Their joint residence and close relationship were the catalysts, but not the cause, of these changes in their views. Köppen had been working on climate systems since the 1880s, and his interest in climate had begun much earlier, when he was a thirteen-year-old child growing up in Russia. His father, already seventy-six in 1860, found the raw climate of Petersburg increasingly hard to bear and had bought an estate in the Crimea in Karabagh. Köppen, in a memoir written in 1931 (when he was eighty-five), remarked that the transition from north to south in Russia was much more stark than in Western Europe, and that as a child he had marveled, on rail trips from St. Petersburg to the Crimea, at the constantly changing vegetation outside the windows of the train.1 Near St. Petersburg there was boreal coniferous forest, and in the Crimea a Mediterranean, arid, subtropical climate; in between were a variety of woodland and steppe climates.
Köppen was certainly a founder of the field of climatology. Julius Hann’s Handbook of Climatology (1883), Aleksandr Ivanovich Voiekov’s Climates of Our Globe and Particularly Russia (1884), and Alexander Georg Supan’s (1847–1920) climate province scheme all appeared at about the same time as Köppen’s initial (1884) article in Meteorologische Zeitschrift: “The Heat Zones of the Earth Considered with Reference to the Length of Hot, Temperate, and Cold Seasons and the Effect of Heat on the Organic World.”2
Köppen took as the basis of his system the temperatures in different regions during the warmest months of the year and prepared isothermal maps based on them. “Isothermal maps” are maps that show lines of equal temperature (isotherms). He showed that the 10°C (50°F) isotherm was the boundary of forest and tundra (the Baumgrenze or tree limit), and he classified places where the mean monthly temperature never went below 20°C (68°F) as “tropical.”
The system that Köppen chose began from his discovery that the northernmost latitude for the growth of trees is well defined by the temperature of the warmest month. Where the mean (diurnal) temperature is 10°C or less in the warmest month, trees cannot grow: this is the Baumgrenze. It actually exists, and it exists independently of knowing why it exists. It is a description of a climate phenomenon rather than an explanation of why it occurs. Having decided to proceed in this way, Köppen superimposed his isotherms on existing maps of vegetation and showed that definable suites of vegetation showed up when the mean monthly temperature remained above 10°C for one, four, and twelve months. He developed the idea extensively, mapping temperature distributions by drawing maps for the 10°C isotherm and for the 20°C isotherm, for periods of one, four, and twelve months. This allowed him to develop a latitude zone system of climate representing the whole globe.
In 1901 Köppen republished this in book form: Versuch einer Klassifikation der Klimate Vorzugsweise nach ihren Beziehungen zur Pflanzenwelt (An attempt at a classification of climate chiefly with respect to its relationship to the world of plants).3 In this extremely important and influential book, Köppen attempted to meld his work with standard botanical classifications. Some climatologists thought this a step backward from a modern and scientized (read “thermodynamic”) approach to climate, which would be much closer to dynamic meteorology, but it was a sound choice. The identifying mark of a climate is its vegetation, seen as an integral response to temperature, rainfall, orientation to the continental interior or the oceans, and soil type. Climate zones were, Köppen argued, differentiated by this full ensemble of factors: not just temperature, but the entire weather picture; not just the amount of precipitation, but its type and its seasonal distribution.
Because Köppen was a founding editor of Meteorologische Zeitschrift, he could publish revisions to his scheme almost on a monthly basis, and he interspersed these modifications (at decadal intervals) with bound book publications and summaries. In 1890 he published a cloud atlas, along with Neumayr and Hildebrandsson, and was able to insert his climate scheme into what quickly became a widely used reference book.
By 1918, all the other founders of climatology were either dead or nearly so. Voeikov had died in 1916, Supan would die in 1920, and Hann would die in 1921. Hann had published his last great work, his three-volume Handbuch der Klimatologie, in 1908–1911. Hann and Köppen were close colleagues and friends, and Köppen was strongly influenced by Hann’s inclusion, in his later work, of geology in the system of climate indicators: the use of coral reefs and limestone, salt and gypsum, peat, coal, and glacial tills, in addition to plants or plant fossils—the latter in mapping the climates of the ice age. In 1918, Köppen had published a new edition of his standard work on climate, Klimakunde I: Allgemeine Klimalehre (Climatology I: general theory), incorporating some of this material.4
In that same year, he had worked out a simplified five-climate-zone scheme: polar, boreal, temperate rain, desert, and tropical rain; he had included this as a brief addendum in the 1918 edition of his work. Collaborating daily with Wegener on the climates of the past, in order to understand the relationship of continental displacements and displacements of the pole, he had further simplified this five-zone scheme to four zones, consolidating the boreal and polar zones into a single zone.
Köppen had also, from the beginning, been deeply interested in the causes of climate change. The theory of continental glaciation, or of the “Great Ice Age,” had only come into general acceptance during his university years in the 1870s. By the later 1880s there was incontrovertible evidence of a much earlier Carboniferous/Permian Ice Age in the Southern Hemisphere, as well as suggestions that in that Achaean past (known then generically as the Algonkian) there had also been ice ages. The theory of ice ages went far to explain climate change at different latitudes, even if there was (yet) no explanation of ice ages.
