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Holocene variations of monsoon rainfall in Rajasthan
QUATERNARY RESEARCH 16, 135-- 145 (1981)
Holocene Variations of Monsoon Rainfall in Rajasthan R. A. BRYSON AND A. M. SWAIN Center for Climatic Research, Institute for Environmental Studies, University of Wisconsin, 1225 W. Dayton Street, Madison, Wisconsin 53706 Received N o v e m b e r 10, 1980 T w o r e c o n s t r u c t e d histories of the m o n s o o n rainfall in R a j a s t h a n s h o w that the m o n s o o n was w e a k or a b s e n t in latest glacial time. With the a d v e n t of H o l o c e n e climatic patterns, fresh water lakes f o r m e d in dune fields and the pollen rain p r e s e r v e d in t h e s e reservoirs provides a basis for the r e c o n s t r u c t i o n of the m o n s o o n history. The two r e c o n s t r u c t i o n s , separated by only 150 km, have s o m e features in c o m m o n and s o m e striking differences. Both s h o w m a x i m u m m o n s o o n a m o u n t s in the early Holocene, with a roughly two-thirds d e c r e a s e to the present. Both s h o w salinization in sub-Boreal time. Both s h o w long intervals o f near complete desiccation in the last four millennia. The shorter term variations, h o w e v e r , are not closely parallel. T h e s e shorter t e r m variations m a y be explained in t e r m s o f the behavior of the p r e s e n t day interannual variability.
INTRODUCTION
sonally dry lake beds. The details of these profiles are given in Singh et al. (1972, 1974). In addition, 114 surface samples of pollen were collected at 22 different sites in northwestern India for which climatic data were available (Singh et at., 1973). A transfer function for relating presentday pollen rain to present-day climate was then constructed using the method of Webb and Bryson (1972), and this function was applied to fossil pollen assemblages obtained from the sediment monoliths. The variables used and their weightings are given in Table 1, as well as the percentage of the variance of the climatic variables explained by the pollen assemblage.
The scientific traveller to northwestern India cannot fail to notice the impressive aerosol loading of the atmosphere in the dry season. This aerosol loading is (a) deflated soil material (Bryson et al., 1964; Bryson, 1972) and (b) significant to the climate of "he region (Bryson and Baerreis, 1967). These facts in turn raise questions as to the history of the terrigenous aerosol in terms of its relation t o cultural history (since it appears to be associated with human activity). In response to these questions, a program of paleoclimatic reconstruction for Rajasthan was undertaken through a contract with Dr. Gurdip Singh of the Birbal Sahni Institute of Paleobotany in Lucknow, U.P., India. 1 Some results of this investigation will be described below, placing the reconstructed climate of Rajasthan in a broader global context where possible.
THE LUNKARANSAR CLIMATE PROFILE Figure 1 shows the reconstructed history of the rainfall in the Lunkaransar Lake vicinity, 65 km from Bikaner in northwestern Rajasthan at 28° 30' N, 73° 45' E from the time of lake formation to the present. This lake formed in a dune field at the time of the major transition to Holocene climates about 10,000- 10,800 yr B.P., as did Didwana (to be considered below). The rising water table at that time, necessary to produce permanent fresh-water lakes in the low areas, implied that significant m o n s o o n
METHOD The basic data for the study consisted of pollen profiles from two sediment monoliths excavated from the bottoms of dry or seai Present address: D e p a r t m e n t of Biogeography and G e o m o r p h o l o g y , R e s e a r c h School o f Pacific Studies, The Australian National University, Canberra, Australia. 135
0033-5894/81/050135-11 $02.00/0 Copyright © 1981by the University of Washington. All rights of reproduction in any form reserved.
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LUNKARANSAR, RAJASTHAN, INDIA FIG. 1. Summer (June-October) and annual precipitation at Lunkaransar Lake, Rajasthan reconstructed from pollen percentages by the canonical correlation method of Webb and Bryson (1972). The lake appears to have disappeared during periods with annual precipitation less than about 200 ram. The " × " represents present annual rainfall.
rains did not appear prior to about 10,800 yr B.P. This supports the conclusion, obtained by general circulation modeling of the 18,000 yr B.P. climate, that a northwestern Indian summer monsoon was not characteristic of late Pleistocene conditions (Manabe and Hahn, 1977). Though fluctuating with a crude 650-750 yr rhythm, the summer monsoon rainfall appears to have averaged around 500 mm per season for the first 6000 yr of the history of the lake. This, plus the winter rains ranging up to 500 mm or so per season, seems to have maintained fresh water in the lake and stabilized the dunes (Singh, 1971, p. 184). Indeed, agriculture appears to have arrived very early in the various parts of Rajasthan including the area of Lunkaransar Lake, as shown by the appearance of
cultigens in the pollen rain and charcoal fragments from what may be stubble burning (Singh, 1971). Dates of first evidence of cultivation range from 9400 yr B.P. at Lunkaransar to about 8000 yr B.P. at Sambhar. By 5000 yr B.P. the Indus Culture was well established in the region, with a large number of upland occupation sites where dominantly rain-fed agriculture would seem to be indicated. Location of such major cities as Harappa and Mohenjo-Daro on the Indus floodplain suggests to some that the culture was essentially riverine (Agrawal, 1971). However, most of the sites are not on f l o o d p l a i n s , at least in R a j a s t h a n (Ghosh, personal communication, 1966), and the Lunkaransar reconstruction suggests not only far more summer rainfall than the present but maximum winter pre-
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BRYSON AND SWAIN
cipitation as well during the Indus Culture interval of 5000-3500 yr B.P. Using the Lunkaransar pollen profile, Singh (1971) interpreted the 5000-3500 yr B.P. period as the wettest in the past 10,000 yr. The reconstruction shows total annual rainfall not much different at that time than in the previous centuries. Singh's interpretation was based on an increase in the percentages of several pollen types, including syzygium, Pinus, and Artemisia, that are normally found in relatively moist environments. He did not note that these taxa have spatial distributions that have high positive correlations with the winter rainfall pattern rather than the summer monsoon (Table 1). The Indus period may have thus had higher precipitation efficiency than earlier or later, due to reduced net loss of water during the winter. Certainly during the intervals of maximum winter rain during Indus time there must have been a net gain during the winter. Around 3700 yr B.P. the lake became saline, shown by h a l o p h y t e s replacing Typha and other freshwater aquatic species in the pollen record (Singh, 1971; Singh et al., 1972), and within a few centuries dried up entirely. At this same time the dunes once more became mobile, suggesting a significant r e d u c t i o n of the v e g e t a t i v e cover, and the Indus Culture disappeared (Singh, 1971; Agrawal et al., 1964). A number of the old towns were buried under sand dunes as at Osian, near Jodhpur. There appear to be no dated occupation sites during the next 700 yr or so in the whole region. Indeed, the last Indus Cult u r e d ate is within the u n c e r t a i n t y o f radiocarbon dating of the time of desiccation of Lunkaransar and Sambhar Lakes and the remobilization of the dune fields. The first Painted Grey Ware or Aryan sites were occupied about seven centuries after the last Indus dates and were not generally reoccupation of the Indus sites (Ghosh, 1966, personal communication). Osian, where the main temple was built about 900 B.C. on dune sand over a previous town
site, is a case belonging to the minority. Since this appearance of a new culture appears before the reestablishment of a seasonal salt lake about 2000 yr B.P., one might reason that the aridity had diminished enough for at least nomadic herdsmen to occupy much of the region. After the deposition of the sterile saline s e d i m e n t s o f the " l o n g d r o u g h t " (ca. 3600-2000 yr B.P.), the pollen rain into the lake indicates, when translated into seasonal rainfall amounts, a summer monsoon only half as copious as the p r e d r o u g h t value, and negligible winter rain. Within this era, the highest computed amounts occurred between 700 and 1100 yr B.P., plus or minus the error of interpolation between radiocarbon dates.
