At www.eurekalert.org/pub_releases/2015-04/wsu-ctc042815.php … this is a story on Tibet and it seems there was a major change at the end of the third millennium BC – at the period when the Akkadian Empire dramatically collapsed. We learn that climate change may have been responsible – and the cooling period between 2300 and 2000BC appears to be what they are finger pointing (but I may be wrong). Later, at some stage, farmers in Tibet began growing wheat and barley – and it is suggested this was as a result of the cool weather. Wheat and barley are to a certain extent, frost tolerant, but the trouble here is that these cereals were being grown from around 1700BC (why the gap between 2000 and 1700BC). The Tibetan culture is described as collapsing in late third millennium (as societies were equally in crisis elsewhere in the ancient world).
The same story was posted at http://wattsupwiththat.com/2015/04/29/claim-climate-change-may-be-respon… … which more or less seals the collapse as contemporary with that of Akkad and Old Kingdom Egypt.
On the subject of the Akkadians, EM Smith has been looking at them – go to http://chiefio.wordpress.com/2011/11/03/akkadians-and-chad/ … but it is a wider field than just the collapse.