Full-glacial summer temperatures in eastern North America as inferred from Wisconsinan vegetational zonation

Palaeogeography, Palaeoclimatology, Palaeoecology, 79 (1990): 305 312

305

Elsevier Science Publishers B.V., Amsterdam

Full-glacial summer temperatures in eastern North America as inferred from Wisconsinan vegetational zonation D. W . W o o d c o c k a'l a n d P. V. W e l l s b

aNational Center for Atmospheric Research, P.O. Box 3000, Boulder, CO 80307-3000, U.S.A. bDepartment of Botany, UniversiO, of Kansas, Lawrence, KS 66045, U.S.A. (Received August 14, 1989; revised and accepted May 1, 1990)

ABSTRACT Woodcock, D. W. and Wells, P. V., 1990. Full-glacial summer temperatures in eastern North America as inferred from Wisconsinan vegetational zonation. Palaeogeogr., Palaeoclimatol., Palaeoecol., 79: 305-312. A variety of types of fossil evidence- - pollen, macrofossils, and snails - - document the occurrence of boreal forest species at low latitudes of eastern North America during the last full glacial (18,000 yr B.P.). Both the northern and southern boundaries of boreal coniferous forest are determined by limiting values of summer warmth. Knowledge of these limiting factors permits construction of a gradient of Wisconsinan summer temperatures. The reconstructed temperature departures are 5-12°C for the eastern part of the continent south of the ice, with temperature departures of no less than 5-6°C along the Gulf coast. Information regarding occurrence of tundra and depression of treeline in the Appalachian mountains provides another estimate of summer temperature depression that is in agreement with these results. Comparison with proxy data from other parts of the world shows that the degree of temperature depression is of about the same degree as that postulated for the tropics (Rind and Peteet, 1985; Lui and Colinvaux, 1985) on the basis of the elevational shifts in vegetational zones and snowline. Quantitative estimates of past temperatures (or temperature departures) of the type produced here are valuable because they permit comparison with model simulations of the full-glacialclimate. The depression in summer temperatures indicated is lower than most recent modeling results using CLIMAP boundary conditions and adds to suggestions that the reconstructed seasurface temperatures used as boundary conditions in simulations of glacial and Holocene climates may be in error.

Introduction Q u a n t i t a t i v e estimates of past climates are particularly valuable in the o n g o i n g a t t e m p t to d o c u m e n t the n a t u r a l variability o f the climate system. Vegetation exhibits a sensitive response to climate b u t the climatic significance of records o f past vegetation is often difficult to evaluate. A m o n g the factors that can complicate analysis are the variety o f climatic a n d n o n c l i m a t i c factors that can limit p l a n t distributions, the incomplete unders t a n d i n g of the way in which climate is limiting for p l a n t growth, interactions o f precipitation a n d temperature with respect to effects o n vegetation, 1Present address: Department of Geosciences, University of Missouri-Kansas City, Kansas City, MO 64110, U.S.A. 0031-0182/90/$03.50

© 1990 Elsevier Science Publishers B.V.

a n d the effects o f presence or absence of dispersal agents or other historically d e t e r m i n e d factors o n d i s t r i b u t i o n s or even the characteristics o f entire v e g e t a t i o n a l zones. T h e r e do exist, however, certain c i r c u m s t a n c e s in which the response of v e g e t a t i o n to l i m i t i n g climatic factors is relatively clearcut. It is p r o p o s e d that the r e l a t i o n s h i p between the full-glacial p a t t e r n of v e g e t a t i o n a l zones in eastern N o r t h A m e r i c a a n d limiting values o f s u m m e r t e m p e r a t u r e is one such example. Wolfe (1979) has s h o w n that the mesic forests of eastern Asia fall into p h y s i o g n o m i c units b o u n d e d by t e m p e r a t u r e p a r a m e t e r s a n d that similar p a t t e r n s can be recognized in o t h e r parts of the world where p r e c i p i t a t i o n is n o t an i m p o r t a n t l i m i t i n g factor for forest growth. The h u m i d east coasts o f the c o n t i n e n t s , i n c l u d i n g