It seemed to Wegener and Köppen that climate change, as well as the resulting long-term changes in vegetation, depended on ice ages, migration of the pole of rotation, continental displacements, and nothing else. These explained the facts of climate change as no other theory could, particularly when the distribution of identical animals and plants separated by great oceans was taken into account. If no one knew the cause of ice ages in 1920, there was, with Wegener’s hypothesis, a causal story connecting migrations of the poles and the movement of the continents: the latter caused the former, and then the former restarted the latter. The distribution of ice age remains was intimately connected with the dynamic of continental displacement and migration of the poles. This was indeed the Ariadne’s thread through the labyrinth.
Köppen and Wegener were excited and pleased with their hypothesis, but at the time they were writing there was no shortage of theories of climate change. Some of these were theories of ice ages alone, and others were more general. There were cosmic theories, of Earth’s climate being changed by passing through different gaseous regions of space, or passing through cosmic dust. There were theories that climate change was caused by variation in solar radiation. There were astronomical theories: that ice ages were caused by changes in Earth’s orbital parameters. There were theories of variations in Earth’s internal heat, variations in elevation, variation in land and sea distribution, variation in ocean currents, changes in the composition of the atmosphere, episodes of volcanic dust, changes in atmospheric circulation, and movements of the poles and displacement of the crust. Some of these theories went back to the beginning of the nineteenth century, but most of them had been proposed or updated since the 1890s, and the majority of them had been developed during or after the First World War.5
What separated Köppen and Wegener from this welter of theoretical propositions was that they were actually attempting to demonstrate something, support it geophysically, sequence it in time with fossil evidence, map it geographically, measure it with astronomical position finding, and in general integrate it as fully as possible with the existing literature of the earth sciences. Theories of variations in Earth’s orbital parameters were supported by calculation, but none of the other theories were anything but an idea. Changes in ocean currents or atmospheric composition left no
signature; there was no global, rhythmic record of volcanic eruptions to make the air dustier or less dusty, let alone a record of cosmic dust; there was no record of long-term variation in incoming solar radiation. While the bulk of these theories emphasized one or another factor, the consensus was that “they all played a role,” which is a way of saying either “I don’t know” or “We may never know.”
Wegener had surrendered the manuscript for the second edition of his book to Vieweg in April, not because it was done but because he could not afford the time to work on it any longer. He had lectures to prepare for his course in climatology at the University of Hamburg; these were given at the Geographical Institute on the west side of the Außen Alster, one of the two great artificial lakes in the center of Hamburg. His daytime job at the observatory was some distance away from this, adjacent to the harbor. The Geological Institute, which held all the publications in modern geology and geophysics which he required for his geophysical work, was on the east side of the Alster. He was constantly running from one place to another, and Else said that one day he came home so exhausted that he had remarked, to her considerable consternation, “Ten years in Hamburg, and I’m done for” (ich bin erledigt).6
Moreover, he had to turn his mind back to meteorology, not just to the management of the maritime meteorology part of the observatory but to theoretical issues. Bjerknes had announced that he would hold an international conference in Bergen, Norway, in early August. It was a move to standardize the collection and dissemination of weather data and the format of weather maps, something that Wegener was keenly interested in. It was also a plan for the establishment of a circumpolar weather service. Actually, Bjerknes had announced two conferences. One would be held in July for the Allies—Britain, France, and the United States—along with scientists from Iceland, Norway, and Sweden. He then had planned a second conference for the Central Powers—Austria, Finland, and Germany—to meet with Norwegian and Swedish meteorologists.7
This pressure from meteorology was real, though Wegener continued work on the maps he had wanted in the book but had not had time to finish. He had started two maps of the changing continental position between the Carboniferous and the Quaternary, as well as maps of the path of the poles in both the Northern and Southern Hemispheres since the Carboniferous. For the book he had had to settle instead for a map from Kreichgauer’s book and another borrowed from Dacqué.8
Köppen continued on with the work as well. He was preparing two articles for Petermanns, summarizing different aspects of their joint work. The choice of venue was important for him; it would be his first public declaration of support for Wegener’s hypothesis, and he wanted it to have the greatest possible visibility and reach the widest possible audience. Petermanns was, of course, also the place where Wegener’s hypothesis had first appeared, as well as the place where the widely read criticisms by Andrée (and, via him, by Diener) had appeared in 1917.
The first of these two articles was the easier one to write: “Polwanderung, Verschiebungen der Kontinente und Klimageschichte” (Migration of the pole, displacement of the continents, and the history of climate).9 Köppen and Wegener’s work on the history of climate now fell into an established tradition, which included Reibisch, Simroth, Kreichgauer, and Karl Löffelholz von Colberg (1840–1917), all of whom explained changes in climate by a shift in latitude zones following a displacement of Earth’s pole. Since Reibisch and Kreichgauer were both still alive and still active (Löffelholz had died in 1917, Simroth in 1920), Köppen thought it especially important to have a detailed discussion of the similarities and differences between the work of these predecessors and his own and Wegener’s work. There was neither room nor time in the second edition of the book for such an extensive summary, and Wegener had handled the matter in a few dismissive sentences. Köppen’s relaxed, ample, and discursive style made a much better job of it and put their work in context, while managing to show, without any sting, that what they had just done was much better than anything that had come before.
