The Continental Record of Environmental Conditions at 18,000 yr B.P.: An Initial Evaluation

Abstract

The development of reliable paleoclimatic maps at a global scale requires data at the following three levels of analysis: (1) well-recorded observations of evenly positioned, well-dated geological evidence (Level I), (2) paleoclimatic estimates derived from this evidence by well-defined quantitative repeatable methods (Level II), and (3) maps synthesizing the estimates from several independent sources of geological evidence (Level III). Our paper describes much of the currently available paleoclimatic data from unglaciated terrestrial areas at ca. 18,000 yr B.P. and illustrates the quantity and quality of the data at both the Level I and the Level II stages of analysis. Although the scarcity of well-dated evidence for this time period precluded any major Level III syntheses of the information, comparisons were drawn where possible between the geological evidence and the climatic conditions simulated by general-circulation model experiments of Gates (1976a, b) and Manabe and Hahn (1977). Of the more than 320 sites with data from 18,000 yr B.P., only 65 are well-dated with bracketing dates within the interval of 23,000 to 13,000 yr B.P., whereas about 100 are undated or poorly dated. We concentrated our survey on palynological and paleobotanical evidence and also thoroughly reviewed the evidence for water levels in lakes at 18,000 yr B.P. In areas with few of these sources of evidence, data on former snowlines, periglacial features, and eolian deposits were included, but the survey of these data is far from complete. Maps of the assembled data reveal the consistency of the paleoclimatic estimates in “data-rich” areas and also show which areas required additional information. The maps show that conditions were colder than present at 18,000 yr B.P. for all sites with temperature estimates. Estimated temperature depressions varied from ca. 1° to 12°C or more, depending on the location of the sample, the type of geological evidence, and the method of temperature estimation. Interpreted hydrological conditions were more variable spatially than the temperature estimates. The southwestern U.S. was moister than present, whereas the southeast may have been drier. Europe and the northern Mediterranean across to Afghanistan were drier than present, but northwest Africa was wetter. Australia was mainly drier than present, but several sites there as well as in Africa show significant climatic changes between 21,000 year period and 16,000 yr B.P. This latter evidence suggests that considerable variability may have occurred during the several thousand-centered on 18,000 yr B.P. Accurate time control is therefore required for the geological data used to study the climate dynamics of 18,000 yr ago. Large portions of South America and Asia as well as significant portions of the other continents lack the data base, or at least the well-dated base, required to define the 18,000 yr B.P. climate. In the few areas where comparisons were made with the Ice Age climates simulated by general-circulation models, general agreement existed between the geological evidence and the model simulations. Many critical comparisons were thwarted, however, by the lack of model simulations for all seasons at 18,000 yr B.P. Difficulty in validating precipitation anomalies in the tropics also arose because surface-albedo values, which are a vital input to the general circulation models, are estimated from the same evidence that is used to validate the results of the models.

Late Cenozoic changes in atmospheric circulation deduced from North Pacific eolian sediments

“…Global climate has also become drier since the Early Cenozoic with increasing aridity through the Neogene {Doff, 1964; Wolfe and Hopkins, 1967;Kemp, 1978;Wolfe, 1978;Frakes, 1979). The world overall apparently was more arid during the last glacial maximum 18,000 yrs ago than during the present interglacial although that generalization does not hold for all regions (Sarnthein, 1978;Peterson et al, 1979;Rea, 1982).…”

Reconstruction of past terrestrial carbon storage in the Northern Hemisphere from the Osnabrück Biosphere Model and palaeodata

“…The palaeoclimatic data (CLIMAP Project Members 1976, Peterson et al 1979) suggest that the unglaciated land surface at 18000 yr BP was generally colder and drier than at present. The differences in temperature were from -1 to -12°C and even larger, while precipitation differences were more variable.…”

“…Possible low-fertilization effects of low atmospheric CO2 concentration on the net primary production cannot be taken into account. Therefore, the combined use of palaeodata (Peterson et al 1979, CLIMAP Project Members 1981, Frenzel et al 1992 and of a biosphere carbon cycle model is required to reconstruct the dynamics of vegetation and soil carbon storage under changing global boundary conditions.…”

Bi̇r Genel Bakis: Günümüzden 150.000 İle 25.000 Yillar Öncesi̇nde Paleoli̇ti̇k Döneme De Karsilik Gelen Zaman Araliginda Avrupa Cografyasinin Manzarasi

“…OIS 2 safhası ve onun gösterdiği, buzulların giderek çekilmesi sonucu iklimin ılımanlaşmasıyla kendisini gösteren iklimsel değişim, pek çok çalışmada anlatılmış olması nedeniyle (bakınız CLIMAP Project Members, 1976Peterson et al, 1979;COHMAP Members, 1988 Çalışmamızın Tarihsel Derinliğinin Alt Sınırını Oluşturan 150.000 Yıl Öncesinde Gelişmiş Buzullaşma OIS 6 safhasına (Saalian/Warthe Karmaşası) karşılık gelen süreçte buzullar, bütün Pleistosen boyunca kuzey yarım küredeki en geniş yayılımlarını gerçekleştirdiler (Şekil 6) ve günümüze yakın tarihte gelişen buzullaşmaya oranla (OIS 2), varlıklarını çok daha uzun bir zaman süresince koruyabildiler. Kuzey Atlantik'ten, OIS 6 buzullaşma safhasıyla ilgili tatmin edici miktarda ölçümler elde edilmemekle birlikte, söz konusu safha süresince buzulların, OIS 2 buzullaşma safhasının ulaştığı enlemlerin ancak 2°-3° daha altındaki enlemlere kadar yayılabilmiş olmaları nedeniyle, Atlantik Okyanusu ve Akdeniz'in koşulları, OIS 2 safhası boyunca egemen olan koşullara bakarak uyarlanmıştır.…”