Climatic variations in historic and prehistoric time.
By Otto PETTERSONPublished in 10913 in
UR Svenska Hydrografisk-Biologiska Kommisionens Skrifter
In the last centuries of the Middle Ages a series of political and economic catastrophes occurred all over the then-known world.
They synchronise with the occurrences of a startling and unusual kind in the kingdom of Nature. The coast of Iceland and Greenland became blocked by Polar ice. Frequent volcanic eruptions occurred in Ice-land and the surrounding seas. Violent storm-floods devastated the coast of the North Sea and Baltic. In certain Cold winters Öresund and the Baltic were frozen over and the lucrative Hanseatic herring fishery of the early middle ages which had been carried on in the Baltic and Öresund ceased altogether. All these events are recorded in ancient chronicles which also depict the social and economic state of the communities which were greatly influenced by the violent climatic variations and their consequences: famine and disease.
The ancient Sagas and the convent chronicles give no hint of any supposed connection between the catastrophes in nature and the human world. The Icelandic chronicles from the 14th and 15th century abound in the descriptions of catastrophes such as:
“shallaeri micit vm allt hwd . hafis vmhaerlis Island . landskialfte mikill vm allt land . elldz uppkoma j heklu fialli . elldeyar – myrkr mikit sva at fal sol – bolnasot mandaudi
” a. e. o.
Simultaneously there occurred violent floods and inundations of the European continent and winters of unexampled severity. Such was that of the year of 1322-1333 thus described in the history of Olaus Magnus.
“ait Albertus Crantsius diligentissimus omnium regionum scripter anno MCCCXXIII gelidissimo frigore constringebatur mare at pedestri itinere per glace de littere Lubicense in Daniam in Prussiam rnare tran-siretur, dispositis per loca opportuna in glacie hospícii,
We are told in the Chronica Guthilandorum that in that winter it was possible to drive over the ice between Sweden and Gothland.
The climatic variations recorded in mediaeval chronicles have given rise to much speculation lately, especially in Sweden. I here give the names of three well known Swedish papers on the subject. Ehren-heims speech on his resignation or the presidency of the Royal Academy of Science in 1824 “Om Climaternas Rörlighet"
; the chapter on climatic variations pag. 562-572 in “Lehrbuch der kosmischen Fysik”
by S. Arrhenius and N. Ekholm's paper "On the variations of climate”
, etc. The historic material at hand is much more extensive than generally supposed (Only part of it is published, for inst. the 10 Icelandic annual-series by G. Storm and Hennig's “Katalog bemerkenswerther Vittterungser scheinungen von den altesten Zeite bis rum Jahre 1800”
The unpublished Danish and Norwegian records are being prepared for publication, the former by Captain C. J. Speerschneider of the Danish Met. Inst., the latter by Dr. E. Bull of Christiania.
Till quite recently the opinion has prevailed among meteorologists and geographers that the old records are unreliable and exaggerated, and that no real variation in the climate has occurred in historic time. This opinion is concisely expressed in Nansen's book “Paa ski over Grönland”. The physical condition of Greenland remained all through the middle ages approximately the same as they are today. The same view characterizes Nansen's latest publication "Nord i Taskeheimen”
Of late, however, dissenting opinions have been advanced in various countries, in Sweden by Ekholm and Sernandel', in Germany by Brückner and in America by E. Hunntington.
In Ekholm, opinion the climate of Scandinavia has changed since medieval time and this change he attributes to a gradual transition from a continental to a more maritime climate. Such a change would manifest itself by variations in the mean temperature and precipitation at Stockholm, Lund, Copenhagen, Petersburg a o places when compared with earlier observations made in the 18th and 19th centuries and with the oldest observations we possess i. e. those made by Tycho Breahe, on the island of Hven in Öresund at the end of the sixteenth century. Such a comparison made by Ekholm will be referred to later.
E. Huntington approaches the question from an archeological point of view. In his extensive travels in Central Asia and the interior of North America he studied the former extension and gradual exsiccation of steppe-lakes as shown by ancient shorelines and remnants of cities abandoned in periods of drought. The climatic variations which caused floods, inundations and ice-blockades in Greenland, Iceland and Europe manifested themselves in the interior of the continents by devastating periods of drought, forcing the population to migrate.
Huntington holds the invasion of the Tartars into China and Europe in the 14th century as well as that of the Aztecs into Mexico to be caused by such variation in the climate.
We possess no actual observation of climatic variations in the countries on the western and northern shores of the Atlantic. Here we have only the ancient Sagas to go by, and what they have to say on the subject I will recapitulate later. It is necessary, however, that I should say some words now as to my own standpoint in this matter.
In my opinion the ancient records of variations in the climate indicate that a change in the oceanic circulation and in the conditions of the Atlantic has occurred. No geologic alteration that could influence the climate has occurred for the past six or seven centuries. The changes due to cultivation of land, clearing of forests, draining of marshes and regulation of river-beds are too insignificant to explain these phenomena. Their very nature, i.e floods, inundations, ice-blockades, suggest a dislocation in the oceanic circulation, the ultimate cause of which must be ascribed to cosmic agents.