Köppen’s presentation of their joint theory for Petermanns provides an opportunity to reemphasize how much the fate of Wegener’s ideas depended, after 1920, on Köppen’s collaboration, coauthorship, reputation, and support, all rendered more powerful still via his persuasive, clear, readable prose. Köppen wrote fluidly, well, and often. In the period from 1918 to 1920 (for half of that time fully employed at the observatory) he wrote and published forty-two notes and articles in scientific and popular scientific periodicals.10 He was as famous as he was relentless; his name and his work were everywhere. He had been publishing scientific papers in refereed periodicals since 1868; he spoke German, Russian, French, and Esperanto (he was a great promoter of the last as an instrument of international scientific cooperation). In addition to these languages, he read English, Italian, and Spanish and kept abreast of scientific developments in all seven languages.
Köppen understood the importance of seeing novelty in context and in clarifying the issues at hand. In the very beginning of this article for Petermanns he laid out the “most obvious causes of climate change”: changes in solar radiation, changes in the proportion of land and water, and the shift of Earth’s pole. Solar radiation could of course change Earth’s climate, but because the whole Earth is illuminated by the Sun, the climate of the whole Earth must change everywhere in the same direction if the amount of solar radiation changes; this can’t explain regional variation. Similarly, the proportion in each hemisphere of land and water and the possible alternation of land and water must influence climate, but a look at the temperature in the Northern and Southern Hemispheres shows that the mean sea level temperatures measured in them vary by only a few degrees, in spite of the fact that the former has most of the land and the latter has most of the water; the influence is only perhaps a third or a quarter of that of latitude alone. Moreover, no theory of alternation of land and water could explain the fossil remains of deciduous trees above latitude 80° north.
This means, Köppen argued, that we must consider the displacement of the pole of rotation and a shift of latitude zones. The possibility of displacement of Earth’s pole, he continued, does not come from geophysics, but from the data of paleontology. Here he quoted Rudzki: “in case the paleontologists ever come to the conclusion that the distribution of climatic zones in one of the past geological epochs points to an axis of rotation totally different from the present axis, there will be nothing left for the geophysicists but to accept this contention.”11
While Köppen continued to work enthusiastically on that paper and a second paper on the causes and effects of continental drift and polar wandering, the latter had to stay on hold throughout the spring and early summer of 1920. The intuition of a “pole-fleeing force” had been Köppen’s, but the mathematics and physical working out were clearly something for Wegener to do, and he was simply overwhelmed; it was all he could do to finish the maps.
With the coming of spring, Wegener made some attempt to return to his family life, if not a more relaxed schedule. Else loved their sailboat and was anxious to get back on the water. She pointed out to Alfred that their newborn, Lotte, could sleep in a basket as they sailed, while the older girls remained at home with their grandparents. So, beginning in May, they spent every weekend they could on the water, often with Kurt. Kurt had sailed the boat almost every weekend all winter long; he loved being in a boat or in an airplane—his restless nature demanded more movement. Else later wrote that in the Hamburg years Kurt’s pessimism about the future, as well as his (from her vantage point) pointless longing for the prewar world, left both her and Alfred often depressed; the solution to this problem was to see him mostly on Saturday afternoons and Sundays when they might sail together, during which his pessimistic attitude evaporated.12
With the end of the “Easter term” in July, Alfred and Else were able to take a longer sailing trip, but Alfred’s mind was very much on the coming conference in Bergen. While he had been able to catch up with t
he literature, the events of 1918 and 1919—leaving the army, returning to Marburg, leaving Marburg for Hamburg, plunging into the second edition of his book—had severed him as thoroughly as the war years from active conversation with colleagues; the conference would be his best opportunity in a decade. Moreover, most of the senior scientists of the prewar period were now retired or dead, and Wegener would be in the presence of contemporaries and younger men, with the exception of Vilhelm Bjerknes, who, though twenty years Wegener’s senior, was always full of youthful energy and pushing strenuously forward to recruit the next generation to his scheme for streamlining and unifying weather forecasting.
As is the case with most scientific meetings, the real action in Bergen that August took place away from the lecture hall, and here there was much of interest for Wegener. One surprising development was Wegener’s meeting with Felix Exner. It was a long time now since Exner’s damaging review of Wegener’s thermodynamics text, and a few years since Wegener’s response in 1917 to Exner’s work on dynamic meteorology. They discovered, once they were face to face, that they admired one another’s intelligence, that they enjoyed one another’s conversation, and that they were probably going to become friends.13 Exner was still involved in an acrimonious dispute with Bjerknes, as he had his own scheme for weather maps and thought, as did most of the Austrians, that Bjerknes had undervalued their contribution to meteorology.14 Wegener had no role in this controversy and did not want one. Perhaps, like his contemporary Ficker (trained in Germany and working in Austria), he could see Exner’s pain at the collapse of a huge and beautiful imperial scientific establishment and the impoverished and restricted future that appeared to face Austrian science.15
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