306

eastern North America, can generally be considered to fall under this category. This paper presents a reconstruction of summer temperatures (warm-month mean) for North America east of the Rockies based on data from pollen, plant macrofossils, and snails. The analysis is based on the climatic requirements of zonal vegetation and has some similarity to methods employed by Soviet researchers (Savina and Khotinsky, 1983) in reconstructing Quaternary climates. One of the goals of this research is to arrive at a coherent picture of climatic conditions on a global scale. Toward this end, the reconstruction for eastern North America will be compared with results of similar analyses for other parts of he world. Although a variety of indicators have been used to reconstruct climatic conditions over the continents, the discussion here focuses on botanical indicators (or snails, which are a proxy indicator of vegetation) and locations in which areal estimates of climate variables have been made based on botanical indicators and using techniques similar to those used here. The quantitative estimates generated and the global patterns that emerge should be helpful in evaluating mathematical simulations of ice age climates. Some generalized comparisons will be made to recent modeling results. Summer temperatures are chosen as the focus of this study for the following reasons. First, the relationship between limiting values of summer warmth and vegetation can be seen fairly easily in the area under consideration. Other climatic entities generally prove more problematic. Winter tempertures are one example, since many temperate-latitude taxa appear to have wider physiological tolerances to low-temperature extremes than their ranges suggest (George et al., 1982; Velichko, 1983). The assumption is made here that precipitation is not an important limiting factor in the humid eastern part of the North American continent. However, even where precipitation is limiting for vegetation, estimates of this variable are complicated by the nonindependence of precipitation and temperature and the fact that seasonal occurrence of precipitation may be another important consideration. Precipitation is also, coinci-

D.W. WOODCOCK AND P. V. WELLS

dentally, the most difficult quantity to simulate in the mathematical models - - another reason that temperatures may be a more useful entity to work with. In addition, knowledge of temperatures on an areal basis may permit construction of global temperature gradients, which are important determinants of global circulation and precipitation patterns.

The full-glacial record Occurrence of boreal forest A summary of the pollen and macrofossil record for eastern North America that includes maps of the reconstructed full-glacial vegetation can be found in Delcourt and Delcourt (1987). Other treatments are Watts (1983), Kutzback and Wright 0985), and Jacobson et al. (1987). Boreal-forest species occurred far to the south of their present range (Fig.l; Watts, 1980; Delcourt and Delcourt, 1987). Tundra is recorded at high altitudes of the Appalachian mountains and probably occupied a narrow zone south of the ice sheet. Pinus banksiana and Picea were the most abundant taxa in the boreal forest, with P. banksiana predominating east of the Appalachians and Picea to the west.

~tJ~nc~lfsrous

~

limit of forest

Fig.1. Location of North American full-glacial recqrds discussed in text. Dots represent locations of pollen and macrofossil records. The western Kansas fossil snail assemblages have been radiocarbon-dated and are full-glacial or equivalent (Wells and Stewart, 1987); the Texas records are dated by stratigraphic means. Location of the Laurentide ice sheet indicated in outline.

FULL-GLACIAL SUMMER TEMPERATURES IN EASTERN NORTH AMERICA INFERRED FROM WISCONSINAN VEGETATIONAL ZONAIION

Boreal forest is recorded at sites in the southern Appalachians in Alabama and Georgia and at a location in the Coastal Plain of South Carolina at about the same latitude (34°N; Watts, 1980). The southernmost full-glacial 1 records of boreal species are in west-central Georgia (Watts, 1980) and north-central Louisiana (Kolb and Fredlund, ! 983). At the Louisiana site, deciduous-forest taxa dominate, indicting that the boreal-forest boundary was somewhat to the north. At a location only slightly farther south, in Alabama, deciduous species with no spruce are recorded at this same time period (Delcourt, 1980). The boundary between boreal and deciduous forest is thus projected as extending across the southern Coastal Plain to the Mississippi Valley at a latitude of approximately 33°N. The southern limit of boreal forest farther west, in the central Great Plains is not as yet firmly established. (These records are presented in somewhat more detail since they have not been summarized elsewhere.) There are full- to lateglacial pollen and/or macrofossil records of Picea, Larix, and, locally, P. banksiana (toward the southeast) along the eastern margin of the Plains from Saskatchewan and the Dakotas south to northeastern Kansas and southwestern Missouri. Recently, peat samples from south-central Kansas (near Wichita) have yielded abundant macrofossils and pollen of Picea and a cone of P. banksiana (Fredland and Jaumann, 1987; Jaumann, unpublished). Far to the west, in northwestern Kansas and adjacent Nebraska, abundant charcoal, leaves, and cones of Picea (mostly P. glauca) and some needles of the cordilleran Pinusflexilis have been uncovered in full-glacial loessal deposits dated from 14,000 to 18,000 yr ago (Wells and Stewart, 1987). Directly associted fossil landsnail and smallmammal assemblages are decidedly boreal in character at various sites across the northern half of Kansas. In southwestern Kansas, however, lMacrofossils of white spruce [Piceaglauca (Moench.) Voss.] have been recovered in deposits in southeastern Louisiana (Delcourt and Delcourt, 1977) that data to the late glacial (12,740 yr B.P.). The authors attribute this extreme southerly occurrenceto localizedclimaticconditions involvingpersistent summer fog associated with flowof glacial meltwater down the Mississippi.

