Climates of Hunger: Mankind and the World's Changing Weather

3. Weather of lndian Times

CHAPTER 3 Weather of Indian Times WHENWE PROPOSED a drought for the Great Plains 800 years ago, we made, in effect, a prediction, though one for the past rather than for the future. But the prediction was not specific enough to be tested. It had to be refined: where, precisely, would slightly expanded westerlies cause a drought, and how serious would it be? The weather map for A.D. 1200 We used the same technique described in chapter I-that of past climate analogs. We found modern times of expanded westerlies and checked the resulting rainfall patterns. While such expanded westerlies have not dominated decades in modern time, they have governed the weather ofindividual months. We compared the weather data from those months to data from months with a more contracted pattern, and mapped our findings (figure 3.1). Such a map shows the details of a dry month in modern times, as a model of a dry decade or centlilfY in the past. As we 31 Decreases in precipitation D 0-25070 Em 25-50070 !iliilill More than 50070 o 500 mi ! ! ! i i i o 800km Figure 3.1. July precipitation decreases to be expected with a slightly expanded flow of westerlies, based on 20 years of modern weather records. Shaded areas have less precipitation when westerlies are expanded. Unshaded areas have various percentages of increased precipitation at the same time. Arrow indicates migration of Upper Republican Indians, discussed later in this chapter. Adapted from Bryson and Baerreis, 1968, p. 7. Weather ofIndian Times 33 pointed out in the case of Mycenae, such :an analogy can be drawn because the same atmospheric pattern produces both times of dryness ; the difference is in how long the pattern persists. We focused on conditions in July, which is a crucial month for plains farmers; dry Julys damage agriculture there, just as dry Januaries are crucial in Greece. Our map's most interesting feature was an area already known to students of the plains' ecology and climate, the prairie peninsula. This finger of dry climate and corresponding vegetation extends eastward from the Dakotas through Iowa and into Illinois (Borchert, 1950). This is the dry shadow of the Rockies, and it lengthens and becomes drier as west winds blow more strongly. Our map showed that in times of increalsed westerlies, the prairie peninsula received little precipitation. Rainfall there was 25 percent or more below normal. We had, then, a prediction of drought in a specific area. To test the hypothesis, Baerreis and Bryson determined to excavate villages within this peninsula for evidence of climatic change. As an anthropologist, David Baerreis brought to this work a specialized knowledge that was essential. Our cooperation points up an important trait of research on past climates: it must be interdiSCiplinary . The climatologist cannot afford to overlook any possible clues or techniques simply because they are outside his topics of work in the past. An ecotone in Iowa Usually, archaeology does not employ the scientific method of hypothesis, prediction, and testing. Archaeollogists excavate promising sites, then analyze and interpret what they find in light of other evidence . The full scientific method was a fa~rly unusual approach to such a case. We knew several sites of ghost villages within the prairie peninsula , including those of the Mill Creek culture in northwestern Iowa, first excavated in the 1930s, but not thoroughly explored. We knew that the villages had been occupied about A.D. 1200 by a hunting and farming society; we could hope for a variety of climatic signals among the thick piles of debris left behind (see figure 3.2). That area today averages about 25 inches of precipitation a year. Generally it produces good corn and soybean yields, but the rainfall 34 1. Two Tales ofFamine Wooded w1(1860) o 10 I '" ' o Figure 3.2. Area occupied by the Mill Creek people about A.D. 900-1400. A, B, and C were the principal research sites discussed; unlabeled dots indicate other known sites. After Bryson and Baerreis, 1968, p. 291. is not always enough to compensate for hot summer winds, and agriculture is frequently moisture-limited. A drop of 25 percent or more in moisture will show up dramatically in cornfields; it would also show up in the hunting and agricultural economy of the Mill Creek people. Refuse of their diets would reflect a drought-as would the pollen scattered by plants growing 800...