed.), p. (Table A2), but also in the eastern Mediter- 37-56. Yale Univ. Press, New Haven. ranean (Cita et al., 1977). It is also repre- Dansgaard, W., Johnson, S. J., Clausen, H. B., and sented by a sequence of lake Sediments and Langway, C. C., Jr. (1972). Speculation about the intermediate tills (numbered 6 and 7) in next glaciation. Quaternary Research 2, 396—398. western Wyoming (Richmond). Duplessy, J. C., Labeyrie, J., Lalou, C., and Nguyen, H. V. (1970). Continental Climatic variations be- ACKNOWLEDGMENTS tween 130,000 and 90,000 years BP. Nature (Lon- The author is indebted to several colleagues for don) 226, 631-633. stimulating discussions; among them are J. M. Mitch- Eddy, J. A. (1976). The Maunder Minimum. Science ell, Jr. and O. A. Kukla (äs reviewers), A. Berger, R. 192, 1189-1202; cf. also Science 198, (1977), Bryson, M. J. Budyko, F. K. Hare, W. W. Kellogg, J. 824-829. Kutzbach, H. Müller, and H. H. Lamb. Emiliani, C., and Shackleton, N.J. (1974). The Brunnes : Isotopic Paleotemperatures and REFERENCES Geochronology. Science 183, 511—514. Andrews, J. T., and Mahaffy, M. A. W. (1976). Fink, J., and Kukla, G. J. (1977). Pleistocene climates Growth rate of the Laurentide ice sheet and sea level in Central Europe: At least 17 interglacials after the lowering (with emphasis on the 115,000 BP sea level Olduvai Event. Quaternary Research 7, 363—371. low). Quaternary Research 6, 167—183. Flohn, H. (1974a). Instabilität und anthropogene B6, A. W. H., Damuth, J. E., Lott, L., and Free, R. Modifikation des Klimas. Annalen der Meteo- (1976). Late Quaternary climatic record in western rologie, Neue Folge 9, 25—31. equatorial Atlantic Sediments. Geological Society of Flohn, H. (1974b). Background of ageophysical model America Memoir 145, 165—200. of the initiation of the next glaciation. Quaternary Berger, A. L. (1977). Support for the astronomical Research 4, 385-404. theory of climatic change. Nature (London) 269, Flohn, H. (1978). Abrupt events in climatic history. In 44-54. "Climatic Changes and Variability, a Southern Berger, A. L. (1978). Long-term variations of caloric Perspective" (A. B. fitlocket al., Eds.), Vol. 23, p. insolation resulting from the earth's Orbital ele- 124-134. Cambridge Univ. Press, London/New ments. Quaternary Research 9, 139 — 167. York. Bloom, A. L., Broecker, W. S., Chappell, J. M. A., Frenzel, B. (1967). "Die Klimaschwankungen des Eis- Matthews, R. K., and Mesolella, K. J. (1974). zeitalters." Vieweg, Braunschweig. Quaternary sea-level fluctuations on a tectonic Gardener, J. V., and Hays, J. D. (1976). Responses of coast: New 230 Th/234 U dates from the Huon sea-surface temperature and circulation to global Peninsula, New Guinea. Quaternary Research 4, climatic change during the past 200,000 years in the 184-205. eastern equatorial Atlantic Ocean. Geological Soci- Bray, J. R. (1974). Volcanism and glaciation during the ety of America Memoir 145, 221—246. last 40 mülenia. Nature (London) 252, 679-680. GARP (1975). Global Atmospheric Research Pro- Bray, J. R. (1977). Pleistocene volcanism and glacia- gramme: The Physical Basis of Climate and Climate tion. Science 197, 251-254. Modelling. ICSU-WMO, GARP Publication Series Butler, R. F., Lindsay, E.H., Jacobs, L. L., and No. 16. Johnson, N. M. (1977). Magnetostratigraphy of the Gow, A. J., Ueda, H. T., and Garfield, D. E. (1969). Cretaceous/Tertiary boundary in the San Juan Antarctic ice sheet: Preliminary results of first core Basin, New Mexico. Nature (London) 267, hole to bedrock. Science 16l, 1011-1013. 318-323. Hays, J. D., Imbrie, J., and Shackleton, N. J. (1976a). 148 HERMANN FLOHN

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