307

there is a notable contrast. The pleniglacial (18,000 yr B.P.) landsnail fauna from southwestern Kansas and adjacent Oklahoma records the absence of three hyperboreal species present in northern Kansas; also, many of the boreal species are lacking in the mammalian fauna and a few southern taxa are present that are absent farther north (Stewart, 1987). These data suggest a climatic boundary at about 36 38°N on the central Great Plains, corresponding to a shift in vegetation from a more boreal spruce-aspen taiga north of 38 ° possibly to a more cordilleran pine-aspen woodland in southwestern Kansas and the "Panhandle" Plains of Oklahoma and Texas. Late-Wisconsinan landsnail faunas from the Llano Estacado of Texas definitely indicate cool, moist woodlands south to at least 3 4 N ; Populus sp. successional to cordilleran conifers is a possibility (Frye and Leonard, 1957; Wendorf, 1961). Thus the pattern of latitudihal zonation in the Great Plains involves a shift from boreal (Picea-dominated) forest to cordilleran conifers with an admixture of Populus (P. tremuloides), rather than from boreal taiga to eastern deciduous forest as in eastern North America. Forests with dominant conifers (Picea or cordilleran pines) thus extended to quite low latitudes of the Great Plains (to 34°N). The deciduous forests that are recorded in areas of the eastern part of the Gulf Coast probably extended west across Texas, although their presence has not been documented. The boundary between deciduous tbrest and needle leaf evergreen (taiga-like) association is thus projected as extending west across the southern Plains at a latitude of approximately 34°N.

Summer temperatures Although mean annual temperature is the most important climatic factor limiting vegetational zones in the humid tropics, at higher latitudes (and at higher elevations of the tropics) summer warmth becomes an important limiting factor for tree growth. Thus the shift in dominance from broadleaf deciduous trees to the needleleaved evergreens of the boreal forest corresponds to a warm-month mean of 20°C (Wolfe, 1979). Likewise, summer temperatures are the factor that limits boreal forest

308

D.W. WOODCOCK AND P. V. WELLS

in its northern extent. A warm-month mean of 10°C approximates the temperature below which tree growth is not possible, a relationship that has been recognized by many authors (Walter, 1973; Wolfe, 1979; Box, 1981). By taking the northern and southern limits of boreal-type forest to represent the warm-month isotherms of l0 ° and 20°C and comparing the generated temperature gradient to the current average July temperatures using graphical means, it is possible to generate a map of the departure of summer temperatures for the full-glacial (Fig.2). The indicated temperature departures are minus 5-12°C for the eastern part of the continent and no less than 5-6°C for the southern Coastal Plain. Summer temperatures were thus significantly lower than at present. An independent check of the estimates given here can be made by reference to the records of tundra vegetation in the mountains of Maryland and West Virginia (Maxwell and Davis, 1972; Watts, 1980). Pieea pollen is recorded in both cases, indicating that these two sites were not very far from the treeline. The limit of tree growth along an elevational gradient again generally approximates the 10°C warm-month mean (Billings, 1988). Comparison with current average July temperture (approximately 21 °C) at these two sites 1, which are at elevations of 800 and 1000 m, yields an estimate of depression of summer temperatures relative to

the present of ll°C. This is a minimum estimate and is in agreement with temperature departures derived from the shift of zonal vegetation.

