Reid A. Bryson and Christine Padoch

On the Climates of History The development of objective, quantitative evidence of how climates or climata, and the associated biota, have changed significantly (even during postglacial and historical times) has expanded the possibility of the rational inclusion of the climatic factor in the study of history. Climatic variation has produced variation in both the quantitative and qualitative character of the economic base of cultures, nations, and societies. This new recognition is not a revival of environmental determinism; it implies neither that all environmental changes have a climatic cause, nor that all cultural changes have an environmental cause, and it does not rest on an assumption that the links between climatic and human history are simple or straightforward. This new appreciation of the role of climates and climatic change is rather an extension of well-known ecological principles.

One of these principles predicts that changes in community composition will result from shifts in relative competitive advantage when environmental factors change. In the human context, this principle suggests that the physical environment, and particularly the climate, gives a bias to the direction and success of the near-infinite series of decisions that make up the course of history. That is, with a shift in temperatures, or of amounts and timing of rainfall, the particular mix of resource use techniques characteristic of a population may well change, some occupations supplanting others as they become more profitable, less risky, and therefore more important. Clearly the size and rapidity of climatic change is crucial to the ability of societies to adapt or to cope.

Another of these principles is that of limiting factors. To use an example from limnology, nitrogen may be limiting in one lake, phosphorus in another. Lack of heat is limiting in the Arctic, lack of water in the desert. Climatic fluctuations may move temperature, precipitation, or other climatic thresholds and change the absolute limits of particular economic activities, altering previous patterns. New possibilities may also be opened or old patterns eliminated.

What then do we know about the climates of history?

DOCUMENTARY EVIDENCE There are scattered references to climatic change, and its impact on human activities, that go back to Greek and Roman times. Cyprian, about 250 A.D., commented on the diminution of winter rains and summer heat in Tunisia. Less clear is Avienus' description of the deserted and desert nature of the east coast of Spain in the fourth century A.D. It is not until the eighth or ninth century, however, that the documentary evidence becomes abundant enough to make possible a nearly continuous account for some regions.

As recently as 1962, reconstruction of past climates was largely qualitative, or at best semi-quantitative. The relative abundance of comments on the severity or mildness of winters, as gleaned from such sources as chronicles and diaries, gave a rough measure of the character of a decade. Similar relative measures of summer wetness were possible. More quantitative but less direct were the records of grain prices, numbers of prayers said for rain per year, duration and extent of sea ice, outbreaks of weather-related disease, famines, and the like. Interpretation and calibration were necessary, and one was never sure whether some of the variation from one year to the next might not be due to non-climatic economic or political factors.

At a conference on the climates of the eleventh and sixteenth centuries, the climatologists were surprised by, and enthusiastic about, the wealth of quantitative data on climate-related parameters that historians could provide for a reconstruction of climate, but none knew exactly how to interpret the data, even when gathered by the most skillful historians. The historians, on the other hand, wanted to know what the climates had been in order to interpret history.

A primary problem with the use of documentary evidence (other than documented instrumental data) in the reconstruction of past climates is the problem of compound parameters. Most of the documented data are derived from such items as grain prices, which depend on a variety of factors, both economic and climatic (and their interaction). The reconstruction of the variations of individual factors, such as summer temperature or rainfall, require as many different sets of data as there are interacting factors. This requirement is simply a restatement of the rule that the simultaneous solution of a set of linear equations requires as many equations as there are unknown quantities.

A second problem is that of critical lacunae. In a discussion of evidence of drought at the time of the disappearance of a particular culture, a palynologist once argued that a core from a nearby lake contained no evidence, in the pollen record, of drought at that time. When the original paper on the pollen analysis of that core was checked, it was found that the lake had totally dried up during that period and that there was thus no pollen evidence because there was no pollen preserved. It is likely that this problem may also be a function of historical records. The "times of troubles" have more fragmentary records than stable times.

Another problem arises with subjective records of climate such as "this has been the coldest winter in memory" — the data filter of human recollection. Such references are to the perceived normal and thus might record short-term, but probably not long-term changes. For example, Lamb's analysis of the chronicles from Russia shows a great variation in the number of severe winters prior to the time of Napoleon's invasion, but not after. Did the...