IMPLICATIONS AND VALIDATION OF THE LUNKARANSAR PROFILE There is no in situ data to test the validity of the monsoon reconstruction other than the mobility of the dunes and absence of occupation during the aridity shown by the desiccation of the lake. However, there are some c o n s i s t e n c y tests t hat might be applied to the profile. Because of the interconnectedness of the climatic system, there must be climatic events elsewhere that are related to the monsoon fluctuations. One qualitative measure has already been mentioned. During late glacial times, i.e., prior to 10,800 yr B.P., the monsoon was suppressed. This is consistent with the modern climatic condition in that the monsoon rains are a summer phenomenon and appear in India after the spring contraction of the circumpolar vortex has taken the jet stream north of the Himalayas (Yin, 1949). Certainly the late glacial time was winterlike. It would be reasonable, then, to expect that one should be able to find evidence of high latitude cooling and an expansion of the A r c t i c c l i m a t e s during the " l o n g drought." There is such evidence in the paleosols of northern Canada. These fossil soils show a marked expansion of tundra at the expense of boreal
MONSOON R A I N F A L L IN RAJASTHAN
139
forest starting about 3600 yr B.P. (Bryson rate of change for oceans should be slower et al., 1965; Sorenson et al., 1971). B y 2000 than f o r c o n t i n e n t s , h e n c e rising hemiyr B.P. the forest had readvanced to near spheric mean temperatures should enhance its present position but only half way to the the l a n d - s e a contrast that drives the monposition it had occupied prior to 3600 yr soon. Using the 1000-yr reconstruction of B.P. This failure of the Arctic climatic re- I c e l a n d i c t e m p e r a t u r e s o f B e r g t h 6 r s s o n gion to shrink to its f o r m e r size (as indi- (1969), we may then construct an estimated cated by tundra extent) is consistent with m o n s o o n rainfall series for Bikaner and the failure o f the monsoon to regain its for- Agra (Fig. 2). This reconstruction, though m e r i n t e n s i t y . A s e c o n d r e t r e a t a b o u t the details may not be valid, follows quite 7 0 0 - 8 0 0 yr B.P. followed a readvance o f well the trend of the last 1000 yr as shown in the forest halfway from its present and 2000 Figure 1, i.e., generally high ca. 1000- 1200 yr B.P. position to the pre-3600 yr B.P. po- A.D. then low ca. 1600-1900 A.D. Both sition. This would imply scanty monsoon may be verified in part by the historical rains at 2000 yr B.P. and the present, with r e c o r d o f f a m i n e s c o m p i l e d b y N a q v i larger amounts around 800 yr B.P. This is (1962), and indicated by " F " in Figure 2, what the Lunkaransar profile in Figure 1 since famines in the region largely reflect shows. For 2000 yr B.P. we must assume failures of the monsoon. less than present day rainfall to explain the The n u m e r i c a l r e c o n s t r u c t i o n (Fig. 1) complete desiccation of the lake. agrees with Singh's qualitative estimates Modern climatic data may be used to cor- of total precipitation in the period 10,000roborate the above reasoning. One may re- 3500 yr B.P., being 2 5 0 - 5 0 0 mm greater gress modern rainfall records from north- than the present (Singh, 1974). His estiwest India o v e r the past h u n d r e d years mates were based on relatively high peragainst Icelandic annual mean temperature centages o f f r e s h - w a t e r aquatics and o f data, which may in turn be taken as a p r o x y terrestrial shrubs and herbs that are usually for Arctic temperatures in general (Grove- f o u n d in semi-humid to h u m i d environman and Landsberg, 1979). A fairly good ments. r e l a t i o n m a y be o b t a i n e d b e t w e e n the THE DIDWANA LAKE RECORD northwestern India m o n s o o n decadal rainfall average, say of Bikaner and Jodhpur, At D i d w a n a , in the e a s t e r n p a r t o f and a combination of the Icelandic annual Rajasthan (27° 20' N, 74 ° 35' E), the record mean temperature and its rate of change. is somewhat different (Fig. 3). The time of This regression yields an explained vari- formation of the lake lags L u n k a r a n s a r by ance of 82%. 3 The rationale for this re- about a millennium, and after sub-Boreal lationship is that cold decades are more time the lake stays desiccated to the preswinter-like, thus drier, while warm decades ent. The variability o f the r e c o n s t r u c t e d are more summer-like and hence should monsoon rainfall is also greater, though the have better developed monsoons. This on a variability of both profiles is entirely conshort time scale is equivalent to the absence sistent with the present coefficient of variof m o n s o o n rains in late-glacial time, and ability (Das, 1968, p. 137). the presence of m o n s o o n rains during the The ratio of r e c o n s t r u c t e d mean monHolocene, especially during mid-Holocene soon rainfall to the m o d e r n precipitation is when it is assumed there was a maximum o f about the same for the 10,000-4000 yr B.P. N o r t h e r n H e m i s p h e r e t e m p e r a t u r e . T h e period at Didwana Lake as the ratio of reconstructed m o n s o o n rainfall for the same period at L u n k a r a n s a r to the m o d e r n rain2 T h i s r e s u l t w a s o b t a i n e d by Prof. W. A. R. fall at Bikaner. H o w e v e r , the Didwana reBrinkmann of the University of Wisconsin's Center for Geographic Analysis and Department of Geography. construction shows an irregular but pro-
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gressive decrease throughout the first 60% of the Holocene, accentuated around the end of the Atlantic climatic episode (ca. 5000 yr B.P.), whereas L u n k a r a n s a r does not have such a clear decline. Another significant difference in the profiles must be noted. The six to eight century long variations of the L u n k a r a n s a r record do not m a t c h closely the fluctuations of comparable period at Didwana.