The third part of this paper will contain the proofs in support of the theory here propounded as regards the countries west and north of the Atlantic. The fourth part will contain those pertaining to the European seas and coasts. In the first part I will endeavor to point out the cause of the climatic variations.
My own research and those made during the past thirteen years by the International Cooperation for the investigation of the sea, have enabled me to study the variations in the seas surrounding Sweden since 1891. The material collected by myself and by Dr. G. Ekman who in 1876 with A. W. Cronander and in 1877 with F. L. Ekman investigated the Baltic and Cattegat, and later by himself carried out the first fundamental investigation of the conditions of the Bohuslän coast in the first years of the present herring period, have enabled me to study the causes of these variations. They appear to be of a periodic nature and ultimately ruled by cosmic agents, i.e. variations in the tide-generating force exercised by the sun and moon upon the waters of the ocean. I have also ascertained that the tide-generating force exhibits secular variations and that an absolute maximum occurred 500 years ago accompanied by a series of secondary maxima and minima in the centuries immediately before and after.
The seas surrounding Sweden are particularly well adapted for the study of periodicities in the oceanic circulation and its dependence on the tide-generating force. The tidal movement, almost imperceptible at the surface reappears at a vastly enlarged scale as great submarine waves in the border layer between the surface and the deepwater. The Swedish expedition in August 1907 to the Belts and Öresund, first discovered the submarine semidiurnal waves which for the last 4 years have been registered daily at the marine station on Bornö.
As director of the work of Svenska Hydrografisk-Biologiska Kommisionen, I have been able to concentrate my attention on these problems and to obtain expert help in such questions of astronomy and mathematics, as are beyond my own reach. Reports by the experts J. W. Sandström.N. Zeilon, H. Petterson, G. Strömberg, are published separately in Sv. Hydr.-Biol. Kom:s Skrifter.
The material compiled from self registering instruments and from the analytical work of our assistant V. Söderberg is too large to be published in extenso. All observations, however are recorded in the archives of the station of Bornö and available to experts.
In publishing this paper I have to acknowledge the aid of numerous contributors. The historic dates I have obtained partly by studying original authors, partly from personal communication both verbal and by letter from historians, archeologists, and philologists, in this and other countries. It is impossible to enumerate all those from who I have obtained information. I shall, however, always give the name of my informant when making use of a statement and wish I were here to express my grateful acknowledgment of the information so courteously and readily given.
1. Cosmic Causes of Climatic Variations of Short Periods
As mentioned in introduction, the floods on the North Sea coast are among the vents on which the mediaeval chronicles put the greatest stress. In the last centuries these seem to have been of an unprecedented violence and much more frequent than now. The devastating force of the flood is governed by two agents. ie., high water level and force of wind. It is impossible to ascertain now whether the wind force was greater in those ages, but it is beyond doubt that the water level in some occasions and especially in winter was abnormally high and at other times much below the average or, in other words, that the range of the tide attained an absolute maximum in those centuries. This assertion is borne out by the following facts.
The tide generating force of the sun and moon, which governs the range of the tide, increases in our latitudes with their declination and proximity to the earth and is greatest when each of de heavenly bodies attains its maximum declination and proximity to the earth simultaneously.
This happened to the sun about 1328 (Bohlin's calculation) when the Perihelion of the earth and the winter solstice occurred on the same day. At the time of the winter solstice of 600 years ago the tide generating force of the sun must have had an absolute maximum.
The node and apside line of the moon's orbit coincide (on an average) on a certain day every third year. At present that day is approximately the 26th of September. In 1796 it was the 19th of October and at the end of the 14th century or beginning of the 15th century. It is that evident in all probability the moon (at new moon), at least every ninth year, attained a position nearer to the sun at winter-solstice than it has since attained or will attain for thousands of years to come. At full moon, both the sun and moon approached nearer to the Earth than usual thereby increasing their disturbing influence on its gravitation.
The position of the moon's orbit with the apside-axis pointing to the moon is indicated by the ellipse. Three years later the apside-axis has revolved 3 x 40º 645 = 121º 935 (40º 465 annually) in the one direction and the node axis 3 x 19º 355 = 58º 065 (19º 355 annually) in the other direction. Thus they again coincide forming a node-episode in the position a
III. Six years later the node-apside no longer points straight to the sun at perihelion. It now forms an angle of 5º7 with it is original position at perihelion nine years ago, 18 years later the angle will be 11º4, after 36 years 22.8 a.s.o.
