difference in growth in the different ages of the tree). Fig. 20. 

 The growth of course depends on the climate. Huntington writes: 

 »The Sequoia grew in a region whose climate resembles that 

 of New Mexico except that it is colder and has no rain-period in 

 summer. Long winters with much snow and rains that last till 

 the beginning of the hot and dry summer arc conditions which 

 promote a rapid growth. » 



1000 1100 1200 1300 MOO 1500 1600 1700 1800 1900 



H 





















































































t 









































\ 





























































































Fig. 20. Uncorrected growth of the Sequoia Gigantea in California,. 



Judging from the diagram such climatic conditions have pre- 

 vailed during the 13th and 14th centuries. 



To me it seems probable that the abundant precipitation 

 which favoured the rapid growth of the Galifornian pinetrees 

 was due to the influence of the tide-generating force on the cir- 

 culation in the Pacific. In any case it is interesting to find evidence 

 of exceptional climatic conditions at the end of the Middle-ages 

 on the Pacific coast as well as on the Atlantic. In the interior 

 of Asia these variations were still more fateful. The Asiatic steppe- 

 lakes alternately expanded and dried up causing destruction to 

 the districts and cities on their shores which compelled the popu- 

 lation to migrate. Huntington's investigation of ancient ruins 

 and shore-lines brings him to the following conclusion: the 

 lakes appear to have experienced a period of contraction in the early 

 part of the Christian era followed by expansion in the Middle-ages 

 and by renewed but less marked, contraction in modern time»> 



The expansion of the steppe-lakes in Asia, which indicates 

 a wet period occurred, according to Huntington, simultaneously 

 with the moist period in California or about 1400 a. G. It was pre- 

 ceded by a dry and warm period which turned the kingdoms of 

 central Asia into deserts and caused the invasion of Europe by 

 the steppe-peoples under Genghis Khan a. o. According to Hun- 

 tington several such periods of drought have occurred since the 

 beginning of the Christian aera both in America and in Central Asia. 

 Huntington's opinion has recently been confirmed by Bruckner who is 

 a great authority on this matter; Bruckner writes: »ebenso ist der 

 Mongolinvasion eine Klimave,rschlechterung in Asian vorausgegangen. 

 Fur diese liegt ein sicheres Symptom vor: das Kaspisehe Meer hatte, 

 nach Bauwerken an seiner Uferzone deren Alter sich bestimmen liess, 

 im XII Jahrhundert einen Tief stand wie niemals nachher und 

 wie lange Zeit nicht vorher. Ein solcher Tiefstand kann nur durch 

 grosse Trockenheit hervorgerufen worden sein. Diese Trockenheit 

 kann aber nicht lokal gewesen sein sondern muss, wie das ganze 



Gebiet der Wolga, so auch weite Gebiete Zentral-Asiens mitbetroffen 

 haben. In ihr mo elite ich die Veranlassung zum Einbruch der 

 Mongolenscharen nach Europa wie nach Indien und China sehen. 

 So spricht denn gar manches dafur dass auch die Volkerwander- 

 ungen der Vergangenheit durch grosse Klimaschwankungen ver- 

 ursacht worden sind. Huntington hat geradezu von einem Puls- 

 schlag Asiens gesprochen. Rhytmisch, wie der Puis schlagt, so wirft 

 Asien entsprechend den Schwankungen des Klimas von Zeit zu 

 Zeit Volkerwellen in die peripheren Gebiete der alten Welt. » 



Bruckner also points out how the effects of these wet or dry 

 periods differ according to the geographical position of the countries. 

 A period of draught which turns vast areas in Asia and North 

 America into desertland may bring fertility and opulence to the 

 countries on the Atlantic shores: 



»in Europa verhalten sich die feuchten dem Atlantischen 0- 

 zean naheliegenden Lander, so vor allem Norwegen, Danemark, Ir- 

 land und Grossbrittanien, aber auch Schweden und Mitteleuropa, 

 gerade umgekehrt wie das trockne Innere des Kontinents ». 



Bruckner's investigations concerning the 35 — 36 years variation 

 of the climate, the rise and fall in the amount of precipitation, 

 the prices of corn, and the emigration within the last century in- 

 dicate that the problem started by Ehrenheim in his paper 

 »0m climaternas rorlighet» has once more become actual. 



The very extent of these variations which encompass both 

 America and Central-Asia indicates that they are of cosmic origin. 