THE VALIDITY OF THE DIDWANA PROFILE That the apparent time of formation of Didwana Lake and the time of its more recent desiccation does not match the corresponding times for L u n k a r a n s a r is not surp r i s i n g . T h e r e is no w a y o f k n o w i n g whether the monoliths excavated at each site actually included the very first sedi-
ments deposited in each lake. One does not know, either, where the deepest parts of the lake basins were two to four millennia ago in this region of mobile sediments. H o w e v e r , the shorter term variations of reconstructed m o n s o o n rainfall amounts, and their differences b e t w e e n the two sites must be of concern: are they r a n d o m errors of reconstruction or could they be the result of real differences in climate? Since the reconstructions were derived from relationships between pollen rain and m o d e r n climate, it is reasonable to expect that the reconstructed records might be as disparate as the m o d e r n climates at the sites. This assumes that the climatic processes m a y have changed in relative intensity but not in kind. H o w e v e r , r a n d o m errors would be expected to p r o d u c e different fluctuations in the two profiles. H e n c e , we
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will examine those large-scale processes which would be operative at both locations, but to which each local response would be different. The climatic stations closest to Lunkaransar and Didwana are Bikaner and Jodhpur, respectively. The monsoon rains normally arrive at Jodhpur about the first of July and reach Bikaner about a week later. June then is n o r m a l l y p r e - m o n s o o n in Rajasthan, but isolated, often heavy premonsoon showers yield about 30 mm or less of rainfall in June at both locations. July is a different matter. After the " b u r s t " of the monsoon, the usual rains are quite light, falling from clouds with tops at 3 - 4 km, but with imbedded deep convective cells associated with heavy rains. These deep cells are re-
lated to several synoptic processes which vary from year to year in location and timing (Rahmatullah, 1952). Consequently, the c o r r e l a t i o n b e t w e e n the time series o f monthly rainfall at Bikaner and Jodhpur is essentially nil, a X2 test of association for July being very close to the random value. However, there is a systematic relationship between June rainfall and July rainfall at each station, stronger at Jodhpur than at Bikaner. In the 31 days of July, Jodhpur receives, normally, 122 mm of monsoon rain. At Bikaner with about three-quarters as many days of monsoon, the normal July rainfall is about three-quarters as great, or 87 mm. It is widely recognized that the date of onset of the monsoon is important to the total amount of monsoon rain (Das, 1968). A
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v e r y e a r l y , v i g o r o u s o n s e t is o f t e n associated with a " b r e a k " or rainless spell several w e e k s later, h o w e v e r (Trewartha, 1981). Thus, at J o d h p u r the rank correlation b e t w e e n J u n e rainfall a m o u n t s and July amounts is - 0 . 7 b a s e d on a 60 yr record. During the same period the corresponding correlation at Bikaner was - 0 . 3 . Despite their proximity the two stations show different r e s p o n s e s e v e n to regional variations in the m o n s o o n onset. I n d e e d the v a r i a n c e spectra of the m o d e r n rainfall time series at the two locations are quite different, as the reader m a y verify f r o m standard sources. Having established that the L u n k a r a n s a r and Didwana reconstructions should look different b e c a u s e of differing r e s p o n s e to climatic p r o c e s s e s , we are still faced with the question of w h e t h e r the variations of each reconstruction can be u n d e r s t o o d as n o n r a n d o m r e s p o n s e to k n o w n forcings. Modern data and climatological reasoning m a y be used to a n s w e r this question in part. Annual m o n s o o n rainfall for the instrumental period in Rajasthan was found to be r e l a t e d to the t e m p e r a t u r e a n d r a t e o f change of t e m p e r a t u r e in the Arctic, as represented b y Icelandic temperatures. Table 2 r e p r e s e n t s an e x t e n s i o n o f this i d e a to TABLE 2. SENSITIVITY OF RAJASTHAN MONSOON RAINFALL TO NORTHERN HEMISPHERE MEAN ANNUAL SURFACE TEMPERATURE AND ITS RATE OF CHANGE Station
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specific stations and specific months. T h e entire 84 yr of r e c o r d e d rainfall for J o d h p u r and 88 yr for Bikaner was r e g r e s s e d m o n t h by month, against the previous 5 yr m e a n N o r t h e r n H e m i s p h e r e surface t e m p e r a t u r e , the two 5 yr periods previous to that, and s e v e n periodic t e r m s (0.898, 0.854, 0.835, 0.807, 0.422, 0.444, 0.488 cy/yr) after first transforming the rainfall data b y calculating the normalized cuberoot. This transformation was to r e m o v e m o s t of the s k e w n e s s and m a k e various records easily c o m p a r able. The regression coefficients m a y then be c o n v e r t e d into sensitivities, i.e., the percentage change of m o n t h l y rainfall, estim a t e d f r o m the regression equation, that would result from a I°C change in N o r t h e r n H e m i s p h e r e 5 yr m e a n t e m p e r a t u r e or a I°C change of that m e a n t e m p e r a t u r e per 5 yr. Since m o s t of the variation of H e m i s p h e r i c m e a n t e m p e r a t u r e is due to variation o f A r c t i c t e m p e r a t u r e , the r e g r e s s i o n s are c o m p a r a b l e to those discussed a b o v e for " R a j a s t h a n m e a n annual rainfall" [(Bikaner + Agra)/2] versus Icelandic m e a n annual temperature. F r o m T a b l e 2 we see t h a t w h e n the N o r t h e r n H e m i s p h e r e is w a r m ( m o r e s u m m e r - l i k e ) b o t h the L u n k a r a n s a r and D i d w a n a areas should have m o r e m o n s o o n rainfall, but the effect should be a b o u t twice as large in the latter area. Turning to Figure 3 we see that the r e c o n s t r u c t e d m a x i m u m m e a n rainfall at Didwana was during the Atlantic episode, which in turn is regarded as the w a r m e s t part of the H o l o c e n e hemispherically. L u n k a r a n s a r (Fig. 1) does not s h o w an A t l a n t i c e p i s o d e m a x i m u m o f s u m m e r rainfall, which m a y be due in part to the half-as-great sensitivity to N o r t h e r n H e m i s p h e r e t e m p e r a t u r e s h o w n in Table 2 for Bikaner. A n o t h e r factor is the relationship s h o w n in Part B of Table 2. Table 2B shows that w h e n the northern h e m i s p h e r e is w a r m i n g , the July rainfall goes up at L u n k a r a n s a r but d o w n at Didwana. The reverse is true w h e n the hemispheric t e m p e r a t u r e is f a l l i n g - - D i d w a n a rainfall increases; at L u n k a r a n s a r rainfall
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decreases. In August both locations have decreased rainfall following decades of rising temperature. We may infer, then, that the early Holocene rising temperatures simply shifted the rains from August to July at Lunkaransar, while the small sensitivity to absolute hemispheric temperature produced little net change, and post-Atlantic decreasing temperatures shifted the rain back to August. The net effect was little general trend. At Didwana the rising early Holocene temperatures would have suppressed both July and August precipitation for a smaller monsoon total. On the other hand, when the rate of change of hemispheric temperature leveled out during the Atlantic, the suppression due to rising temperatures would have ceased and the small enhancement due to higher absolute hemispheric temperature would have prevailed, giving a maximum of precipitation at Didwana. The lack of sensitivity, both to absolute hemispheric temperature and its rate of change, at Lunkaransar might explain the dissimilarity of the general millenia-long trends at the two sites. From the above discussion we see that qualitatively there is some reason to believe that the differences of the Lunkaransar and Didwana reconstructions are real, i.e., in accord with the climatic differences today. We must consider whether the proposed mechanism is quantitatively adequate. The Didwana reconstruction shows the Atlantic episode wetter than the Boreal and subBoreal by about 150 mm per year or about 18%. If the intercentury sensitivity of Didwana area rainfall to hemispheric mean temperature is approximated by the interpentadal sensitivity, the implication is that the hemispheric mean temperature during the Atlantic was about I°C higher than during the Boreal or sub-Boreal. This we believe is a reasonable number. We will not, however, make a linear extrapolation of the present day interpentadal sensitivity to the general trend over the past 10,000 yr. Table 2 also provides a rationale for the