Evidently a constellation like that represented by position a
1 offers the greatest chance that the moon revolving in its orbit will approach nearest to the Earth and exercise its greatest influence simultaneously with the sun( at the Earth's perihelion). In this constellation the maximum of the tide-generating force will occur at full moon
. The disturbing influence on the sun's photosphere and corona, exercised by the moon and Earth together, will attain this maximum at new moon
. Both maxima are absolute and will not regain their full force till 2 thousand years have passed, though in the meantime secondary maxima will appear. This situation may be described as perihelion-node-apside.
According to a rough calculation I made, the last occurrence of the perihelion-node-apside ought to have taken place in 1369. The interval does not quite amount to 2000 years. In order to get the time and period more accurately determined I requested Mr. Strömberg, assistant to the observatory of Stockholm, to revise my calculations using the latest value of the lunar constants. I also requested Dr. Hans Pettersson to calculate quantitatively the variation of the disturbance in the gravitation.
The calculation of Mr. Strömberg gave the year 1433 a.C. as the year of perihelion-node-apside.
The report on the variation by Mr. Petterssson is published in the “Publications de Circumstances” of the International Council for the Study of the Sea and is called “Long Periodic Variations of the Tide-generating Force”
. According to the research by Strömberg and Petterson the absolute maximum of the tide-generating force must occur about 3500 b.C., 1900 b.C., 250 b.C. 1433 a.C 3.300 a.C o.s.o., or with an interval of about 18 centuries (the intervals are not quite equal).
The secondary maxima which occur in the intervals between absolute maxima are now more or less developed in proportion to their distance from an absolute maximum. Their appearance is regulated by the occurrence of the constellations node-perihelion and apside-perihelion. These coincidences occur at intervals varying between 84 and 93 years (= secondary maxima). The coincidence will be less perfect every ninth year (= tertiary maxima). Finally, the proximity of the apside line Earth-sun at perihelion will cause the tide generating force to attain still weaker maxima in the interval between two tertiary maxima, vis. between the 4th
year before or after a tertiary maximum.
The amplitude of these different periods are approximately calculated:
For secondary (84-93 years period) 2.0%
For tertiary (9 years period) 2.1%
When taking as unit the value of the force at the absolute maxima.
In calculating these values, the eccentricity of the lunar orbit is assumed to be constant. The eccentricity, however, is affected by the sun and is greatest when the apside-line coincides with the direction towards the sun and greatest of all when the apside-line at the Earth's perihelion coincides with the direction of the major axis of the Earth's orbit. The absolute maxima therefore are noted by a high eccentricity of the lunar orbit and are strengthened by the growth of eccentricity in perigee the moon approaches the earth and in apogee it withdraws from the earth and approaches the sun. It is impossible on account of the complicated nature of the lunar disturbances to get any approximate estimate of the influence of the eccentricity on the value of the maximum of the tide generating force 500-600 years ago.
In contrast to the maximum-years mentioned 3500, 1900, 250 b.C. and 1.433, 3.300 a.C. We get the years 2.800 and 1.200 b.C. and 550 and 2.400 a.C. when the constellation perihelion-node-apside is changed into its opposite aphelion-node-apside.
If we represent the variation in the tide generating force graphically by a continuous curve it would appear like the succession of big oceanic waves with intervals of some thousand years, their crests rippled by numerous lesser wave-systems each with their proper crests and hollows.
The amplitude of all wave systems increase as they approach years of absolute maximum and decrease near the minimum years 1250 b.C. a.s.o. The difference in springtide and neap-tide, as well as of the sea level in general at ebb and flood, in winter and in summer must also be greatest at the time of maxima and smallest at the time of minima. This variation in the tide-water at the surface need not necessarily be great to work destruction on low lying coast-line guarded by sand dunes. Formed by the action of the waves on the beach in centuries of weak ebb and flood these sand dunes may in succeeding centuries of a growing tide become gradually undermined till at least easily broken through by the slight increase in the destructive force.
If we find that the absolute maxima are followed by catastrophes such as floods, outbursts of polar ice, and climatically by sharp contrasts of temperature, excessive drought and excessive precipitation, we should expect the minimum-years to be noted for the reverse conditions. In such an investigation the sea necessarily comes first, for it is far more susceptible to the influence of the tide-generating force than it is the atmosphere or the solid earth's crust. What we should expect from the tide-generating force at the time of absolute maxima is this:
- In the surface: Greater tidal phenomena with bigger floods-waves and a greater difference between spring-tide and neap-tide capable of breaking through dikes and flooding lowlying coasts.
- In the border-layer (when a less heavy water layer superposes saltier and heavier water, as is always the case in arctic seas and in Skagerak, Cattegat and Baltic): strong pulsations in the undercurrent appearing as submarine waves, which enter into fjords and inlets like de s.c. “moon-waves” we have observed in the “Baltic” the Cattegat and Gullmarfjord.
This diagram shows how submarine waves, 25-30 m. in height enter the fjord in the border-layer between the “Baltic” water of the surface and the salt deepwater which rises and falls causing the surface layer to retract and expand in thickness.
