Mongolia and Central Asia began turning into a desert 23 million years ago. The rise of lesser-known mountain ranges, such as the Tian Shan and the Altai, sealed off moisture from the west and north.

See the source article by following the link below:
Mongolia and Central Asia began turning into a desert 23 million years ago. The rise of lesser-known mountain ranges, such as the Tian Shan and the Altai, sealed off moisture from the west and north.