What thickness a sand layer in the desert

@Ferra.ru
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Today it is considered that deserts make 33% of all land on a planet. But it taking into account the Antarctic and Arctic deserts, and so this indicator equals 14,2%. At the same time sandy deserts borrow 1/5 from the last number, that is about 4,26 million square kilometers. But whether you reflected, what thickness of sand in that district? .