why have deserts developed in south western part of north america
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Answer:
During the Paleocene, some time after the dinosaurs’ extinction, most of North America (including Alaska) was covered in evergreen tropical rain forests. But after some time, due to a heavy decline in precipitations, these jungles were replaced by dry deciduous forests as well as arid grasslands. During the latest Miocene and early Pliocene, geological forces altered landscapes, causing a reversal to more tropical climates. Sea level rose so much that the Gulf of California expanded into what is today the area of Los Angeles. But the Pliocene’s warmth ended abruptly at the beginning of the Pleistocene (about 1.8 million years ago), when the Earth entered a new climatic era that surpassed the Miocene in continental conditions. As such, warm desert communities were restricted to well below 300 m in North America, and they did not reform until about 9000 years ago, as the last displaced woodland plants retreated towards higher altitudes. However, the modern assemblages must have recurred many times during the Pleistocene interglacials, only to retreat to warmer climates in the south once the Ice Age returned. Consequently, modern desert communities (like the Mojave, the Colorado…) in the southwest of North America have been present for only about 5% of the 2.4 million years of the Pleistocene, while Ice Age woodlands in the lowlands persisted for about 90% of this period!
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