Modern and full-glacial vegetational zonation Comparison of the vegetational zones expected under the present and full-glacial temperature regimes using the temperature parameters established by Wolfe for mesic forests (1979) yields the following results. If the temperature parameters are plotted on a graph of generalized warm- vs. cold-month mean temperatures for eastern North America (Fig.3), the sequence of forest types mixed broadleaf deciduous/mixed northern hardwood/ mixed coniferous/taiga occurs going north along a latitudinal gradient. (Mixed coniferous forest and taiga together comprise boreal forest.) These categories can be generally recognized in the vegetation of North America. The most evident difference between the zonational pattern in North America and that of Asia, which Wolfe uses as the basis for his analysis, is that, in North America, deciduous forest occurs in some warm-climate zones which are in Asia occupied by broadleaf evergreen forests (a phenomenon attributed to outbreaks of

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Fig.2. Summer temperature departures (18,000yr B.P.present).

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Fig.3. ModernvegetationalzonationofeasternNorthAmerica. Warm- and cold-monthmeans are generalizedvalues for the eastern part of the continent.

F U L L - G L A C I A L S U M M E R T E M P E R A T U R E S IN E A S T E R N N O R T H A M E R I C A I N F E R R E D F R O M W I S C O N S I N A N V E G E T A T I O N A L Z O N A l ION

cold Arctic air and the susceptibility of broadleaf evergreens to below-freezing temperatures; Wolfe, 1979). When, however, estimated Wisconsinan tempertaures are plotted in a similar fashion, a different vegetational zonation results (Fig.4). Here warmmonth mean temperatures are values derived from the analysis presented in this paper and coldmonth means are generalized values from a general-circulation-model simulation of full-glacial conditions carried out using C L I M A P boundary conditions (Kutzbach and Guetter, 1986). Apart from the southward shift of vegetational zones relative to the present, the most evident difference is the absence of the zone of mixed northern hardwood forest. Mixed northern hardwood and mixed coniferous forest are often treated as one vegetational zone since many tree species occur in both associations; they differ as to whether evergreen or deciduous trees are dominant. Although there is no record of mixed northern hardwood assemblages per se in the pollen record, Delcourt and Delcourt (1981, 1987) have maped a narrow zone of mixed coniferous-northern hardwood forest between boreal and mixed deciduous forest. The steepness of the glacial-age ecotone between boreal forest, with evergreens

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dominant, and mixed deciduous forest, with Quercus and Carva, together with southern pine, predominating, has been remarked on (Delcourt, 1980). The occurrence of this abrupt boundary has been related to the positioning of the glacial-age Polar Front (Delcourt and Delcourt, 1984). Analysis presented here suggests that (a) over the range of mean annual temperatures in which mixed northern hardwoods could have existed (3-10°C), summer temperatures were too low and (b) the abruptness of the transition between boreal forest and mixed deciduous forest is related to the absence of the mixed northern hardwoods zone (and its climatic niche). This analysis also suggests that mixed coniferous forest should have existed at a range of latitudes during the full glacial. This vegetational type has conifers dominant but differs from taiga in including a significant component of deciduous trees (Wolfe, 1979). The diversity of some of the borealforest assemblages, particularly from the site in western Tennessee (Delcourt et al., 1980), suggests that mixed coniferous forest might be an appropriate categorization of the more southerly borealforest assemblages.

Comparison to other regions Europe and Soviet Eurasia

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Fig.4. Wisconsinan vegetational zonation. Values for warmmonth means are generalized from the vegetational results presented here. Cold-month means are from the GCM simulations for 18,000yr B.P. by Kutzbach (1987).

Vegetational records from northern Eurasia document the prevalence of dry-climate vegetation and the existence of marked differences between the full-glacial vegetational zones and those of the present (Velichko, 1983). The latter consideration has made it difficult to base climate reconstructions on occurrence of zonal vegetation. Instead, the technique of floristic indicators, a type of analysis that involves incorporation of the climatic tolerances of all the represented species, has been used. The maps of summer temperature departures that have been produced on the basis of these reconstructions (Velichko, 1983) show strong negative departures near the Scandinavian ice sheet and lack of significant departures across most of the Siberian landmass. To the south, at the latitude of Soviet Georgia, summer temperatures are reconstructed as 5°C or more below present values.