The instrument which records these movements of the deep layer consists of a float of ice-copper of 500 l. capacity filled with water and so constructed as to float en the border later and partake its movements.
The undulations of the border layer are evidently of tidal nature and have hitherto been overlooked. This is not the time to settle whether they should be classed as “wave movements”
. In any case they are not local oscillations originating within the fjord, for their period is 13-14 days (or 7 days) and follows the lunar periods, while the longest independent oscillation of the fjord is 1 hour 49 min. for the surface seiche and 2-3 days for the unimodal seiche of the deepwater. In all probability theses submarine waves are formed by the impact of the oceanic tidal wave on the submarine ridges in the North Atlantic, f. inst. The Faroe-Shetland ridge of the North sea bank in lat. 60º1, rising out the depth of the Norwegian Sea. The phenomenon is akin to be reproduced Dr. Zeilon experiment.
The advancing wave is stopped and broken by the wall in the experiment (the suboceanic ridge) and pours over the tip of the wall in little cascades of water then to reveal along the bottom on the other side as a succession of submarine “solitary-waves”.According to Zeilon these submarine waves preserve their original period of undulation, though otherwise modified as to length, face and amplitude. If the original undulations contains several periods as in tidal phenomena some of these may be suppressed while others develop and become dominant. The total of energy in the wave motion will remain constant, of course, but it may be unequally divided among the induced wave-systems. This seems to be the case in the Cattegat where the diurnal tidal waves are blotted out by the long periodic waves (the “moon-waves”), the amplitude of which is so great that they measure up to 30 m. between crest and hollow in the Gullmarfjord and even more at the Skagen light ship.
When these giant waves enter the fjord they appear as an influx of the water at 33 m. depth while there is an outflow of the surface-water. The strength of both currents is registered in cm/sec. by two propeller-wheels at Bornö Station. The velocity of the under-current may in occasions (as seen in diagram fig. 7) amount to 8-17 cm/sec. When the submarine currents observed were those of the 16th
Nov. 1919.The submarine wave, which had entered in the 3 previous constellation which caused this was the total lunar eclipse the night between the 16th
Nov. or, in other words, the combination of full moon with perigee and node-apside of the lunar orbit, which constellation for the past 4 ½ years observations in the Gullmarfjord has been found to bring strong ebb in the “moon-waves.”
Zeilon, on the seiches of the Gulmarfjord Sv Hydrogr. Biol. Komm:s Skrifter.
The diagram shows the upheaval of the undermost water layers represented by the isohalines of 33, 32, and 30% (which occurred 3 times in November 1910) and the sudden subsidence of the water layers the 16-17 Nov. (eclipse day) and the 29 Nov. -1 Dec. (new moon).
This instance alone shows that the submarine waves are not incidental movements of the water caused by meteorological agents such as changes in the atmospheric pressure, winds, o.s.a.
The real proof of their tidal nature, however, is that they reappear on the same date in a succession of years. Thus the moon-waves of 1909 reappear and can be identified in the wave series of 1910, and 1911. Not so, however, that the wave of, say 7th Feb. 1909, will return on the 7th Feb. of 1910 and 1911. The next year's wave appears 10-11 days earlier than the wave of the previous year. In the diagram representing the moon-waves of the Gullmarfjord in 1909, 1910, and 1911 the dates are so arranged that the wave of Febr. 7 1901 is placed above the wave of 29 Jan. 1910 observed in the Gullmar-fjord during the winter-months 1909, 1910, and 1911. The fulldrawn line represents the declination of the moon. The dotted line represents the daily variation of the tide-generating force of the sun and the moon at the Lat. of Station Bornö.
a, b, c, d, etc. denote the corresponding boundary waves in the years 1909, 1910, & 1911.50.000 H L; 35.000 H L etc, the weekly catch of herrings in the Cattegat.The isohalines of 34%, 33%, 32% and 30% are represented by full drawn lines.
Wave of Febr. 7 1901 is placed above the wave of 29 Jan. 1910.
In the second diagram we see this wave placed above that of 19 Jan 1911. Arranged in this manner the analogy is quite obvious. The difference in the dates at first puzzled me until I found the simple solution.
The lunar year is 10 days shorter than the solar. It comprises 12 synodic lunar revolutions, each of 29,531 days, and 13 tropic periods of declinations of 27,399 each. Thus the same lunar phase will recur after nearly 355 days and the moon will attain almost the same declination north or south.
12 x 29,531 (synodic revolutions) = 354,57 days
13 x 27,399 (periods of declination) = 355,18 days
- That he phase and declination of the moon influences the movements in the oceanic border-layer.
- That if the moon's synodic period of revolution and period of declination at a certain season unite in raising a submarine wave somewhere in the sea, it is not to be expected that the new waves will appear regularly every 14th (or 7th) day except for a limited space of time on either side of the epoch. One or two before or after the epoch the two lunar periods will counteract each other.