apparently erratic behavior of the reconstructed monsoon rainfall amounts at the time scale of centuries. The general variability of the individual reconstructed values is about 35-40% of the mean value for the whole period, compared to the 45-50% present-day interannual coefficient of variability in Rajasthan. Thus larger fluctuations might be expected at Didwana where the mean precipitation is higher. It is known that the climate has not varied smoothly and uniformly during the Holocene. There have been a number of shorter term climatic events such as the stillstand or readvance of the continental glaciers of the northern hemisphere ca. 8300 yr B.P. If Table 2 is applicable, the rapid cooling at the onset of this glacial event, followed by rapid warming to the Atlantic episode, should have produced increased monsoons at Didwana prior to 8300 yr B.P., followed by reduced rainfall ca. 8000 yr B.P. Such is observed in Figure 3, but there is no strong excursion apparent for Lunkaransar (Fig. 1), and this is also in accord with the low sensitivity to hemispheric temperature rate of change indicated in Table 2B. Similar effects of other Holocene climatic events may be seen in the reconstructed monsoon climate, leading one to believe that the variability of the reconstructions is real, and not a statistical artifact. Only hypotheses may be offered for the general two-thirds decrease of rainfall at both sites during the Holocene. At least four reasonable factors may be considered: (1) The obliquity, or inclination of the earth's axis to the plane of the ecliptic, was about 24.4° at the beginning of the record (10,000 B.P.) and has decreased to about 23.3 ° . It is generally believed by climatologists that high obliquity is associated with strong seasonal contrasts, i.e., hot summers and cold winters. Hotter summers hemispherically m i g h t b e reasonably associated with a strong monsoon, and thus decreasing monsoon~ in Rajasthan during the Holocene. The Milankovich es¢
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BRYSON AND SWAIN
timates of about 5°C change in summer temperatures since 10,000 yr B.P. would imply a sensitivity (as used in Table 2) of about 0.13 per degree. This is a very reasonable figure (Lamb, 1972, pp. 31-33). (2) The precession of the equinoxes makes a small change in the length of the summer, from summer solstice to autumnal equinox. One may calculate about 5 days shortening from 10,000 yr B.P. to the present. This might shorten the length of the monsoon season by 5 or 6 days, but at the present per-day monsoon rainfall rate in Rajasthan (ca. 4.7 mm/day), this effect would probably be too small. There may be strong nonlinearity, however. (3) The secular change in the height of the Himalayas may have affected the intensity of the monsoon. (4) The decline in apparent monsoon rainfall in the past few millennia may be acc e n t u a t e d by the v e g e t a t i o n - s o i l def l a t i o n - atmospheric a e r o s o l - r a d i a t i o n rainfall and dew feedback mechanism proposed by Bryson and Baerreis (1967). This was the proposition that led to the research described in this paper. Which, if any, of these possibilities is realistic cannot be determined without further research.
summer rainfall was not higher than the period preceding the florescence of the Indus Culture. There are two features of the reconstructed monsoon history pertinent to und e r s t a n d i n g the agricultural base of Indus society. The winter rains appear to have been maximum during Indus time, decreasing the winter desiccation now characterizing the region, and increasing the overall precipitation efficiency. In addition, the rapid salinization of the lakes followed by their disappearance coincided with the disappearance of the culture. This strongly suggests that the collapse of the culture was causally linked to a collapse of their agricultural base associated with climatic change. The indicated drought of many centuries duration and remobilization of the dune fields at the end of Indus time shows that the monsoon may not be as stable a feature of the climate as we would like it to be with present population densities. Further refinement of this research will probably require a broader geographical distribution of present-day pollen rain samples in order to utilize the available pollen profiles located near the edges of the existing surface data array.
SUMMARY
ACKNOWLEDGMENTS
We have shown that a Holocene history of monsoon rainfall in Rajasthan can be objectively reconstructed from pollen profiles, and that the histories derived from two profiles are mostly consistent with independent data and with what we know of the dynamics of the monsoon. The largest problem emerging from the study seems to be the explanation of the irregular decrease of the rainfall from early Holocene time to the present. The cultural significance of this monsoon history is considerable, being applicable to the region occupied by the Indus Culture between 5000 and 3500 yr B.P., roughly. This time was one with at least double the present summer rainfall, if the reconstruction is correct. However, the
We wish to thank Dr. Gurdip Singh for the production of a high-grade pollen data base for this study and Dr. Waltraud Brinkmann for her contribution to the understanding of the present climatic teleconnections. This material is based partially upon work supported by the National Science Foundation under Grant No. ATM79-26039. The authors wish to thank Thompson Webb III for his constructive review of the manuscript.
REFERENCES Agrawal, D. P. (1971). "The Copper Bronze Age in India." Munshiram Manoharlal, New Delhi. Agrawal, D. P., Kusumgar, S., and Sarna," R. P. (1964). Radiocarbon dates of archaeological samples. Current Science 33, 226. Bergth6rsson, P. (1969). An estimate of drift ice and temperature in Iceland in 1000 years. JOkull 19, 94-101. Bryson, R . A . (1972). Climatic modification by air
MONSOON RAINFALL IN RAJASTHAN pollution. In " T h e Environmental F u t u r e " (N. Polunin, Ed.), pp. 133-154. Macmillan, New York. Bryson, R. A., and Baerreis, D. S. (1967). Possibilities of major climatic modification and their implications: Northwest India, a case for study. Bulletin of the American Meteorological Society 48, 136-142. Bryson, R . A . , Irving, W . M . , and Larson, J . A . (1965). Radiocarbon and soils evidence of former forest in the southern Canadian tundra. Science 147, 4 6 - 48. Bryson, R. A., Wilson, C. A., III, and Kuhn, P. M. (1964). Some preliminary results of radiation sonde ascents over India. Proceedings WMO-IUGG Symposium on Tropical Meteorology, Rotorua, New Zealand, November, 1963, Wellington, New Zealand Meteorological Service. Das, P. K. (1968). "The Monsoons." National Book Trust, New Delhi. Ghosh, A. (1966). Personal communication. Groveman, B. S., and Landsberg, H. E. (1979). Reconstruction of northern hemisphere temperature: 1579- 1880. Publ. No. 79-181, June, 1979, Meteorology Program, University of Maryland, College Park, MD 20742. Lamb, H. H. (1972). "Climate: Present, Past and Future," Vol. 1. Methuen, London. Manabe, S., and Hahn, D. G. (1977). Simulation of the tropical climate of an Ice Age. Journal of Geophysical Research 82, 3889-3911. Naqvi, S. N. (1962). Data presented at Conference on the Climate of the Eleventh and Sixteenth Centuries, Aspen, Colorado, June 16-24, 1962.
