The Physical Geography of North America

By Antony R. Orme | Go to book overview
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4
The Holocene Environment
Louis A. Scuderi

Holocene environmental conditions in North America are closely linked to the prevailing atmospheric circulation pattern. Climatically, North America can be divided into several distinct regions on the basis of each region's relationship to large-scale features of the general circulation such as the Arctic Front and the average polar jet location. Although the meteorology of these regions varies on an annual basis, they may be viewed as relatively stable when averaged over decadal and centennial timescales. However, over longer intervals, the position and extent of these regions varies in response to large-scale changes in the general circulation pattern.

Long-term variations in the location and extent of climatic regions in North America are, in turn, responses to variations in energy balance conditions (Berger, 1978; Kutzbach, 1987; Kutzbach and Webb, 1993; Felzer et al., 1998). For example, changes related to Milankovitch cycles alter solar insolation receipt, modify the flow of energy over Earth's surface, and in the North American sector initiate changes in ice-cap size with resultant variation in the position of the Arctic Front and jet stream. An understanding of these interactions can help explain many of the disparate findings that result from time-transgressive changes in circulation patterns over North America (Davis, 1984).


4.1 Environmental Determinants
of North American Climate

Solar radiation changes relative to the present, as derived from equations that describe Earth's orbital eccentricity, precession, and obliquity (Berger, 1978), indicate that summer insolation (June, July, and August) in the Northern Hemisphere has varied in a predictable way from the end of the Pleistocene to the present (fig. 4.1). Summer insolation was approximately 8% greater than present at 9000 B.P. (radiocarbon years before A.D. 1950), and overall was at least 4% greater than present between 12,000 and 6000 B.P. This enhanced summer energy receipt was due to the combination of a maximal tilt of Earth's axis (24.5°) and a change in the timing of perihelion to July at 9000 B.P. Winter insolation (December, January, and February) had the opposite sign at the same times (fig. 4.1). The results of these combined orbital factors and the contrast between summer and winter insolation produced an enhanced seasonality at 9000 B.P.

As summer insolation increased to its Northern Hemisphere maximum at 9000 B.P., the Laurentide Ice Sheet melted and retreated. This had a profound effect on atmospheric circulation over North America. Earlier, during the height of the Wisconsinan Glaciation at 18,000 B.P., the

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