陕北植被变迁在地质时期以及历史时期早期，主要由气候条件所控制，植被类型随气候的冷暖干湿变化而变迁。

During the early geological and historic stage, it was the climate that mainly influenced the change of the vegetation in the north of Shaanxi.

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至少在过去的5亿年中的一部分时间内，地球上的物种的50%到90%以上已经在地质时期的一瞬间内消失了。

At least a handful of times in the last 500 million years, 50 to more than 90 percent of all species on Earth have disappeared in a geological blink of the eye.

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在整个地质时期，它们随着自然地气候循环减小或涨大。

Over geologic time, they have ebbed and flowed with natural climate cycles.

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