145
Rahmatullah, M. (1952). Synoptic aspects of the monsoon circulation and rainfall over Indo-Pakistan. Journal of Meteorology 9, 176-179. Singh, G. (1971). The Indus Valley Culture seen in the context of postglacial climatic and ecological studies in n o r t h w e s t India. Archaeology and Physical Anthropology in Oceania 6, 177-189. Singh, G., Chopra, S. K., and Singh, A . B . (1973). Pollen-rain from the vegetation of northwest India. New Phytologist 72, 191-206. Singh, G., Joshi, R . D . , and Singh, A. B. (1972). Stratigraphic and radiocarbon evidence for the age and development of three salt lake deposits in Rajasthan, India. Quaternary Research 2, 496-505. Singh, G., Joshi, R . A . , Chopra, S . K . , and Singh, A. B. (1974). Late Quaternary history of vegetation and climate of the R a j a s t h a n D e s e r t , India. Philosophical Transactions of the Royal Society of London B 267, 467-501. Sorenson, C. J., Knox, J. C., Larson, J. A., and Bryson, R. A. (1971). Paleosols and the forest border in Keewatin. Quaternary Research 1, 468-473. Trewartha, G. T. (1981). "The Earth's Problem Climates," 2nd Ed., pp. 183- 190. University of Wisconsin Press, Madison, WI. Webb, T., III, and Bryson, R. A. (1972). Late- and postglacial climatic change in the northern Midwest, USA: Quantitative estimates derived from fossil pollen spectra by multivariate statistical analysis. Quaternary Research 2, 7 0 - 115. Yin, Maung Tun (1949). A synoptic-aerologic study of the onset of the summer monsoon over India and Burma. Journal of Meteorology 6, 393-400.
Holocene Variations of Monsoon Rainfall in Rajasthan R. A. BRYSON AND A. M. SWAIN Center for Climatic Research, Institute for Environmental Studies, University of Wisconsin, 1225 W. Dayton Street, Madison, Wisconsin 53706 Received N o v e m b e r 10, 1980 T w o r e c o n s t r u c t e d histories of the m o n s o o n rainfall in R a j a s t h a n s h o w that the m o n s o o n was w e a k or a b s e n t in latest glacial time. With the a d v e n t of H o l o c e n e climatic patterns, fresh water lakes f o r m e d in dune fields and the pollen rain p r e s e r v e d in t h e s e reservoirs provides a basis for the r e c o n s t r u c t i o n of the m o n s o o n history. The two r e c o n s t r u c t i o n s , separated by only 150 km, have s o m e features in c o m m o n and s o m e striking differences. Both s h o w m a x i m u m m o n s o o n a m o u n t s in the early Holocene, with a roughly two-thirds d e c r e a s e to the present. Both s h o w salinization in sub-Boreal time. Both s h o w long intervals o f near complete desiccation in the last four millennia. The shorter term variations, h o w e v e r , are not closely parallel. T h e s e shorter t e r m variations m a y be explained in t e r m s o f the behavior of the p r e s e n t day interannual variability.
INTRODUCTION
sonally dry lake beds. The details of these profiles are given in Singh et al. (1972, 1974). In addition, 114 surface samples of pollen were collected at 22 different sites in northwestern India for which climatic data were available (Singh et at., 1973). A transfer function for relating presentday pollen rain to present-day climate was then constructed using the method of Webb and Bryson (1972), and this function was applied to fossil pollen assemblages obtained from the sediment monoliths. The variables used and their weightings are given in Table 1, as well as the percentage of the variance of the climatic variables explained by the pollen assemblage.
The scientific traveller to northwestern India cannot fail to notice the impressive aerosol loading of the atmosphere in the dry season. This aerosol loading is (a) deflated soil material (Bryson et al., 1964; Bryson, 1972) and (b) significant to the climate of "he region (Bryson and Baerreis, 1967). These facts in turn raise questions as to the history of the terrigenous aerosol in terms of its relation t o cultural history (since it appears to be associated with human activity). In response to these questions, a program of paleoclimatic reconstruction for Rajasthan was undertaken through a contract with Dr. Gurdip Singh of the Birbal Sahni Institute of Paleobotany in Lucknow, U.P., India. 1 Some results of this investigation will be described below, placing the reconstructed climate of Rajasthan in a broader global context where possible.
THE LUNKARANSAR CLIMATE PROFILE Figure 1 shows the reconstructed history of the rainfall in the Lunkaransar Lake vicinity, 65 km from Bikaner in northwestern Rajasthan at 28° 30' N, 73° 45' E from the time of lake formation to the present. This lake formed in a dune field at the time of the major transition to Holocene climates about 10,000- 10,800 yr B.P., as did Didwana (to be considered below). The rising water table at that time, necessary to produce permanent fresh-water lakes in the low areas, implied that significant m o n s o o n
METHOD The basic data for the study consisted of pollen profiles from two sediment monoliths excavated from the bottoms of dry or seai Present address: D e p a r t m e n t of Biogeography and G e o m o r p h o l o g y , R e s e a r c h School o f Pacific Studies, The Australian National University, Canberra, Australia. 135
0033-5894/81/050135-11 $02.00/0 Copyright © 1981by the University of Washington. All rights of reproduction in any form reserved.
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LUNKARANSAR, RAJASTHAN, INDIA FIG. 1. Summer (June-October) and annual precipitation at Lunkaransar Lake, Rajasthan reconstructed from pollen percentages by the canonical correlation method of Webb and Bryson (1972). The lake appears to have disappeared during periods with annual precipitation less than about 200 ram. The " × " represents present annual rainfall.
