This seems an appropriate point at which to interrupt the story of ecology in order to present some aspects of the development of climatology in relation to the problems of biology. The daily weather record was kept traditionally in terms of rainfall, temperature, and wind movement. Meteorology attempted to explain in terms of science the laws governing weather behavior, with a view to immediate usefulness as in navigation and agriculture. The climatologist dealt with the series of daily weather records and he assumed that the mean or average of these daily variations determined the general prevailing condition called climate. The stress upon usefulness emphasized the idea of forecasting the weather changes, either for short periods or over long periods of time. This latter aspect stimulated attempts to discover some definite rule of periodicity in recurrence. Extremists yearned to reduce the whole subject to a fixed law and order that would govern man and civilization. In this pursuit of a goal of wishful thinking, theories outdistanced facts.
The weather data in the traditional form of averages, deviations from the mean, maxima and minima, etc., did not provide adequate material for the ecologist. In the study of physiological reactions to climate, humidity and evaporation were important and little data of that nature were available. Questions arose whether the methods of recording such data were adequate. Doubt arose whether the averages of so-called normal conditions were determining factors in the survival and competition of living organisms. It was experience with such data and methods that caused Livingston (1921) to call for new approaches and Shelford and Flint (1943) to point out that for practical chinch bug control studies weather data were still inadequate.
After the beginning made by Transeau, there were many attempts to formulate a single index which would express by a strictly quantitative procedure the several factors which enter into the mapping of climatic regions. In Germany, W. Koppen presented one, first in 1918, but more fully in 1923, and revised successively through 1936, that recognized rainfall, temperature, humidity, and used the coldest period of the year as a basis of division between hot and cold climates. R. J. Russell (1926, 1931) devised some modification and applied it to the western United States. W. Van Royen (1927) made different adjustment and applied it to the United States and Canada east of the Rockies. Thornthwaite (1931), devised a four-factor procedure: a precipitation-evaporation (P-E) index; a temperature-efficiency (T-E) index, and seasonal distribution of the P-E index, and summer concentration of the T-E index. In his map of the North American climates and of the world climates he applied only the first three. Trewartha (1937) made other modifications of Koppen, applying them to the mapping of world climates. There were valid objections to all of these procedures because the exact relations of the factors to plant growth were not fully understood. Purely quantitative methods broke down and resort was made to practical adjustments which necessarily were subjective rather than quantitative. One of Thornthwaite's seasonal distribution types, was rainfall deficiency for all seasons, which was applied to the great plains. A critic, S. B. Jones (1932), queried, deficient for what?; certainly not for native vegetation. However, too much emphasis should not be directed at the deficiencies of these systems because, as P. R. Crowe (1936) pointed out, they were new departures in climate analysis and as such opened a new era.
As a further step in avoiding subjective concepts such as the ideas of adequacy and deficiency, the present author suggests a new set of terms, quantitative in their implications, to designate climates; wet, high rainfall, mid rainfall, low rainfall, and dry. These words substitute for super-humid, humid, sub-humid, semi-arid, and arid, and at the same time, express the purpose of classification in simple common names (Malin, 1947).
Another kind of challenge to old procedures is found in the concept of climatic years and the significance of extremes and variability. At various times the idea had been advanced that the effect of extremes of climate might be more decisive in relation to vegetation than the normals of mean temperatures and rainfall. Shimek (1911) had given it some stress in his analysis of why the prairies were treeless. Jenny (1941) summarized the results of a fourteen-year record of rainfall and erosion in Missouri during which time there had been 420 rains of sufficient volume to cause run-off. In 28 of these two inches or more of water fell during 24 hours, and were responsible for over half of the soil erosion during the whole fourteen-year period. As a system for dealing with climate classification, however, R. J. Russell (1932, 1934) led in this new departure, pointing out that especially in erosion, although occupying a short time and infrequent in occurrence, extremes might have a greater significance than normal conditions. He then proposed the idea of climatic year, using the concept of desert year, steppe year, etc., in mapping climates, emphasizing the frequency of recurrence of each in contrast with the system of normal climates which smoothed out the exceptional years in the averages. In respect to the climatic year and vegetation, he said:
Mature plants are less likely to succumb to the vicissitudes of climate than are young ones. Though an occasional desert or tundra year may upset vegetational balances temporarily, recurrence at such a rate as once a century very likely has little bearing on the distribution of forest or grassland. Yet, if the recurrence is so frequent that individual plants are prevented time after time from reaching maturity to withstand such extremes, it becomes highly significant. The westward extent of the forest toward the Great Plains of the United States is more probably related to the reoccurrence of desert years than it is to the distribution of normal precipitation.