The reason why the harmonic analysis, as usual in these cases, gave an obscure result will be explained later on. The moon-waves are undulations in the border-layer of the sea and subject to all circumstances that affect boundary-waves. The phenomenon would disappear if the seawater became homogenous. On encountering submarine ridges the waves change into cascades or breakers. If the ridge rises to or above the level of the border-layer as in the middle and south part of the Cattegat, then the oceanic tidal waves is broken by innumerable phase-ruptures just as the flood-wave of the surface breaks when encountering an obstacle. In other words: the character of the medium precludes the use of the analytical method.
I found the insufficiency of the analytic method when trying to reconstruct the sunspot curve for 150 years by the aid of the harmonic analysis. I found that the sunspot curve could not be reconstructed on the basis of any given period for more than 34 years.
Then a new periodicity sets in. In reading up the subject I found that Schuster before me had had the same experience with the same problem. He found that no single period dominated the sunspot frequency permanently: one period would prevail for a time and then be supplanted by another. He discovered a number of such periods, 4.7 years, 8.34, 13.5 and finally the most important of them all, 11.25.
He says: "The existence of a number of definite periods cannot be doubted whatever we may think of their numerical relationship. The recurrence of the maximum activity of each period seems to take place with an accuracy which may be equal to that of orbital revolution, but the characteristic property of these periods is the greater variability of the activity."
In other words, the periods succeed one another.The one will attain its full activity when the influence of the other abates or ceases (become latent). The recurrence of the sunspots gives an instance of periodic phenomena with short terms of existence.The recurrence in the Saros period of occultations (the eclipse-cycle) is the best defined and most lasting of the phenomena which depend on the lunar periods. The coincidence of the lunar periods in Saros is as follows:
The tropic lunar period does not fit so well into the Saros as the others. Because the otherwise high conformity the Saros period of the eclipse-phenomenon, however, obtains a long term of existence. The lunar eclipse repeats itself 48-49 times in regular succession with a very small displacement in the time and place 865 1/2 years once in 223 months or 19 eclipse years (of 346.62 days); a solar eclipse repeats itself 68 - 75 times till the end of its term of existence in 1260 years.
Probably the "longevity
" of the eclipse-series, known of old in China and Caldea, caused the belief that all phenomena originating in celestial causes possess unlimited periodicity and that every such phenomenon in Nature could by the aid of the harmonic analysis be theoretically reconstructed with a higher accuracy the longer the chain of observations.
By demanding too much of permanency many phenomena investigated by meteorologists and oceanographers have been classed as irregular and incidental, though in reality they are bound by laws and periodical though their periodicity has but a brief existence. Often the apparent periodicity of a phenomenon gets obscure and as if blotted out by other periods somewhat in the manner as a sea wave when watched for a time appears to dissolve and merge into other waves ystems. Yet no one doubts that every particle of water will perform its orbital revolution with perfect though very complex periodicity.
An analysis of the variation of the tide-generating force shows its period to be very complex. Not less complex will its effect be on the sea and the atmosphere be. The ordinary way of reconstructing complex periods by adding a number of sinus-functions into a Fourier's series with constant coefficients, requires that all periodically acting agents shall act continuously and uniformly through the ages. This is not the case with the constellations of the sun, moon and earth in their action on the photosphere and the corona of the sun and on the hydrosphere and atmosphere of the earth.
Phenomena which cannot by harmonic analysis be recognized as periodic are otherwise disposed of. They are treated as incidental and eliminated from the calculation by the ordinary meteorological way of elimination by averages using the method of least squares, which method presupposes the phenomena to be independent of one another. In this way the relation between many hydrographical and meteorological phenomena is blotted out and the Chance is accorded far too important a part. The individuality of a phenomenon and its relation to similar phenomena are both underrated by allowing it to be represented by an average which is the exact expressive of a situation unknown to Nature.
Both the harmonic analysis and the calculation by averages are used quite indiscriminately in the Hydrography. I do not mean that they should be left out in any case where they are of real advantage but I wish to point out that they are not infallible. The harmonic analysis was incapable of showing the relationship between the undulations in the border-layer of the sea and the lunar periods when analyzing the 41/2
years series of daily observations on the basis of the same lunar period. Yet the relationship is quite evident in the diagram which depicts the real situation from the daily observations put together.
In the Swedish oceanographic work, of which G. Ekman and I are the leaders, we have sought to collect such "snapshots" of the situation in the sea are different seasons in a succession of years in order to study the periodicity of the oceanic circulation and its causes. This too was the object of the joint investigation of the Swedish and Danish Commissions in 1891 - 1899 carried out by seasonal cruises 4 times annually which have a general view of the North Sea, the Cattegat and the Skagerak. In 1893 and 1893 other countries partook in this investigation and at the Stockholm Conference 1988 these seasonal cruises were put as chief article on the hydrographical program of research.
