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.
在过去我们习惯了每十年一次大的干旱的气候循环周期。
In the past we used to have regular 10-year climatic cycles which were always followed by a major drought.
在过去的250万年里欧洲的冰河时期气候循环加速了这个侵蚀的过程。
The climatic cycles of the glacial period in Europe over the past 2.5 million years have accelerated this erosion process.
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