310

The equatorial regions Although the low-latitude areas have been interpreted variously with regard to precipitation regime during the full-glacial, the proxy data relevant to temperature are relatively unambiguous. Records from several high-elevational areas in the low latitudes (Hawaiian Islands, Colombian Andes, New Guinea, and East Africa) show that the snowline and the upper limit of the treeline were considerably depressed relative to today (Rind and Peteet, 1985). Although the snowline can be affected by temperature and amount of precipitation, the only factor that can explain depression in both treeline and snowline is lower temperature. Temperature departures of minus 5-6°C have been postulated based on the degree of depression seen (approximately 1000 m; Rind and Peteet, 1985). One of the very few lower-elevation records from the New World tropics documents the presence, during the last glacial period, of Andean conifers at elevations 700 m lower than they presently occur (Lui and Colinvaux, 1985); these data have been interpreted as indicating a minimum temperature departure of 4.5°C, a figure consistent with above-cited estimates.

Summary and conclusions The degree of reduction in summer tempertures inferred for eastern North America contrasts with results for Soviet Eurasia. The contrast between the proposed temperature departures for North America (5-12°C) and northern Eurasia (0°C) may relate to a series of zonal asymmetries between the Western and Eastern hemispheres during the last full glacial involving extent and location of continental glaciers and permafrost, occurrence of glacial-age lakes, and characteristics of vegetation (Velichko, 1980). The explanation advanced by Velichko for these zonal asymmetries is an increase in easterly transport in the lower layers of the atmosphere at the time of the last glacial maximum associated with intensification of surface high pressure over the continents. The effect of this proposed change in circulation patterns would have been to increase aridity over large areas of the Siberian landmass, whereas conditions would have

D.W. WOODCOCK AND P. V. WELLS

remained wet in eastern North America because of easterly transport of humid air from the Atlantic and the existence of a warm current in the lower latitudes of the North Atlantic. Apart from the Siberian landmass, summer temperatures that were lower by at least 5°C appear to have been characteristic of the Northern Hemisphere. The negative departures at the middle latitudes (5°C for southern Europe, 5-6°C for southeastern North America) are of about the same magnitude as those proposed in the equatorial areas (minus 5°C). The reconstructed temperature gradient across the low and middle latitudes would have been approximately the same as at present. With these general indications as to summer temperatures in mind, comparison can be made to the results of general circulation models. In these treatments, boundary conditions (extent of land and sea ice, land-surface albedo, sea-surface temperatures) at 18,000 yr B.P. are used to simulate full-glacial climatic conditions. A series of modeling experiments of this sort have been run, beginning with that of Gates in 1976. Comparison is made here to two of the more recent modeling attempts. Variations exist in treatment of boundary conditions, surface parameterizations, and various other aspects of model construction. In Manabe and Broccoli (1984), sea ice and seasurface temperature are determined interactively; in Kutzbach and Guetter (1986) and Kutzbach (1987), these quantities are specified by the CLIMAP data set. Summer temperature departures are in both cases constructed with respect to control runs of the respective models. Comparisons can be made most directly by examination of maps of surface temperature departures; estimates of these quantities are presented in Table 1. Model estimates of summer temperature departures are in general less than that arrived at using reconstruction approaches. Note that these are departures over an area, rather than a range at any one point; for example, the difference between modeled and reconstructed temperatures in the tropics is 5°C over a wide area. The only area in which estimated departures are not lower than empirically derived estimates is eastern Siberia; in this region, the models are

FULL-GLACIAL S U M M E R T E M P E R A T U R E S IN EASTERN NORTH AMERICA I N F E R R E D FROM WISCONSINAN VEGETATIONAL ZONATION

311

TABLE 1 Departures in summer temperature CC): modeling results versus reconstructions Eastern North America South of ice Manabe and Broccoli (1984)

2 to

Kutzbach and Guetter (1986)

3 to - 7

S. Europe

E. Siberia

Southeasl

12

+ 2 to + 4

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Rind and Peteet (1985) Reconstructionsb