A general view of the Atlantic surface-water, its salinity, temperature and plankton-life was obtained in 1987.1989 by monthly observations 
The synoptic method is valuable and necessary because through it we trace and locate the relationship of different groups of phenomena, such as f. inst. the relationship of plankton- and fish life to the salinity, temperature and movement of the sea. This fact is accentuated by the recommendation for the future reconnoitring of the Atlantic of the three Geographical Congresses: of London, Berlin and Rome, and also by being taken up by the Austrian-Italian investigation of the of the Adriatic whose program is based on the pattern of the Stockholm Conference. It is all the more deplorable that the seasonal cruises of the international Investigation have been discontinued and that we have fallen back on a pedantic computation of averages from the observation-series far too defective to serve this purpose. Fortunately the fish-biologic investigation which is carried on in a less abstract manner has given so brilliant results that they make up for the want of general interest in certain other fields or research.
From the beginning the oceanic periodicity and its causes has been the problem of greatest interest to Swedish investigators. As has been shown the cause of our present periodic phenomenon, the moon-waves, is cosmic and must be ascribed to the changes in the lunar constellations. Recent observations made by the Danish commission with the "Tor" and by Michael Sars Expedition to Strait of Gibraltar in 1910, show that the waves similar to those we found in the Belt and Skagerak, though far greater, appear on the sub-oceanic ridges: the Wyville Thomson Ridge, the Faroe-Shetland, a. o.
They may be regarded as breakers of the oceanic tidal wave impinging on submarine ridges. Their amplitude is very great and they make the interchange of water between the Atlantic basins pulsate in accord with the periods of the tidal wave. The submarine breakers which enter into the Cattegat are forced to dissolve into irregular eddies when the deep-channel shoals out, as the Anholt, or changes into narrow grooves as in lat. 57º10'. Here the border-layer in which they travel thins out when meeting the solid sea bottom. When General Carp denies the existence of the moon-waves, because they were not found in South Cattegat (and the Baltic!) he expects too much from the transmitting power of the tidal phenomenon.
The localities where they can be observed are probably restricted to Skagen, the Gullmarfjord and possibly the Christiania-fjord though as yet we know nothing of the latter. They are the only to be found where the water is deep and stratified. In front of the great submarine ridges they dissolve into breakers in the manner of surface-waves. The herring shoals travelling with these cascades generally stop at the point in the Cattegat where the wave-motion breaks up.
Sometimes, if the undercurrent is very intense, the herring may pass across the south plateau of the Cattegat and will then collect in the sounds where the different current-branches unite. Later it will be shown that this must have happened in the centuries 1,100-1,600 a. C. when the great Hanseatic herring fishery flourished and the maximum of the tide generating force occurred. Of this lucrative fishery only faint traces now remain in the years of good fishery on the coast of Scania and in the Belt.
2 A similar instance is mentioned in my paper: The connection between hydrographical and meteorological phenomena. Quart. Journ, R. Meteorol. Soc. July 1912
3 On the other side it is obvious that the harmonic analysis must comprise a number of periods - the more the better. For a short lived or cyclic periodic phenomenon - as the moonwaves, the sunspots a. O. - it consequently must give vague and indistinct results if indiscriminately employed. It seems possible that a number of meteorological phenomena (as f. inst. the variations of high and low pressure, of dry and wet periods, etc) are in reality periodic and of cosmic origin although their real nature is obscured because the medium (the ocean & the atmosphere) is subject to alterations (of stratification and otherwise, etc.)
II. Outburst from polar ice from Arctic and Antarctic Seas
Submarine waves also enter the polar basin. They were first traced by Nansen, who at a depth of some hundred meters found irregular variations in the temperature of the Gulf stream branch which enters the Polar Sea. In periods of maxima this influx will be stronger and able to break up the ice, the effects of which will be seen in an increase of the ice transported by the polar current. In periods of minima the ice, no longer subject to the melting influence of the undercurrent will increase in thickness by the cooling from the atmosphere which very slowly penetrates the ice cover.
It is a well known fact that "outbursts" of drift-ice from the polar seas occur periodically and it is significant that the last great outburst from the Antarctic which culminated in 1894-1895 occurred under the constellation I have described as "perihelion-apside" (fig. 10).
The following charts illustrate this great outburst of Antarctic icebergs. In 1888 no icebergs were reported by the Australian liners. In the following years icebergs were sighted more and more frequently and in the years 1892-1897 they became a serious danger to navigation.
Figure 11. Icebergs in the Antarctic ocean.
The charts are compiled from H. C. Russels paper Proc. R. Soc. Nw S. Wales 1896. The signs /\ /\ denote groups of iceberg sighted.