rains did not appear prior to about 10,800 yr B.P. This supports the conclusion, obtained by general circulation modeling of the 18,000 yr B.P. climate, that a northwestern Indian summer monsoon was not characteristic of late Pleistocene conditions (Manabe and Hahn, 1977). Though fluctuating with a crude 650-750 yr rhythm, the summer monsoon rainfall appears to have averaged around 500 mm per season for the first 6000 yr of the history of the lake. This, plus the winter rains ranging up to 500 mm or so per season, seems to have maintained fresh water in the lake and stabilized the dunes (Singh, 1971, p. 184). Indeed, agriculture appears to have arrived very early in the various parts of Rajasthan including the area of Lunkaransar Lake, as shown by the appearance of
cultigens in the pollen rain and charcoal fragments from what may be stubble burning (Singh, 1971). Dates of first evidence of cultivation range from 9400 yr B.P. at Lunkaransar to about 8000 yr B.P. at Sambhar. By 5000 yr B.P. the Indus Culture was well established in the region, with a large number of upland occupation sites where dominantly rain-fed agriculture would seem to be indicated. Location of such major cities as Harappa and Mohenjo-Daro on the Indus floodplain suggests to some that the culture was essentially riverine (Agrawal, 1971). However, most of the sites are not on f l o o d p l a i n s , at least in R a j a s t h a n (Ghosh, personal communication, 1966), and the Lunkaransar reconstruction suggests not only far more summer rainfall than the present but maximum winter pre-
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BRYSON AND SWAIN
cipitation as well during the Indus Culture interval of 5000-3500 yr B.P. Using the Lunkaransar pollen profile, Singh (1971) interpreted the 5000-3500 yr B.P. period as the wettest in the past 10,000 yr. The reconstruction shows total annual rainfall not much different at that time than in the previous centuries. Singh's interpretation was based on an increase in the percentages of several pollen types, including syzygium, Pinus, and Artemisia, that are normally found in relatively moist environments. He did not note that these taxa have spatial distributions that have high positive correlations with the winter rainfall pattern rather than the summer monsoon (Table 1). The Indus period may have thus had higher precipitation efficiency than earlier or later, due to reduced net loss of water during the winter. Certainly during the intervals of maximum winter rain during Indus time there must have been a net gain during the winter. Around 3700 yr B.P. the lake became saline, shown by h a l o p h y t e s replacing Typha and other freshwater aquatic species in the pollen record (Singh, 1971; Singh et al., 1972), and within a few centuries dried up entirely. At this same time the dunes once more became mobile, suggesting a significant r e d u c t i o n of the v e g e t a t i v e cover, and the Indus Culture disappeared (Singh, 1971; Agrawal et al., 1964). A number of the old towns were buried under sand dunes as at Osian, near Jodhpur. There appear to be no dated occupation sites during the next 700 yr or so in the whole region. Indeed, the last Indus Cult u r e d ate is within the u n c e r t a i n t y o f radiocarbon dating of the time of desiccation of Lunkaransar and Sambhar Lakes and the remobilization of the dune fields. The first Painted Grey Ware or Aryan sites were occupied about seven centuries after the last Indus dates and were not generally reoccupation of the Indus sites (Ghosh, 1966, personal communication). Osian, where the main temple was built about 900 B.C. on dune sand over a previous town
site, is a case belonging to the minority. Since this appearance of a new culture appears before the reestablishment of a seasonal salt lake about 2000 yr B.P., one might reason that the aridity had diminished enough for at least nomadic herdsmen to occupy much of the region. After the deposition of the sterile saline s e d i m e n t s o f the " l o n g d r o u g h t " (ca. 3600-2000 yr B.P.), the pollen rain into the lake indicates, when translated into seasonal rainfall amounts, a summer monsoon only half as copious as the p r e d r o u g h t value, and negligible winter rain. Within this era, the highest computed amounts occurred between 700 and 1100 yr B.P., plus or minus the error of interpolation between radiocarbon dates.
IMPLICATIONS AND VALIDATION OF THE LUNKARANSAR PROFILE There is no in situ data to test the validity of the monsoon reconstruction other than the mobility of the dunes and absence of occupation during the aridity shown by the desiccation of the lake. However, there are some c o n s i s t e n c y tests t hat might be applied to the profile. Because of the interconnectedness of the climatic system, there must be climatic events elsewhere that are related to the monsoon fluctuations. One qualitative measure has already been mentioned. During late glacial times, i.e., prior to 10,800 yr B.P., the monsoon was suppressed. This is consistent with the modern climatic condition in that the monsoon rains are a summer phenomenon and appear in India after the spring contraction of the circumpolar vortex has taken the jet stream north of the Himalayas (Yin, 1949). Certainly the late glacial time was winterlike. It would be reasonable, then, to expect that one should be able to find evidence of high latitude cooling and an expansion of the A r c t i c c l i m a t e s during the " l o n g drought." There is such evidence in the paleosols of northern Canada. These fossil soils show a marked expansion of tundra at the expense of boreal
MONSOON R A I N F A L L IN RAJASTHAN
139
forest starting about 3600 yr B.P. (Bryson rate of change for oceans should be slower et al., 1965; Sorenson et al., 1971). B y 2000 than f o r c o n t i n e n t s , h e n c e rising hemiyr B.P. the forest had readvanced to near spheric mean temperatures should enhance its present position but only half way to the the l a n d - s e a contrast that drives the monposition it had occupied prior to 3600 yr soon. Using the 1000-yr reconstruction of B.P. This failure of the Arctic climatic re- I c e l a n d i c t e m p e r a t u r e s o f B e r g t h 6 r s s o n gion to shrink to its f o r m e r size (as indi- (1969), we may then construct an estimated cated by tundra extent) is consistent with m o n s o o n rainfall series for Bikaner and the failure o f the monsoon to regain its for- Agra (Fig. 2). This reconstruction, though m e r i n t e n s i t y . A s e c o n d r e t r e a t a b o u t the details may not be valid, follows quite 7 0 0 - 8 0 0 yr B.P. followed a readvance o f well the trend of the last 1000 yr as shown in the forest halfway from its present and 2000 Figure 1, i.e., generally high ca. 1000- 1200 yr B.P. position to the pre-3600 yr B.P. po- A.D. then low ca. 1600-1900 A.D. Both sition. This would imply scanty monsoon may be verified in part by the historical rains at 2000 yr B.P. and the present, with r e c o r d o f f a m i n e s c o m p i l e d b y N a q v i larger amounts around 800 yr B.P. This is (1962), and indicated by " F " in Figure 2, what the Lunkaransar profile in Figure 1 since famines in the region largely reflect shows. For 2000 yr B.P. we must assume failures of the monsoon. less than present day rainfall to explain the The n u m e r i c a l r e c o n s t r u c t i o n (Fig. 1) complete desiccation of the lake. agrees with Singh's qualitative estimates Modern climatic data may be used to cor- of total precipitation in the period 10,000roborate the above reasoning. One may re- 3500 yr B.P., being 2 5 0 - 5 0 0 mm greater gress modern rainfall records from north- than the present (Singh, 1974). His estiwest India o v e r the past h u n d r e d years mates were based on relatively high peragainst Icelandic annual mean temperature centages o f f r e s h - w a t e r aquatics and o f data, which may in turn be taken as a p r o x y terrestrial shrubs and herbs that are usually for Arctic temperatures in general (Grove- f o u n d in semi-humid to h u m i d environman and Landsberg, 1979). A fairly good ments. r e l a t i o n m a y be o b t a i n e d b e t w e e n the THE DIDWANA LAKE RECORD northwestern India m o n s o o n decadal rainfall average, say of Bikaner and Jodhpur, At D i d w a n a , in the e a s t e r n p a r t o f and a combination of the Icelandic annual Rajasthan (27° 20' N, 74 ° 35' E), the record mean temperature and its rate of change. is somewhat different (Fig. 3). The time of This regression yields an explained vari- formation of the lake lags L u n k a r a n s a r by ance of 82%. 3 The rationale for this re- about a millennium, and after sub-Boreal lationship is that cold decades are more time the lake stays desiccated to the preswinter-like, thus drier, while warm decades ent. The variability o f the r e c o n s t r u c t e d are more summer-like and hence should monsoon rainfall is also greater, though the have better developed monsoons. This on a variability of both profiles is entirely conshort time scale is equivalent to the absence sistent with the present coefficient of variof m o n s o o n rains in late-glacial time, and ability (Das, 1968, p. 137). the presence of m o n s o o n rains during the The ratio of r e c o n s t r u c t e d mean monHolocene, especially during mid-Holocene soon rainfall to the m o d e r n precipitation is when it is assumed there was a maximum o f about the same for the 10,000-4000 yr B.P. N o r t h e r n H e m i s p h e r e t e m p e r a t u r e . T h e period at Didwana Lake as the ratio of reconstructed m o n s o o n rainfall for the same period at L u n k a r a n s a r to the m o d e r n rain2 T h i s r e s u l t w a s o b t a i n e d by Prof. W. A. R. fall at Bikaner. H o w e v e r , the Didwana reBrinkmann of the University of Wisconsin's Center for Geographic Analysis and Department of Geography. construction shows an irregular but pro-
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gressive decrease throughout the first 60% of the Holocene, accentuated around the end of the Atlantic climatic episode (ca. 5000 yr B.P.), whereas L u n k a r a n s a r does not have such a clear decline. Another significant difference in the profiles must be noted. The six to eight century long variations of the L u n k a r a n s a r record do not m a t c h closely the fluctuations of comparable period at Didwana.
