The environmental changes produced on a global scale by uplift involve more complicated processes. Important simulations have been produced by researchers who used the National Center for Atmospheric Research's Community Climate Model with variables adjusted for three different orographic conditions. The three variations are plotted for no elevation, half elevation, and full mountain elevation in two separate areas: the extensive high ground that spreads from the Rockies to the Sierra Nevada and the Cascades in the western United States, and the Tibetan Plateau bordered to the south by the lofty Himalaya. The three variations of uplift correspond respectively to conditions that dominated 40-million-years ago, 8-million-years ago, and during modern times. The results suggest that the uplift of those two areas during the past 50-million-years modified the Northern Hemisphere's atmospheric circulation, in a domino-like series of events that could have promoted the glaciations of the current ice age. The uplifted plateaus would have rerouted dominant air circulation, establishing violent monsoonal weather systems. Those weather systems, in turn, would have accelerated chemical weathering from the mountains—weathering that draws carbon dioxide from the atmosphere. An atmosphere depleted in carbon dioxide would have a global cooling effect and would encourage ice buildup in the higher latitudes.

A differing hypothesis points out that increased weathering, induced by frost, could remove material from an already high plateau. Traditionally, geologists have considered coarse massive deposits as evidence of uplift. They have tended to assume that uplift exposes more rock to weathering and produces piles of boulders and cobbles downstream. The new hypothesis attributes the piles of rubble to climate change—unloading the upstream regions generates uplift because of isostatic compensation.

Another tectonic mechanism that may have contributed to the ice age in the Northern Hemisphere is the closing of the gap between North and South America by the uplift of the Isthmus of Panama. Changes in plankton preserved in deep-sea sediments date the uplift of the Isthmus to 3-million-years ago. Blockage of the westward-flowing equatorial currents from the Atlantic to the Pacific would deflect warm equatorial waters out of the Gulf of Mexico. The Gulf Stream delivers large masses of warm water to the northern Atlantic, where it evaporates and increases humidity. The humidity precipitates as snow at high latitudes. The hypothesis that a strong Gulf Stream engendered the ice age attributes the accumulation of continental ice sheets to that increase in precipitation (Figure 3.12).

The cause of the current ice age probably incorporated facets of all these changes and more. The implication is quite astounding. Small tectonic events plunged the entire Northern Hemisphere into a persistent ice age. This current ice age has persisted for the past 2.5-million-years. All human civilizations have occurred within one geologically brief interglacial interval, and there is no indication when this ice age will come to an end.

Throughout earth history, lateral plate movements have caused profound environmental changes. Paleolatitudinal data indicate that between about 450 million and 400-million-years ago, the great supercontinent of Gondwana encroached on the south pole and underwent major climatic changes that appear to have influenced all latitudes. The polar region of Gondwana, which now constitutes northern Africa, was the center of a vast glaciation. Today, in the Sahara Desert we find massive deposits 450-million-years old that bear striations produced by grinding along the base of glaciers and that contain boulders. Many species failed to adapt to the new thermal regimes or to migrate successfully to hospitable ones. This was the time of one of the most severe mass extinctions to have taken place in the marine realm during the past 600-million-years.

Anomalous distributions of fossils have been related to plate movement in quite a different way. The distribution of some fossil species simply does not make sense. During the 1970s such abnormal distributions provided the first clues in both eastern and western North America to the existence of exotic terranes—large blocks and fragments of lithosphere that have been sutured to the edge of the continent (Figure 3.13). The indication that these terranes had been rafted into place from far away came from the observation that their fossil marine faunas represented paleobiogeographic provinces quite different from those of neighboring continental regions. In this way a major mechanism of continental growth was demonstrated.

History of Life

Life on Earth develops in an ever-changing environment, and paleobiologists are reaching out to other disciplines for help in assessing the effects of environmental changes on ancient life. Prominent questions are those about the kinds of environmental change that influence the evolution and extinction of life and about the time scales on which the

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