1. O. T. Cleve, G. Ekman and O. Petterson: Les variations annuelles de l'eau de l'ocean Atlantique. Göteborg 1901.
Nine years afterwards there occurred a remarkable outburst of drift from the Arctic sea which will be in fresh memory in the Scandinavian countries because it was followed by a general failure in fisheries of cod, herring, etc. along our coasts from Finnmarken and Lofoten to the Skagerak and Cattgat. The greater part of the Barents Sea was covered with pack ice up to May, the ice-border approaching to the Marman and the Finmark coasts nearer than ever before. Herds of arctic seals visited these coasts and some specimens on the arctic white fish extended their wanderings to the Christian fjord and even entered into de Baltic. The position of the moons orbits was shown in the following diagram.
Nine years afterwards, in 1912, the last great ice-year of the Labrador Current, the situation was figured in the following diagram. (fig.13.)
All these constellations are of the type which I have denoted as perihelion-apside which brings a secondary maximum of the tide-generating force and -if we may judge from the late experience –outbursts of ice from the polar seas. If this so it seems worthwhile to discuss the question: what happens at the epochs of maxima of the tide-generating force ruled by the constellation perihelion node-apside akin to that which occurred at the beginning of the 15th
century. (fig. 14)?
The earliest information we possess regarding the climate of Iceland is derived from the record of the monk Dicuil of Ireland in 825. He describes a visit some 30 years earlier by some Irish ecclesiastics to the Island of "Thyle" (Iceland). At that time, about a century before its colonisations by the Norsemen, Iceland, was visited and inhabited by the Irish. The Sagas call them "Papar" which indicates that they were monks or hermits and that before the time of the "Landnama" or Viking-age intercourse was kept up between these anchorites and the monasteries of their mother-country. Dicuil narrates the description of the island given by his fellow-monks, who had been there from February to August, and add: "... because of this I believe that those authors
(Plinius, Solinus a .o.) who have written that there is a frozen sea (mare concretum) about Thyle have erred in as much as those who sailed thither have been on that island in the natural season of severe cold ... But after a days journey to the north of the island they found a frozen sea (congelatum mare)"
Like Nansen most geographers of our time take it for granted that the climate of Iceland has not altered in historic time. In order to reconcile Dicuil's description with this view, Nansen makes the totally unwarranted supposition that Dicuil's "Thyle" was in reality part of the Norwegian coast about the lat:s of Iceland, perhaps the coast outside Romsdale. Nansen writes: "All the information preserved regarding "Thyle" fits in on the Norwegian coast, but on no other country."
For my part, I own, I cannot see why Dicuils description should pass for something else than what it claims to be, i e. an account of a visit by some Irish monks to their fellow-monks in Iceland, who, as is actually known, at the time lived on the island as hermits or as missionaries among the Celtic settlers there. As far as we know, no Irish hermits or anchorites settled in Norway in the seventh or eighth century. There was no inducement then for the monks to sail to Norway and, if we assume that they were storm driven thither, it remains to be explained how, in sailing a days journey from the coast of Romsdale, they could come upon a frozen sea, a problem that may offer difficulties to those who with Nansen hold that the climate has not varied in historic time.
According to most of the Sagas, the island was discovered in 874 by Ingolf (Are Frode, Isléndingbók 1120-1130):
"Ingolf built (his house) in Reykiavik. Upon that time Iceland was covered with woods from mountain to shore. Then there were Christian men which the Norsemen call Papar..."
According to another version the island first visited by Gardar Svavarson, a man born in Sweden after whom the island was called "Gardarsholm" (Tjodrik Munk:s Historia de antiquitate regum Norwegiensium, about 1180, and Sturla's Landnámabók about 1250). The text runs: "Gardar sailed round the land and proved it to be an island. He spent a winter at Husavik in Skialfande and built a house there. (in 864 according to Arngrim Jonsson).
In the spring when he was ready to sail a boat drifted from him... Gardar sailed to Norway and said much in praise of the land. After him the land was called Gardarsholm and there was wood then from mountain to shore."
There was snow on the mountains and because of that Iceland was also called "Snölandet"
The name Iceland was given to it by a third vikingm Floki Vilgerdarson. He sailed south of the island and landed at Vatsfjord on the northwestern shore. "The spring was rather cold. Then Floki went north on the mountain and saw a fjord which was full of sea-ice. Therefore they called the island Iceland."
This is the only statement I can find from the Landnama-time which speaks of the ice is the polar current having reached Iceland. It is nowhere mentioned that the drift ice hindered the norsemen in their journeys to and from the island. Nowadays the drift-ice is the cause of the bad years in Iceland when, as frequently happens, the ice of the polar current blocks the coast. Its absence in old times must have favoured the cultivation and farming in Iceland even if the climate did not differ much from what it is now in iceless years.
A. Jonsson 
(in 1593) shows that the early settlers in Iceland were successful in the tillage of the ground and that laws existed for the harvesting of the corn. Local names were often derived from agricultural terms, “hence from the fields there are proper name of certayne places… all which are manifest takens of the tillage of the ground amongst the first Islanders which also even unto this day, I heare, is practiced by some Inhabitants of South Island but with less increase, the ground and temper of the ayre degenerating from the first goodness thereof after so many ages….”