THE VALIDITY OF THE DIDWANA PROFILE That the apparent time of formation of Didwana Lake and the time of its more recent desiccation does not match the corresponding times for L u n k a r a n s a r is not surp r i s i n g . T h e r e is no w a y o f k n o w i n g whether the monoliths excavated at each site actually included the very first sedi-
ments deposited in each lake. One does not know, either, where the deepest parts of the lake basins were two to four millennia ago in this region of mobile sediments. H o w e v e r , the shorter term variations of reconstructed m o n s o o n rainfall amounts, and their differences b e t w e e n the two sites must be of concern: are they r a n d o m errors of reconstruction or could they be the result of real differences in climate? Since the reconstructions were derived from relationships between pollen rain and m o d e r n climate, it is reasonable to expect that the reconstructed records might be as disparate as the m o d e r n climates at the sites. This assumes that the climatic processes m a y have changed in relative intensity but not in kind. H o w e v e r , r a n d o m errors would be expected to p r o d u c e different fluctuations in the two profiles. H e n c e , we
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will examine those large-scale processes which would be operative at both locations, but to which each local response would be different. The climatic stations closest to Lunkaransar and Didwana are Bikaner and Jodhpur, respectively. The monsoon rains normally arrive at Jodhpur about the first of July and reach Bikaner about a week later. June then is n o r m a l l y p r e - m o n s o o n in Rajasthan, but isolated, often heavy premonsoon showers yield about 30 mm or less of rainfall in June at both locations. July is a different matter. After the " b u r s t " of the monsoon, the usual rains are quite light, falling from clouds with tops at 3 - 4 km, but with imbedded deep convective cells associated with heavy rains. These deep cells are re-
lated to several synoptic processes which vary from year to year in location and timing (Rahmatullah, 1952). Consequently, the c o r r e l a t i o n b e t w e e n the time series o f monthly rainfall at Bikaner and Jodhpur is essentially nil, a X2 test of association for July being very close to the random value. However, there is a systematic relationship between June rainfall and July rainfall at each station, stronger at Jodhpur than at Bikaner. In the 31 days of July, Jodhpur receives, normally, 122 mm of monsoon rain. At Bikaner with about three-quarters as many days of monsoon, the normal July rainfall is about three-quarters as great, or 87 mm. It is widely recognized that the date of onset of the monsoon is important to the total amount of monsoon rain (Das, 1968). A
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v e r y e a r l y , v i g o r o u s o n s e t is o f t e n associated with a " b r e a k " or rainless spell several w e e k s later, h o w e v e r (Trewartha, 1981). Thus, at J o d h p u r the rank correlation b e t w e e n J u n e rainfall a m o u n t s and July amounts is - 0 . 7 b a s e d on a 60 yr record. During the same period the corresponding correlation at Bikaner was - 0 . 3 . Despite their proximity the two stations show different r e s p o n s e s e v e n to regional variations in the m o n s o o n onset. I n d e e d the v a r i a n c e spectra of the m o d e r n rainfall time series at the two locations are quite different, as the reader m a y verify f r o m standard sources. Having established that the L u n k a r a n s a r and Didwana reconstructions should look different b e c a u s e of differing r e s p o n s e to climatic p r o c e s s e s , we are still faced with the question of w h e t h e r the variations of each reconstruction can be u n d e r s t o o d as n o n r a n d o m r e s p o n s e to k n o w n forcings. Modern data and climatological reasoning m a y be used to a n s w e r this question in part. Annual m o n s o o n rainfall for the instrumental period in Rajasthan was found to be r e l a t e d to the t e m p e r a t u r e a n d r a t e o f change of t e m p e r a t u r e in the Arctic, as represented b y Icelandic temperatures. Table 2 r e p r e s e n t s an e x t e n s i o n o f this i d e a to TABLE 2. SENSITIVITY OF RAJASTHAN MONSOON RAINFALL TO NORTHERN HEMISPHERE MEAN ANNUAL SURFACE TEMPERATURE AND ITS RATE OF CHANGE Station
July
August
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+8%
B. Percentage change of monthly rainfall per degree Celsius change of the 5 yr mean Northern Hemisphere surface temperatures in the 10 preceding yr Lunkaransar + 24% - 22% Didwana - 24% - 37%
specific stations and specific months. T h e entire 84 yr of r e c o r d e d rainfall for J o d h p u r and 88 yr for Bikaner was r e g r e s s e d m o n t h by month, against the previous 5 yr m e a n N o r t h e r n H e m i s p h e r e surface t e m p e r a t u r e , the two 5 yr periods previous to that, and s e v e n periodic t e r m s (0.898, 0.854, 0.835, 0.807, 0.422, 0.444, 0.488 cy/yr) after first transforming the rainfall data b y calculating the normalized cuberoot. This transformation was to r e m o v e m o s t of the s k e w n e s s and m a k e various records easily c o m p a r able. The regression coefficients m a y then be c o n v e r t e d into sensitivities, i.e., the percentage change of m o n t h l y rainfall, estim a t e d f r o m the regression equation, that would result from a I°C change in N o r t h e r n H e m i s p h e r e 5 yr m e a n t e m p e r a t u r e or a I°C change of that m e a n t e m p e r a t u r e per 5 yr. Since m o s t of the variation of H e m i s p h e r i c m e a n t e m p e r a t u r e is due to variation o f A r c t i c t e m p e r a t u r e , the r e g r e s s i o n s are c o m p a r a b l e to those discussed a b o v e for " R a j a s t h a n m e a n annual rainfall" [(Bikaner + Agra)/2] versus Icelandic m e a n annual temperature. F r o m T a b l e 2 we see t h a t w h e n the N o r t h e r n H e m i s p h e r e is w a r m ( m o r e s u m m e r - l i k e ) b o t h the L u n k a r a n s a r and D i d w a n a areas should have m o r e m o n s o o n rainfall, but the effect should be a b o u t twice as large in the latter area. Turning to Figure 3 we see that the r e c o n s t r u c t e d m a x i m u m m e a n rainfall at Didwana was during the Atlantic episode, which in turn is regarded as the w a r m e s t part of the H o l o c e n e hemispherically. L u n k a r a n s a r (Fig. 1) does not s h o w an A t l a n t i c e p i s o d e m a x i m u m o f s u m m e r rainfall, which m a y be due in part to the half-as-great sensitivity to N o r t h e r n H e m i s p h e r e t e m p e r a t u r e s h o w n in Table 2 for Bikaner. A n o t h e r factor is the relationship s h o w n in Part B of Table 2. Table 2B shows that w h e n the northern h e m i s p h e r e is w a r m i n g , the July rainfall goes up at L u n k a r a n s a r but d o w n at Didwana. The reverse is true w h e n the hemispheric t e m p e r a t u r e is f a l l i n g - - D i d w a n a rainfall increases; at L u n k a r a n s a r rainfall