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5to -6

~'Annual temperature. hSee text for references. consistent in estimating negative temperture departures, whereas reconstructions yield temperatures similar to the present. Kutzbach and Wright (1985) compared modeling results for 18,000 yr B.P. with geological evidence from the Northern Hemisphere, noting that the slight depressions in summer temperature suggested by the models are not consistent with the depression of vegetational zones (to 1200 1500 m in Death Valley; Woodcock, 1987) seen in western North America. Interpretation of these elevational depressions is complicated by difficulties in sorting out the effects of precipitation and temperture on vegetation; however, estimates of depressions in summer temperatures for the southwestern United States have ranged as high as 10°C (Galloway, 1983) and 8 14 C (Woodcock, 1987), Discrepancies between model and reconstruction estimates in the tropics have been treated in some detail by Rind and Peteet (1985). At the low latitudes, sea-surface temperatures are the most important of the boundary conditions in influencing atmospheric temperatures. In view of the significant differences between temperatures simulated using C L I M A P boundary conditions and those indicated by proxy data, Rind and Peteet (1985) carried out a modeling run in which the C L I M A P sea-surface temperatures were lowered by 2':'C globally. The results produced are in better agreement with reconstructed temperatures. In addition, as these authors note, lower temperatures are associated with increased aridity in the equatorial regions, where many estimates of drier conditions have been made.

Rind and Peteet (1985) advance the possibility that the C L I M A P sea-surface temperatures may not be accurate, especially in the area of the tropical oceans. The issue is important since sea-surface temperatures are used as boundary conditions in many model simulations of glacial and Holocene climates (see C O H M A P , 1988). The results presented here for North America are consistent with interpretations of Rind and Peteet. Also relevant in this context are the results of Manabe and Broccoli (1985): in their model simulations, sea-surface temperatures were determined interactively and temperture estimates are 1 - 2 ' C lower than the C L I M A P figures for the tropical areas. Two points are particularly important to stress in conclusion: (1) Reason exists to reexamine the C L I M A P techniques and the assumptions involved in reconstructing sea-surface temperatures. (2) Further analyses that use paleobotanical records to derive quantitative estimates of the fullglacial climate are both possible and needed.

References

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312 Delcourt, P. A., 1980. Goshen Springs: Late Quaternary Vegetation record for southern Alabama. Ecology, 56: 371-386. Delcourt, P. A. and Delcourt, H. R., 1977. The Tunica Hills, Louisiana-Mississippi: Late glacial locality for spruce and deciduous forest species. Quat. Res., 7: 218-237. Delcourt, P. A. and Delcourt, H. R., 1981. Vegetation maps for eastern North America: 40,000 yr B.P. to the present. In: R. Romans (Editor), Geobotany 2. Plenum Press, New York, N.Y., pp. 123-166. Delcourt, P. A. and Delcourt, H. R., 1984. Late Quaternary paleoclimates and biotic responses in eastern North America and the western Atlantic Ocean. Palaeogeogr., Palaeoclimatol., Palaeoecol., 48: 263-84. Delcourt, P. A. and Delcourt, H. R., 1987. Long-term Forest Dynamics of the Temperate Zone: A Case Study of LateQuaternary Forests in Eastern North America. Springer, New York, N.Y. Delcourt, P. A. et al., 1980. Quaternary vegetation history of the Mississippi Embayment. Quat. Res., 13:111-32. Fredlund, G. G. and Jaumann, P. J., 1987. Late Quaternary palynological and paleobotanical records from the central Great Plains. In: W. C. Johnson (Editor), Quaternary Environments of Kansas (Kansas Geol. Surv. Guidebook Ser., 5). Kansas Geol. Surv., Lawrence, Kans. Frye, J. C. and Leonard, A. B., 1957. Studies of Cenozoic geology along eastern margin of Texas High Plains. Tex. Bur. Econ. Geol. Dep. Invest., 32, pp. 1-62. Galloway, R. W., 1983. Full-glacial southwestern United States: Mild and wet or cold and dry? Quat. Res., 19: 236-248. Gates, W. L., 1976. Modeling the ice-age climate. Science, 191: 1131-41.

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