The second concept proposed by Russell (1934) was that of nuclear and transitional climates. The former was one that experienced no exceptional years denoting a different climate class. Between two nuclear climates was an area of transition within which occurred exceptional years of extremes. Applied to the eastern United States the mesothermal nuclear climate occupied the southern Atlantic and Gulf states and the microthermal climate occupied the Great Lakes states and those lying westward The line dividing the intervening transitional zone on a 50 per cent frequency of climatic years ran from New York City southwestward down the Ohio river and across the plains to central New Mexico. This system abandoned the traditional assumption of fixed climatic boundaries based upon statistical averages. It revealed the more important fact of variability in a wide transitional zone between major climatic regions and the general conformities of both natural and domestic vegetation with such climatic designations.
The idea of variability was applied in a different manner by Kendall (1935) employing a modified Koppen formula to the mapping of annual climate for each year, 1914-1931, and then making a composite map by superimposing all the boundary lines of the year-climates on one map. This procedure presented vividly the fact that climatic regions were not bounded by a fixed line, but by a maze of lines occupying a wide transitional zone. As applied to the North American Grassland and the Prairie Peninsula the maze of north-south lines intertwined up and down the Prairie Plains, and the east-west lines between the Great Lakes and the Ohio river and eastward across southern New England. The Prairie Peninsula lay in the area of right-angle intersection of these two sets of lines.
The Scotch climatologist, P. R. Crowe (1933) introduced another new concept which avoided the ideas of adequacy and critical, both of them subjective terms, and dealt with variability, frequency, and "indices of probability." After applying it first to European material, he applied it to the great plains of the United States: "The essential feature is the degree of change experienced between one period and another" - contrasts in distribution and frequency - "rainfall relativity."
Still another innovation was that of E. E. Lackey applied to rainfall in Nebraska in 1935 and to the great plains in 1937. He based his system on the median rather than the mean in relation to maxima and minima and on the frequency of variability in percentage with the median at 50 per cent. Five maps in the annual variability series were made, one for each frequency interval of 20, 40, 50, 60, 80 per cent, and lines were drawn from the Canadian boundary to southern Texas connecting specified rainfall amplitudes. Thus the line showed nineteen inches of rainfall 80 per cent of the time fell in central Kansas; 60 per cent near Dodge City; 50 per cent between Dodge City and Garden City; 40 per cent near Garden City, and 20 per cent in eastern Colorado.
A project somewhat similar to Kendall's was executed by Thornthwaite (1941) using his own system as applied to annual and crop-season climates for each of the years, 1900-1939, but he did not make the composite map. Instead he applied the idea of frequency of occurrence over a forty-year period (1900-1939) of six types of climate years, one map for each type; arid, semi-arid and drier, sub-humid and drier, humid, and super humid. Each map showed in colors the areas included, in six degrees of frequency. To all residents of the grasslands, variability of climate was axiomatic, but the climatologists had at long last given it both mathematical and cartographical demonstration.
Many climatologists concluded that the weather ran in cycles, but they did not agree on the length or even the definition. Brunt (1937) insisted upon the definition of cycle given in the Oxford English Dictionary: "A period in which a certain round of events or phenomena is completed, recurring in the same order in succeeding periods of the same length." Others thought of cycles as recurring events over periods of varying length. The important point is that the first definition would eliminate definitely practically all argument over weather cycles, while the second definition rendered the whole argument virtually meaningless because of the introduction of the unpredictable factors of approximation and coincidence. Furthermore the weather trends of one region did not necessarily coincide with some other region, even an adjoining one. A climate that was favorable to one form of plant or animal life was not necessarily so to others. The concept of universal optimum efficiency climate collapsed with the simple question, for what plant or animal?