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decreases. In August both locations have decreased rainfall following decades of rising temperature. We may infer, then, that the early Holocene rising temperatures simply shifted the rains from August to July at Lunkaransar, while the small sensitivity to absolute hemispheric temperature produced little net change, and post-Atlantic decreasing temperatures shifted the rain back to August. The net effect was little general trend. At Didwana the rising early Holocene temperatures would have suppressed both July and August precipitation for a smaller monsoon total. On the other hand, when the rate of change of hemispheric temperature leveled out during the Atlantic, the suppression due to rising temperatures would have ceased and the small enhancement due to higher absolute hemispheric temperature would have prevailed, giving a maximum of precipitation at Didwana. The lack of sensitivity, both to absolute hemispheric temperature and its rate of change, at Lunkaransar might explain the dissimilarity of the general millenia-long trends at the two sites. From the above discussion we see that qualitatively there is some reason to believe that the differences of the Lunkaransar and Didwana reconstructions are real, i.e., in accord with the climatic differences today. We must consider whether the proposed mechanism is quantitatively adequate. The Didwana reconstruction shows the Atlantic episode wetter than the Boreal and subBoreal by about 150 mm per year or about 18%. If the intercentury sensitivity of Didwana area rainfall to hemispheric mean temperature is approximated by the interpentadal sensitivity, the implication is that the hemispheric mean temperature during the Atlantic was about I°C higher than during the Boreal or sub-Boreal. This we believe is a reasonable number. We will not, however, make a linear extrapolation of the present day interpentadal sensitivity to the general trend over the past 10,000 yr. Table 2 also provides a rationale for the
apparently erratic behavior of the reconstructed monsoon rainfall amounts at the time scale of centuries. The general variability of the individual reconstructed values is about 35-40% of the mean value for the whole period, compared to the 45-50% present-day interannual coefficient of variability in Rajasthan. Thus larger fluctuations might be expected at Didwana where the mean precipitation is higher. It is known that the climate has not varied smoothly and uniformly during the Holocene. There have been a number of shorter term climatic events such as the stillstand or readvance of the continental glaciers of the northern hemisphere ca. 8300 yr B.P. If Table 2 is applicable, the rapid cooling at the onset of this glacial event, followed by rapid warming to the Atlantic episode, should have produced increased monsoons at Didwana prior to 8300 yr B.P., followed by reduced rainfall ca. 8000 yr B.P. Such is observed in Figure 3, but there is no strong excursion apparent for Lunkaransar (Fig. 1), and this is also in accord with the low sensitivity to hemispheric temperature rate of change indicated in Table 2B. Similar effects of other Holocene climatic events may be seen in the reconstructed monsoon climate, leading one to believe that the variability of the reconstructions is real, and not a statistical artifact. Only hypotheses may be offered for the general two-thirds decrease of rainfall at both sites during the Holocene. At least four reasonable factors may be considered: (1) The obliquity, or inclination of the earth's axis to the plane of the ecliptic, was about 24.4° at the beginning of the record (10,000 B.P.) and has decreased to about 23.3 ° . It is generally believed by climatologists that high obliquity is associated with strong seasonal contrasts, i.e., hot summers and cold winters. Hotter summers hemispherically m i g h t b e reasonably associated with a strong monsoon, and thus decreasing monsoon~ in Rajasthan during the Holocene. The Milankovich es¢
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BRYSON AND SWAIN
timates of about 5°C change in summer temperatures since 10,000 yr B.P. would imply a sensitivity (as used in Table 2) of about 0.13 per degree. This is a very reasonable figure (Lamb, 1972, pp. 31-33). (2) The precession of the equinoxes makes a small change in the length of the summer, from summer solstice to autumnal equinox. One may calculate about 5 days shortening from 10,000 yr B.P. to the present. This might shorten the length of the monsoon season by 5 or 6 days, but at the present per-day monsoon rainfall rate in Rajasthan (ca. 4.7 mm/day), this effect would probably be too small. There may be strong nonlinearity, however. (3) The secular change in the height of the Himalayas may have affected the intensity of the monsoon. (4) The decline in apparent monsoon rainfall in the past few millennia may be acc e n t u a t e d by the v e g e t a t i o n - s o i l def l a t i o n - atmospheric a e r o s o l - r a d i a t i o n rainfall and dew feedback mechanism proposed by Bryson and Baerreis (1967). This was the proposition that led to the research described in this paper. Which, if any, of these possibilities is realistic cannot be determined without further research.
summer rainfall was not higher than the period preceding the florescence of the Indus Culture. There are two features of the reconstructed monsoon history pertinent to und e r s t a n d i n g the agricultural base of Indus society. The winter rains appear to have been maximum during Indus time, decreasing the winter desiccation now characterizing the region, and increasing the overall precipitation efficiency. In addition, the rapid salinization of the lakes followed by their disappearance coincided with the disappearance of the culture. This strongly suggests that the collapse of the culture was causally linked to a collapse of their agricultural base associated with climatic change. The indicated drought of many centuries duration and remobilization of the dune fields at the end of Indus time shows that the monsoon may not be as stable a feature of the climate as we would like it to be with present population densities. Further refinement of this research will probably require a broader geographical distribution of present-day pollen rain samples in order to utilize the available pollen profiles located near the edges of the existing surface data array.
SUMMARY
ACKNOWLEDGMENTS
We have shown that a Holocene history of monsoon rainfall in Rajasthan can be objectively reconstructed from pollen profiles, and that the histories derived from two profiles are mostly consistent with independent data and with what we know of the dynamics of the monsoon. The largest problem emerging from the study seems to be the explanation of the irregular decrease of the rainfall from early Holocene time to the present. The cultural significance of this monsoon history is considerable, being applicable to the region occupied by the Indus Culture between 5000 and 3500 yr B.P., roughly. This time was one with at least double the present summer rainfall, if the reconstruction is correct. However, the
We wish to thank Dr. Gurdip Singh for the production of a high-grade pollen data base for this study and Dr. Waltraud Brinkmann for her contribution to the understanding of the present climatic teleconnections. This material is based partially upon work supported by the National Science Foundation under Grant No. ATM79-26039. The authors wish to thank Thompson Webb III for his constructive review of the manuscript.
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