 I have suggested that one of their causes may be the variations 

 in the tide-generating force, which in the first place depends 

 on the changing of the moon periods. To be valid this expla- 

 nation must be shown to apply to the great periodic variations 

 encompassing 1800 years as well as to the smaller changes which 

 are accomplished within a month. It is possible that by using 

 the harmonic analysis in a less summary manner than hitherto 

 we may be able to distinguish between the effects of heat and 

 gravity upon the meteorological variation of short periods. Mr. 

 Stromberg who assisted me in the calculations of astronomic values 

 has brought forward a new hypothesis which is the subject of a 

 special paper and which shows the influence of the variation in 

 the tidegenerating force on the changes of the meantemperature 

 in the course of a month. It is a common belief that the moon 

 influences the weather. This belief is founded on the actual experi- 

 ence of mankind through countless ages. 



Before starting weather-forecasts of long range, however, it would 

 be well to see how far the experience of the past bears out 

 the theory here advanced as to the connection between climatic 

 changes and the variation in the tide-generating force. The great- 

 est of the periods I have indicated, the period for the occurrence 

 of the absolute maxima, attained its latest epoch in 1434 and has 

 a periodicity of about 1800 years. The maximum previous to that 

 of 1434 must have occurred about 366 b. C. In the interval there 

 must have been an absolute minimum of the period, somewhere 

 about 500—600 a. C. What happened then? 



IX 



Climatic Variations in Prehistoric Time. 



As already mentioned an absolute maximum of the tide-genera- 

 ting force occurred 3 or 4 centuries before the Christian aera. We 

 must now see if this maximum also had effects resembling those in 

 the 14th century a. C, e. g: the devastating stormfloods on the 

 North Sea and the Baltic coats, the inundations and cold periods 

 in the northern countries with their consequences, famine and mi- 

 gration of the population. We possess a few historic notes from which 

 we may infer something relating to the climate. The best proofs 

 however the study of archeology and quartenary geology will 

 furnish. The 3rd and 4th ce tury b. C. denote the earliest epoch 

 of the Iron age or rather the transition from the Bronze-age to the 

 Iron age. During the later Stone-age and the first stage of the Bronze- 

 age, which, according to Montelius, may be put at about 1600 b. 

 C, the Littorina epoch still lasted. The oceanic water had then 

 freer access to the Cattegat and the Baltic, partly because the 

 headland of Skagen was not yet formed, partly because the great 

 depression with which the Littorina-epoch commenced had made 

 Oresund deeper than it is now by some 5 or 6 meter. The salt water 

 then entered the Baltic in larger quantities and the North Sea fishes 

 found the same conditions of life in the middle of the Baltic as 

 nowadays in southern Cattegat (see Munthe's map p. 24). 



To the geologist of the quaternary period the tresholds of the 

 Baltic in the Belts and the Sund constitute a point of van- 

 tage in estimating the influence of post-glacial changes. The 

 numerous careful measurements of the fossil deposits on the coast 

 of Scania and Sjsellancl have given the following results. 



During the epoch of the Littorina sea Oresund was 5 or 6 meter de- 



eper than it is now. The salt occeanic water could then enter the 

 Baltic more easily than now, and this influenced the hydrographic 

 state and the animal life of the Baltic, which then differed greatly 

 from that of the present. From the former and present spread 

 of the mollusc fauna Munthe has mapped out the salinity limit 

 (the isohalines) of the surfacelayer of the Baltic at the Littorina 

 period. A comparison between Munthe's surface map and that of the 

 present time which I have compiled from F. L. Ekman's observa- 

 tions in 1877 shows the influence on Baltic hydrography caused 

 b}^ a livelier interchange with the waters of the ocean due to 

 the lowering of these thresholds of the Baltic. The state of the 

 Baltic in the neighbourhood of Gothland during the Littorinaperiod 

 corresponds to that of southern Cattegat at the present time. 



The salt water had, consequently, a far freer access to the interior 

 of the Baltic, and with the salt water came the sea-fish whose migrations 

 at that time ex-tended far into the Central and Northern Baltic. Munthe 

 has been able to reconstruct the hydrographical conditions existing 

 during the Littorina-period, in a surface-map of the salt-percentage exis- 

 ting at that time. If we compare Munthe's chart of the salt-proportion 

 in the Baltic during the Littorina period, i. e., for about 5,000 — 6,000 

 years ago, with a similar map for the present day we shall find that 

 the limit for 10 °/ 00 salt, which now lies at the Sund and Gedser, 

 during the Littorina period lay in the gulf of Bothnia, north of Aland 

 The annual herring migration from the North Sea, which now extends 

 to the neighbourhood of the Sund, extended during the Littorina 

 period through the whole of the Baltic and into the Gulf of Finland 

 and the Gulf of Bothnia. The same state of things must have exi- 



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