See https://tallbloke.wordpress.com/2023/07/18/climate-change-fostered-the-rise-and-fall-of-the-tibetan-empire-from-600-to-800-ad-researchers-find/ … and the story can also be found at https://phys.org/news/2023-07-climate-fostered-fall-tibetan-empire.html … a study by scientists from the Institute of Tibetan Plateau Research, the Chinese Academy of Sciences. They obtained a high resolution climate record over the last 2000 years, using varves caught in sediments of a lake on the Tibetan Plateau. A warm and humid period kicked in during the 7th to 9th centuries AD and coincided with the rise of the Tibetan Empire, we are told. Its fall coincides with a return to cold weather and aridification. This dateline is consistent with the wider world – and the rise and fall of civilisations [or hiccups in their existence]. In the Americas for example, or the Middle East [the Abbasids], as well as in various parts of Europe. The Tibetan Empire period refers to a phase of unification on the plateau.
One may wonder what happened in the Roman Warm Period, or the Medieval Warm Period. Did uification occur at those times – or is something else at work behind the scenes. We know what was going on in China – the rise and fall of dynasties coinciding with ups and downs in the environment. Did new arrivals enter Tibet during the very cold episode preceding unification – in mid 6th century AD. Were they responsible for unification – or colonisation [when times were good]. Were the shackles lifted once the bad weather came back to bit. Lots of questions left unanswered.