Arngrim Jonsson, the contemporary of Björn Jonsson, evidently did not share the modern view that the climate in Iceland had not changed since its first colonization in the 8th
century up to his own time, the end of the 16th
century. In the Icelandic annals and in Thoroddsen's paper “Den Grönlädska drifisen vid Island” (tidskr. Ymer) we can follow the gradual deterioration of the Icelandic climate.
Thoroddson writes: “The drift causes the bad years in Iceland 
. The north coast is most exposed to the ice and here is very seldom that we do not in some way or another and in some season or other, experience its unpleasant neighbourhood. In the older Icelandic annals the drift-ice is often mentioned, but only when it has proved especially disastrous to the country…
Although weather conditions are often mentioned in the older annals and Sagas I cannot find that the annual ice-drift to the shores of Iceland is spoken before de 13th century…”
In the 13th
century Iceland began to get blocked by the drift ice from Greenland. The blockade was much more severe then than now; although even now the north coast frequently blocked and sometimes, though not often, even the east-end south coasts. Owing to the influence of Irminger current the west coast in our time is nearly always free from ice. Thoroddsen enumerates the worst ice-years beginning with:
“1233: The drift-ice lay off the shore all summer.
1260:  “Drift-ice all round Iceland so that every fjord was packed with it.”
1279:  “Very severe cold in the winter and so much ice that it was possible to drive for miles out the coast. The ice continued far into the summer and from many fishing harbours it was impossible to get out to sea and fish."
1290-91: Ice north of the country all summer measuring 15 ells in thickness.
1306: The spring was called “the ice spring”
Drift-ice on the east coast down to Sjoa. The ice went away at Easter.
1347-48: Much ice. The sea was frozen all round the land so that one could ride from one head-land to the next.
1360: The drift-ice lay off the shore till 24th
1375: The drift-ice lay off the shore to the 17th
Ice blockades of this kind which were unknown in the Viking-age have since occurred several times in every century and have naturally put back the cultivation of the land. To show what and “ice-year” in Iceland means I will quote the description by Thoroddsen of the year 1695
“The ice encompassed the whole country except Ingolfsnaes, which is unexampled in history. There were such quantities of ice in most places that the open sea was invisible even from the loftiest mountain peaks and the merchant ships could not land. The ice drifted from the north-country to the east-coast and thence to the south-coast. As early as in April it had reached Thorlakshavn whence it continued to Hitaros. On the northwest side the ice drifted past Latraberg into Bredebugten. In the beginning of May it was possible to ride or drive outside every fjord in the north-country.”
It is interesting to compare the series if ice-years in Iceland with the description in Swedish records (f. inst. in Scriptores rerum Suecicarum
), of the sever winters in Scandinavia in the 12th
centuries, when the Baltic was frozen over several times between Sweden, Denmark and Germany. Later I shall collocate the ice years in Iceland and the alteration in the old direction for navigation between Iceland and Greenland as described by Ivar Bårdson, stewart to the bishop of Österbygden in Greenland 1341. His description is recorded by Björn Jonsson (1574-1656) in his “Grönlands Annaler”.
The ice conditions in Greenland are intimately connected with those in Iceland. The advance of the ice out of Nordbotn in the 12-13th
centuries of which Bårdson speaks proved fatal to the old Norse colonies in Greenland, because it cut off the communication with their mother-country. Thus, the Vestbygd settlement was destroyed about 1342 and the Österbvgd about 1418
. In discussing these catastrophes so often described I will begin by stating my own opinion as to the cause of it and later discuss the facts on which my theory is founded.
1. Purchas, his Pilgrims, published 1670.
2. From 800 to 1250 Iceland seems to have had a prosperous time without calamities caused by volcanic eruptions, earthquakes, frost & ice blockades. Between 1291 and 1348, however, a catastrophic period set in. At least 29 of these 58 years are noted for terrible catastrophes according to G. Storms “Icelandic Annals unto 1578” ("hallaeri micit om allt land, etc"). The volcanic eruption in 1300 and following years was preceded by a series of earthquakes and upheaval of volcanic islands ("elldeyar") on the Reykianaes submarine ridge.
3. Refers probably to the following year. In annal. Regii we read “ MCCLXI Hafiss umhvarfiss island”.
4. Also the year 1275 was an ice year: “Kringdi hafis naer vm allt Islands" (Gottschalks Ann).
5. The exact time cannot be determined with security.
6. Nordenskiölds intention (see p. 403 of Den andra Dicksonska expeditionen till Grönland 1883) was, according to his own words, to follow the sailing directions of Ivar Bårdsson: “Steer straight westward from Iceland: there is Gunnbjörnskär”! Thus Nordenskiöld found Gunnbjörnskär of the Sagas which had been lost 500 years ago.
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