The idea of cycles came to be most generally associated with sun-spot frequency of approximately eleven year (11.13 years) periods. Here the popular idea was one of absolute cycles of exactly eleven years, without realization that no such periodicity existed. The sun-spot frequency varied from seven to seventeen years, and the eleven-year figure was a mathematical average - a fiction - which would have about as much practical value to agricultural planning as the tradition of planting in the light or the dark of the moon. The whole question was subject to controversy but the fact remained that the burden of proof rested upon the advocates of periodicity and cause-and-effect relationship. Among other things it should be conclusive that when climatologists cannot predict from one year to the next even approximately the weather behavior, its relation to crops, or its relation to insect pests, there cannot be any scientific basis upon which to predicate a climatic theory of civilization. As applied to particular problems, MacLulich (1936) challenged the sunspot periodicity as applied to the fluctuation of numbers of lynx and varying hares in Canada, and Cross (1940) went further in demonstrating not only that there was no such periodicity in the red fox in Ontario, but that the numbers of foxes fluctuated on a basis of natural regions within the province.
A revision of the viewpoint expressed in some of the agricultural publications of the federal government is found in R. J. Russell's chapter "Climatic change through the ages" in the departmental yearbook for 1941, Climate and Man. In this, Russell rejected explicitly the cycle hypothesis substituting the term fluctuation:
Man had observed that climatic conditions fluctuate rather widely from time to time at a given place, and in seeking to understand such natural phenomena he has been tempted to explain such fluctuations on the basis of recurring cycles. As yet, however, no definite proof has been advanced to contradict the opinion that all such relatively short-term climatic changes are nothing more than matters of chance. The world pattern of climates today is the product of climatic variations, not the expressions of recurring mean, or normal, conditions. The extent of desert climate will not be the same next year as this. The humid margin of the desert is the product of an ever-changing distribution of extreme aridity. The time may come when such changes will be well enough understood to be of definite forecast and economic value, but it is likely that such information will be the fruit of long-continued and patient research.Interest in changes of geologic proportions will remain intellectual. ...
Many years must pass, however, before the careful presentations such as R. J. Russell's, will rectify the influence of the misrepresentations of the drouth of the 1930s. The most recent attempt at a theoretical explanation of the climate of North America is that of I. R. Tannehill, Drought, its causes and effects (1947). The thesis is that on a broad scale the Pacific ocean controls the weather of the continent. The Pacific high pressure area off the California coast in winter is associated with rainfall; while the shift northwestward of the enlarged Pacific high pressure area in summer is related to a dry California coast. Not actual temperatures but relative or contrasting temperatures over the oceans and the continents control air mass movements. This basic principle, applied consistently, imposes a substantial rethinking about the uses of weather data. The task is not easy. Only an over-simplified version can be indicated here. In winter, the northern North American continent is colder relatively than the oceans west and east. The eastward moving Pacific air mass passes rapidly over the mountains into the Canadian sink, while its continued movement eastward is retarded by the resistance of the Atlantic-continental temperature contrast. The outcome is a movement of the Canadian air mass southward, where it meets normally the Gulf and the Atlantic air masses moving northward and into the continental area, producing rain. In summer, the temperature contrasts are opposite, the Canadian air mass moves eastward, reinforced by hot air from the southwest, tending to deflect the moist Gulf air mass eastward and to leave the great plains dry. Unusual temperature contrasts react to create extreme drouth conditions.
What controls the northern Pacific high pressure area? Tannehill relates this problem to solar radiation. As more is known about sunspots than about other aspects of solar radiation, the influence of the sun is discussed primarily, but tentatively, in those terms. But Tannehill is not naive about sunspots, and points out that there is as much confusion about them as about drouths; rainfall variations do not follow exactly the sunspot cycles; local changes in temperature and rainfall appear quite irregular and do not coincide with sunspot numbers. The author repudiates, however, any form of single factor explanations. As the rainfall of each local area is controlled by its own peculiar conditions taken as a whole explanations of drouth must vary from place to place. He admits the inadequacy of the data and the weaknesses in the explanations, and calls for data collected on a different basis suitable to the purpose in hand. Tannehill's general hypothesis is the first that appears to be at all tenable, and pending further research and testing, it merits serious consideration, but expectations must be kept within the limits indicated.